Ricardo Gonçalo - RHUL Higgs WG meeting - Physics Week - Oct.04 1 Analysis of Invisible Higgs production in the tth channel Introduction Analysis overview

Ricardo Gonçalo - RHUL Ricardo Gonçalo - RHUL Higgs WG meeting - Physics Week - Higgs WG meeting - Physics Week - Oct.04Oct.04

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Analysis of Invisible Higgs Analysis of Invisible Higgs production in the tth channelproduction in the tth channel

IntroductionIntroductionAnalysis overviewAnalysis overview

Results so farResults so farComparison to referenceComparison to reference

Kinematic fitKinematic fitConclusionConclusion

Ricardo Goncalo

Pedro Teixeira-Dias


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IntroductionIntroduction Motivation:Motivation:

– Several scenarios for physics BSM predict a Several scenarios for physics BSM predict a significant invisible branching ratio significant invisible branching ratio

– Complementary to the VBF channel with invisible Complementary to the VBF channel with invisible HiggsHiggs

Build upon work done at Royal Holloway:Build upon work done at Royal Holloway:(E.Brambilla, talk at Higgs WG meeting, May 2002; T.L.Cheng, (E.Brambilla, talk at Higgs WG meeting, May 2002; T.L.Cheng,

MSc.Thesis, available at: http://www.pp.rhul.ac.uk/~ctehlee/)MSc.Thesis, available at: http://www.pp.rhul.ac.uk/~ctehlee/)

Aim to reproduce and build upon results from Aim to reproduce and build upon results from previous analysis: previous analysis:

(B.Kersevan, M.Malawski, E.Richter-Was, Eur. Phys. J C29 (B.Kersevan, M.Malawski, E.Richter-Was, Eur. Phys. J C29 (2003) 541, ATL-COM-PHYS-2003-016; M.Malawski, MSci Thesis, (2003) 541, ATL-COM-PHYS-2003-016; M.Malawski, MSci Thesis,

hep-ph/0407160)hep-ph/0407160)


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Analysis overviewAnalysis overviewCut-based analysisCut-based analysisOnly signal and tt backg so farOnly signal and tt backg so farDifficulties:Difficulties: Two components of missing Two components of missing

momentum: can’t reconstruct momentum: can’t reconstruct ttbbl l

ttbar is the most significant ttbar is the most significant background and is very similar background and is very similar to signalto signal

Signal/Background ~10Signal/Background ~10-3-3

ProcessProcess xBRxBR

tth 330 (*) fb

tt 490 000 fb

bbW, W l 73 000 fbbbZ, Z l+l- 61 400 fbttW, W l 420 fbttZ, Z 190 fb (*) =520 fb in reference

analysis, with SM couplings


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CutsCuts

Reference analysis cuts:Reference analysis cuts: 1 electron (p1 electron (pTT>25GeV; |>25GeV; ||<2.5)|<2.5)or 1 muon (por 1 muon (pTT>20GeV; |>20GeV; ||<2.5)|<2.5) Veto on additional electron (pVeto on additional electron (pTT>10GeV)>10GeV)or additional muon (por additional muon (pTT>6GeV)>6GeV) 2 b-tagged jets2 b-tagged jets 2 or more un-tagged jets2 or more un-tagged jets ttbjj reconstruction: bjj reconstruction:

– |m|mjjjj-m-mWW|<15GeV; ||<15GeV; ||<2.0 for jets in W|<2.0 for jets in Wjjjj– |m|mbjjbjj-m-mtt|<25GeV|<25GeV

mmTT > 120 GeV > 120 GeV Missing EMissing ETT > 150GeV > 150GeV Scalar sum of pScalar sum of pTT of reconstructed l j j b b: of reconstructed l j j b b: EETT>250GeV>250GeV In reconstructed W In reconstructed W jj: R jj: Rjjjj = =((22

jjjj++22jjjj) < 2.2 (to reject lep-tau decays) ) < 2.2 (to reject lep-tau decays)

2

2

lepton

T

miss

TleptonT

missTT ppEEm


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SimulationSimulation Channel: Higgs (mChannel: Higgs (mhh = 120 GeV) decaying to neutralinos in = 120 GeV) decaying to neutralinos in

MSSM MSSM – (tan(tan= 5, m= 5, mAA= 1 TeV; M= 1 TeV; M11= 44 GeV, M= 44 GeV, M22= 220 GeV, M= 220 GeV, M33= 1 TeV)= 1 TeV)

PYTHIA 6.203 for signal and backgroundPYTHIA 6.203 for signal and background Generated 60 M tt + 10 M tthGenerated 60 M tt + 10 M tth Atlfast simulation, ATLAS release 7.0.2Atlfast simulation, ATLAS release 7.0.2

– Low luminosity settingLow luminosity setting– Cone jets (RCone jets (Rconecone = 0.4) = 0.4)– Jet tagging: b jets 60%; c mistag 10%; u,d,s,Jet tagging: b jets 60%; c mistag 10%; u,d,s, mistag 1% mistag 1%– CTEQ5L PDFsCTEQ5L PDFs– mmtoptop = 175 GeV (..historical) = 175 GeV (..historical)

Interfaced code to Atlfast within Athena to produce Interfaced code to Atlfast within Athena to produce dedicated ntuplededicated ntuple


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The truth!The truth! At parton level: At parton level:

– tops back-to-back in background tops back-to-back in background – more “mercedes star”-like in signalmore “mercedes star”-like in signal– Most missing pMost missing pTT from Higgs decay (especially in had-had channel) from Higgs decay (especially in had-had channel)

tt

t

t

th

(,ETmiss)

(h,ETmiss)


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Signal and tt backgroundSignal and tt background Transverse mass, Transverse mass, EET T

and Eand ETTmissmiss are good are good

discriminating variablesdiscriminating variables mmTT has sharp edge for tt has sharp edge for tt

background at ~mbackground at ~mWW

But background But background xsection ~1000 times xsection ~1000 times higher than signalhigher than signal

Tails of background Tails of background distributions very largedistributions very large

No cuts; Lumi = 1fb-1

2

2

lepton

T

miss

TleptonT

missTT ppEEm

mT(GeV)

ttbartth

ETmiss (GeV) ET (GeV)


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Results so far…Results so far… Relaxed ERelaxed ETT

missmiss cut to 120GeV wrt reference analysis cut to 120GeV wrt reference analysis Background much higher than tthBackground much higher than tth Signal and background normalized to 30 fbSignal and background normalized to 30 fb-1-1


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Results so far…Results so far… Most background comes Most background comes

from lep-tau and lep-lep from lep-tau and lep-lep decays of tt, as decays of tt, as concluded in reference concluded in reference paperpaper

decays increase the decays increase the missing Emissing ETT

WWjj reconstructed jj reconstructed from ISR/FSR jets in from ISR/FSR jets in lep-tau and lep-lep lep-tau and lep-lep eventsevents

lep-

lep

lep-

tau

tau-

tau

lep-

had

had-

had


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Results so far…Results so far… Main problem in tt Main problem in tt

background is the “lep-background is the “lep-tau” channel decays tau” channel decays with fake Wwith fake Wjjjj

tth signal much more tth signal much more pure wrt Wpure wrt Wjj (we’re jj (we’re looking at the tails of tt looking at the tails of tt background)background)

Not much point in Not much point in rejecting taus in rejecting taus in Atlfast, must look for Atlfast, must look for other possibilities other possibilities


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Results so far…Results so far… No cut found so far that can be targeted at lep-tau and lep-No cut found so far that can be targeted at lep-tau and lep-

lep channels in tt production in addition to what was found lep channels in tt production in addition to what was found in the reference analysisin the reference analysis


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Results so farResults so farAccepted events for 30 fbAccepted events for 30 fb-1 -1

tth x-section scaled to SM value as used in tth x-section scaled to SM value as used in reference analysisreference analysis

tth: x-sectiontth: x-section = 520 fb= 520 fb-1-1

Signal Signal = 44.3 events= 44.3 events

tt: x-sectiontt: x-section = 490000.0 fb= 490000.0 fb-1-1

BackgroundBackground = 812 events= 812 events

S/B = 1.55


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Comparison with reference analysisComparison with reference analysisCumulative efficiency of cuts:Cumulative efficiency of cuts: Reasonable agreement with reference analysis: ~10-20% for signal Reasonable agreement with reference analysis: ~10-20% for signal

(“check” column)(“check” column) Agreement wrt tt background efficiency becomes worse (factor 1.35 – Agreement wrt tt background efficiency becomes worse (factor 1.35 –

2.8) for cuts after m2.8) for cuts after mTT cut cut Testing all tTesting all tbjj combinations against mbjj combinations against mWW and m and mtt gives efficiency ~30% gives efficiency ~30%

better wrt “cross check” (“this analysis” column) both for signal and better wrt “cross check” (“this analysis” column) both for signal and background background (small) net gain in significance (small) net gain in significance

Cut Reference tth

Cross check tth

this analysis tth

Reference tt Cross check tt

this analysis tt

Lepton 22% 23.6% 23.6% 22% 23.6% 23.7%

Jets/b jets 5.0% 5.7% 5.7% 4.9% 5.01% 5.02%

t bjj 2.6% 2.9% 3.4% 2.4% 2.60% 2.98%

mT 0.87% 1.12% 1.33% 0.041% 0.055% 0.063%

ETmiss 0.41% 0.51% 0.67% 2.0x10-5 3.8x10-5 ±0.6 4.9x10-5

ET 0.40% 0.50% 0.67% 2.0x10-5 3.7x10-5 ±0.6 4.6x10-5

Rjj 0.28% 0.33% 0.44% 7.5x10-6 2.1x10-5 ±0.5 2.8x10-5


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mWW

bt

e/

mt

Kinematic fitKinematic fitTried something different: Assume pT

miss comes from and h only

Build grid of points in pT

and (l,) and calculate pZ

pZ can be found from pT

and plep assuming W on mass-shell

From pb , p and plep, calculate mt for each point

Propagate errors in mt from grid spacing to obtain mt and calculate 2

2

2

2

mt

PDGt

rect mm

T

HiggsT

totalT ppp


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Kinematic fit resultsKinematic fit results

Kinematic fit ~works for Kinematic fit ~works for signal: signal: (p(pTT

hh)~85GeV)~85GeV

Will try to use fit results for Will try to use fit results for discrimination against tt discrimination against tt backgroundbackground

The hope is that this The hope is that this allows other cuts to be allows other cuts to be relaxed relaxed

Correlations to mCorrelations to mTT and and

EETTmissmiss may be important may be important


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ConclusionsConclusions

Major background to tth has been studiedMajor background to tth has been studied Reasonable agreement with reference Reasonable agreement with reference

analysis – still work to be done to find analysis – still work to be done to find remaining differencesremaining differences

Some improvement in significance may be Some improvement in significance may be achieved by different reconstruction of tachieved by different reconstruction of tbjj bjj

Simple kinematic fit to semileptonic top Simple kinematic fit to semileptonic top decay may be useful to discriminate against decay may be useful to discriminate against backgroundbackground

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Ricardo Gonçalo - RHUL Higgs WG meeting - Physics Week - Oct.04 1 Analysis of Invisible Higgs production in the tth channel Introduction Analysis overview