Backgrounds to New Physics Signals Peter Ratoff Lancaster University 2003 CTEQ Summer School, Saint Feliou de Guixols, Catalonia.

Backgrounds to New Physics Signals

Peter RatoffLancaster University

2003 CTEQ Summer School, Saint Feliou de Guixols, Catalonia

An experimental physicist’s perspective ...

• Signatures of new physics• Importance of backgrounds - lessons from history• Some of the basic background processes• Theoretical review - ME/MC - W+jets comparison• The “Qaero” program • Tevatron Run II results:

• SM backgrounds processes• Examples of new physics searches

... illustrated with recent examples from Tevatron Run II

Or, why am I here ?

…and thanks to Joey Huston for use of some of hisCTEQ 2002 Summer School slides!

Signatures of New Physics

• Ws, jets, s, b quarks, missing ET

• … pretty much the same as signatures for SM physics

• How do we find new physics? By showing that its not ‘old’ physics!

– can be modifications to the rate of production

– … or modification to the kinematics, e.g.angular distributions

• Crucial to understand the QCD dynamics and normalization of both backgrounds to any new physics and to the new physics itself

• Some backgrounds can be measured in situ

– … but may still want to predict in advance, e.g. QCD backgrounds to H• For some backgrounds, need to rely on theoretical calculations, e.g. ttbb

backgrounds to ttH

… look at some examples

J.Huston, CTEQSS’02

Importance of Backgrounds?

A few lessons from history …– UA1 Monojets– CDF Run I inclusive jet cross-section– SM Higgs potential at the Tevatron

#1 : Monojets in UA1• UA1 monojets (1983-1984)

– Possible signature of new physics (SUSY, etc)

– A number of backgrounds were identified, but each was noted as being too small to account for the observed signal

• pp->Z + jets

|_ • pp->W + jets

|_ |_ hadrons +

• pp->W + jets

|_ l + • pp->W + jets

|_ |_ l +

jet

• …but the sum was not– “The sum of many small things is a big

thing.” G. Altarelli

• Can calculate from first principles or calibrate to observed cross sections for Z->e+e- and W->e

• Ellis, Kleiss, Stirling PL 167B, 1986.

J.Huston CTEQSS02

Consistent over 7 orders of magnitude deviation at high Et

Vital to understand QCD in order to perform precision/search physics

BUT

Run 1 inclusive jet cross section

#2 : CDF Inclusive Jet Cross-section

T. Shears IOP/Durham ‘03

Exotic explanations

• Composite quarks - Eichten, Lane and Peskin (1983) – contact term added to LO QCD

Lagrangian increased cross-section for high Et jets

s stops running: conspiracy between new SUSY particles, colour sextet

• new particle: (leptophobic) Z'

SM explanation

Important gluon-gluon and gluon-quark contributions at high EtGluon PDF @ high x not well known.

Run 2 - more high Et jets:

Test QCD at high Et

Discriminate between new physics and gluon PDF

New bins for Run 2

T.Shears IOP/Durham ‘03

but WH/ZH production more accessible ...

Associated production WH or ZH

Gluon fusion

#3 : SM Higgs searches at the Tevatron

For MH 135 GeV• H0 bb dominates … but rate falling rapidly• QCD background precludes gg H bb

For MH 135 GeV• Gauge boson decays dominate ( H0 WW )

SM Higgs decay branching ratios

Low mass Higgs sensitivity depends on• the integrated luminosity collected • b-quark jet tagging performance• mass resolution of reconstructed bb jets• a good understanding of all backgrounds

For MH 135 GeV: use the same basic strategy as LEP …… study associated production of ZH and WH

To the standard leptonic HZ channels add W l with H bb ... N.b. the qqbb channel is very difficult as the

QCD backgrounds are severe

Tevatron: low mass Higgs searches

SM Backgrounds to light Higgs productionWH (MH < 135 GeV) lbb

W+jets Wg* Wbb lbb, W Z/* lbb

Wg* Wjj ljj, W Z/* ljj (fake b jets)tt pairs

tt (W)Wbb (l)lbbsingle top

W tb Wbb lbb qg q’tb q’Wbb q’lbb

ZH (MH < 135 GeV) llbb/bbW/Z+jets

Zg* Zbb llbb/bb, Z Z/* llbb/ bbZg* Zjj lljj/jj, Z Z/* lljj/ jj (fake b jets)

Wg* Wbb (l)bb, W Z/* (l)bbWg* Wjj (l)jj, W Z/* (l)jj (fake b jets)

tt pairstt WWbb l()l()bb, (l)(l)bb

single top (bb only)W tb Wbb (l)bb

qg q’tb q’Wbb q’(l)bbQCD jets (bb only)

gg bb, gg jj (fake b jets)

Low mass Higgssearch at the

Tevatron

Need to understand:• W/Z+jets• top (tt pairs, single top)• QCD jets

Trilepton final states~ low bgnds but small rateGolden Modes: like-sign,like-flavour leptons

Like sign dileptons + jets~ many SM bgnds (VVV, Vtt, VVjj, tt, Vjjj)

Dileptons + ET

~ large SM bgnds(VV, tt, , tW)

Tevatron: high mass Higgs searches

gg fusion Assoc. prod

H VV(V=W,Z)

MH > 135 GeV

SM Backgrounds to heavy Higgs production

W/Z+H (MH > 135 GeV) W/Z VV l± l±jjVVV

e.g. W+W-W+ l+() jj l+()ttV

e.g. ttZ WWbbZ l±() jj bb l(l) l± l±jj XVVjj

e.g. WZjj l±() l(l) jj l± l±jj Xtt pairs

tt WWbb l ±()jjb(b) l±()jj l(q)(b) l± l±jj XVjjj + fake j e

e.g. Wjjj l±() jj j l±() jj “e” l± l±jj

gg H (MH > 135 GeV) ll VV

WW ll, Z Z/* ll, W Z/* l l(l) tt pairs

tt WWbb ll(bb) pairs

Z/* l()l()

High mass Higgssearch at the

Tevatron

Need to understand:• VVV, VV• VV+2 jets, V+3 jets• top (tt pairs, Vtt)• pairs

Tevatron: low mass Higgs searchesRun II Higgs/SUSY Working Group, October 2000Simplified generic detector, “unsophisticated” analysissmall S/B essential to understand all backgrounds!

ZH bb + llbb

• Generic QCD jets bgnd in bb (gg bb) cannot be reliably simulated• Study assumed = 50% of total bgnd from the other sources (CDF Run I)• Must be determined from real data!

Mangano, Nason, RidolfiPeterson frag.

The basic background processesNeed to understand:-

• W/Z boson production (+jets) • VV • V + 2 jets • VVV• VV+2 jets, V+3 jets

• top production • tt pairs • single top• Vtt

• Drell-Yan pairs (qq * ee, ,)• QCD jets• ...

Can investigate thesebackground processes:

• theoretically• experimentally• both (ideally)

Theoretical Review

• Tree level calculations

• Monte-Carlo interfaces – Les Houches Accords

• W + jets

• Parton showering

• Resummation of Large Logs

• Higher orders

Theoretical Predictions for New (Old) Physics

There are a variety of programs available for comparison of data to theory and/or predictions.– Tree level

Les Houches accord

– Leading log Monte CarloMC@NLO

– NnLO

– Resummed

Important to know strengths/weaknesses of each.

In general, agree quite well…but before you appeal to new physics, check theME. (for example using CompHEP)Can have ME corrections to MC or MC corrections to ME. (in CDF->HERPRT)

Perhaps biggest effort…include NLO MEcorrections in Monte Carlo programs…correct normalizations. Correct shapes. NnLO needed for precision physics.

Resummed description describes soft gluoneffects (better than MC’s)…has correct normalization (but need HO to get it); resummed predictions include non-perturbative effects correctly…may have to be put in by hand in MC’sthreshold kT

W,Z, Higgsdijet, direct

b space(ResBos)

qt space

Where possible, normalize to existing data.J.Huston, CTEQSS’02

• Good testing ground for parton showers, matrix elements, NLO• Background for new physics or old physics (e.g. top production)• Reasonable agreement for the leading order comparisons using VECBOS (but large scale dependence)

W + jets at the Tevatron

• Good agreement with NLO (and smaller scale dependence) for W + 1 jet


W + jets• For W + n jet production, typically

use Herwig (Herprt) for additional gluon radiation and for hadronization

• Can also start off with n-1 jets and generate additional jets using Herwig


) More Comparisons (VECBOS and HERWIG)

• Start with W + n jets from VECBOS • Start with W + (n-1) jets from VECBOS


More Comparisons• Start with W + n jets from VECBOS • Start with W + (n-1) jets from VECBOS


Tree Level Calculations• Leading order matrix

element calculations describe multi-body configurations better than parton showers

• Many programs exist for calculation of multi-body final states at tree-level

• CompHep– includes SM Lagrangian and several other

models, including MSSM– deals with matrix elements squared– calculates leading order 2-->4-6 in the final

state taking into account all QCD and EW diagrams

– color flow information; interface exists to Pythia

– great user interface

• Grace– similar to CompHep

• Madgraph– SM + MSSM– deals with helicity amplitudes– “unlimited” external particles (12?)– color flow information– not much user interfacing yet

• Alpha + O’Mega– does not use Feynman diagrams – gg->10 g (5,348,843,500 diagrams)


Monte Carlo Interfaces• To obtain full predictability for a

theoretical calculation, would like to interface to a Monte Carlo program (Herwig, Pythia, Isajet)– parton showering (additional jets)

– hadronization

– detector simulation

• Some interfaces already exist– VECBOS->Herwig (HERPRT)

– CompHep->Pythia

• A general interface accord was reached at the 2001 Les Houches workshop (“Physics at TeV Colliders”)

• All of the matrix element programs mentioned will output 4-vector and color flow information in such a way as to be universally readable by all Monte Carlo programs

• CompHep, Grace, Madgraph, Alpha, etc, etc

->Herwig, Pythia, Isajet


• The Les Houches accords will be implemented in all ME/MC programs that experimentalists and theorists use

• They will make it easy to generate the multi-parton final states crucial to much of the Run 2/HERA/LHC physics program and to compare the results from different programs

• experimentalists/theorists can all share common MC data sets

• They will make it possible to generate the pdf uncertainties for any cross sections

The Les Houches Accords 2001

• Accord #1 (MEMC):• PYTHIA, CompHEP, Wbbgen,• Madgraph, Herwig, Grace, AcerMC

• Accord #2 (PDFs in ME/MC):• Interface is as easy to use as PDFLIB (and easier to update) • First version has CTEQ6M, CTEQ6L, all CTEQ6 error PDFs and MRST2001 PDFs• Available in MCFM• See pdf.fnal.gov


Parton Showering

• Determination of the Higgs signal requires an understanding of the Higgs pT distribution at both LHC and Tevatron

– for example, for gg->HX->X, the shape of the signal pT distribution is harder than that of the background; this can be used to advantage

• To reliably predict the Higgs pT distribution, especially for low to medium pT region, have to include effects of soft gluon radiation

– can either use parton showering a la Herwig, Pythia, ISAJET or kT resummation a la ResBos

– parton showering resums primarily the (universal) leading logs while an analytic kT resummation can resum all logs with Q2/pT

2 in their arguments; but expect predictions to be similar and Monte Carlos offer a more useful format

• Where possible it’s best to compare pT predictions to a similar data set to ensure correctness of formalism; if data is not available, compare MC’s to a resummed calculation or at least to another Monte Carlo

– all parton showers are not equal!

Note the large difference between PYTHIA versions5.7 and 6.1. Which one is correct?

Higgs Pt case study


Changes in PYTHIA

• Older version of PYTHIA has more events at moderate pT

• Two changes from 5.7 to 6.1– A cut has been placed on the combination of z and Q2

values in a branching: u=Q2-s(1-z)<0 where s refers to the subsystem of hard scattering plus shower partons

• corner of emissions that do not respect this requirement occurs when Q2 value of space-like emitting parton is little changed and z value of branching is close to unity

• necessary if matrix element corrections are to be made to process

• net result is substantial reduction in amount of gluon radiation

• In principle affects all processes; in practice only gg initial states

– Parameter for minimum gluon energy emitted in space-like showers is modified by extra factor corresponding to 1/ factor for boost to hard subprocess frame

• result is increase in gluon radiation

• The above are choices, not bugs; which version is more correct?

Compare to ResBos

S. Mrenna80 GeV Higgs generated at the Tevatron with Pythia

Higgs Pt case study


Comparison of PYTHIA and ResBos for Higgs Production at LHC

• ResBos agrees much better with the more recent version of PYTHIA

– Suppression of gluon radiation leading to a decrease in the average pT of the produced Higgs

– Affects the ability of CMS to choose to the correct vertex to associate with the diphoton pair

• Note that PYTHIA does not describe the high pT end well unless Qmax

2 is set to s (14 TeV)

– Again, ResBos has the correct matrix element matching at high pT; setting Qmax

2=s allows enough additional gluon radiation to mimic the matrix element

Higgs Pt case study


Comparisons with Herwig at the LHC

• HERWIG (v5.6) similar in shape in PYTHIA 6.1 (and perhaps even more similar in shape to ResBos)

• Is there something similar to the u-hat cut that regulates the HERWIG behavior?

– Herwig treatment of color coherence?

Higgs Pt case study


Resummation of Large Logs

• A1, B1 and (a bit of) A2 are effectively in Monte Carlos (especially Herwig)

• A1,A2 and B1 for Higgs production are in current off-the-shelf version of ResBos– …as are C0 and C1 which

control the NLO normalization

• The B2 term has recently been calculated for ggH


The need for higher order…

John Campbell, FNAL

What would we like?Bruce Knuteson’s wish-list from the Run 2 Monte Carlo workshop

…all at NLO

What are we likely to get ?

Single top production• Harris et al - fully differential final states

Harris, Laene, Phaf, Sullivan and Weinzierl (2000)

Diboson prod’n e.g. pp WW leptons• Baur et al - lepton correlation only partially included

Baur, Han and Ohnemus (1995, 1996)• Dixon et al - full correlations, anomalous couplings

Dixon, Kunszt and Signer (1999)• MCFM - full correlations, singly resonant contributions

Campbell and Ellis (1999)

Inclusive jets• JETRAD - 1 and 2 jets only

Giele, Glover and Kosower (1993)• Giele, Kilgore - 3 jet production

Giele and Kilgore (2000)

NLO QCD Simulations

Drell-Yan + heavy flavours• MCFM - Wg* bb

Ellis and Veseli (1998)• MCFM - Zg* bb

Drell-Yan + jets• DYRAD - vector boson + 0 or 1 jets

Giele, Glover and Kosower (1993)• VECBOS - vector boson + 3 Z jets or 4 W jets

Berends, Kuijf, Tausk and Giele (1991)

John Campbell, FNAL

MCFM (Monte Carlo for Femtobarn Processes) J. Campbell and K. Ellis

• Goal is to provide a unified description of processes involving heavy quarks, leptons and missing energy at NLO accuracy

• There have so far been three main applications of this Monte Carlo, each associated with a different paper.

– Calculation of the Wbb background to a WH signal at the Tevatron. R.K.Ellis, Sinisa Veseli, Phys. Rev. D60:011501 (1999), hep-ph/9810489.

– Vector boson pair production at the Tevatron, including all spin correlations of the boson decay products.

J.M.Campbell, R.K.Ellis, Phys. Rev. D60:113006 (1999), hep-ph/9905386.

– Calculation of the Zbb and other backgrounds to a ZH signal at the Tevatron.

J.M.Campbell, R.K.Ellis, FERMILAB-PUB-00-145-T, June 2000, hep-ph/0006304.

The last of these references contains the most details

of our method.

MCFM Process List(included at NLO)

ppbar W/ZW+WW+ ZZ + ZW/Z + HW/Z + 1 jetW/Z + g* bb

Various leptonic and/or hadronic decays of thebosons are included as further sub-processes

n.b. No NLO prediction for W/Z + 2 jets isavailable, but this is under construction in MCFM

John Campbell, FNAL

MCFM study for Tevatron Higgs sensitivity

John Campbell, FNAL

“Qaero (Sleuth)” Strategy• Consider recent major discoveries in hep

– W,Z bosons CERN 1983

– top quark Fermilab 1995

– tau neutrino Fermilab 2000

– Higgs Boson? CERN 2000

• In all cases, predictions were definite, aside from mass

• Plethora of models that appear daily on hep-ph

• Is it possible to perform a “generic” search?

Transparencies from Bruce Knuteson talk at Moriond 2001

W2j

We consider exclusive final statesWe consider exclusive final states

We assume the existence of standard object definitions

These define e, μ, , , j, b, ET, W, and Z fi

All events that contain the same numbers of each of these objects belong to the same final state

Step 1: Exclusive final statesSleuth Bruce Knuteson

eμET

Z4j

eET jj eE

T 3j

W3jeee

ZWμμjj eμE

T j

μμμeee

Results

Results agree well with expectationNo evidence of new physics is observed

DØ data

Search for regions of excess (more data Search for regions of excess (more data events than expected from background)events than expected from background) within that variable spacewithin that variable space

probability to be SM

Tevatron Run IIW/Z, top, searches

p-p collisions at s = 1.96 TeV_

CDF & D0

New tracking

SVT displaced track trigger

Particle ID (TOF)

Muon, Calor coverage extended

New tracking (+solenoid)

SST displaced track trigger

Preshower detectors

Improved shielding + muon triggering

Tevatron operating parameters

396 – 132 ns

2 x 1032 cm-1 s-1

1.96 TeV

6.5 – 11 fb-1

2001 - 2009

Run 2

396 ns3.5 sBunch spacing

4.5 x 1031 cm-2 s-1

2 x 1031 cm-2 s-1Luminosity

1.96 TeV1.8 TeVc.m. energy

~190 pb110 pb-1Integrated Luminosity

20031992 – 1996Date

NowRun 1

Physics in Run 2

W: mass, width, gauge couplings

Top: mass, cross-section, branching ratios

Electroweak

Jet cross-section, shapes multijet events

QCD

Higgs, SUSY, extra

dimensions, leptoquarks compositeness, etc.

Searches

Lifetimes, cross-section, Bc, B, Bs studies, CP violation, xs

Heavy flavour

# events in 1 fb-1

1014

1011

104

107

Electroweak:W, Z

Run 2 benefits:

(W), (Z) 12 %

(WW), (ZZ) 13 - 22%

W, Z essential calibration signals for high Et physics

Measurements in 2fb-1:

m(W) measured to 40 MeV (sys. dominated - theory)

(W) measured to 30 MeV

couplings measured to ~0.3

Selecting W’s & Z’s

WWee

ZZ00++--

W Event selection One isolated high pT central e, or Large ET

Z Event selection Two isolated high pT e’s

One isolated high pT central

A second isolated high pT track (minimum ionizing)

OR

W, Z results

We

Z

Data/theory agree so far ..

Z0eh

We have a clear Z0eh signal.Further study of backgrounds is underway.Our goal is to have a preliminary cross section measurement by summer.

Not only interesting as an EWK measurement, it is important for Higgs and SUSY searches.

W+jets production (1)• Selection

– W( e)

• Isolated e : pT > 20 GeV

• || < 0.8

• Missing ET > 25 GeV

– W( )

• Isolated : pT > 25 GeV

• || < 1.5

• missing ET> 20 GeV

– Jets

• pT > 20 GeV

• || < 2.5

• Compare PYTHIA MC with DATA

• Normalized by area

• Error includes stat. error and dominant syst. error from JES

2nd leading jets

1st leading jets

GeV

GeV

W(e)+jets

QCD BKG

QCD BKG

: Data

: MC

: Data

: MC

W+jets production (2)

Di-jet Mass R between di-jetsW(e)+jets W(e)+jets

• Reconstructed di-jet mass and R(= 2 + 2 ) between jets

– MC reproduces jet distributions well

– First step towards study of W(leptons)H( bb) decay process

QCD BKGQCD BKG

Mjj (GeV) Rjj

: Data

: MC

: Data

: MC

W+jets production (3)

• Di-jet mass and Rjj distribution for W( ) + jets event

Mjj (GeV) Rjj

Di-jet Mass R between di-jets W()+jets

: Data

:MC

QCD BKG

W()+jets

: Data

:MC

Z+jets production (1)• Selections

– 2 muons from Z( )

•pT > 15 GeV

•|| < 2

– 2 electrons from Z( ee)

•pT > 20 GeV

•|| < 2.3– Jets

•pT > 20 GeV

•|| < 2.5

1st leading jets 2nd leading jets

• Compare PYTHIA MC

with DATA • Normalized by area

Combined Z(ee)+jets and Z()+jets

• Error includes stat. error

and dominant syst. error

from JES

Z+jets production (2)• Number of jets in Z + jets final states

• Reconstructed di-jet mass and R(= 2 + 2 ) between jets– MC describes jet distributions well

– First step towards Z(leptons)H( bb) study

Di-jet Mass R between di-jets#jets in Z+jets

Combined Z(ee)+jets and Z()+jets

Dibosons: WWll

CDF Run II Preliminary

Event selection Two isolated high pT e or with opposite charge

Fakes ET>25 GeV

(ET,l/j)>200

or ET>50 GeV DrellYan,Z

Z veto Jet veto

tt

//

2 Candidates in ~ 2 Candidates in ~ 72 pb72 pb-1-1

-

Important background for Higgs Search

WWll Candidate Event

e

ET(e)=41.8 GeVPT()=20.5 GeV/c2

ET=60.2 GeV

WW candidate:WW candidate: Dec 14 2002 run: 155364 event: 3494901e event

CDF Run II Preliminary

Diboson: WWll

CDF Run II PreliminaryIn ~ 72 pb-1:

(Backgrounds: QCD, Drell Yan, WZ, tt)-

Source ee e ll Background 0.290.13 0.470.19 0.770.60 1.530.64 WW ll 0.550.13 0.660.15 1.580.36 2.790.62 Data 1 0 1 2

Cross section to come with more statistics …Cross section to come with more statistics …

WW results also coming soon … results also coming soon …

Single Top: W tb, g*W tbSM process not yet observed experimentally!

Run 1

Important background to W/Z H

(a) s-channel annihilation (b) t-channel W-gluon fusion

12%

l q q

44%44 %

l l q

Electroweak: topRun 2 benefits:

(tt) 40%

More luminosity

Increased b tagging efficiency + lepton acceptance

Tevatron only place to study top until LHC startup

Measurements in 2 fb-1:m(top) ~ 1.2% (cf. 2.9%)

(tt) ~ 10% (cf. 25%)

(single top) ~ 20% (1st!)

|Vtb| ~ 12% (1st!)

q

qq

e+

-

Muon

Track in

the

Calorimet

er

(MTC)

Jet 1

Jet 2

tt->ejj

Top Results

NLO: 6.7+0.71-0.88 pb (m(top)=175 GeV/c2)

CDF l+l

D0 l+jets

CDF l+jets

D0 l+l

(tt) PRELIMINARY

Run 1: m(top) =174.3 ±3.2 ± 4.0 GeV/cGeV/c22

Exotic Physics Program

New Particles & DimensionsNew Particles & Dimensions

Signature Based SearchesSignature Based Searches

HiggsHiggs

CDF Physicist

New Gauge Bosons

New Excited Fermions

SUSY

We’ve established a baselineusing W’s and Z’s. Can we now search for more exoticexoticparticles? Yes!

H++

Searchesfor new resonances

Run 2 expectations:

Higgs: Exclude M(H)110 GeV (2 fb-1)

Exclude M(H)130 GeV (6.5 fb-1)

SUSY:

Extend reach to most natural SUSY masses

Search/Higgs

Tevatron only place to find Higgs + (high mass) new physics until LHC

(Savoy-Navarro, EPS 99)

(under review)

Search for New Resonances in High Mass Dileptons

High Mass Dileptons allow searches for new particle production.

Neutral Gauge Boson Z’Neutral Gauge Boson Z’SM Coupling assumed

Randall-Sundrum GravitonRandall-Sundrum GravitonExcited Graviton in 5 dimensionsFree parameters:

•Mass MG

•Coupling k/MPL

Main background from Drell-Yan pairs

Search for New Resonances in High Mass Dileptons

Data consistent with SM background. No excess observed.No excess observed.

Dielectron Invariant Mass Dimuon Invariant Mass

Large Extra Dimensions: ee/

Fit to 2-D distributions to extract SM, interference, and direct gravity terms; use topologies w/ at least 1 EM obj in central calorimeter

• Fit value of G : expected to be zero in SM

di-EM analysis: G = 0.0 0.27 TeV-4

di- analysis: G = 0.02 1.35 TeV-4

• Extract 95% CL upper limits on G

• Translate to 95% CL lower limits on Planck scale MS , in TeV,

using different assumptions about F

Large Extra Dimensions Results

GRW HLZ for n=: 2 7

Hewett

diEM 1.12 1.16 0.89 1.00

diMU 0.79 0.68 0.63 0.71

diEM limit close to Run Idi limit new channel

2int

2

cosM GKKGSM fffdd

d

4F SG Mwhere

Searches: diphotonDiphoton:

GMSB: radiative decay to LSP (gravitino)

If neutralino NLSP:

Search for + Et

pp i +j

-

01 0

1 + X

G G + X ???

Motivated by Run 1 CDF event

/

Search for First Generation scalar LQ

ProductionProduction qg LQ + LQbar gg LQ + LQbar qqbar LQ + LQbar

Decay Decay LQLQ l+l-qq, LQLQ l±qq, LQLQ qq

Experimental signatureExperimental signature 2 high pt isolated leptons + jets one isolated lepton + MET + jets MET + jets

We search for di-electrons + jets.

Search for First Generation scalar LQ

Event Selection:Event Selection: 2 central electrons with ET > 25 GeV

2 jets with ET(j1) > 30 and

ET(j1) > 15 GeV

Z veto Cuts on sum of jet and electron ET’s

to reject SM backgrounds Expected Bkg: Expected Bkg: 3.4 3.4 3.2 events 3.2 events

(DY+2jet events, tt)

0 events observed in 72 pb-1.0 events observed in 72 pb-1.M(LQ) > 230 GeV/cM(LQ) > 230 GeV/c22 @ 95% CL @ 95% CL(Run I 220 GeV/c(Run I 220 GeV/c22 ) )

Summary and Conclusions• Can’t find new physics at hadron colliders without a very good understanding of

the background from old physics (n.b. lessons from history) • Some of the ‘background processes’ are SM processes not yet observed at

hadron colliders!– QCD jets, Drell-Yan, W/Z (+jets) very old physics (~20+ years)

– tt pairs not so old physics (~ 8 years)

– Dibosons: WW,WZ,ZZ ‘current’ physics

– VVV, Vtt, single top not yet observed!

• Good theoretical understanding of these processes at NLO in Matrix Element calculations and Monte Carlo programs– Les Houches accords (MC interfaces)

– MCFM, etc

– Qaero (model independent NP search)

• It is essential to observe and measure as many as possible of these processes experimentally before we can claim to have seen new physics!

Backgrounds to New Physics Signals Peter Ratoff Lancaster University 2003 CTEQ Summer School, Saint Feliou de Guixols, Catalonia.

Documents

qcd backgrounds

ttbb backgrounds

old physics

nnppw jets

w jets comparison

signatures of new physicsws

new susy particles

hadrons nppw jets