Study of Top properties at LHC

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Study of Top properties at LHC

• Stan Bentvelsen
• Moriond QCD week
• La Thuile, Mar 28 - Apr 4, 2004

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ATLAS CMS top studies

• Top mass determination
• Commissioning phase
• Top properties
• Single top production

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LHC and experiments

• pp (mainly) at ?s 14 TeV
• Startup in April 2007

• Initial/low lumi L?1033 cm-2 s-1
• ? 2 minimum bias/x-ing ? Tevatron-like
  environment
• 10 fb-1 /year
• Design/high lumi L1034 cm-2 s-1
• after 3 years
• 20 minimum bias/x-ing ? fast (? 50 ns) radhard
  detect
• 100 fb-1 /year

TOTEM
27 km ring 1232 dipoles B8.3 T
ATLAS and CMS pp, general purpose
ALICE heavy ions, p-ions
LHCb pp, B-physics
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ATLAS cavern
CMS yoke
Huge activities in construction of both
CMS and ATLAS
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Motivation

• Top quark exists and will be produced abundantly!
• In SM top- and W-mass constrain Higgs mass
• Sensitivity through radiative corrections
• Scrutinize SM by precise determination top mass

• Beyond SM New Physics?
• Many heavy particles decay in tt
• Handle on new physics by detailed properties of
  top
• Experiment Top quark useful to calibrate the
  detector
• Beyond Top Top quarks will be a major source of
  background for almost every search for physics
  beyond the SM

Summer 2003 result
direct
EXCLUDED
indirect
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Top production

• Cross section determined to NLO precision
• Total ?NLO(tt) 834 100 pb
• Largest uncertainty from scale variation
• Compare to other production processes

90 gg10 qq
Low lumi
Process N/s N/year Total collected before start LHC
W? e? 15 108 104 LEP / 107 FNAL
Z? ee 1.5 107 107 LEP
tt 1 107 104 Tevatron
bb 106 1012-13 109 Belle/BaBar ?
H (130) 0.02 105 ?

• Top production cross section approximately 100x
  Tevatron

LHC is a top factory!
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Golden-plated MTop from leptonjet

• Golden channel
• Clean trigger from isolated lepton
• The reconstruction starts with the W mass
• different ways to pair the right jets to form the
  W
• jet energies calibrated using mW
• Important to tag the b-jets
• enormously reduces background (physics and
  combinatorial)
• clean up the reconstruction

• Br(tt?bbjjl?)30for electron muon

• Typical selection efficiency 5-10
• Isolated lepton PTgt20 GeV
• ETmissgt20 GeV
• 4 jets with ETgt40 GeV
• gt1 b-jet (?b?40, ?uds?10-3, ?c?10-2)

Background lt2 W/Zjets, WW/ZZ/WZ
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Lepton jet reconstruct top

• Hadronic side
• W from jet pair with closest invariant mass to MW
• Require MW-Mjjlt20 GeV
• Assign a b-jet to the W to reconstruct Mtop
• Kinematic fit
• Using remaining lb-jet, the leptonic part is
  reconstructed
• ml?b -ltmjjbgt lt 35 GeV
• Kinematic fit to the tt hypothesis, using MW
  constraints

W-mass

• Selection efficiency 5-10

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Top mass systematics

• Method works
• Linear with input Mtop
• Largely independent on Top PT
• Biggest uncertainties
• Jet energy calibration
• FSR out of cone give large variations in mass
• B-fragmentation
• Verified with detailed detector simulation and
  realistic calibration

Source of uncertainty Hadronic ?Mtop (GeV) Fitted ?Mtop (GeV)
Light jet scale 0.9 0.2
b-jet scale 0.7 0.7
b-quark fragm 0.1 0.1
ISR 0.1 0.1
FSR 1.9 0.5
Comb bkg 0.4 0.1
Total 2.3 0.9
Challenge determine the mass of the top around
1 GeV accuracy in one year of LHC
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Alternative mass determination

• Select high PT back-to-back top events
• Hemisphere separation (bckgnd reduction, much
  less combinatorial)
• Higher probability for jet overlapping
• Use the events where both Ws decay leptonically
  (Br5)
• Much cleaner environment
• Less information available from two ?s
• Use events where both Ws decay hadronically
  (Br45)
• Difficult jet environment
• Select PTgt200 GeV

Various methods all have different systematics
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Top mass from J/?

• Use exclusive b-decays with high mass products
  (J/?)
• Higher correlation with Mtop
• Clean reconstruction (background free)
• BR(tt?qqb??J/????) ? 5 10-5
• ? 30 ? 103 ev./100 fb-1 (need high lumi)

MlJ/?
Different systematics (almost no sensitivity to
FSR) Uncertainty on the b-quark fragmentation
function becomes the dominant error
M(J/?l)
M(J/?l)
Mtop
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Consequences from Mtop
Thanks to M.Grunewald

• Assuming total uncertainty on W-mass of 15 MeV
• Combined LHC prospect
• Very challenging measurement!
• Repeat the Electro-Weak fit changing the
  uncertainties
• ?Mtop1 GeV
• ?MW 15 MeV
• Same central values
• SM constraints on MHiggs
• Summer 2003 values

direct
EXCLUDED
(?mH/mH ? 32)

• Chances to rule out SM!

(?mH/mH ? 53)
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Commissioning the detectors

• Determination MTop in initial phase
• Use Golden plated leptonjet
• Selection
• Isolated lepton with PTgt20 GeV
• Exactly 4 jets (?R0.4) with PTgt40 GeV
• Reconstruction
• Select 3 jets with maximal resulting PT

Calibrating detector in comissioning phase Assume
pessimistic scenario -) No b-tagging -) No jet
calibration -) But Good lepton identification
Period Stat ?Mtop (GeV) Stat ??/?
1 year 0.1 0.2
1 month 0.2 0.4
1 week 0.4 2.5
No background included

• Signal can be improved by kinematic constrained
  fit
• Assuming MW1MW2 and MT1MT2

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Commissioning the detectors

• Most important background for top W4 jets
• Leptonic decay of W, with 4 extra light jets
• Alpgen, Monte Carlo has hard matrix element for
  4 extra jets(not available in Pythia/Herwig)

ALPGEN W4 extra light jets Jet PTgt10, ?lt2.5,
?Rgt0.4 No lepton cuts Effective ? 2400 pb

• Signal plus background at initial phase of LHC

L 150 pb-1 (2/3 days low lumi)
With extreme simple selection and reconstruction
the top-peak should be visible at LHC
measure top mass (to 5-7 GeV) ? give feedback on
detector performance
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Search for resonances

• Many theoretical models include the existence of
  resonances decaying to top-topbar
• SM Higgs (but BR smaller with respect to the
  WW and ZZ decays)
• MSSM Higgs (H/A, if mH,mAgt2mt, BR(H/A?tt)1 for
  tanß1)
• Technicolor Models, strong ElectroWeak Symmetry
  Breaking, Topcolor, colorons production,

• Study of a resonance ? once known s?, G? and
  BR(??tt)
• Reconstruction efficiency for semileptonic
  channel
• 20 mtt400 GeV
• 15 mtt2 TeV

1.6 TeV resonance
Mtt
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Couplings and decays

• Does the top quark behaves as expected in the SM?
• Yukawa coupling to Higgs from ttbarH events
• Electric charge
• Top spin polarization
• CP violation
• According to the SM
• Br(t ?Wb) ? 99.9, Br(t ? Ws) ? 0.1, Br(t ?
  Wd) ? 0.01
  (difficult to
  measure)
• Can probe t ?Wnon-b by measuring ratio of
  double b-tag to single b-tag
• Statistics more than sufficient to be sensitive
  to SM expectation for Br(t ? W s/d)
• need excellent understanding of b-tagging
  efficiency/purity

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Rare decays FCNC

• In the SM the FCNC decays are highly suppressed
  (Brlt10-13-10-10)
• Any observation would be sign of new physics
• Sensitivity according to ATLAS and CMS studies
• t ? Zq (CDF Brlt33 _at_ 95CL, ALEPH Brlt17, OPAL
  Brlt13.7)
• Reconstruct t ? Zq ? (ll-)j
• Sensitivity to Br(t ? Zq) 1 X 10-4 (100 fb-1)
• t ? ?q (CDF Brlt3.2 _at_ 95CL)
• Sensitivity to Br(t ? ?q) 1 X 10-4 (100 fb-1)
  
• t ? gq
• Difficult identification because of the huge QCD
  bakground
• One looks for like-sign top production (ie. tt)
• Sensitivity to Br(t ? gq) 7 X 10-3 (100 fb-1)

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Top charge determination

• Can we establish Qtop2/3?
• Currently cannot exclude exotic possibility
  Qtop-4/3
• Assign the wrong W to the b-quark in top decays
• t?W-b with Qtop-4/3 instead of t?Wb with
  Qtop2/3 ?
• Technique
• Hard ? radiation from top quarks
• Radiative top production, pp?tt? cross section
  proportional to Q2top
• Radiative top decay, t?Wb?
• On-mass approach for decaying top two
  processes treated independently
• Matrix elements havebeen calculated and fed
  intoPythia MC

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Top Charge determination

• Determine charge of b-jet andcombine with lepton
• Use di-lepton sample
• Investigate wrong combination b-jet charge and
  lepton charge
• Effective separation b and b-bar possible in
  first year LHC
• Study systematics in progress

• Yield of radiative photons allows to distinguish
  top charge

Q2/3 Q-4/3
pp?tt? 101 10 295 17
pp?tt t?Wb? 6.2 2.5 2.4 1.5
Total background 38 6 38 6
10 fb-1 One year low lumi
events
pT(?)
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Top spin correlations

• In SM with Mtop?175 GeV, ?(t) ? 1.4 GeV ?QCD
• Top decays before hadronization, and so can study
  the decay of bare quark
• Substantial ttbar spin correlations predicted in
  pair production
• Can study polarization effects through helicity
  analysis of daughters
• Study with di-lepton events
• Correlation between helicity angles ? and
  ?-for e/? and e-/?-

ltCosT CosT-gt
ltCosT CosT-gt
With helicity correlation
No helicity correlation
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Top spin correlations

• Also study spin correlations in hadronic decays
  (single lepton events)
• Least energetic jet from W decay ? 0.5
• Ratio between with and without correlations

30 fb-1
ltCosT CosT-gt

• Able to observe spin correlations in asymmetry
  C?
• 30 fb-1 of data
• 0,035 statistical error
• 0,028 systematic error
• 10? statistical significance for a non-zero
  value with 10 fb-1

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Single top production
1) Determination of Vtb 2) Independent mass
measurement

• Three production mechanisms
• Main Background ?xBR(W?l?), le,µ
• tt s833 pb 246 pb
• Wbb s300 pb 66.7 pb
• Wjj s18103 pb 4103 pb

16.6
Wg fusion 24527 pb S.Willenbrock et al.,
Phys.Rev.D56, 5919
Wt 62.2 pb A.Belyaev, E.Boos,
Phys.Rev.D63, 034012
W 10.20.7 pb M.Smith et al., Phys.Rev.D54,
6696
-3. 7

• Direct determination of the tWb vertex (Vtb)
• Discriminants
• - Jet multiplicity (higher for Wt)
• More than one b-jet (increase W signal over W-
  gluon fusion)
• 2-jets mass distribution (mjj mW for the Wt
  signal only)

Wg 54.2 pb Wt 17.8 pb W 2.2 pb
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Single top results

• Detector performance critical to observe signal
• Fake lepton rate
• b and fake rate id ?
• Reconstruction and vetoing of low energy jets
• Identification of forward jets
• Each of the processes have different systematic
  errors for Vtb and are sensitive to different new
  physics
• heavy W ? increase in the s-channel W
• FCNC gu ? t ? increase in the W-gluon fusion
  channel

• Signal unambiguous, after 30 fb-1
• Complementary methods to extract Vtb
• With 30 fb-1 of data, Vtb can be determined to
  -level or better(experimentally)

Process Signal Bckgnd S/B
Wg fusion 27k 8.5k 3.1
Wt 6.8k 30k 0.22
W 1.1k 2.4k 0.46
Process ?Vtb(stat) ?Vtb(theory)
Wg fusion 0.4 6
Wt 1.4 6
W 2.7 5
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Conclusions
LHC is top factory ?(tt)830 pb-1 107 events in
first year

• Precise determination Mtop is waiting
• Challenge to get ?Mtop 1 GeV
• Confirmation that top-quark is SM particle
• Measure Vtb, charge, CP, spin, decays
• Top quarks for commissioning the detectors
• Top peak should be visible with eyes closed
• Todays signal, tomorrows background
• Top quarks as main background for many new
  physics channels

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• Backup

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Top decay

• In the SM the top decays to Wb
• All decay channels investigated
• Using fast parameterized detector response
• Checks with detailed simulations

• Di-leptons (e/?)
• BR4.9 ? 0.4x106 ev/y
• No top reconstructed
• Clean sample
• Single Lepton (e/?)
• BR29.6 ? 2.5x106 ev/y
• One top reconstructed
• Clean sample
• Fully Hadronic
• BR45 ? 3.5x106 ev/y
• Two tops reconstructed
• Huge QCD background
• Large combinatorial bckgnd

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Top mass from di-leptons

• Use the events where both Ws decay leptonically
  (Br5)
• Much cleaner environment
• Less information available due to two neutrinos
• Sophisticated procedure for fitting the whole
  event, i.e. all kinematical info taken into
  account (cf D0/CDF)
• Compute mean probability as function of top mass
  hypothesis
• Maximal probability corresponds to top mass

Source of uncertainty Di-lepton ?Mtop (GeV)
statistics 0.3
b-jet scale 0.6
b-quark fragm 0.7
ISR 0.4
FSR 0.6
pdf 1.2
Total 1.7
80000 events ?(tt) 20 S/B 10
Selection 2 isolated opposite sign leptons Ptgt35
and Ptgt25 GeV 2 b-tagged jets ETmissgt40 GeV
Mean probability
mass
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Top mass from hadronic decay

• Use events where both Ws decay hadronically
  (Br45)
• Difficult jet environment
• ?(QCD, Ptgt100) 1.73 mb
• ?(signal) 370 pb
• Perform kinematic fit on whole event
• b-jet to W assignment for combination that
  minimize top mass difference
• Increase S/B
• Require pT(tops)gt200 GeV

Selection 6 jets (?R0.4), Ptgt40 GeV 2 b-tagged
jets Note Event shape variables like HT, A, S,
C, etc not effective at LHC (contrast to Tevatron)
Source of uncertainty Hadronic ?Mtop (GeV)
Statistics 0.2
Light jet scale 0.8
b-jet scale 0.7
b-quark fragm 0.3
ISR 0.4
FSR 2.8
Total 3.0
3300 events selected ?(tt) 0.63
?(QCD) 210-5 S/B 18
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High Pt sample

• The high pT selected sample deserves independent
  analysis
• Hemisphere separation (bckgnd reduction, much
  less combinatorial)
• Higher probability for jet overlapping
• Use all clusters in a large cone ?R0.8-1.2
  around the reconstructed top- direction
• Less prone to QCD, FSR, calibration
• UE can be subtracted

Mtop
Statistics seems OK and syst. under control
?R
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Jet scale calibration

• Calibration demands
• Ultimately jet energy scale calibrated within 1
• Uncertainty on b-jet scale dominates ?Mtop light
  jet scale constrained by mW
• At startup jet-energy scale known to lesser
  precision

Uncertainty On b-jet scale Hadronic
1 ? ?Mt 0.7 GeV 5 ? ?Mt
3.5 GeV 10 ? ?Mt 7.0 GeV
Uncertainty on light jet scale Hadronic
1 ? ?Mt lt 0.7 GeV 10 ?
?Mt 3 GeV
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Alternative methods

• Determining Mtop from ?(tt)?
• huge statistics, totally different systematics
• But Theory uncertainty on the pdfs kills the
  idea
• 10 th. uncertainty ? ?mt ? 4 GeV
• Constraining the pdf would be very precious
• (up to a few might not be a dream !!!)

• Luminosity uncertainty then plays the game (5?)

Luminosity uncertainty then plays the game (5?)

• Continuous jet algorithm
• Reduce dependence on MC
• Reduce jet scale uncertainty
• Repeat analysis for many cone sizes ?R
• Sum all determined top massrobust estimator
  top-mass

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Monte Carlo tools
(Frixione, Nason, Webber)

• MC_at_NLO NLO Monte Carlo
• Matching NLO calculations of QCD process with
  parton shower MC
• Total rates are accurate to NLO
• Hard emissions treated as in NLO computations
• Soft/collinear emissions handled by MC shower
• No double counting between these
• Based on NLO subtraction method
• Interfaced to Herwig

?(pb)

• Pt(tt system)
• Herwig MC_at_NLO agree at low Pt
• At large Pt MC_at_NLO harder

Log10(PT)
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Investigation concequences
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Associated Higgs production

• One of the promising channels for low-mass Higgs

• Reconstruct ttbar H ? WWbbbb ? (l?)(jj)bbbb
• 6 jets, 4 b-tagged lepton
• reconstruct top decays
• select bjets giving m(l?b), m(jjb) closest to
  m(t)
• look for H ? bb peak using remaining 2 bjets

gg?ttH H?bb s?BR300 fb for MH115 GeV
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Rare decays top?WbZ

• Interesting branching ratio depends strongly on
  Mtop
• Since MtopMWMbMZ
• With present error ?mt ? 5 GeV, BR varies over
  a factor ? 3
• B-jet too soft to be efficiently identified ?
• ? semi-inclusive study for a WZ near
• threshold, with Z ? ll- and W -gtjj
• Requiring 3 leptons reduces the Zjets
  background
• Sensitivity to Br(t ? WbZ) ?? 10-3 for 1 year at
  low lumi.
• Even at high L cant reach Sm predictions (??
  10-7 - 10-6)

G. Mahlon hep-ph/9810485
G(t?WbZ)/G(t?Wb)
M(top) (GeV)
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top?Hq

• Various approaches studied
• Previously ttbar?Hq Wb?(b-bbar)j(l?b) for m(H)
  115 GeV
• Sensitivity to Br(t ? Hq) 4.5 X 10-3 (100
  fb-1)
• New results for
• t tbar?Hq Wb?WWq Wb?(l? l?j) (l?b)
• 3 isolated lepton with pT(lep) gt 30 GeV
• pTmiss gt 45 GeV
• 2 jets with pT(j) gt 30 GeV,
• incl. 1 jet con b-tag
• Kinematical cuts making use of
• angular correlations
• Sensitive to Br(t ? Hq) 2.4 X 10-3
• for m(H) 160 GeV (100 fb-1)

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