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ATLAS DETECTOR ANDPHYSICS PERFORMANCE
1
10
10 2
102
103
mH (GeV)
Sig
nal s
igni
fican
ce
H → γ γ + WH, ttH (H → γ γ ) WH, ttH (H → bb) H → ZZ(*) → 4
l
H → ZZ → llνν H → WW → lνjj
H → WW(*) → lνlν
Total significance
5 σ
100 fb-1
(no K-factors)
Technical Design Report
Issue: 1Revision: 0Reference: ATLAS TDR 15, CERN/LHCC
99-15Created: 25 May 1999Last modified: 25 May 1999Prepared By:
ATLAS Collaboration
Volume II
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ATLAS detector and physics performance Volume IITechnical Design
Report 25 May 1999
All trademarks, copyright names and products referred to in this
document are acknowledged as such.
For this edition typing and typograhical errors have been
corrected. Layout and pagination may thereforediffer slightly with
respect to the first, limited edition.
ii
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ATLAS Collaboration
ArmeniaYerevan Physics Institute, Yerevan
AustraliaResearch Centre for High Energy Physics, Melbourne
University, MelbourneUniversity of Sydney, Sydney
AustriaInstitut für Experimentalphysik der
Leopold-Franzens-Universität Innsbruck, Innsbruck
Azerbaijan RepublicInstitute of Physics, Azerbaijan Academy of
Science, Baku
Republic of BelarusInstitute of Physics of the Academy of
Science of Belarus, MinskNational Centre of Particle and High
Energy Physics, Minsk
BrazilUniversidade Federal do Rio de Janeiro, COPPE/EE/IF, Rio
de Janeiro
CanadaUniversity of Alberta, EdmontonDepartment of Physics,
University of British Columbia, VancouverUniversity of
Carleton/C.R.P.P., CarletonGroup of Particle Physics, University of
Montreal, MontrealDepartment of Physics, University of Toronto,
TorontoTRIUMF, VancouverUniversity of Victoria, Victoria
CERNEuropean Laboratory for Particle Physics (CERN), Geneva
ChinaInstitute of High Energy Physics, Academia Sinica, Beijing,
University of Science and Technology ofChina, Hefei, University of
Nanjing and University of Shandong
Czech RepublicAcademy of Sciences of the Czech Republic,
Institute of Physics and Institute ofComputer Science,
PragueCharles University, Faculty of Mathematics and Physics,
PragueCzech Technical University in Prague, Faculty of Nuclear
Sciences andPhysical Engineering, Faculty of Mechanical
Engineering, Prague
DenmarkNiels Bohr Institute, University of Copenhagen,
Copenhagen
FinlandHelsinki Institute of Physics, Helsinki
FranceLaboratoire d’Annecy-le-Vieux de Physique des Particules
(LAPP), IN2P3-CNRS, Annecy-le-VieuxUniversité Blaise Pascal,
IN2P3-CNRS, Clermont-FerrandInstitut des Sciences Nucléaires de
Grenoble, IN2P3-CNRS-Université Joseph Fourier, GrenobleCentre de
Physique des Particules de Marseille, IN2P3-CNRS,
MarseilleLaboratoire de l’Accélérateur Linéaire, IN2P3-CNRS,
OrsayLPNHE, Universités de Paris VI et VII, IN2P3-CNRS, Paris
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CEA, DSM/DAPNIA, Centre d’Etudes de Saclay, Gif-sur-Yvette
Republic of GeorgiaInstitute of Physics of the Georgian Academy
of Sciences and Tbilisi State University, Tbilisi
GermanyPhysikalisches Institut, Universität Bonn, BonnInstitut
für Physik, Universität Dortmund, DortmundFakultät für Physik,
Albert-Ludwigs-Universität, FreiburgInstitut für Hochenergiephysik
der Universität Heidelberg, HeidelbergInstitut für Physik,
Johannes-Gutenberg Universität Mainz, MainzLehrstuhl für Informatik
V, Universität Mannheim, MannheimSektion Physik,
Ludwig-Maximilian-Universität München, MünchenMax-Planck-Institut
für Physik, MünchenFachbereich Physik, Universität Siegen,
SiegenFachbereich Physik, Bergische Universität, Wuppertal
GreeceAthens National Technical University, AthensAthens
University, AthensHigh Energy Physics Department and Department of
Mechanical Engineering, Aristotle University ofThessaloniki,
Thessaloniki
IsraelDepartment of Physics, Technion, HaifaRaymond and Beverly
Sackler Faculty of Exact Sciences, School of Physics and Astronomy,
Tel-AvivUniversity, Tel-AvivDepartment of Particle Physics, The
Weizmann Institute of Science, Rehovot
ItalyDipartimento di Fisica dell’ Università della Calabria e
I.N.F.N., CosenzaLaboratori Nazionali di Frascati dell’ I.N.F.N.,
FrascatiDipartimento di Fisica dell’ Università di Genova e
I.N.F.N., GenovaDipartimento di Fisica dell’ Università di Lecce e
I.N.F.N., LecceDipartimento di Fisica dell’ Università di Milano e
I.N.F.N., MilanoDipartimento di Scienze Fisiche, Università di
Napoli ‘Federico II’ e I.N.F.N., NapoliDipartimento di Fisica
Nucleare e Teorica dell’ Università di Pavia e I.N.F.N.,
PaviaDipartimento di Fisica dell’ Università di Pisa e I.N.F.N.,
PisaDipartimento di Fisica dell’ Università di Roma ‘La Sapienza’ e
I.N.F.N., RomaDipartimento di Fisica dell’ Università di Roma ‘Tor
Vergata’ e I.N.F.N., RomaDipartimento di Fisica dell’ Università di
Roma ‘Roma Tre’ e I.N.F.N., RomaDipartimento di Fisica dell’
Università di Udine, Gruppo collegato di Udine I.N.F.N. Trieste,
Udine
JapanDepartment of Information Science, Fukui University,
FukuiHiroshima Institute of Technology, HiroshimaDepartment of
Physics, Hiroshima University, Higashi-HiroshimaKEK, High Energy
Accelerator Research Organisation, TsukubaDepartment of Physics,
Faculty of Science, Kobe University, KobeDepartment of Physics,
Kyoto University, KyotoKyoto University of Education,
Kyoto-shiDepartment of Electrical Engineering, Nagasaki Institute
of Applied Science, NagasakiNaruto University of Education,
Naruto-shiDepartment of Physics, Faculty of Science, Shinshu
University, MatsumotoInternational Center for Elementary Particle
Physics, University of Tokyo, TokyoPhysics Department, Tokyo
Metropolitan University, TokyoDepartment of Applied Physics, Tokyo
University of Agriculture and Technology, Tokyo
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MoroccoFaculté des Sciences Aïn Chock, Université Hassan II,
Casablanca, and Université Mohamed V, Rabat
NetherlandsFOM - Institute SAF NIKHEF and University of
Amsterdam/NIKHEF, AmsterdamUniversity of Nijmegen/NIKHEF,
Nijmegen
NorwayUniversity of Bergen, BergenUniversity of Oslo, Oslo
PolandHenryk Niewodniczanski Institute of Nuclear Physics,
CracowFaculty of Physics and Nuclear Techniques of the University
of Mining and Metallurgy, Cracow
PortugalLaboratorio de Instrumentação e Física Experimental de
Partículas (University of Lisboa, University ofCoimbra, University
Católica-Figueira da Foz and University Nova de Lisboa), Lisbon
RomaniaInstitute of Atomic Physics, National Institute of
Physics and Nuclear Engineering, Bucharest
RussiaInstitute for Theoretical and Experimental Physics (ITEP),
MoscowP.N. Lebedev Institute of Physics, MoscowMoscow Engineering
and Physics Institute (MEPhI), MoscowMoscow State University,
Institute of Nuclear Physics, MoscowBudker Institute of Nuclear
Physics (BINP), NovosibirskInstitute for High Energy Physics
(IHEP), ProtvinoPetersburg Nuclear Physics Institute (PNPI),
Gatchina, St. Petersburg
JINRJoint Institute for Nuclear Research, Dubna
Slovak RepublicBratislava University, Bratislava, and Institute
of Experimental Physics of the Slovak Academy ofSciences,
Kosice
SloveniaJozef Stefan Institute and Department of Physics,
University of Ljubljana, Ljubljana
SpainInstitut de Física d’Altes Energies (IFAE), Universidad
Autónoma de Barcelona, Bellaterra, BarcelonaPhysics Department,
Universidad Autónoma de Madrid, MadridInstituto de Física
Corpuscular (IFIC), Centro Mixto Universidad de Valencia - CSIC,
Valencia
SwedenFysiska institutionen, Lunds universitet, LundRoyal
Institute of Technology (KTH), StockholmUniversity of Stockholm,
StockholmUppsala University, Department of Radiation Sciences,
Uppsala
SwitzerlandLaboratory for High Energy Physics, University of
Bern, BernSection de Physique, Université de Genève, Geneva
TurkeyDepartment of Physics, Ankara University, AnkaraDepartment
of Physics, Bogaziçi University, Istanbul
ATLAS Collaboration v
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United KingdomSchool of Physics and Astronomy, The University of
Birmingham, BirminghamCavendish Laboratory, Cambridge University,
CambridgeDepartment of Physics and Astronomy, University of
Edinburgh, EdinburghDepartment of Physics and Astronomy, University
of Glasgow, GlasgowDepartment of Physics, Lancaster University,
LancasterDepartment of Physics, Oliver Lodge Laboratory, University
of Liverpool, LiverpoolDepartment of Physics, Queen Mary and
Westfield College, University of London, LondonDepartment of
Physics, Royal Holloway and Bedford New College, University of
London, EghamDepartment of Physics and Astronomy, University
College London, LondonDepartment of Physics and Astronomy,
University of Manchester, ManchesterDepartment of Physics, Oxford
University, OxfordRutherford Appleton Laboratory, Chilton,
DidcotDepartment of Physics, University of Sheffield, Sheffield
United States of AmericaState University of New York at Albany,
New YorkArgonne National Laboratory, Argonne, IllinoisUniversity of
Arizona, Tucson, ArizonaDepartment of Physics, The University of
Texas at Arlington, Arlington, TexasLawrence Berkeley Laboratory
and University of California, Berkeley, CaliforniaDepartment of
Physics, Boston University, Boston, MassachusettsBrandeis
University, Department of Physics, Waltham, MassachusettsBrookhaven
National Laboratory (BNL), Upton, New YorkUniversity of Chicago,
Enrico Fermi Institute, Chicago, IllinoisNevis Laboratory, Columbia
University, Irvington, New YorkDepartment of Physics, Duke
University, Durham, North CarolinaDepartment of Physics, Hampton
University, VirginiaDepartment of Physics, Harvard University,
Cambridge, MassachusettsIndiana University, Bloomington,
IndianaUniversity of California, Irvine, CaliforniaMassachusetts
Institute of Technology, Department of Physics, Cambridge,
MassachusettsUniversity of Michigan, Department of Physics, Ann
Arbor, MichiganMichigan State University, Department of Physics and
Astronomy, East Lansing, MichiganUniversity of New Mexico, New
Mexico Center for Particle Physics, AlbuquerquePhysics Department,
Northern Illinois University, DeKalb, IllinoisOhio State
University, Columbus, OhioDepartment of Physics and Astronomy,
University of OklahomaDepartment of Physics, University of
Pennsylvania, Philadelphia, PennsylvaniaUniversity of Pittsburgh,
Pittsburgh, PennsylvaniaDepartment of Physics and Astronomy,
University of Rochester, Rochester, New YorkInstitute for Particle
Physics, University of California, Santa Cruz, CaliforniaDepartment
of Physics, Southern Methodist University, Dallas, TexasState
University of New York at Stony Brook, Stony Brook, New YorkTufts
University, Medford, MassachusettsHigh Energy Physics, University
of Illinois, Urbana, IllinoisDepartment of Physics, Department of
Mechanical Engineering, University of Washington,
Seattle,WashingtonDepartment of Physics, University of Wisconsin,
Madison, Wisconsin
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Acknowledgements
The Editors would like to thank Mario Ruggier for his continuous
help and competent adviceon all FrameMaker issues. The Editors also
warmly thank Michèle Jouhet and Isabelle Canonfor the processing of
the colour figures and the cover pages. Finally they would like to
expresstheir gratitude to all the Print-shop staff for their
expertise in printing this document.
Acknowledgements vii
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viii Acknowledgements
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Table Of Contents
14 Physics overview . . . . . . . . . . . . . . . . . . . . . .
45914.1 Introduction . . . . . . . . . . . . . . . . . . . . . .
45914.2 Theoretical picture . . . . . . . . . . . . . . . . . . .
46014.3 Challenges of new physics . . . . . . . . . . . . . . . . .
46214.4 Simulation of physics signals and backgrounds. . . . . . .
. . . 463
14.4.1 Event generators. . . . . . . . . . . . . . . . . .
46414.4.2 Signal observability. . . . . . . . . . . . . . . . .
466
14.5 Outline . . . . . . . . . . . . . . . . . . . . . . .
46814.6 References . . . . . . . . . . . . . . . . . . . . . .
468
15 QCD processes at the LHC. . . . . . . . . . . . . . . . . . .
47115.1 Introduction . . . . . . . . . . . . . . . . . . . . . .
47115.2 Knowledge of the proton structure . . . . . . . . . . . . .
. 472
15.2.1 Global parton analyses and parton kinematics at the LHC .
. . 47215.2.2 Properties and uncertainties of parton distribution
functions . . 47315.2.3 Expected improvements before the LHC
start-up . . . . . . 47715.2.4 The role of data from ATLAS . . . .
. . . . . . . . . 477
15.3 Properties of minimum−bias events. . . . . . . . . . . . .
. 47815.3.1 Importance of minimum−bias studies . . . . . . . . . .
47815.3.2 Selection of minimum−bias events . . . . . . . . . . .
47815.3.3 Modelling of minimum-bias events . . . . . . . . . . .
47815.3.4 Measurements . . . . . . . . . . . . . . . . . . 479
15.4 Measurements of hard diffractive scattering . . . . . . . .
. . . 48215.4.1 Overview . . . . . . . . . . . . . . . . . . . .
48215.4.2 Existing studies of hard diffraction . . . . . . . . . .
. 48415.4.3 Models for hard diffractive scattering . . . . . . . .
. . 48515.4.4 Trigger and event selection . . . . . . . . . . . . .
. 48615.4.5 Single hard diffractive dissociation . . . . . . . . .
. . 48815.4.6 Double Pomeron exchange . . . . . . . . . . . . . .
49115.4.7 Colour-singlet exchange . . . . . . . . . . . . . . .
49315.4.8 Diffractive W and Z production . . . . . . . . . . . .
49415.4.9 Diffractive heavy flavour production . . . . . . . . . .
49415.4.10 Summary on hard diffractive scattering . . . . . . . . .
. 495
15.5 Jet physics . . . . . . . . . . . . . . . . . . . . . .
49615.5.1 Overview . . . . . . . . . . . . . . . . . . . .
49615.5.2 Inclusive jet cross-section . . . . . . . . . . . . . . .
49615.5.3 Jet shape and fragmentation . . . . . . . . . . . . . .
50115.5.4 Di-jet production . . . . . . . . . . . . . . . . .
50215.5.5 Multi-jet production . . . . . . . . . . . . . . . .
50615.5.6 Double parton scattering . . . . . . . . . . . . . . .
507
15.6 Photon physics . . . . . . . . . . . . . . . . . . . . .
50815.6.1 Overview . . . . . . . . . . . . . . . . . . . .
50815.6.2 Inclusive photon production . . . . . . . . . . . . .
50815.6.3 Photon pair production . . . . . . . . . . . . . . .
510
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15.6.4 Photon + jet production . . . . . . . . . . . . . . .
51315.6.5 Photon + charm and photon + beauty production . . . . . .
514
15.7 Drell-Yan physics and gauge-boson production . . . . . . .
. . 51515.7.1 Overview . . . . . . . . . . . . . . . . . . . .
51515.7.2 Drell-Yan production . . . . . . . . . . . . . . . .
51515.7.3 W production. . . . . . . . . . . . . . . . . . .
51715.7.4 Z production . . . . . . . . . . . . . . . . . . .
52115.7.5 Gauge boson pair production . . . . . . . . . . . . .
523
15.8 Heavy flavour physics . . . . . . . . . . . . . . . . . .
52715.8.1 Overview . . . . . . . . . . . . . . . . . . . .
52715.8.2 Charm production . . . . . . . . . . . . . . . . .
52815.8.3 Bottom production . . . . . . . . . . . . . . . . .
53015.8.4 Top production . . . . . . . . . . . . . . . . . .
534
15.9 Conclusion . . . . . . . . . . . . . . . . . . . . . .
53615.10 References . . . . . . . . . . . . . . . . . . . . . .
537
16 Physics of electroweak gauge bosons . . . . . . . . . . . . .
. . 54516.1 Measurement of the W mass . . . . . . . . . . . . . . .
. 545
16.1.1 The method . . . . . . . . . . . . . . . . . . .
54616.1.2 W production and selection . . . . . . . . . . . . . .
54716.1.3 Expected uncertainties . . . . . . . . . . . . . . .
54716.1.4 Results . . . . . . . . . . . . . . . . . . . . . 551
16.2 Gauge-boson pair production. . . . . . . . . . . . . . . .
55316.2.1 Wγ Production . . . . . . . . . . . . . . . . . .
55416.2.2 WZ Production . . . . . . . . . . . . . . . . . .
55516.2.3 Determination of Triple Gauge Couplings . . . . . . . . .
55516.2.4 Systematic uncertainties . . . . . . . . . . . . . . .
55716.2.5 Results . . . . . . . . . . . . . . . . . . . . . 558
16.3 Conclusions . . . . . . . . . . . . . . . . . . . . . .
55916.4 References . . . . . . . . . . . . . . . . . . . . . .
559
17 B-physics . . . . . . . . . . . . . . . . . . . . . . . . .
56117.1 Introduction. . . . . . . . . . . . . . . . . . . . . .
561
17.1.1 General features of beauty production in ATLAS . . . . .
. 56217.1.2 Model used for simulation studies . . . . . . . . . . .
56217.1.3 Trigger . . . . . . . . . . . . . . . . . . . . . 563
17.2 CP-violation studies . . . . . . . . . . . . . . . . . . .
56417.2.1 Overview . . . . . . . . . . . . . . . . . . . .
56417.2.2 Measurement of asymmetry in B0d → J/ψK0s . . . . . . . .
56517.2.3 Measurement of asymmetry in B0d → π+π- . . . . . . . . .
. 57717.2.4 Analysis of the decay B0s → J/ψ φ . . . . . . . . . . .
. 58217.2.5 Analysis of the decay B0d → D0K*0 . . . . . . . . . . .
. . . 59017.2.6 Conclusions on CP violation . . . . . . . . . . . .
. 591
17.3 Measurements of B0s oscillations . . . . . . . . . . . . .
. . 59217.3.1 Introduction . . . . . . . . . . . . . . . . . . .
59217.3.2 Event reconstruction . . . . . . . . . . . . . . . .
593
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17.3.3 Background analysis . . . . . . . . . . . . . . . .
59717.3.4 Evaluation of signal and background statistics . . . . .
. . 59717.3.5 Determination of the proper-time resolution . . . . .
. . . 60017.3.6 Extraction of reach . . . . . . . . . . . . . . . .
. 60117.3.7 Dependence of reach on experimental quantities . . . .
. . 60317.3.8 Conclusions . . . . . . . . . . . . . . . . . . .
604
17.4 Rare decays B → µµ(X) . . . . . . . . . . . . . . . . . .
60417.4.1 Introduction . . . . . . . . . . . . . . . . . . .
60417.4.2 Theoretical approach . . . . . . . . . . . . . . . .
60517.4.3 Simulation of rare B-decay events . . . . . . . . . . . .
60617.4.4 The measurement of the forward–backward asymmetry . . . .
61017.4.5 Conclusions . . . . . . . . . . . . . . . . . . . 612
17.5 Precision measurements of B hadrons . . . . . . . . . . . .
. 61217.5.1 Measurements with the Bc meson . . . . . . . . . . . .
61217.5.2 Λb polarisation measurement . . . . . . . . . . . . .
613
17.6 Conclusions on the B-physics potential . . . . . . . . . .
. . 61517.7 References . . . . . . . . . . . . . . . . . . . . . .
616
18 Heavy quarks and leptons . . . . . . . . . . . . . . . . . .
. 61918.1 Top quark physics . . . . . . . . . . . . . . . . . . . .
619
18.1.1 Introduction . . . . . . . . . . . . . . . . . . .
61918.1.2 tt selection and event yields . . . . . . . . . . . . . .
62018.1.3 Measurement of the top quark mass . . . . . . . . . . .
62218.1.4 Top quark pair production . . . . . . . . . . . . . .
63918.1.5 Top quark decays and couplings . . . . . . . . . . . .
64318.1.6 Electroweak single top quark production . . . . . . . . .
65218.1.7 Conclusions of top quark physics studies . . . . . . . .
. 662
18.2 Fourth generation quarks . . . . . . . . . . . . . . . . .
66318.2.1 Fourth family up quarks . . . . . . . . . . . . . . .
66418.2.2 Fourth family down quarks . . . . . . . . . . . . . .
66618.2.3 Bound states of fourth family quarks. . . . . . . . . . .
667
18.3 Heavy leptons . . . . . . . . . . . . . . . . . . . . .
66818.4 Conclusions . . . . . . . . . . . . . . . . . . . . . .
66918.5 References . . . . . . . . . . . . . . . . . . . . . .
669
19 Higgs Bosons . . . . . . . . . . . . . . . . . . . . . . .
67319.1 Introduction . . . . . . . . . . . . . . . . . . . . . .
67319.2 Standard Model Higgs boson . . . . . . . . . . . . . . . .
674
19.2.1 Introduction . . . . . . . . . . . . . . . . . . .
67419.2.2 H → γγ . . . . . . . . . . . . . . . . . . . . .
67519.2.3 H → Zγ . . . . . . . . . . . . . . . . . . . . .
68419.2.4 H → bb . . . . . . . . . . . . . . . . . . . . .
68519.2.5 H → ZZ* → 4l. . . . . . . . . . . . . . . . . . .
69319.2.6 H → WW(*) → lνlν . . . . . . . . . . . . . . . . .
70419.2.7 WH with H→ WW* → lνlν and W → lν . . . . . . . . . .
70919.2.8 Sensitivity to the SM Higgs boson in the intermediate
mass range 712
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19.2.9 H → ZZ → 4l . . . . . . . . . . . . . . . . . . .
71419.2.10 Heavy Higgs boson . . . . . . . . . . . . . . . .
71619.2.11 Overall sensitivity to the SM Higgs searches . . . . . .
. . 72919.2.12 Determination of the SM Higgs-boson parameters . . .
. . . 730
19.3 Minimal Supersymmetric Standard Model Higgs boson . . . . .
. 73619.3.1 Introduction . . . . . . . . . . . . . . . . . . .
73619.3.2 Scenarios with heavy SUSY particles. . . . . . . . . . .
73719.3.3 Overall sensitivity . . . . . . . . . . . . . . . . .
77319.3.4 Determination of the MSSM Higgs parameters . . . . . . .
77719.3.5 SUGRA scenarios . . . . . . . . . . . . . . . . . 781
19.4 Strongly interacting Higgs sector . . . . . . . . . . . . .
. 79519.4.1 Detector performance issues . . . . . . . . . . . . .
79519.4.2 Vector boson scattering in the Chiral Lagrangian model .
. . . 796
19.5 Conclusions on the Higgs sector . . . . . . . . . . . . . .
. 80119.6 References . . . . . . . . . . . . . . . . . . . . . .
803
20 Supersymmetry . . . . . . . . . . . . . . . . . . . . . .
81120.1 Introduction. . . . . . . . . . . . . . . . . . . . . .
81120.2 Supergravity models . . . . . . . . . . . . . . . . . . .
816
20.2.1 Inclusive SUGRA measurements . . . . . . . . . . . .
81820.2.2 Exclusive SUGRA measurements for moderate tan β . . . . .
82220.2.3 l+l- SUGRA signatures. . . . . . . . . . . . . . . .
82520.2.4 More complex leptonic SUGRA signatures . . . . . . . .
82920.2.5 h → bb SUGRA signatures . . . . . . . . . . . . . .
83520.2.6 Thresholds and model-independent SUGRA masses . . . . .
83920.2.7 Other signatures for SUGRA Points 1 – 5 . . . . . . . . .
84220.2.8 Exclusive SUGRA measurements for large tan β. . . . . . .
84720.2.9 Fitting minimal SUGRA parameters . . . . . . . . . . .
85320.2.10 Non-universal SUGRA models . . . . . . . . . . . .
860
20.3 Gauge mediated SUSY breaking models . . . . . . . . . . . .
86320.3.1 GMSB Point G1a . . . . . . . . . . . . . . . . .
86520.3.2 GMSB Point G1b . . . . . . . . . . . . . . . . .
87020.3.3 GMSB Point G2a . . . . . . . . . . . . . . . . .
87320.3.4 GMSB Point G2b . . . . . . . . . . . . . . . . .
87720.3.5 Fitting GMSB parameters . . . . . . . . . . . . . .
883
20.4 R-Parity breaking models . . . . . . . . . . . . . . . . .
88720.4.1 Baryon number violation: χ10 → qqq . . . . . . . . . . .
88820.4.2 Lepton number violation: χ10 → l+l-ν . . . . . . . . . .
89520.4.3 Lepton number violation: χ10 → qql, qqν . . . . . . . . .
906
20.5 Conclusion . . . . . . . . . . . . . . . . . . . . . .
91020.6 References . . . . . . . . . . . . . . . . . . . . . .
911
21 Other physics beyond the Standard Model . . . . . . . . . . .
. . 91521.1 Introduction. . . . . . . . . . . . . . . . . . . . . .
91521.2 Search for technicolor signals . . . . . . . . . . . . . .
. . 915
21.2.1 Technicolor signals from qq fusion . . . . . . . . . . .
916
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21.2.2 Signals from vector boson fusion . . . . . . . . . . . .
92421.2.3 Conclusion. . . . . . . . . . . . . . . . . . . . 925
21.3 Search for excited quarks . . . . . . . . . . . . . . . . .
92521.3.1 The widths of excited quarks . . . . . . . . . . . . .
92621.3.2 Simulation of the signal and backgrounds . . . . . . . .
. 92721.3.3 Conclusion. . . . . . . . . . . . . . . . . . . .
930
21.4 Leptoquarks . . . . . . . . . . . . . . . . . . . . . .
93121.5 Compositeness . . . . . . . . . . . . . . . . . . . . .
932
21.5.1 High-pT jets . . . . . . . . . . . . . . . . . . .
93221.5.2 Transverse energy distributions of jets. . . . . . . . .
. . 93321.5.3 Jet angular distributions. . . . . . . . . . . . . .
. . 93521.5.4 Dilepton production . . . . . . . . . . . . . . . .
939
21.6 Search for new gauge bosons and Majorana neutrinos . . . .
. . . 93921.6.1 Search for new vector bosons . . . . . . . . . . .
. . 94021.6.2 Search for right-handed Majorana neutrinos . . . . .
. . . 944
21.7 Monopoles . . . . . . . . . . . . . . . . . . . . . .
94921.8 References . . . . . . . . . . . . . . . . . . . . . .
952
A Members of the ATLAS Collaboration . . . . . . . . . . . . . .
955
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14 Physics overview
14.1 Introduction
The ATLAS physics programme has been already discussed in
several documents, the mostcomprehensive ones being the Letter of
Intent [14-1] and the Technical Proposal [14-2]. Thegoals which
have been defined there and which have guided the detector
optimisation proce-dure remain essentially the same, the most
important one being measurements that will lead toan understanding
of the mechanism of electroweak symmetry breaking.
The high energy and luminosity of the LHC offers a large range
of physics opportunities, fromthe precise measurement of the
properties of known objects to the exploration of the high ener-gy
frontier. The need to accommodate the very large spectrum of
possible physics signatureshas guided the optimisation of the
detector design. The desire to probe the origin of the elec-troweak
scale leads to a major focus on the Higgs boson; ATLAS must be
sensitive to it over thefull range of allowed masses. Other
important goals are searches for other phenomena possiblyrelated to
the symmetry breaking, such as particles predicted by supersymmetry
or technicol-our theories, as well as new gauge bosons and evidence
for composite quarks and leptons. Theinvestigation of CP violation
in B decays and the precision measurements of W and top-quarkmasses
and triple gauge boson couplings will also be important components
of the ATLASphysics programme.
As discussed in the previous volume, and as also will be
illustrated several times throughoutthis one, excellent performance
of the detector is needed to achieve these physics goals.
• The various Higgs boson searches, which resent some of the
most challenging signatures,were used as benchmark processes for
the setting of parameters that describe the detectorperformance.
High-resolution measurements of electrons, photons and muons,
excellentsecondary vertex detection for τ−leptons and b-quarks,
high-resolution calorimetry forjets and missing transverse energy
(ETmiss) are essential to explore the full range of possi-ble Higgs
boson masses.
• Searches for SUSY set the benchmarks on the hermeticity and
ETmiss capability of the de-tector, as well as on b-tagging at high
luminosity.
• Searches for new heavy gauge bosons provided benchmark
requirements for high-resolu-tion lepton measurements and charge
identification in the pT range as large as a few TeV.
• Signatures characteristic for quark compositeness set the
requirements for the measure-ment of very high-pT jets.
• The precision measurements of the W and top-quark masses,
gauge boson couplings, CPviolation and the determination of the
Cabibbo-Kobayashi-Maskawa unitarity triangleyielded benchmarks that
address the need to precisely control the energy scale for jetsand
leptons, determine precisely secondary vertices, reconstruct fully
final states with rel-atively low-pT particles and trigger on
low-pT leptons.
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14.2 Theoretical picture
The Standard Model (SM) [14-3] is a very successful description
of the interactions of the com-ponents of matter at the smallest
scales (10-18 m) and highest energies (~200 GeV) accessible
tocurrent experiments. It is a quantum field theory which describes
the interaction of spin-1/2point-like fermions, whose interactions
are mediated by spin-1 gauge bosons. The bosons are aconsequence of
local gauge invariance applied to the fermion fields and are a
manifestation ofthe symmetry group of the theory, i.e.
SU(3)xSU(2)xU(1) [14-3] [14-4].
The fundamental fermions are leptons and quarks. The left-handed
states are doublets underthe SU(2) group, while the right-handed
states are singlets. There are three generations of fermi-ons, each
generation identical except for mass: the origin of this structure,
and the breaking ofgenerational symmetry (flavour symmetry), remain
a mystery. There are three leptons withelectric charge -1, the
electron (e), muon (µ) and tau lepton (τ) and three electrically
neutral lep-tons, the neutrinos νe, νµ and ντ. Similarly, there are
three quarks with electric charge 2/3, up(u), charm (c) and top
(t), and three with electric charge -1/3, down (d), strange (s) and
bottom(b). The quarks are triplets under the SU(3) group and thus
carry an additional ‘charge’, referredto as colour. There is mixing
between the three generations of quarks, which is parametrised
bythe Cabibbo-Kobayashi-Maskawa (CKM) [14-5] matrix whose origin is
not explained by theStandard Model.
The SU(2)xU(1) symmetry group (which describes the so-called
electroweak interaction) isspontaneously broken by the existence of
a (postulated) Higgs field with non-zero expectationvalue [14-6].
This leads to the emergence of massive vector bosons, the W and Z,
which mediatethe weak interaction, while the photon of
electromagnetism remains massless. One physical de-gree of freedom
remains in the Higgs sector, which should manifest as a neutral
scalar bosonH0, which is presently unobserved. The SU(3) group
describes the strong interaction (quantumchromodynamics or QCD)
[14-4]. Eight vector gluons mediate this interaction. They carry
col-our charges themselves, and are thus self-interacting. This
implies that the QCD coupling αs issmall for large momentum
transfers but large for small momentum transfers, and leads to
theconfinement of quarks inside colour-neutral hadrons. Attempting
to free a quark produces a jetof hadrons through production of
quark-antiquark pairs and gluons.
The success of the SM of strong, weak and electromagnetic
interactions has drawn increased at-tention to its limitations. In
its simplest version, the model has 19 parameters, the three
couplingconstants of the gauge theory SU(3)xSU(2)xU(1), three
lepton and six quark masses, the mass ofthe Z boson which sets the
scale of weak interactions, and the four parameters which
describethe rotation from the weak to the mass eigenstates of the
charge -1/3 quarks (CKM matrix). Allof these parameters are known
with varying errors. Of the two remaining parameters, a
CP-vio-lating parameter associated with the strong interactions
must be very small. The last parameteris associated with the
mechanism responsible for the breakdown of electroweak SU(2)xU(1)
toU(1)em. This can be taken as the mass of the, as yet
undiscovered, Higgs boson. The couplings ofthe Higgs boson are
determined once its mass is given.
The gauge theory part of the SM has been well tested, but there
is no direct evidence either foror against the simple Higgs
mechanism for electroweak symmetry breaking. All masses are tiedto
the mass scale of the Higgs sector. Although within the model there
is no guidance about theHiggs mass itself, some constraints can be
delivered from the perturbative calculations withinthe model
requiring the Higgs couplings to remain finite and positive up to
an energy scale Λ[14-7]. Such calculations exists at the two-loop
level for both lower and upper Higgs massbounds. With present
experimental results on the SM parameters, if the Higgs mass is in
the
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range 160 to 170 GeV [14-8] then the renormalisation-group
behaviour of the Standard Model isperturbative and well behaved up
to Planck scale ΛPl ~ 1019 GeV. For smaller or larger values ofmH
new physics must set in below ΛPl.
As its mass increases, the self couplings and the couplings to
the W and Z bosons grow [14-9].This feature has a very important
consequence. Either the Higgs boson must have a mass lessthan about
800 GeV, or the dynamics of WW and ZZ interactions with
centre-of-mass energiesof order 1 TeV will reveal new structure. It
is this simple argument that sets the energy scale thatmust be
reached to guarantee that an experiment will be able to provide
information on the na-ture of electroweak symmetry breaking.
The presence of a single elementary scalar boson is
unsatisfactory to many theorists. If the theo-ry is part of some
more fundamental theory, which has some other larger mass scale
(such as thescale of grand unification or the Planck scale), there
is a serious ‘fine tuning’ or naturalnessproblem. Radiative
corrections to the Higgs boson mass result in a value that is
driven to thelarger scale unless some delicate cancellation is
engineered ((m02 − m12) ~ mW2 where m0 and m1are order 1015 GeV or
larger). There are two ways out of this problem which involve new
phys-ics on the scale of 1 TeV. New strong dynamics could enter
that provides the scale of mW, or newparticles could appear so that
the larger scale is still possible, but the divergences are
cancelledon a much smaller scale. In any of the options, Standard
Model, new dynamics or cancellations,the energy scale is the same;
something must be discovered at the TeV scale.
Supersymmetry [14-10] is an appealing concept for which there is
so far no experimental evi-dence. It offers the only presently
known mechanism for incorporating gravity into the quan-tum theory
of particle interactions and provides an elegant cancellation
mechanism for thedivergences, provided that at the electroweak
scale the theory is supersymmetric. The successesof the Standard
Model (such as precision electroweak predictions) are retained,
while avoidingany fine tuning of the Higgs mass. Some
supersymmetric models allow for the unification ofgauge couplings
at a high scale and a consequent reduction of the number of
arbitrary parame-ters.
Supersymmetric models postulate the existence of superpartners
for all the presently observedparticles: bosonic superpartners of
fermions (squarks and sleptons), and fermionic superpart-ners of
bosons (gluinos and gauginos). There are also multiple Higgs
bosons: h, H, A and H±.There is thus a large spectrum of presently
unobserved particles, whose exact masses, couplingsand decay chains
are calculable in the theory given certain parameters.
Unfortunately these pa-rameters are unknown. Nonetheless, if
supersymmetry is to have anything to do with elec-troweak symmetry
breaking, the masses should be in the region below or order of 1
TeV.
An example of the strong coupling scenario is ‘technicolour’ for
models based on dynamicalsymmetry breaking [14-11]. Again, if the
dynamics is to have anything to do with electroweaksymmetry
breaking we would expect new states in the region below 1 TeV; most
models predicta large spectrum of such states. An elegant
implementation of this appealing idea is lacking.However, all
models predict structure in the WW scattering amplitude at around 1
TeV centre-of-mass energy.
There are also other possibilities for new physics that are not
necessarily related to the scale ofelectroweak symmetry breaking.
There could be new neutral or charged gauge bosons withmass larger
than the Z and W; there could be new quarks, charged leptons or
massive neutrinos,or quarks and leptons could turn out not to be
elementary objects. While we have no definitiveexpectations for the
masses of these objects, the LHC experiments must be able to search
forthem over the available energy range.
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Results on precision measurements within the Standard Model, as
well as limits on new phys-ics, from present experiments are
presented, case by case, in the relevant chapter of this
volume.
14.3 Challenges of new physics
This volume presents examples of the physics programme which
should be possible with theATLAS detector. The channels studied in
previous documents [14-1][14-2] are re-examined andmany new
strategies proposed.
In the initial phase at low luminosity, the experiment will
function as a factory for QCD process-es, heavy flavour and gauge
bosons production. This will allow a large number of
precisionmeasurements in the early stages of the experiment.
A large variety of QCD related processes will be studied. These
measurements are of impor-tance as studies of QCD ‘per se’ in a new
energy regime with high statistics. Of particular inter-est will be
jet and photon physics, open charm and beauty production and gauge
bosonsproduction. A study of diffractive processes will present
significant experimental challenges it-self, given the limited
angular coverage of the ATLAS detector. Several aspects of
diffractiveproduction of jets, gauge bosons, heavy flavour partons
will be nevertheless studied in detail.LHC will extend the
exploration of the hard partonic processes to large energy scales
(of fewhundred GeV2), while reaching small fractional momentum of
the proton being carried by ascattered partons (of 10-5). Precise
constraints on the partonic distribution functions will be de-rived
from measurements of Drell-Yan production, of W and Z bosons
production, of produc-tion of direct photons and high-pT jets,
heavy flavours and gauge boson pairs. Deviation fromthe theoretical
predictions for QCD processes themselves might indicate the onset
of new phys-ics, such as compositeness. Measurement and
understanding of these QCD processes will be es-sential as they
form the dominant background searches for new phenomena.
Even at low luminosity, LHC is a beauty factory with 1012 bb
expected per year. The availablestatistics will be limited only by
the rate at which data can be recorded. The proposed
B-physicsprogramme is therefore very wide. Specific B-physics
topics include the search for and meas-urement of CP violation, of
Bs0 mixing and of rare decays. ATLAS can perform
competitivehigh-accuracy measurements of Bs0 mixing, covering the
statistically preferred range of theStandard Model predictions.
Rare B mesons such as Bc will be copiously produced at LHC.
Thestudy of B-baryon decay dynamics and spectroscopy of rare B
hadrons will be also carried out.
LHC has a great potential for performing high precision top
physics measurements with abouteight million tt pairs expected to
be produced for an integrated luminosity of 10 fb-1. It wouldallow
not only for the precise measurements of the top-quark mass (with a
precision of ~2 GeV)but also for the detailed study of properties
of the top-quark itself. The single top productionshould be
observable and the high statistics will allow searches for many
rare top decays. Theprecise knowledge of the top-quark mass places
strong constraints on the mass of the StandardModel Higgs boson,
while a detailed study of its properties may reveal as well new
physics.
One of the challenges to the LHC experiments will be whether the
precision of the W-massmeasurement can be improved. Given the 300
million single W events expected in one year ofdata taking, the
expected statistical uncertainty will be about 2 MeV. The very
ambitious goalfor both theory and experiment is to reduce the
individual sources of systematic errors to less
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than 10 MeV, which would allow for the measurement of the W mass
with precision of betterthan 20 MeV. This would ensure that the
precision of the W mass is not the dominant source oferrors in
testing radiative corrections in the SM prediction for the Higgs
mass.
The large rate of gauge boson pair production at the LHC enables
ATLAS to provide criticaltests of the triple gauge-boson couplings.
The gauge cancellations predicted by the StandardModel will be
studied and measurements of possible anomalous couplings made.
These probeunderlying non-standard physics. The most sensitive
variables to compare with Standard Mod-el predictions are the
transverse momentum spectra of high-pT photons or reconstructed Z
bos-ons.
If the Higgs boson is not discovered before LHC begins
operation, the searches for it and itspossible supersymmetric
extensions in the Minimal Supersymmetric Standard Model (MSSM)will
be a main focus of activity. Search strategies presented here
explore a variety of possiblesignatures, being accessible already
at low luminosity or only at design luminosity. Althoughthe
cleanest one would lead to reconstruction of narrow mass peaks in
the photonic or leptonicdecay channels, very promising are the
signatures which lead to multi-jet or multi-τ final states.In
several cases signal-to-background ratios much smaller than one are
expected, and in mostcases detection of the Higgs boson will
provide an experimental challenge. Nevertheless, theATLAS
experiment alone will cover the full mass range up to 1 TeV for the
SM Higgs and alsothe full parameter space for the MSSM Higgs
scenarios. It has also a large potential for searchesin alternative
scenarios.
Discovering SUSY at the LHC will be straightforward if it exists
at the electroweak scale. Copi-ous production of squarks and
gluinos can be expected, since the cross-section should be aslarge
as a few pb for squarks and gluinos as heavy as 1 TeV. Their
cascade decays would lead toa variety of signatures involving
multi-jets, leptons, photons, heavy flavours and missing ener-gy.
In several models, discussed in detail in this volume, the
precision measurement of themasses of SUSY particles and the
determination of the model parameters will be possible. Themain
challenge would be therefore not to discover SUSY itself, but to
reveal its nature and de-termine the underlying SUSY model.
Other searches beyond the Standard Model have been also
investigated. Throughout this vol-ume are presented strategies for
searching for technicolour signals, excited quarks, leptoquarks,new
gauge bosons, right-handed neutrinos and monopoles. Given the large
number of detailedmodels published in this field, the task of
evaluating each of them is beyond the scope of thisdocument. Rather
an exploratory point of view is taken, examples are used and in
some cases adetailed study is performed.
14.4 Simulation of physics signals and backgrounds
In the process of evaluation of the physics potential of the
ATLAS experiment, Monte Carloevent generators were used to simulate
multiparticle production in physics processes appearingin the pp
collisions. Detailed or parametrised simulation of the detector
response to this multi-particle stream was then used to evaluate
the possible observability of the signal.
In the full detector simulation, described in Section 2.2, the
detailed geometry of the detector isimplemented and the
interactions of particles with the material of the detector are
modelled.Results from full-simulation studies have been described
in Chapters 3-10 for several crucial
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benchmark signatures and physics processes, e.g. mass
resolutions, acceptances and identifica-tion efficiencies for H →
γγ, H → ZZ∗ → 4l, H → bb, H → ττ decays, ETmiss resolution, b-jet
and τ-jet identification capability.
However, in most of the cases presented in this volume,
evaluation of the expected signals andbackgrounds has been done
with the fast simulation described in Section 2.5. This
simulationincludes, in a parametrised way, the main aspects related
to the detector response: jet recon-struction in the calorimeters,
momentum/energy smearing for leptons and photons, reconstruc-tion
of missing transverse energy and charged particles. It is tuned to
reproduce as well aspossible the expected ATLAS performance, and
this tuning has been verified with severalbenchmark processes as
described in Section 2.5.
The fast simulation was used very extensively for estimating the
expected backgrounds fromphysics processes. Such approach was
particularly useful for channels requiring large eventsamples,
which one could not process with the much more time-consuming full
simulation.Many of these studies are presented in this volume, e.g.
for Higgs searches in Chapter 19, where,in some cases, both
irreducible and reducible backgrounds required simulations of
several mil-lion events.
14.4.1 Event generators
There are several available Monte Carlo event generators for pp
collisions, the most exhaustiveones, with respect to available
physics processes and complexity in modelling hadronic
interac-tions, being: HERWIG [14-14], ISAJET[14-12] and
PYTHIA[14-13]. Each of these simulates ahadronic final state
corresponding to some particular model of the underlying physics.
The de-tails of the implementation of the physics are different in
each of these generators, however theunderlying philosophy of the
generators is the same.
• The basic process is a parton interaction involving a quark or
gluon from each of the in-coming protons. Elementary particles in
the final state, such as quarks, gluons or W/Z/γ-bosons, emerge
from the interaction. The fundamental process is calculated in
perturba-tive QCD, and the initial momentum of the quarks or gluons
is given by structure func-tions.
• Additional QCD (gluon) radiation takes place from the quarks
and gluons that partici-pate in the basic scattering process. These
parton showers are based on the expansionsaround the soft and
collinear limits and can be ascribed to either the initial or final
state.The algorithm used by HERWIG includes some effects due to
quantum interference andgenerally produces better agreement with
the data when detailed jet properties are stud-ied. The showering
continues down to some low energy cut-off. For some particular
casesthe matrix element calculations involving higher-order QCD
processes are used. Theevents that have more energy in the parton
process have more showering, and conse-quently more jet
activity.
• The collection of quarks and gluons must then be hadronised
into mesons and baryons.This is done differently in each of the
event generators, but is described by a set of (frag-mentation)
parameters that must be adjusted to agree with experimental
results. HER-WIG looks for colour singlet collections of quarks and
gluons with low invariant massand groups them together; this set
then turns into hadrons. PYTHIA splits gluons intoquark-antiquark
pairs and turns the resulting set of colour singlet quark-antiquark
pairsinto hadrons via a string model. ISAJET simply fragments each
quark independently pay-ing no attention to the colour flow. In
ISAJET the underlying event that arises from the re-
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maining beam fragments must be added. The other generators tie
these fragments backinto the partonic system in order to neutralise
the colour.
Matrix elements are likely to provide a better description of
the main character of the events, i.e.the topology of well
separated jets, while parton showers should be better at describing
the in-ternal structure of these jets.
The above model(s) describe events where there is a
hard-scattering of the incoming partons; ei-ther a heavy particle
is produced or the outgoing partons have large transverse
momentum.While these are the processes that are of most interest,
the dominant cross-section at the LHCconsists of events with no
hard scattering. There is little detailed theoretical understanding
ofthese minimum-bias events and the event generators must rely on
data at current energies.These minimum-bias events are important at
LHC, particularly at design luminosity, as theyoverlap interesting
hard-scattering events such as the production of new particles. The
genera-tors use a different approach in this case. ISAJET uses a
pomeron model that has some theoreti-cal basis. HERWIG uses a
parametrisation of data mainly from the CERN pp Collider.
PYTHIAuses a mini-jet model where the jet cross-section is used at
very low transverse momenta, i.e thehard scattering process is
extrapolated until it saturates the total cross-section. Whenever
rele-vant, ATLAS has used the PYTHIA approach with dedicated
modifications that agree withpresent data from Tevatron [14-17].
The multiplicity in minimum-bias events predicted by thisapproach
is larger than that predicted by ISAJET or HERWIG (see Chapter 15),
hence issues as-sociated with pile-up are treated
conservatively.
The generators differ in the extent to which non-standard
physics processes are included. Themost complete implementation of
the Standard Model processes are available in PYTHIA, whileISAJET
has the most complete implementation of SUSY scenarios.
In the physics evaluation presented in this volume, the Standard
Model physics and Higgssearches were mostly simulated with PYTHIA.
ISAJET was used extensively for the supersym-metry studies but some
analyses have been done also with the supersymmetric extension of
PY-THIA [14-23]. HERWIG has been used for some of the QCD studies.
The model of the hadronicinteractions implemented in the physics
generator has a direct impact on physical observablessuch as jet
multiplicity, their average transverse momentum, internal structure
of the jets andtheir heavy flavour content. That was one of the
reasons why, whenever possible, PYTHIA wasused enabling a
consistent set of signal and background simulations to be
generated.
Theoretical precision of the existing Monte Carlo generators is
far from adequate for the chal-lenging requirements of the LHC
experiments. Despite the huge efforts which have been putinto
developing of physics generators for hadron colliders over the last
years, the precision withwhich e.g. present data can be reproduced
is not better than 10-30%, and in some cases is notbetter than a
factor of two.
Table 14-1 shows a few examples of important signal and
background processes with their pre-dicted cross-sections, as used
in the simulations discussed in this volume. If not explicitly
statedotherwise, these are calculated using leading-order QCD as
implemented in PYTHIA 5.7, usingthe CTEQ2L set of structure
functions as the reference one. Whenever better or more
appropri-ate calculations were available, the production
cross-section from PYTHIA was suitably nor-malised, or a different
Monte Carlo generator was used.
• The QCD multi-jet production is a dominant background for e.g.
Higgs searches in themulti-jet final state. The production of
events with three or more high-pT jets is not wellmodelled by
lowest-order di-jet processes convoluted with parton showers. To
illustratethe large discrepancy between exact matrix element
calculations and parton shower ap-
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proaches in this case, Table 14-1 gives rates for one, three and
four jet final states as givenby the exact multi-parton matrix
element NJETS Monte Carlo [14-15] and PYTHIA. Onthe other hand,
heavy flavour content of jets is not modelled with the NJETS Monte
Carlo.Simulation of four b-jet final states has been therefore only
possible with the PYTHIA gen-erator, which has the heavy flavour
content of the partonic shower implemented.
• In the case of di-jet production in association with a W or Z,
the VECBOS Monte Carlo[14-16], dedicated to this process, has been
used. Exact matrix-element calculations wereused also for
estimating the expected cross-section in the case of Wbb [14-18]
and Zbb [14-19] production. In the first case a modified version of
HERWIG [14-18] was used, while inthe second case the EUROJET Monte
Carlo [14-21] was adopted.
• The leading order tt cross-section is quoted in Table 14-1
since it has been used for all thebackground studies to new
physics. For the specific case of top physics studies inChapter 18,
a more accurate NLO calculation of 833 pb has been used, except for
the caseof single-top production, for which the NLO terms are not
yet known.
• The total bb cross-section is also quoted in Table 14-1. For
the B-physics studies, muchmore detailed work reported in Chapter
17 has shown that for high-pT b-quark produc-tion, which can
provide a Level-1 trigger with a high-pT muon, the PYTHIA model
asused by ATLAS [14-20] reproduces quite well the bb production as
measured at the Teva-tron [14-21] [14-22]. In this case, only a
small fraction of the total cross-section quoted inTable 14-1 is
relevant for physics, and many of the large theoretical
uncertainties inherentto the calculations of the total bb
production are very significantly reduced. A more de-tailed
discussion of bb production at the LHC is discussed in Section
15.8.
The list above collects some relevant examples of the attempts
which have been made to esti-mate as correctly as possible the
expected production rates at the LHC. More details can befound in
the specific Chapters of this volume discussing particular physics
processes
Large uncertainties in the signal and background production
cross-sections, due to missinghigher-order corrections, structure
function parametrisations, energy scale for the QCD evolu-tion, as
well as models used for full event generation, remain. In addition,
despite the existenceof many higher-order QCD correction (K-factor)
calculations, not all processes of interest at theLHC have
benefited from this theoretical effort. In most cases they have
also not been embodiedin the Monte Carlo generator, so that proper
studies of their impact on the observed rates can-not be
undertaken. Therefore, the present studies consistently and
conservatively avoided theuse of K-factors, resorting to Born-level
predictions for both signal and backgrounds.
14.4.2 Signal observability
In the following sections, most of the results will be given for
integrated luminosities of 30 fb-1and 100 fb-1, which are expected
to be collected in three years of data taking at the initial
(low)luminosity and one year of data taking at the design (high)
luminosity respectively. The ulti-mate discovery potential is
evaluated for an integrated luminosity of 300 fb-1.
In most cases, the event selection for signal and background has
been performed as it might beexpected for off-line analysis. The
foreseen trigger LVL1/LVL2 menus were used for the dis-cussed
channels. The possible irreducible and reducible backgrounds are
extensively discussed.Given that the presently available tools for
physics modelling have inherent uncertainties, anal-yses in most
cases are straightforward; sophisticated statistical methods and
very detailed opti-misation of cuts are not applied.
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The observation of a given signal will be considered as possible
if a significance of five standarddeviations, defined according to
the naive estimator S/ , where S (B) is the expected numberof
signal (background) events, can be obtained. This includes the
relevant systematic uncertain-ties. If the number of expected
signal and background events is smaller than 25, Poisson
statis-tics has been used to compute the equivalent Gaussian
significance.
Table 14-1 Leading order cross-sections for some typical
processes at the LHC. Unless stated otherwise, thesenumbers have
been obtained by using PYTHIA 5.7 with CTEQ2L structure
functions.
Process Cross-section Comments
Inclusive HmH = 100 GeV
27.8 pb
WH with W → lνmH = 100 GeV
0.40 pb
ttH with one W → lνmH = 100 GeV
0.39 pb
Inclusive SUSY, ~ 1 TeV
3.4 pb ISAJET or PYTHIA
Inclusive bb 500 µb All di-jet processes used
Inclusive tt (mt = 175 GeV) 590 pb
Di-jet processes:1 jet pTj > 180 GeV, |η| < 3.2
3 jets pTj > 40 GeV, |η| < 3.2
4 jets pTj > 40 GeV, |η| < 3.2
13 µb
2.0 µb (0.7 µb)
0.4 µb (0.1 µb)
PYTHIA
NJETS (PYTHIA)
NJETS (PYTHIA)
Inclusive W 140 nb
Inclusive Z 43 nb
Wjj with W → lνwith 2 jets pTj > 15 GeV, |η|< 3.2
4640 pb VECBOS
Wbb with W → lν 69.3 pb Matrix element [14-18]+ HERWIG
Zjj with Z → llwith 2 jets pTj > 15 GeV, |η|< 3.2
220 pb VECBOS
Zbb with Z → ll 36 pb EUROJET + [14-19]
WW 71 pb
WZ 26 pb
Wγ with W → lνwith pTγ > 100 GeV, |η| < 2.5
210 fb
mg̃ mq̃
B
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14.5 Outline
This volume reviews the potential of the ATLAS detector for the
observability of a variety ofphysics processes, starting from the
studies of hadronic physics, precision measurements in theStandard
Model sector and CP-violation phenomena, continuing through the
searches for theHiggs boson(s) and supersymmetry, and ending with a
discussion of physics beyond the Stand-ard Model.
The volume begins with a discussion of QCD processes (Chapter
15), which have the largestrate and represent the dominant
background for new physics searches. Next is a discussion ofthe
properties of the W and Z gauge bosons and how ATLAS can improve
the precision meas-urements of the masses and couplings (Chapter
16). This is followed by a presentation of the B-physics programme;
methods for the measurement of CP violation, mixing and rare decays
arediscussed (Chapter 17). Next, measurements related to the top
quark and searches for otherheavy quarks/leptons are described
(Chapter 18). The Standard Model Higgs boson and its var-iants in
the minimal supersymmetric model provide a benchmark for LHC
physics; the largenumber of possible discovery channels are
analysed in detail (Chapter 19). Physics beyond theStandard Model
is the subject of the final two sections; the most popular
extension to Super-symmetry is discussed in detail and many
signatures that allow precise measurements in thissector are
presented (Chapter 20). Finally, signatures for other extensions to
the Standard Mod-el, such as new gauge bosons and technicolour, are
discussed (Chapter 21).
14.6 References
14-1 ATLAS Letter of Intent, CERN/LHCC/92-4, CERN 1992.
14-2 ATLAS Technical Proposal, CERN/LHCC 94-43, CERN 1994.
14-3 S. Glashow, Nucl. Phys. 22 (1961) 579;S. Weinberg, Phys.
Rev. Lett. 19 (1967) 1264;A. Salam, in: ‘Elementary Particle
Theory’, W. Svartholm,ed., Almquist and Wiksell,Stockholm,1968;H.D.
Politzer, Phys. Rev. Lett 30 (1973) 1346;D.J. Gross and F.E.
Waltzed, Phys. Rev. Lett. 30 (1973)1343.
14-4 H. Fritzsh and M. Gell-Mann, Proc. XVI Int. Conf. on High
Energy Physics, eds. J. D.Jackson and A. Roberts (Fermilab
1972).
14-5 M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49 (1973)
652;N. Cabibbo, Phys. Rev. Lett. 10 (1963) 531.
14-6 P. W. Higgs, Phys. Rev. Lett. 12 (1964) 132; Phys. Rev. 145
(1966) 1156;F. Englert and R. Brout, Phys. Rev. Lett 13 (1964)
321;G. S. Guralnik, C. R. Hagen and T. W. Kibble, Phys. Rev. Lett
13 (1964) 585.
14-7 L. Maiani, G. Parisi and R. Petronzio, Nucl. Phys. B136
(1979) 115;N. Cabbibo, L. Maiani, G. Parisi and R. Petronzio, Nucl.
Phys. B158 (1979) 295;R. Dashen and H. Neuberger, Phys. Rev. Lett.
50 (1983) 1897;D. J. E. Callaway, Nucl. Phys. B233 (1984) 189;M. A.
Beg, C. Panagiatakopolus and A. Sirlin, Phys. Rev. Lett 52 (1984)
883;M. Lindner, Z. Phys. C31 (1986) 295.
14-8 T. Hambye and K. Riesselmann, Phys. Rev. D55 (1997)
7255.
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14-9 C. Quigg, B.W. Lee and H. Thacker, Phys. Rev. D16 (1977)
1519;M. Veltman, Acta Phys. Polon. B8 (1977) 475.
14-10 J. Wess and B. Zumino, Nucl. Phys. B70 (1974) 39.
14-11 For a review, see K.D. Lane hep-9605257 (1996).
14-12 F. Paige and S. Protopopescu, in Supercollider Physics, p.
41, ed. D. Soper (WorldScientific, 1986).
14-13 T. Sjostrand, Comp. Phys. Comm. 82 (1994) 74.
14-14 G. Marchesini et al., Comp. Phys. Comm. 67 (1992) 465.
14-15 F.A. Berends and H. Kuijf, Nucl.Phys.B353 (1991) 59.
14-16 F.A. Berends, H. Kuijf, B. Tausk and W.T. Giele,
Nucl.Phys.B357 (1991) 32;W. Giele, E. Glover, D. Kosower, Nucl.
Phys. B403 (1993) 633.
14-17 The CDF Collaboration, F. Abe et al., Phys. Rev. D41
(1990) 2330; Phys. Rev. Lett. 61 (1988)1819.
14-18 M.L. Mangano, Nucl. Phys. B405 (1993) 536.
14-19 B. van Eijk and R. Kleiss, in [14-24], page 183.
14-20 S. Baranov and M. Smizanska, ‘Beauty production overwiew
from Tevatron to LHC’,ATLAS Internal Note ATL-PHYS-98-133 (1998);P.
Eerola,’ The inclusive muon cross-section in ATLAS’, ATLAS Internal
Note ATL-PHYS-98-120 (1998).
14-21 D0 Collaboration, S. Abachi et al., Phys. Rev. Lett. 74
(1996) 3548; Phys. Lett. B370 (1996)239.
14-22 CDF Collaboration, F. Abe et al., Phys. Rev. Lett. 71
(1993) 500; Phys. Rev. Lett. 71 (1993)2396; Phys. Rev. Lett. 71
(1993) 2537; Phys. Rev. Lett. 75 (1995) 1451; Phys. Rev. D50
(1996)4252; Phys. Rev. D53 (1996) 1051.
14-23 T. Sjostrand, Comp. Phys. Comm. 82 (1994) 74. The
supersymmetry extensions aredescribed in S. Mrenna, Comp. Phys.
Comm. 101 (1997) 232.
14-24 Proceedings of the Large Hadron Collider Workshop, Aachen,
1990, edited by G. Jarlskogand D. Rein, CERN 90-10/ECFA 90-133.
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15 QCD processes at the LHC
15.1 Introduction
The study of QCD processes at the LHC will serve two main goals.
First the predictions of QCDwill be tested and precision
measurements will be performed, allowing additional constraints
tobe established e.g. on the distribution of partons in the proton,
or providing measurements ofthe strong coupling constant αs at
various scales. Second QCD processes represent a major partof the
background to other Standard Model processes and signals of new
physics at the LHCand thus need to be understood precisely in the
new kinematic region available here. Devia-tions from the QCD
expectations might themselves also indicate the occurrence of new
physics,as in the case of compositeness for the jet transverse
energy and di-jet invariant mass and angu-lar distributions.
Furthermore, the production cross-sections for almost all processes
are con-trolled by QCD.
Tests of QCD can be performed by comparing measurements to fixed
order (either LO (leadingorder) or NLO (next-to-leading order))
calculations or to leading-log Monte Carlo programswhich contain LO
matrix elements and approximate higher orders through the use
ofparton showers (and also include the hadronisation of the
partonic system). Perturbative QCDcan also be tested by extracting
(or constraining) the fundamental parameter αs. The
differencebetween a LO and a NLO calculation is quantified in the
K-factor; the K-factor is defined as theratio between the
cross-section at NLO to the one at LO. The K-factor can become
significantlylarger than 1, especially when new sub-processes
appear at next-to-leading order. Calculationsat next-to-leading
order are mostly restricted to parton level and often performed by
numericalintegration of the corresponding matrix elements.
This chapter gives an overview of different measurements of QCD
processes [15-1], [15-2], [15-3] to be performed with ATLAS,
classified by the main characteristics (or main selection
criteria)of the final state. Besides a qualitative overview, a few
examples are given where first quantita-tive investigations of the
potential of ATLAS have been performed. The organisation of
thechapter is as follows: the next section contains a brief summary
on the present knowledge ofparton densities and some perspectives
for improvements before the start of LHC. Then meas-urements of
properties of minimum-bias events (Section 15.3) are discussed,
followed by a de-scription of studies of hard diffractive
scattering (Section 15.4). Next, the information to bededuced from
the measurement of jets (Section 15.5) is described, followed by a
section on pho-ton physics (Section 15.6) and one concerning the
production of Drell-Yan pairs and heavygauge bosons (Section 15.7).
Before concluding, the production of heavy flavours (charm, bot-tom
and top, Section 15.8) is discussed.
Unless stated differently in the corresponding sections, the
standard trigger settings have beenused. The signatures listed in
[15-4] for the first level (and the second level) of the trigger
systemconsist mainly of inclusive signatures. It is foreseen to
accept a fraction of events with lowerthresholds and it is possible
to include specific signatures (esp. at the higher levels of the
triggersystem) combining different objects and thus allowing
lowering of the corresponding thresh-olds. One important exception
is the case of hard diffraction and the case of minimum-biasevents,
where dedicated triggers will have to be employed.
2 2→
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15.2 Knowledge of the proton structure
15.2.1 Global parton analyses and parton kinematics at the
LHC
The calculation of the production cross-section at the LHC both
for interesting physics process-es and their backgrounds relies
upon a knowledge of the distribution of the momentum fractionx of
the partons in the proton in the relevant kinematic range. These
parton distribution func-tions (pdf’s) are determined by global
fits (see [15-5] for a pedagogical overview) to data
fromdeep-inelastic scattering (DIS), Drell-Yan (DY), jet and direct
photon production at current ener-gy ranges. Two major groups, CTEQ
[15-6] and MRS [15-7], provide regular updates to the par-ton
distributions when new data and/or theoretical developments become
available.
Lepton-lepton, lepton-hadron and hadron-hadron interactions
probe complementary aspects ofperturbative QCD (pQCD).
Lepton-lepton processes provide clean measurements of αs(Q2) andof
the fragmentation functions of partons into hadrons. Measurements
of deep-inelastic scatter-ing structure functions (F2,F3) in
lepton-hadron scattering and of lepton pair production
cross-sections in hadron-hadron collisions provide the main source
of information on quark distribu-tions qa(x,Q2) inside hadrons. At
leading order, the gluon distribution function g(x,Q2) enters
di-rectly in hadron-hadron scattering processes with direct photon
production and jet final states.Modern global parton distribution
fits are carried out to next-to-leading order (NLO) which al-lows
qa(x,Q2), g(x,Q2) and the strong coupling αs(Q2) to all mix and
contribute in the theoreticalformulae for all processes.
Nevertheless, the broad picture described above still holds to
somedegree in global pdf analyses. In pQCD, the gluon distribution
is always accompanied by a fac-tor of αs, in both hard scattering
cross-sections and in the evolution equations for the parton
dis-tributions. Thus, the determination of αs and the gluon
distribution is, in general, a stronglycoupled problem. One can
determine αs separately from e+e- interactions or determine αs
andg(x,Q2) jointly in a global pdf analysis. In the latter case,
though, the coupling of αs and thegluon distribution may not lead
to a unique solution for either (see e.g. in [15-8]).
Currently, the world average of αs(MZ) is of the order of 0.118
− 0.119 [15-9]. The average valuefrom LEP is 0.121 while the DIS
experiments prefer a somewhat smaller value (of the order of0.116 −
0.117). Since global pdf analyses are dominated by the high
statistics DIS data, theywould favour the values of αs closer to
the lower DIS values. The more logical approach is toadopt the
world average and concentrate on the determination of the pdf’s.
This is what bothCTEQ and MRS currently do. One can either quote a
value of αs(MZ) or the value of ΛQCD. Forthe latter case, however,
the renormalisation scheme used together with the number of
flavourshas to be clearly specified. Usually the MS scheme is used.
The specification of the number offlavours is important as the
value of αs has to be continuous across flavour thresholds. A
rangeof αs(MZ) of 0.105 to 0.122 corresponds to the range of 100
< ΛQCD < 280 MeV for five flavoursand to 155 < ΛQCD <
395 MeV for four flavours.
The data from DIS, DY, direct photon and jet processes utilised
in pdf fits cover a wide range inx and Q. The kinematic ‘map’ in
the (1/x,Q) plane of the data points used in a recent parton
dis-tribution function analysis is shown in Figure 15-1. The HERA
data (H1 and ZEUS) are predom-inantly at low x, while the fixed
target DIS and DY data are at higher x. There is
considerableoverlap, however, with the degree of overlap increasing
with time as the statistics of the HERAexperiments increases. The
DGLAP equations [15-10] in pQCD describe the change of the par-ton
distributions with Q2. The NLO DGLAP equations should describe the
data over the wholekinematic range shown in Figure 15-1. At very
low x, however, the DGLAP evolution is be-lieved to be no longer
applicable and a BFKL [15-11] description must be used. No clear
evi-
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dence of BFKL physics is seen in the current range of data; thus
all global analyses useconventional DGLAP evolution of the pdf’s.
There is a remarkable consistency between thedata in the pdf fits
and the NLO QCD theory to fit these. Over 1300 data points are
shown inFigure 15-1 and the χ2/DOF for the fit of theory to data is
of the order of 1.
In Figure 15-2 the kinematics appropriate forthe production of a
state with mass M and ra-pidity y at the LHC is shown [15-12]. For
ex-ample, to produce a state of mass 100 GeV atrapidity y = 2
requires partons of x values 0.05and 0.001 at a Q2 value of 104
GeV2. The figurealso shows another view of the kinematic cov-erage
of the fixed target and the HERA experi-ments used in the pdf
fits.
15.2.2 Properties and uncertainties ofparton distribution
functions
Figure 15-3 shows the parton distributions forthe different
quark flavours and the gluon asobtained from the CTEQ4M
distribution [15-8]for a scale of Q2 = 20 GeV2, in Figure 15-4
thecorresponding distributions are shown for ascale of Q2 = 104
GeV2. Clearly visible is thedominance of the gluon distribution for
smallparton momenta. In addition the violation ofthe flavour
symmetry for u and d sea quarkscan be seen.
Figure 15-1 A kinematic map of data points in the (1/x,Q) plane
from different processes used in a global fit ofparton densities
(from [15-5]).
100 101 102 103 104
1/X
100
101
102
Q (
GeV
)DIS (fixed target)HERA (’94)DYW-asymmetryDirect-γJets
Figure 15-2 Parton kinematics at the LHC (from [15-12]) in the
(x,Q2) kinematic plane for the production ofa particle of mass M at
rapidity y (dotted lines).
10-7 10-6 10-5 10-4 10-3 10-2 10-1 100100
101
102
103
104
105
106
107
108
109
fixedtarget
HERA
x1,2 = (M/14 TeV) exp(±y)
Q = M
M = 10 GeV
M = 100 GeV
M = 1 TeV
M = 10 TeV
66y = 40 224
Q2
(G
eV2 )
x
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Parton distribution determined at a given x and Q2 ‘feed-down’
to lower values of x at highervalues of Q2. The accuracy of the
extrapolation to higher Q2 depends both on the accuracy ofthe
original measurement and any uncertainty on αs(Q2). For the
structure function F2, the typi-cal measurement uncertainty at
medium to large x is of the order of 3%. At high Q2 (about105 GeV2)
there is an extrapolation uncertainty of 5% in F2 due to the
uncertainty in αs.
Figure 15-6 shows the gluon distribution as afunction of x for
five different values of Q2, us-ing the CTEQ4M distribution. Most
of the evo-lution takes place at low Q2 and there is onlylittle
evolution for x values around 0.1. In con-trast, at an x value of
0.5, the gluon distribu-tion decreases by a factor of approximately
30from the lowest to the highest Q2.
Global fits can also be performed using lead-ing-order (LO)
matrix elements, resulting inleading-order parton distribution
functions.Such pdf’s are preferred when leading ordermatrix element
calculations (such as in MonteCarlo programs like HERWIG [15-13]
and PY-THIA [15-14]) are used. The differences be-tween LO and NLO
pdf’s, though, areformally NLO; thus the additional error
intro-duced by using a NLO pdf should not be sig-nificant. A
comparison of the LO and NLOgluon distribution is shown in Figure
15-7 forthe CTEQ4 set, where the LO distribution is CTEQ4L and the
NLO distribution is CTEQ4M.The differences get even smaller at
larger Q2 values.
Figure 15-3 Parton distributions for the CTEQ4M pdfat Q2 = 20
GeV2. The gluon distribution has beenreduced by a factor of 10.
Figure 15-4 Parton distributions for the CTEQ4M pdfat Q2 = 104
GeV2. The gluon distribution has beenreduced by a factor of 10.
0
0.5
1
1.5
2
10-4
10-3
10-2
10-1
1x
xf(x
,Q2 )
0
1
2
3
4
10-4
10-3
10-2
10-1
1x
xf(x
,Q2 )
Figure 15-5 Gluon distribution for the CTEQ1L,CTEQ2L, CTEQ3L and
CTEQ4L pdf’s at a value ofQ2 = 5 GeV2 (from [15-5]).
10-4
10-3
10-2
10-1
1
10
10-3
10-2
10-1
x
x*G
(x,Q
2 )
Parton Density of the Nucleon
Gluon Distribution
Q2 = 5 GeV2
CTEQ4LCTEQ3LCTEQ2LCTEQ1L
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Many of the comparisons in this document have been performed
with the CTEQ2L pdf, a pdfthat is on the order of five years old
[15-15]. A comparison of the gluon distribution forCTEQ1L, CTEQ2L,
CTEQ3L and CTEQ4L is shown in Figure 15-5. With increasing amounts
ofdata included from HERA, the tendency has been for the low x
pdf’s to increase. The relative in-creases are reduced at higher
values of Q2.
Figure 15-6 Gluon densities as a function of x fromthe CTEQ4M
parton distribution set for five differentQ2 values: 2, 10, 50, 104
and 106 GeV2 (from [15-5]).
Figure 15-7 Comparison of the gluon distributionfrom the CTEQ4L
(leading order) and the CTEQ4M(next-to-leading order) global fit
(from [15-5]).
Figure 15-8 Normalised quark-gluon luminosity func-tion (as a
function of ) for variations inthe gluon distribution which are
consistent with exist-ing DIS and DY datasets (from [15-17]). The
dottedcurve shows a toy model with more quarks at x > 0.5for
large Q2 than in CTEQ4M.
Figure 15-9 Normalised gluon-gluon luminosity func-tion (as a
function of ) for variations inthe gluon distribution, which are
consistent with exist-ing DIS and DY datasets (from [15-17]).
10-4
10-3
10-2
10-1
1
10
10 2
10-3
10-2
10-1
x
x*G
(x,Q
2 )
Parton Density of the Nucleon
CTEQ4M Gluon Distribution
Q2 = 2 GeV2
Q2 = 10 GeV2
Q2 = 50 GeV2
Q2 = 104 GeV2
Q2 = 106 GeV2
10-4
10-3
10-2
10-1
1
10
10-3
10-2
10-1
x
x*G
(x,Q
2 )
Parton Density of the Nucleon
Gluon Distribution
Q2 = 5 GeV2
CTEQ4MCTEQ4L
τ x1 x2⋅= τ x1 x2⋅=
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In addition to having the best estimate for the values of the
pdf’s in a given kinematic range. itis also important to understand
the allowed range of variation in the pdf’s, i.e. their
uncertain-ties. The conventional method of estimating parton
distribution uncertainties is to compare dif-ferent published
parton distributions. This is unreliable since most published sets
of partondistributions (e.g. from CTEQ and MRS) adopt similar
assumptions and the differences betweenthe sets do not fully
explore the uncertainties that actually exist. Ideally, one might
hope to per-form a full error analysis and provide an error
correlation matrix for all the parton distributions(see e.g.
[15-16]). This goal may be difficult to carry out for two reasons.
Experimentally, only asubset of the experiments usually involved in
the global analyses provide correlation informa-tion on their data
sets in a way suitable for the analysis. Even more important, there
is no estab-lished way of quantifying the theoretical uncertainties
for the diverse physical processes thatare used and uncertainties
due to specific choices of parametrisations. Both of these are
highlycorrelated.
As the LHC is essentially a gluon-gluon collider and many hadron
collider signatures of physicsboth within and beyond the Standard
Model involve gluons in the initial state, it is important
toestimate the theoretical uncertainty due to the uncertainty in
the gluon distribution. The mo-mentum fraction carried by gluons is
42% with an accuracy of about 2% (at Q = 1.6 GeV in theCTEQ4
analysis), determined from the quark momentum fraction using DIS
data. This impor-tant constraint implies that if the gluon
distribution increases in a certain x range, momentumconservation
forces it to decrease in another x range. To estimate the
uncertainty on the gluondistribution, an alternative approach has
been carried out [15-17]: the (four) parameters of thegluon
distribution (based on the CTEQ4 set) have been varied
systematically in a global analy-sis and the resulting parton
distributions have been compared to the DIS and Drell-Yan
datasetsmaking up the global analysis database. Only DIS and
Drell-Yan datasets were used, as the ex-perimental and theoretical
uncertainties for these processes are under good control. Only
thosepdf’s that do not clearly contradict any of the (DIS and
Drell-Yan) data sets in the global analy-sis database were kept.
The variation of the gluon distribution obtained with this
proced