tau jet identification in charged higgs search monoranjan guchait tifr, mumbai india-cms...
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Tau Jet Identification in Charged Higgs Search
Monoranjan Guchait TIFR, Mumbai
India-CMS collaboration meeting 27-28th March,2009 University of Delhi
M.Guchait, R.Kinnunen, M.Kortelainen, Sami Lehti A. Nikitenko, L. Wendland
CMS AN 09/036
Motivation
MSSM: 5 Higgs h, H, A , H+, H-
Standard model: one Higgs, mass is not predictable
Predictable in terms of Parameters
Two parameters: MA and tanβ
Signature of Charged Higgs carry unambiguous Signal of NEW Physics
Charged Higgs Production
RLc
RbLt
Wmscm
btmbtmH
M
gL
tancot
tancot
2
Coupling
High tanβ
Charged Higgs Decay
H → t b is dominant for Higher higgs, but huge contamination
H→ tau + nu is Sub-dominant, useful to find the Higgs signal
Tau decay and Helicity Correlations
~ 2/3
Tau polarization
Angular distributions ~
cos1 P
Helicity Correlations
Fast simulations
Guchait,Kinunnen,Lehti,CMS IN 2008/008
1 prong
3 prong
Event SamplesCMSSW 1_6_12 + TAUOLA for tau decays
Signal for MH=200,300,400,tanβ=30 250 K events
QCD(PYTHIA)Pt_hat=80-230 GeV3.4 M events
tt + 0/1 jets(ALPGEN)1.8 M events
W+3/4 jetsPeak sample:MW<150 GeV, 1.3 M eventsTail sample: MW>150 GeV, BW, 76 K events
Signal
QCD
ttbar
W+3/4 jets
Jet and Track Reconstruction
Calorimeter tau jets(Calotau)are usedMC based jet energy corrections are used CMS IN 2007/029
Iterative tracking used for tracksTracks down to pt>0.3 GeV are used CMS IN 2007/035
Jet energy resolution for MC matched calorimeter tau jets (CaloTau) for mH+=300 GeV/c2
Why to Optimize against QCD BG• [P-TDR II]: The transverse mass
(mT) of the H+ is reconstructed from the tagged tau and the MET
– H+ events acquire mT valuesmainly up to mH+
– W events acquire mT values mainly up to mW
– QCD multi-jet events may faketau jet and MET;
o can contaminate the signal region
o very large cross-section– also off-shell W events can
contaminate the mT signal regio
–but relatively low cross-section
Optimize against QCD multi-jet background anduse helicity correlations to suppress W decays
Selection Strategy for 1 prong
• Jet ET > 119 GeV, jet eta: ||<1.7
• Leading track pT>20 GeV/c
–through R cut, pT>95 GeV/c
• 1 isolated charged track–isolation cone R=0.50; min. track pT>1.0 GeV/c
• Standard track quality cuts–Nhits>=8, normalized track 2<10, IPT<300 m, |IPz|<1 mm
Selection Strategy for 1 prong
Isolated electromagnetical energy deposition–isolation annulus R=0.10-0.50, allowed energy ET
isol<1.8 GeV
• Matching of track momentum to hadronic energy deposition–to reject electrons; ET
HCAL / pTtrack - 1 > -0.90
• Helicity correlations, R=ptrack/Evis. jet > 0.8
–to suppress taus from W decays–suppresses further also hadronic jets with neutral particles
Jet Et threshold• Jet Et threshold of 119 GeV was found to be optimal
against QCD multi-jet events in the 1-prong final state
The high jet ET threshold
viable also for signal with
mH+~mt
Efficiency of the jet ET threshold for MC matched
H+ decays
PAS Figure 2
after allother cuts
Electromagnetic Isolation
• Due to the boost effect, the 0’s are contained within a narrow cone in tau decays; the ECAL energy deposition is calculated in an isolation annulus around this signal cone
[CMS Note 2006/028]
Optimum cut for 1-prong: ET
isol.<1.8 GeV in an annulus of R=0.10-0.5
Electron rejection • Main sources of electrons are
– Wee and Wee
decays• These electrons can be effectively
suppressed by matching the HCAL energy deposition to the momentum carried by the track,
• i.e. ETHCAL/pT
track-1 = Re
– the HCAL energy is summed in a coneof R=0.50 around the leading trackaxis
• Optimum cut was found to be givenby Re> -0.90
Tracker Isolation• Low charged track multiplicity and
isolated track signature in tau jet
• Required 1 charged track in isolation cone of 0.50
• Counted tracks with pT > pTmin to
filter out very soft tracks
• Only tracks from interaction vertexwere considered
– |IPz|<1 mm
• Rejected tracks, which consist of hitsbelonging to different tracks
– IPT<300 m[CMS Note 2006/028]
• Optimum choice for 1 prongs:
– pTmin =1.0 GeV/c (same as at trigger
level)– could use smaller value (e.g. 0.7
GeV/c), if necessary
Tau Helicity CorrelationDifferent polarization effects in tau decay from H and W decay is exploited
R distribution including all 1-prong tau decay modes after all other
cuts
PAS Figure 7
after allother cuts
Signal eff ~ 0.5 Bg ~ 0.2 or less
Tracker
Calorimeter
Selection strategy for 3 prongs• Choose a1
+++- decays (2/3 of 3-prongs)
• Jet ET > 100 GeV, jet eta: ||<1.8
• Leading track pT>20 GeV/c (to mimic single tau trigger)
• 3 isolated charged tracks– isolation annulus R=0.04-0.50; min. track pT>0.8 GeV/c
• Standard track quality cuts– Nhits>=8, norm. track 2<10, Qtrack = ±1, IPT<300 m, |IPz|<1 mm
• Isolated electromagnetical energy deposition– isolation annulus R=0.15-0.50, allowed energy ET
isol<1.8 GeV
• Matching of energy carried by the tracks to calorimeter energy E=Etracks/Ejet-1 > -0.2; to reject neutral hadrons
• Flight path of the tau lepton and tau invariant mass
• Helicity correlations, R=pldg.track/Evis. jet > 0.55
– to suppress taus from W decays– suppresses further also hadronic jets with neutral particles
Tau invariant mass
• tau invariant mass calculated from the tracks (no 0’s)
–robust method–signal distribution is
a distinct peak smeared a little due to the tau neutrino
• Optimum value of m<1.5 GeV/c2 chosen
Rejecting Neutral hadrons
• Energy carried by the tracks is matched to the jet energy to reject jets with considerable neutral particle content–select the a13+decay for the signal (~2/3 of 3-prongs)
• Optimum value of E=Etracks/Ejet-1 > -0.2 chosen
Helicity Correlations
Can be used R variable
More useful is
Optimum cut value >0.5 Rejects good fraction fromW
Summary of Results: 1prong
Signal
QCD
ttbar
W+3/4 jets
Summary of Results: 3 prong
Signal
QCD
ttbar
W+3/4jets
Summary of Results:Signal
H+200
H+300
H+400
1-prong 13.4 2.71 1.73
error ±0.4 ±0.09 ±0.04
Efficiency 1.7 % 3.2 % 4.6 %
purity 99.5 % 98.8 % 99.6 %
3-prong 2.9 0.82 0.37
error ±0.2 ±0.04 ±0.02
Efficiency 0.37 % 0.70 % 0.98 %
purity 99.0 % 99.7 % 99.8 %
Summary of Results:QCD
QCD80-120
QCD 120-170 QCD170-230
<2400 4800 1400±1400 ±400
<7.9e-7 9.7e-6 1.4e-5
<2400 <400 220±150
<7.9e-7 <8.0e-7 2.2e-6
1 prong
Error
Eff.
3 prong
Error
Eff.
Summary of Results:ttbar, W+3/4 jets
ttbar ttbar+1 jet
62 27±5 ±4
1.0e-4 1.5e-4
38 13.8±4 ±2.5
6.1e-5 7.8e-5
1 prongError
Eff.
3 prongError
Eff.
W+3jpeak
W+4jpeak
W+3jtail
W+4jtail
1.6 0.76 0.40 0.07±1.2 ±0.4 ±0.11 ±0.02
2.8e-6 6.2e-6 3.7e-4 3.6e-4
1.6 0.76 0.20 0.022±1.2 ±0.44 ±0.08 ±0.010
2.8e-6 6.2e-6 1.9e-4 1.1e-5
Conclusions• Robust tau jet idenfication presented for the H+ channel
– The tau jet ID part without helicity correlations is basically a standard tau ID with a high pT cut
• Tau-identification successful for 1-prong final states– Signal efficiency 1.7-4.6 % with high signal purity– QCD multi-jet background can be reduced by a factor of ~105 or
better– Also the ttbar and W+jets backgrounds are suppressed strongly,
due to hadronic jets suppression and tau polarization effects on W decay
• 3-prong final states can be used– 21-30 % increase in signal– 10-15 % increase in QCD multi-jet events, but ttbar background
increases with ~3-4 times and W+jets with ~4-6 times as much as the signal
– precise estimation of background events would require factorization (or huge MC production)