Study of VBF Httlep had

1
Study of VBF H-gttt-gtlephad

• Junichi Tanaka
• ICEPP, Univ. of TOKYO
• Discovery of Higgs and Supersymmetry
• to Pioneer Particle Physics in the 21th Century

2
Outline

• Introduction
• Study of 3rd jet by ME event generator
• Study of fake BGs
• Summary
• Prospects Discovery Potential of SM Higgs

3
SM Higgs Production Decay _at_ LHC
Excluded by LEP
Production
Decay
tt-
LEPEWWG2005S
114.4 lt mH lt 219 GeV/c2 _at_95 C.L.

• VBF H-gttt at low-mass region (mH 120GeV)
• Large production cross section and branching
  ratio sxBR 300fb

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Vector Boson Fusion Process
Rapidity gap
f
h
PRD59(1999)14037 hep-ph/9908378

• Characteristics
• High Pt jets in forward region
• Rapidity gapRight fig h3 h of 3rd jet
  average of h of 2 forward jets
• Signal a dip in the center
• BG a peak in the center
• -gt jet activities depend on structure of color
  flow.

(EW)
Treatment of 3rd jet is important in this mode!
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Study _at_ ATLAS
ATLAS Scientific note Eur.Phys.J.C32S219-54,2004
BG by Comphep
VBF H-gttt

• VBF H-gttt is one of promising channels near the
  LEP limit. (left fig)
• Reconstruct M(tt) by using missing Et and observe
  a peak (right fig) on the right side of the Z
  mass peak.
• VBF H-gttt is one of discovery channels at the
  low mass region.

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Motivation

• Improvement and progress of our tools since the
  previous studies (Eur.Phys.J.C32S2,19,2004, the
  pervious slide)
• 1) Events with multi jets can be generated by the
  recent ME event generators.
• 3rd jet was evaluated by Parton Shower (PS) model
  in the previous studies.
• We evaluate 3rd jet by the ME event generator.
• 2) Environment of reconstruction and full
  simulation, which will be used in 2007, is
  available.
• Fake rate can be estimated from the full
  simulation, which is based on Geant4.
• Large QCD backgrounds, which have not been
  considered in the previous studies, are checked
  by using results of the full simulation.

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Type of Simulation

• Full simulation
• Based on Geant4
• Initial layout
• Used for detail studies
• Development of reconstruction tools, for example,
  tracking, reconstruction of missing Et, Particle
  ID tools (e, m, tau, b, g).
• Fake rates, crack effect for missing Et and so on
• Take much time, typically, 15mins/event with
  3GHz CPU.
• Fast simulation
• Based on parameterized performance of full
  simulation results
• Used for the BG study, where we need high
  statistics.
• Most of todays results are obtained by the fast
  simulation.

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Study of 3rd jet by ME event generator
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ME ALPGEN PS

• Alpgen (M.L.Mangano) is one of the ME event
  generators.
• Studies using ME event generators becomes
  popular.
• ME has an advantage for producing high Pt jets.
• To remove collinear and soft divergence, Pt and
  Rjj are required at parton level.
• Soft and collinear kinematical regions are
  covered by PS.
• We produced exclusive samples for
  Znjets(n2,3,4) with modified ALPGEN ver1 (small
  weight events can be produced).
• Znjets is a dominant BG.
• MLM method is used to avoid double counting. The
  similar method is officially introduced from ver2.

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To Avoid Double Counting

• We use the following method (our MLM method) to
  avoid the double counting of events (not CKKW
  method)
• Veto events having jets (Ptgt20GeV) at the
  outside of cone (dR0.4), whose center is defined
  by the direction of a ME-parton.

ME-parton
Rec-jet
Bad
Good
See T.Sasakis talk later
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Pt and h Distributions
Alpgen sample(ver1), after lepton cut, normalized
to 1fb-1.
Pt of 3rd jet
eta of 3rd jet
preliminary
Z2j Z2jZ3j Z2jZ3jZ4j
with MLM method to avoid double counting
preliminary
log10Pt
25GeV
100GeV

• 3rd jets with high Pt increase.
• 3rd jets become more central. It has a large
  effect on Central Jet Veto.
• Pt and h of 3rd jet depend on the number of jets.
  Cannot ignore them.

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CJV Survival Probability

• Central Jet Veto (CJV) Veto events having
  additional jets with a given Pt in the rapidity
  gap.

preliminary
jet
t
t
jet
f
Rapidity gap
h
(additional jets in the rapidity gap)

• Survival probability of Z2j is higher than
  Z3j and Z4j due to smaller jet activity in
  the rapidity gap.
• 40 of Znjets (n2,3,4) remains in total in
  case of Pt20GeV.

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M(tt) Distribution
L30fb-1

• After all the selection criteria
• A amount of remaining Z2j is large due to large
  cross section and high CJV survival probability.
• Z3j and Z4j still remains. They are not
  negligible.
• Since resolution becomes worse as the number of
  jets increases, their tails can contribute to BGs.

Z2j
Z3j
Z4j
preliminary
Signal region
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Study of fake BGs
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Origin of Fake Lepton

• Since cross section (x-sec) of events with QCD
  jets is much larger than that of the signal
  (x-sec300fb), we suffer from these events, which
  have fake leptons from jets. W j j j -gt W t j j
  x-sec gt10nb j j j j -gt e/m t j j x-sec
  gt10mb
• Fake Tau
• 1 or 3 p- with p0, which have most of energy of
  a jet
• Fake Electron
• Pair production of gamma (from p0)
• Dalitz decay
• p- and p0 with E/p1
• B/D semileptonic decay
• Fake Muon
• pion punch through
• pion decay in flight
• B/D semileptonic decay

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Tau Reconstruction and ID

• Hadronic decay 1prong 50 and 3prongs 15
• We need to identify tau-jets from many QCD-jets.

Charged tracks 1 or 3 Photons from p0

• Reconstruction two ways at ATLAS
• 1) Cell based start from cells, which is the
  similar as cone jet algo.
• 2) Track based start from charged track(s)
• Properties of tau-jet for identification
• 1 or 3 charged pion tracks
• Narrowness and Isolation

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Tau Rec/ID Performance
Cell based method
Fake rate of 10-2 means we misidentify one QCD
jet as a tau in 100 QCD jets. Depend on Pt
and type of jet. Gluon fake rate is better than
quark fake rate dueto difference in narrowness.
Light Quark Gluon
fake rate
preliminary
20ltPtlt40GeV 40ltPtlt60GeV 60ltPtlt100GeV 100ltPtlt140GeV
efficiency
This result is parameterized and then is used in
fast simulation. At least we need 50 eff. with
these fake rates for the discovery with L10fb-1.
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e/m Fake Rates

• Fake rates of electron and muon from jets are
  obtained from QCD di-jet samples using the full
  simulation.
• See O.Jinnouchis talk later.
• Typical performance
• Jet -gt e 10-3
• Jet -gt m 10-4
• These results are taken into account in the fast
  simulation to evaluate BG contributions.

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W3jets 4jets
Corresponds to L 70fb-1
Corresponds to L 10fb-1
4jets
W3jets
preliminary
fake(e-gtjet) 10-3
preliminary
(Signal region110-135GeV)

• W j j j -gt W t j j j j j j -gt e t
  j j
• W3j and 4jets remain with the same order of
  Znjets.
• Improvement of tauID for both cases and decrease
  of fake electron for 4jets are critical to
  decrease these BGs.

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M(tt) and Significance
Signal (m120)
very preliminary
Znj(QCD)
L30fb-1
Z/ttjj(EW)
tt, WWjj
a kind of BGs is the same as our paper(page5).
jjjj
A
B
C
W3jets
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sBG0.18fb
sBG0.43fb
sBG0.73fb
10
10
sSIG0.45fb

• We assume that lepton(e,m) efficiency is 85.
• Significance becomes worse by a factor of 2 due
  to the new backgrounds.
• We need to improve tauID/fake electron and to
  optimize cuts.

Signal region110-135GeV
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Summary

• VBF H-gttt-gtlephad is important for the SM Higgs
  discovery in the low mass region (mH120GeV).
• We evaluated 3rd jet by the ME event generator.
• There are differences in Pt and h of 3rd jet by
  the number of jets.
• Central jet veto survival probability and tail of
  M(tt) depend on the number of jets.
• We should use ME generators for multi-jets
  studies as much as possible.
• We studied fake BGs.
• W3jets and 4jets become large BGs for the
  lephad channel.
• We need to improve our tauID and other PID
  performance.
• We must understand QCD BGs for any LHC physics
  since their x-sec is very large in the LHC.
  (maybe beyond our expectation)

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Discovery potential of SM Higgs
ATLAS-PHYS-2004-005
preliminary
NB not include our latest results.
10fb-1 ATLAS
Today
L10fb-1 VBF tt(lllh) 2.6 gg 5.0 _at_
mH120GeV
200
120GeV
Two promising channels at mH120GeV! We are
preparing for this discovery!
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Backup
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ATLAS (A Toroidal LHC ApparatuS)
E,P resolution (P 100GeV) m 2 e, g
1.5 Jets 8

• 40MHz beam crossing
• Readout channel 160M
  channels
• Raw data 320Mbyte/sec
• (1TB/hour)

• Inner tracking system 2T solenoid magnet
• Liq. Ar EM calorimeter
• Muon spectrometer air-core troidal magnet

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Summary of SM Higgs Studies
ATLAS
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Hadronic tau decay mode

• Branching ratio of H-gttt
• BR(lep-lep) 12
• BR(lep-had) 46
• Lep-had mode plays an important role for the
  Higgs discovery.

factor of 4
MH120GeV
Mtt Distributions of VBF H-gttt
ATLAS
(em only)
ATLAS
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Fake rates
very preliminary
in 10-4
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Simulated data

• W(-gtln)3jets
• Alpgen ver1
• Jet condition Ptgt15GeV, etalt5,
  Rjjgt0.7Max(J1,J2)gt30GeV, M(J1,J2)gt200GeV,
  deltaEta(J1,J2)gt2
• x-sec 1.4nb very large!
• 4jets
• Alpgen ver1
• Jet condition Rjjgt0.7
• Forward region Ptgt30GeV, eta1xeta2lt0,
  eta1-eta2gt2.5Mjjgt350GeV, etalt5.0
• Central region Ptgt20GeV, etalt2.7, Mjjgt80GeV
• x-sec 2mb very large!!!

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qqH -gt qqtt -gt qq lep had
tau-gte
FWJet1
FWJet2
tau-gth (1prongpi0)
Missing Et
Well look this type of event in the real data !!!
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MEPS

• Z2j sample
• z2j without MLM method
• Z3j sample
• z2j with MLM method
• z3j without MLM method
• Z4j sample
• z2j with MLM method
• z3j with MLM method
• z4j without MLM method