The Hadronic Final State at HERA

The Hadronic The Hadronic Final State at Final State at HERAHERA

Rainer MankelRainer MankelDESYDESY

for the H1 & ZEUS collaborationsfor the H1 & ZEUS collaborations

Sino-German Workshop on Sino-German Workshop on Frontiers in QCD Frontiers in QCD Hamburg,Hamburg, 21-Sep-200621-Sep-2006

21-Sep-2006 R. Mankel: The Hadronic Final State at HERA 2

HERAHERA HERA ep collider at HERA ep collider at

DESY: a unique machineDESY: a unique machine Presently the only Presently the only

operating high energy operating high energy collider in Europe collider in Europe

HERA collides protons HERA collides protons and electrons/positrons and electrons/positrons at at s=318 GeVs=318 GeV

HERA-II run features HERA-II run features upgraded luminosity upgraded luminosity and polarization and polarization

Colliding beam Colliding beam experiments: H1 and experiments: H1 and ZEUSZEUS

HERAHERA

H1H1

ZEUZEUSS


HERA LuminosityHERA Luminosity HERA-II has been surpassing HERA-II has been surpassing

all previous luminosity all previous luminosity achievementsachievements

Already the HERA-II eAlready the HERA-II e--p run p run (Dec 04-Jun 06) has (Dec 04-Jun 06) has delivered delivered more collisions more collisions than six years of than six years of HERA-IHERA-I (1995-2000) (1995-2000)

Since end Jun 06, machine Since end Jun 06, machine has switched back to ehas switched back to e++p p (~40 pb(~40 pb-1-1 since) since)

O(O(500 pb500 pb-1-1) per expt ) per expt expected by end of data-expected by end of data-taking in mid 2007taking in mid 2007

Also analysis of HERA-I data Also analysis of HERA-I data is still going strongis still going strong


Typical Structure of Hadronic Final Typical Structure of Hadronic Final States at HERAStates at HERA

Current jetCurrent jet

Proton Proton remnant remnant

QQ22: virtuality of : virtuality of exchanged photon exchanged photon (boson)(boson) Q2 > 1 GeV2 : deep-

inelastic scattering (DIS)

Q2 < 1 GeV2 : photo-production (PHP)

x (xx (xBjBj): fraction of ): fraction of proton momentum proton momentum carried by struck carried by struck quarkquark


Comparison of Hadronic Final Comparison of Hadronic Final State StructureState Structure

contains main features of contains main features of energetic hadron interaction energetic hadron interaction ((proton remnantproton remnant))

less complexless complex than hadron- than hadron-hadron interactionhadron interaction

clean reconstruction of clean reconstruction of kinematic variableskinematic variables

ideal laboratory for studying ideal laboratory for studying QCDQCD

Quark jetQuark jet Anti-quark jetAnti-quark jet

ee++ee interaction interactionQuark jetQuark jet

Quark jetQuark jet

Proton Proton remnantremnant


hadron-hadron hadron-hadron interactioninteraction


Quark jetQuark jet

eep interactionp interaction


ee pp

Colliding mode detectors can Colliding mode detectors can generally measure current generally measure current jet & scattered electron very jet & scattered electron very well (“central region”)well (“central region”) in these areas, also in these areas, also

theoretical approaches are theoretical approaches are tested & tuned besttested & tuned best

in PHP, the scattered in PHP, the scattered electron usually escapes electron usually escapes along the beam pipealong the beam pipe

The The proton remnantproton remnant emerges close to beam pipe emerges close to beam pipe & is & is less accessibleless accessible these areas also pose big these areas also pose big

challenges to theorychallenges to theory Additional jets can arise from Additional jets can arise from

more complex processesmore complex processes

Colliding Beam DetectorsColliding Beam Detectors

protonproton

remnantremnant

current jetcurrent jet

scattered electronscattered electron


Some “Frontier” Questions Some “Frontier” Questions Related to Hadronic Final StateRelated to Hadronic Final State

How relevant are higher orders in How relevant are higher orders in perturbative QCD?perturbative QCD?

How well do we understand the How well do we understand the workings of QCD in the workings of QCD in the forward forward areaarea??

Can we distinguish Can we distinguish evolution evolution schemesschemes in parton cascades? in parton cascades?

At which accuracy can we describe At which accuracy can we describe production of production of heavy flavor?heavy flavor?


OutlineOutline

1. Inclusive Jets1. Inclusive Jets

3. Forward Jets3. Forward Jets

2. Multiple Jets2. Multiple Jets 4. D* -Jet 4. D* -Jet CorrelationsCorrelations

5. Beauty Cross 5. Beauty Cross Section from Section from

6. Charm & 6. Charm & Beauty in PHP Beauty in PHP

DijetDijet


1. Inclusive 1. Inclusive JetsJets


Inclusive Jet Inclusive Jet Production in NC Production in NC DISDIS

Jet search in Breit frameJet search in Breit frame virtual photon purely space-like, virtual photon purely space-like,

defines longitudinal directiondefines longitudinal direction optimal separation of proton optimal separation of proton

remnant & recoiling parton remnant & recoiling parton High EHigh Ejetjet

T,BT,B mainly sensitive to hard QCD mainly sensitive to hard QCD

processesprocesses ideal testing ground for pQCDideal testing ground for pQCD

Experiment and NLO calculations Experiment and NLO calculations agree over agree over five orders (!) of five orders (!) of magnitudemagnitude in the Q in the Q22 spectrum spectrum

Impressive success for QCD Impressive success for QCD theorytheory

ExperimentalExperimental uncertainty (mainly uncertainty (mainly jet energy scale) tends to be jet energy scale) tends to be smaller than theoreticalsmaller than theoretical uncertainly of NLO calculationsuncertainly of NLO calculations


Inclusive Jet Production in NC DIS Inclusive Jet Production in NC DIS (cont’d)(cont’d)

EEjetjetT,BT,B dependence dependence

becomes less becomes less steep as Qsteep as Q22 increasesincreases

Measurements Measurements well described by well described by NLO QCDNLO QCD


Inclusive Jet Inclusive Jet Production (cont’d)Production (cont’d)

Differential cross sections vs Differential cross sections vs EET,BT,B

jetjet and Q and Q22 can be used to can be used to extract strong coupling constantextract strong coupling constant

Running of Running of ss clearly seen clearly seen Shape Shape agreesagrees with theoretical with theoretical

expectationexpectation Value of Value of ss(M(MZZ) in accord with ) in accord with

world averageworld average competitive competitive precisionprecision

Measuring whole Q range Measuring whole Q range in one in one analysisanalysis avoids systematic avoids systematic uncertainties that arise when uncertainties that arise when combining different experimentscombining different experiments


2. Multi-Jet 2. Multi-Jet Final StatesFinal States


Multijet Final StatesMultijet Final States Historical note: in 1979, the Historical note: in 1979, the first direct first direct

observation of the gluonobservation of the gluon was made at was made at DESY, as a third jet in eDESY, as a third jet in e++ee-- annihilation annihilation

resulting from resulting from hard gluon radiationhard gluon radiation could estimate could estimate ss from relative rate from relative rate

Three-jet signatures can be seen as the Three-jet signatures can be seen as the modern HERA equivalentmodern HERA equivalent of this of this measurementmeasurement in Breit frame similar quite in Breit frame similar quite

picture as in epicture as in e++ee--

one jet emerging from one jet emerging from hard hard gluon radiationgluon radiation

can measure can measure s s from ratio of from ratio of

3-jet : 2-jet production 3-jet : 2-jet production


Tri-Jet Final State: Jet Energy SpectraTri-Jet Final State: Jet Energy Spectra

Jets classified Jets classified according to according to decreasing decreasing transverse energy transverse energy EET,BT,B

jetjet

Good descriptionGood description by by NLO* in O(NLO* in O(ss

33), even ), even at low Eat low ET,BT,B

jetjet

Effect of Effect of orderingordering

*NLOJET with CTEQ6*NLOJET with CTEQ6


Ratio of Tri-Jet to Di-Jet Ratio of Tri-Jet to Di-Jet ProductionProduction

Correlated uncertainties Correlated uncertainties largely largely cancelcancel in ratio in ratio

Ratio decreases with Ratio decreases with increasing Qincreasing Q22

reflects decreasing reflects decreasing strength of couplingstrength of coupling

well described by theorywell described by theory Absolute ratio can be used Absolute ratio can be used

to determine to determine ss(m(mZZ)) systematics systematics

complementary to complementary to inclusive jet inclusive jet measurementmeasurement

.)(.)(exp.)(0013.01179.0 0064.00046.0

0028.00046.0 thstats


ss Summary Summary

ss measurements from measurements from HERA have reached an HERA have reached an impressive level of impressive level of precisionprecision need help from theoryneed help from theory

Consistent both Consistent both internally & with other internally & with other experimentsexperiments

With more data to come With more data to come from from HERA-II HERA-II further further improvement expectedimprovement expected

C. GlasmanC. Glasman


3. Forward 3. Forward JetsJets


Forward JetsForward Jets Forward area is Forward area is

particularly sensitive to particularly sensitive to details in evolution of details in evolution of parton cascadeparton cascade

At low x, we do not probe At low x, we do not probe the valence structure of the valence structure of the proton, but rather the proton, but rather see universal structure of see universal structure of QCD radiationQCD radiation at work at work signature: signature: forward jetforward jet

This enables us to This enables us to examine different examine different mechanisms of parton mechanisms of parton cascade evolutionscascade evolutions


Dynamics of Parton Dynamics of Parton EvolutionEvolution

DGLAPDGLAPDokshitzer-Gribov-Lipatov-Dokshitzer-Gribov-Lipatov-

Altarelli-ParisiAltarelli-Parisi

Evolution in powers of ln Evolution in powers of ln QQ22

Strongly orderered in kStrongly orderered in kTT

Well established at high x Well established at high x and Qand Q22, but expected to , but expected to break downbreak down at low x at low x

Evolution in powers of Evolution in powers of ln 1/xln 1/x

Strongly orderered in Strongly orderered in xx

May be May be applicableapplicable at at low xlow x

BFKLBFKLBalitsky-Fadin-Kuraev-LipatovBalitsky-Fadin-Kuraev-Lipatov

CCFMCCFMCiafaloni-Catani-Fiorani-Ciafaloni-Catani-Fiorani-

MarchesiniMarchesini

Evolution in both ln QEvolution in both ln Q2 2 and and ln 1/xln 1/x

Bridge between DGLAP Bridge between DGLAP and BFKLand BFKL

Angular orderingAngular ordering May be May be applicableapplicable at low x at low x


Forward Jet Measurements Forward Jet Measurements (DIS)(DIS)

DGLAPDGLAP leading order suppressed leading order suppressed

by kinematicsby kinematics even with NLO, factor 2 even with NLO, factor 2

below databelow data at low x at low x need for higher orders?need for higher orders?

xxBjBj<0.004, 7<0.004, 7oo<<jetjet<20<20oo, x, xjetjet>0.035>0.035

CCFMCCFM distribution distribution too hardtoo hard comparatively poor comparatively poor

description of the description of the datadata

CDM (similar to BFKL)CDM (similar to BFKL) generally goodgenerally good

DGLAP with resolved virtual photonDGLAP with resolved virtual photon similar to CDM, but fails to similar to CDM, but fails to describe forward+dijet sampledescribe forward+dijet sample

Cuts Cuts designed to designed to

enhance enhance BFKL effectsBFKL effects


Forward Jets SummaryForward Jets Summary

Limitations of the pure DGLAP Limitations of the pure DGLAP approach approach clearly seenclearly seen in the in the forward areaforward area higher order parton emissions break higher order parton emissions break

ordering schemeordering scheme Calculations which include such Calculations which include such

processes (CDM) achieve better processes (CDM) achieve better description of the datadescription of the data


4. Charm & 4. Charm & Jets in Photo-Jets in Photo-ProductionProduction


D*-Jet Correlations in Photo-D*-Jet Correlations in Photo-ProductionProduction

Direct photon Direct photon (x(x~1)~1)

Resolved photon Resolved photon (x(x<<1)<<1)

Charm quark mass provides Charm quark mass provides hard scalehard scale even for quasi-real photon (Q even for quasi-real photon (Q22~0)~0) perturbative QCD (pQCD) applicable over perturbative QCD (pQCD) applicable over full phase spacefull phase space

Several basic processes expected to contribute to photo-production of Several basic processes expected to contribute to photo-production of charmcharm

Correlations between Correlations between D*D* and a separate and a separate additional jetadditional jet, or between two , or between two jets (one of them tagged by a D*) allow a very jets (one of them tagged by a D*) allow a very fine-grained comparisonfine-grained comparison of of different theoretical approachesdifferent theoretical approaches


How Models Treat Charm How Models Treat Charm ProductionProduction

PYTHIA: LO direct photon-gluon fusion, charm excitation PYTHIA: LO direct photon-gluon fusion, charm excitation & hadron-like. Higher order contributions simulated with & hadron-like. Higher order contributions simulated with leading-log parton showers in collinear approach. leading-log parton showers in collinear approach.

CASCADE: LO in kCASCADE: LO in kTT factorization approach. Higher order factorization approach. Higher order corrections simulated with initial state parton showers corrections simulated with initial state parton showers (CCFM evolution)(CCFM evolution)

FMNR (FMNR (Frixione-Mangano-Nason-RidolfiFrixione-Mangano-Nason-Ridolfi): NLO calculation ): NLO calculation (O((O(ss

22)), massive scheme in collinear factorization )), massive scheme in collinear factorization approachapproach

ZMVFNS (ZMVFNS (Zero mass variable flavor number schemeZero mass variable flavor number scheme) : NLO ) : NLO calculation (O(calculation (O(ss

22)) in collinear approach, neglecting )) in collinear approach, neglecting charm masscharm mass


D*-Jet Correlations: D*-Jet Correlations: SpectraSpectra

Data show marked Data show marked difference in shape: jets on difference in shape: jets on average more forward than average more forward than D*D*

indicates presence of a indicates presence of a hard non-charm partonhard non-charm parton in in the forward directionthe forward direction

dominant mechanism: dominant mechanism: hard hard gluon radiationgluon radiation from proton from proton

[a PYTHIA variant with [a PYTHIA variant with only only direct photondirect photon does not does not show this difference]show this difference]

All models include this & All models include this & describe effect welldescribe effect well


D*-Tagged Dijets: Transition D*-Tagged Dijets: Transition from Resolved to Direct PHPfrom Resolved to Direct PHP

resolveresolvedd

directdirect resolveresolvedd

directdirect

xxobsobs = fraction of = fraction of

photon energy photon energy participating in hard participating in hard interactioninteraction xx

obsobs ~1 : ~1 : directdirect PHP PHP

xxobsobs <<1 : <<1 : resolvedresolved

PHPPHP Sensitive to gluon Sensitive to gluon

emission in initial stateemission in initial state All calculations All calculations

underestimate relative underestimate relative contribution in xcontribution in x

obsobs <0.6 region<0.6 region


D*-Jet Correlations: Relative D*-Jet Correlations: Relative Azimuth AngleAzimuth Angle

In collinear approximation, In collinear approximation, process process ggcc should lead cc should lead to to back-to-back back-to-back topologytopology

But data show: only 25% of But data show: only 25% of cross section are strictly cross section are strictly back-to-backback-to-back

Remainder can only be Remainder can only be described with significant described with significant contributions from contributions from higher higher order QCD radiationorder QCD radiation

Neither PYTHIA nor Neither PYTHIA nor CASCADE describe full CASCADE describe full rangerange

NLO calculation too low for NLO calculation too low for <120<120oo relevance of relevance of higher order contributionshigher order contributions

Rich testing ground for QCD, Rich testing ground for QCD, challenging for theorychallenging for theory


5. Total 5. Total Beauty Cross Beauty Cross

SectionSection


Total Beauty Cross SectionTotal Beauty Cross Section Very stringent QCD testVery stringent QCD test

large mlarge mbb pQCD reliable in full phase pQCD reliable in full phase space?space?

Measurements in pp, Measurements in pp, , , N and pN have N and pN have shown large discrepanciesshown large discrepancies

Experimental challenge:Experimental challenge: beauty often beauty often tagged tagged with high pwith high pTT electron electron

or muon (secondary vertex, or por muon (secondary vertex, or pT T relative relative to jet) to jet)

measurement restricted to high pmeasurement restricted to high pTT b quark b quark extrapolation uncertaintyextrapolation uncertainty

Alternative: correlation signatureAlternative: correlation signature example:example: di-muon di-muon

Study of di-muon event signatures allows Study of di-muon event signatures allows to use to use low plow ptt

μμ thresholds thresholds measure the total bb cross sectionmeasure the total bb cross section

BB

BB--

DD

DD--

Interaction Interaction vertexvertex

(D)(D)--

--


Extraction of Beauty Extraction of Beauty SignalSignal

Light flavor background Light flavor background similar in (+-) and (similar in (+-) and () ) mass spectra mass spectra exploit exploit for subtractionfor subtraction

Bethe-Heitler and Bethe-Heitler and quarkonia background quarkonia background suppressed by suppressed by non-non-isolationisolation requirement requirement

Bethe-Heitler, quarkonia Bethe-Heitler, quarkonia and cc background and cc background subtracted using MC subtracted using MC (PYTHIA, RAPGAP, (PYTHIA, RAPGAP, HERWIG, GRAPE)HERWIG, GRAPE)


Beauty from Di-Muons: Beauty from Di-Muons: Accessible Quark pAccessible Quark pTT Range Range

Method is sensitive down Method is sensitive down to pto pTT(b)~0(b)~0

Small extrapolation Small extrapolation uncertaintyuncertainty

ppTT distribution of tagged b distribution of tagged b quarksquarks


bb Cross Section from Di-Muon bb Cross Section from Di-Muon EventsEvents

nbsyststatGeVsXbbeptot )(8.43.5)(8.11.16)318)((

PHP: 5.8 nb PHP: 5.8 nb (FMNR,CTEQ5M)(FMNR,CTEQ5M)

DIS: 1.0 nb DIS: 1.0 nb (HVQDIS,CTEQ5F4)(HVQDIS,CTEQ5F4)nbpredictionQCDNLO

7.1

0.38.6:

For muons from b For muons from b decaysdecays

Wide phase Wide phase spacespace

Good Good agreement in agreement in shapeshape

Normalization Normalization underestimated underestimated by theoryby theory

Note: Note: PYTHIA+RAPGAP PYTHIA+RAPGAP scaled by 1.95xscaled by 1.95x


HERA bb Cross Section vs HERA bb Cross Section vs TheoryTheory

Wide range of Wide range of measurements measurements availableavailable

Measurements Measurements tend to be tend to be larger than NLOlarger than NLO

__


6. Charm & 6. Charm & Beauty Di-Beauty Di-

JetJet


Charm & Beauty Di-Jet Cross Charm & Beauty Di-Jet Cross Sections in PHPSections in PHP

Typical topology for charm & beauty Typical topology for charm & beauty production: ≥2 jetsproduction: ≥2 jets

A very elegant way to identify heavy A very elegant way to identify heavy quark production is to use lifetime quark production is to use lifetime tagstags

bc ,

bc ,

c+b lifetime leads to significantly c+b lifetime leads to significantly positive values of positive values of impact impact parameterparameter of charged tracks of charged tracks

can be measured with high can be measured with high resolution resolution silicon vertex detectorssilicon vertex detectors

signedsigned according to jet direction according to jet direction Allows Allows simultaneoussimultaneous determination determination

of charm & beauty rates in PHPof charm & beauty rates in PHP

I.P.I.P.


Charm & Beauty in PHP Di-Jet Charm & Beauty in PHP Di-Jet (cont’d)(cont’d)

Clear Clear excessexcess at positive impact parameter (significance) at positive impact parameter (significance) Since mSince mbb>>m>>mcc, decays of beauty hadrons have significantly , decays of beauty hadrons have significantly

higher number of tracks on averagehigher number of tracks on average Divide into 2 samples according to tracks associated to jet:Divide into 2 samples according to tracks associated to jet:

#tracks=1 : charm enriched#tracks=1 : charm enriched #tracks>1 : beauty enriched#tracks>1 : beauty enriched

)(

ceSignifican

Tracks with Tracks with ppTT>0.5 >0.5 GeV, GeV, 3030oo<<<150<150oo, #CST , #CST hits (rhits (r)≥2)≥2


Charm & Beauty in PHP Di-Charm & Beauty in PHP Di-Jet: Cross SectionsJet: Cross Sections

Cross section:Cross section: NLO (FMNR) agrees for charm, NLO (FMNR) agrees for charm,

but but factor 1.8 too lowfactor 1.8 too low for for beautybeauty

PYTHIA, CASCADE similarPYTHIA, CASCADE similar Shapes of pShapes of pTT

jet jet and and jetjet (not (not shown) reasonably well shown) reasonably well describeddescribed

xxobsobs = fraction of photon energy = fraction of photon energy

participating in hard interaction participating in hard interaction At low xAt low x

obsobs (resolved photon (resolved photon regime), regime), NLO calculation NLO calculation strongly underestimatesstrongly underestimates the the beauty cross sectionbeauty cross section

PYTHIA agrees in shapePYTHIA agrees in shape At xAt x

obsobs >0.85 (direct photon >0.85 (direct photon regime), models work generally regime), models work generally well (well (photon gluon fusion)photon gluon fusion)

QQ22<1 GeV<1 GeV22, 0.15<y<0.8, , 0.15<y<0.8, ppTT

jet1(2)jet1(2)>11(8) GeV, - 0.9<>11(8) GeV, - 0.9<jet1(2)jet1(2)<1.3 <1.3

resolvedresolved directdirect resolvedresolved directdirect


SummarySummary Wealth of measurements from HERA on structure of Wealth of measurements from HERA on structure of

hadronic final statehadronic final state only a small selection presentedonly a small selection presented unique facilityunique facility for QCD studies for QCD studies

NLO largely successful in describing experimental NLO largely successful in describing experimental datadata

Some challenging frontiers identifiedSome challenging frontiers identified QCD dynamics in vicinity of proton remnant (low x regime)QCD dynamics in vicinity of proton remnant (low x regime) resolved photo-productionresolved photo-production beauty cross sectionbeauty cross section

With largeWith large HERA-II data sample HERA-II data sample, and improvements , and improvements in in theorytheory, expect , expect further insightsfurther insights in QCD frontiers in QCD frontiers regarding the hadronic final stateregarding the hadronic final state


The EndThe End


Backup SlidesBackup Slides


Separation of Charm & Separation of Charm & BeautyBeauty

SS11-type events are -type events are dominated by light dominated by light flavors and charmflavors and charm light flavor light flavor

contribution contribution mostly mostly cancelscancels when when “-S“-S11” part is subtracted” part is subtracted

At moderate & large SAt moderate & large S22, , the Sthe S22-type sample is -type sample is dominated by beautydominated by beauty

Contributions can be Contributions can be disentangled by fitting disentangled by fitting corresponding spectra corresponding spectra from PYTHIA MCfrom PYTHIA MC

PPudsuds=1.44=1.440.05, P0.05, Pcc=1.45=1.450.14, 0.14, PPb b =1.98=1.980.22 0.22


The Hadronic Final State at HERA

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