1

A Preliminary Model A Preliminary Model Independent Study of Independent Study of

the Reaction the Reaction ppppqqWWqqWWqqqqℓℓqqqq at CMS at CMS

Gianluca CERMINARAGianluca CERMINARA (SUMMER STUDENT) (SUMMER STUDENT)MUON groupMUON group

25/09/2002 Gianluca CERMINARA 2

The ProjectThe Project

A Study of the WW-Fusion Channel in a A Study of the WW-Fusion Channel in a Model Independent wayModel Independent way

PurposePurpose: : The aim of my work is to verify if The aim of my work is to verify if it’s possible to extract theit’s possible to extract the signalsignal from thefrom the backgroundbackground for the process for the process

ppppqqWqqWLLWWLLqqqqℓℓqqqq ToolsTools::

1. PYTHIA for the event generation

2. CMSJET for the detector simulation

3. ROOT for the data analysis

4. … a complete simulation will follow…

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Why am I Why am I studying this studying this

channel?channel?A Model Independent Study of the A Model Independent Study of the WW FusionWW Fusion in in

order to clarify the order to clarify the Symmetry Breaking Symmetry Breaking MechanismMechanism..

The Standard Model predicts that without a Higgs the scattering amplitude of The Standard Model predicts that without a Higgs the scattering amplitude of the WW fusion process violates unitarity at about 1.5 TeV…the WW fusion process violates unitarity at about 1.5 TeV…

……for this reason something must happen:for this reason something must happen: In the Higgs case we will observe a resonance at MHiggs = MWW

otherwise the cross section will deviate from the SM prediction.

We want to know with which resolution we can study the cross section We want to know with which resolution we can study the cross section as a function of the invariant mass of the two scattered W bosons!as a function of the invariant mass of the two scattered W bosons!

We are looking for everything that may happen in this channel!!!We are looking for everything that may happen in this channel!!!

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What is the signal?What is the signal?

TopologyTopology

Two quarks of the proton emit a W boson, the two bosons interact Two quarks of the proton emit a W boson, the two bosons interact giving in the final state two other W bosons: one of them decays giving in the final state two other W bosons: one of them decays leptonically and the other decays into a pair of quarks.leptonically and the other decays into a pair of quarks.

Signature:Signature:

Two jetsTwo jets in the in the central region central region with high Pt with high Pt coming from coming from the quarks of the quarks of the W decaythe W decay

One One muonmuon in in the barrel with the barrel with high phigh ptt and and missing missing energy of the energy of the neutrino (from neutrino (from the W decay)the W decay)

two jetstwo jets in the in the forwardforward region of the detector (from region of the detector (from the parton hadronisation) .the parton hadronisation) .

6 fermion final state!

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t-tbar backgroundt-tbar background TopologyTopology

From the interaction of a From the interaction of a quark and an anti-quark quark and an anti-quark or of two gluons of the or of two gluons of the proton we have aproton we have a t and t and anti-tanti-t quark quark pairpair.They .They immediately decay into a immediately decay into a b and a W.b and a W.

SignatureSignatureSince the W decays both Since the W decays both leptonically and leptonically and hadronically we have the hadronically we have the same final state as the same final state as the signal:signal: 4 jets4 jets (2 from the (2 from the b and 2 from the W) andb and 2 from the W) and a muona muon..

What are the What are the main main

backgrounds?backgrounds?

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What are the What are the main main

backgrounds?backgrounds?

+

W+jets backgroundW+jets background TopologyTopology

A quark and a gluon/anti-quark give a W and a A quark and a gluon/anti-quark give a W and a quark/gluon.When the W decays leptonically, the final state quark/gluon.When the W decays leptonically, the final state is similar to the one of the signal.is similar to the one of the signal.

SignatureSignatureif during the if during the hadronisation hadronisation the gluon gives the gluon gives origin to more origin to more than one jet this than one jet this background can background can be very be very “dangerous”.“dangerous”.

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What are the What are the main main

backgrounds?backgrounds?WWTTWWTTℓℓqqqq

backgroundbackground TopologyTopology

A quark and an anti-quark A quark and an anti-quark give give two W bosonstwo W bosons in the in the final state. One of them final state. One of them decays hadronically, the decays hadronically, the other decays in a muon and other decays in a muon and a neutrino.a neutrino.

SignatureSignatureThe final state is very The final state is very similar to the one of the similar to the one of the sinal: we have a sinal: we have a muonmuon two two jetsjets and and missing energymissing energy from the neutrino. Only the from the neutrino. Only the two scattered quarks are two scattered quarks are missing.missing.

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Event GenerationEvent Generation

Events generated with Events generated with PYTHIA Monte CarloPYTHIA Monte Carlo package. package.

SignalSignal tt--ttbarbar WW+jets+jets WWWWℓℓqqqq

Cross sectionCross section 18 pb18 pb 378 pb378 pb 46395 pb46395 pb 11 pb11 pb

Number of Number of generated generated eventsevents

35723572 89498949 88858885 27262726

PYTHIA PYTHIA Process code Process code (“Isub”)(“Isub”)

72, 7772, 77 81, 8281, 82 16, 3116, 31 2525

Only a little Monte Carlo statistics was available.Only a little Monte Carlo statistics was available.

PYTHIA 6.158

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Detector SimulationDetector Simulation

CMSJETCMSJET Fast Simulation package. Fast Simulation package.

The leptons can be identified in the range –2.4 < The leptons can be identified in the range –2.4 < < 2.4< 2.4

Jet reconstruction:Jet reconstruction: range –5 < < 5. Cone algorithm with R = 0.5 on the whole range of of the detector. pt >10 GeV.

No No eventevent pile-up pile-up..

CMSJET 4.801

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Signal KinematicsSignal Kinematics

The The two scattered partonstwo scattered partons have pseudo-rapidity above 2.5 and have pseudo-rapidity above 2.5 and they will give origin to they will give origin to forwadforwad jets.jets.

The The quarks from quarks from the Wthe W decay are decay are mainly produced mainly produced in the in the central central regionregion of the of the detector.detector.

AnalysisAnalysis of the of the Parton LevelParton Level kinematics.kinematics.

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Event ReconstructionEvent Reconstruction

MuonMuon

The request of an The request of an isolated isolated muonmuon is unavoidable is unavoidable in order to identify a in order to identify a leptonic W decay.leptonic W decay.

Selection criteria:Selection criteria: Max isolation. pt > 10 GeV

Efficiency:Efficiency:

~96%~96%

Leptons fromLeptons from WW

NeutrinoNeutrino

Reconstruction of neutrino Reconstruction of neutrino four-momentum: four-momentum:

pptt = system missing p = system missing ptt..

ppzz calculated imposing calculated imposing the constraint:the constraint:

mmWW = 80.45 GeV = 80.45 GeV

WW = 2.14 GeV = 2.14 GeV

(p(p + p + p ) )22 = m = mWW22

Smallest pSmallest pzz chosen. chosen. Efficiency:Efficiency:

~82%~82%

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Event ReconstructionEvent Reconstruction

Selection criteria:Selection criteria: || < 3 pt > 10 GeV Combination of 2 or 3

or 4 jets with invariant mass closest to mW = 80.45 GeV

Efficiency:Efficiency:

~96%~96%

Jets from the W decayJets from the W decay : : WWqqqq

The possibility of a good reconstruction of the W boson is basically The possibility of a good reconstruction of the W boson is basically linked to the number of detected jets.linked to the number of detected jets.

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Event ReconstructionEvent Reconstruction

Selection criteria:Selection criteria: || < 3 pt > 10 GeV Combination of 2 or 3

or 4 jets with invariant mass closest to mW = 80.45 GeV

Efficiency:Efficiency:

~96%~96%

Jets from the W decayJets from the W decay : : WWqqqq

The possibility of a good reconstruction of the W boson is basically The possibility of a good reconstruction of the W boson is basically linked to the number of detected jets.linked to the number of detected jets.

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Event ReconstructionEvent Reconstruction

Selection criteria:Selection criteria: pt > 10 GeV; j1xj2< 0; 1.5 < | | < 5; j1–j2> 3.

Efficiency:Efficiency:

~50.8%~50.8%

Forward jet tagging.Forward jet tagging.

Tagging of Tagging of twotwo forward jets forward jets..

(Parton Level)

CUT

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Signal to Signal to Background ratioBackground ratio

Efficiencies of applied cuts for the identification of the Efficiencies of applied cuts for the identification of the signal topology:signal topology:

CutsCuts SignalSignal t-tt-tbarbar W+W+jetsjets WWWWℓℓqqqq

Isolated muon Isolated muon pptt > 10 GeV > 10 GeV 96.0%96.0% 91.5%91.5% 91.6%91.6% 93.6%93.6%

Reconstruction Reconstruction neutrino pneutrino pzz

81.6%81.6% 81.2%81.2% 90.4%90.4% 83.2%83.2%

Reconstruction Reconstruction W from jetsW from jets 96.3%96.3% 99.9%99.9% 35.2%35.2% 83.6%83.6%

Tagging jetsTagging jets 50.8%50.8% 31.7%31.7% 13.0%13.0% 19.6%19.6%

The forward jet tagging is a powerful criterion to improve The forward jet tagging is a powerful criterion to improve the signal to background ratio.the signal to background ratio.

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Signal to Signal to Background ratioBackground ratio

Pseudo-rapidity Pseudo-rapidity WW which decays which decays leptonicallyleptonically

After the first set of cuts all the backgrounds are still After the first set of cuts all the backgrounds are still important. For this reason others variables have been important. For this reason others variables have been used for the discrimination:used for the discrimination:

Applied cut:Applied cut:

||WleptWlept| | 3 3

CUT CUT

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Signal to Signal to Background ratioBackground ratio

Transverse Transverse momentum of the W momentum of the W boson which decays boson which decays leptonicallyleptonically

Applied cut:Applied cut:

ppttWleptWlept > 10 GeV > 10 GeV

Transverse Transverse momentum of the W momentum of the W boson which decays boson which decays hadronicallyhadronically

Applied cut:Applied cut:

ppttWqqWqq > 10 GeV > 10 GeV

CUT

CUT

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Signal to Signal to Background ratioBackground ratio

Transverse Transverse momentum of the momentum of the tagging jet with max Ptagging jet with max Ptt

Applied cut:Applied cut:

ppttjtagjtag > 20 GeV > 20 GeV

Invariant mass of the Invariant mass of the tagging jet systemtagging jet system

Applied cut:Applied cut:

MM(jt1+jt2)(jt1+jt2) > 550 GeV/c > 550 GeV/c22

Cuts on the tagging jets.Cuts on the tagging jets. CUT

CUT

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Signal to Signal to Background ratioBackground ratio

Efficiencies of applied cuts :Efficiencies of applied cuts :

CutsCuts SignalSignal t-tt-tbarbar W+W+jetsjets WWWWℓℓqqqq

||WleptWlept| | 3 3 96.6%96.6% 98.9%98.9% 79.6%79.6% 94.7%94.7%

ppttWleptWlept > 10 GeV > 10 GeV 97.1%97.1% 98.5%98.5% 78.6%78.6% 93.3%93.3%

ppttWleptWlept > 10 GeV > 10 GeV 95.8%95.8% 98.3%98.3% 86.8%86.8% 92.6%92.6%

60 GeV < m60 GeV < mWqqWqq < 100 GeV < 100 GeV 85.9%85.9% 95.9%95.9% 74.2%74.2% 90.9%90.9%

ppttjtagjtag > 10 GeV > 10 GeV 59.7%59.7% 54.8%54.8% 11.3%11.3% 24.5%24.5%

MM(jt1+jt2)(jt1+jt2) > 550 GeV/c > 550 GeV/c22 66.8%66.8% 26.6%26.6% 29.3%29.3% 31.4%31.4%

Total efficiencyTotal efficiency 15.6%15.6% 3.4%3.4% 0.3%0.3% 1.1%1.1%

Again the cuts on the forward tagging system are the most Again the cuts on the forward tagging system are the most effective.effective.

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Preliminary ResultsPreliminary Results

1 year “Low Luminosity” = 10 fb1 year “Low Luminosity” = 10 fb-1-1

1 year “High Luminosity” = 100 fb1 year “High Luminosity” = 100 fb-1-1

Even if theEven if the W+jetsW+jets background isbackground is still importantstill important at high invariant masses the at high invariant masses the signal to background ratio is signal to background ratio is quite good.quite good.

The The high energy regionhigh energy region is the is the most interesting for themost interesting for the new new physicsphysics we are looking for.we are looking for.

S/B = 2.9x10S/B = 2.9x1033/4.3x10/4.3x1033S/B = 2.1x10S/B = 2.1x1044/8.9x10/8.9x1055

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Preliminary ResultsPreliminary Results

Resolution of the WW system invariant mass reconstruction.Resolution of the WW system invariant mass reconstruction.

The The resolutionresolution in the in the reconstruction of the reconstruction of the WW invariant mass at WW invariant mass at this stage is :this stage is :

~15%~15%

Fitting the histogram Fitting the histogram with the sum of two with the sum of two gaussian curves the gaussian curves the resolution in the resolution in the central central peakpeak is: is:

~10%~10%

MMWWWWrecrec - M - MWWWW

gengen

MMWWWWgengenrrWW WW ==

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ConclusionsConclusions

Good signal to background ratio at Good signal to background ratio at high WW invariant masseshigh WW invariant masses

S/B S/B 1 1

Very good resolution on Very good resolution on MMWWWW energy scale of the process energy scale of the process

~10 ~10 15%15%

At this stage the measurement appears At this stage the measurement appears possiblepossible::

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Future ProjectsFuture Projects

A more complete study of this process using a A more complete study of this process using a “Full Simulation” of the CMS detector.“Full Simulation” of the CMS detector.

……are coming soon!are coming soon!

20 k signal20 k signal 30 k W+jet30 k W+jet 20 k t-tbar20 k t-tbar

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A Preliminary Model A Preliminary Model Independent Study of Independent Study of

the Reaction the Reaction ppppqqWWqqWWqqqqℓℓqqqq at CMS at CMS

Gianluca CERMINARAGianluca CERMINARA (SUMMER STUDENT) (SUMMER STUDENT)MUON groupMUON group