Top physics at LHC with tt events

1
Top physics at LHC with
tt events
Fabrice Hubaut (hubaut_at_in2p3.fr)
CPPM/IN2P3Univ. de la Méditerranée (Marseille,
FRANCE)
On Behalf of the ATLAS and CMS Collaborations
Rencontres de Moriond 2006, QCD session, March
18-25
2
Motivations for top quark physics

• Special role in the EW sector and in QCD
• Heaviest elementary particle known ? Yukawa
  coupling close to 1.0
• Top and W masses constrain the Higgs mass
• Short lifetime (lttQCD) unique window on bare
  quarks
• ? A tool for precise SM studies
• Special role in various SM extensions through
  EWSB
• New physics might be preferentially coupled to
  top
• Non-standard couplings between top and gauge
  bosons
• New particles can produce / decay to tops
• ? A sensitive probe to new physics
• Special interest even if it is just a normal
  quark
• ? A major source of background for many
  searches
• ? A tool to understand/calibrate the detector,
  all sub-detectors involved

3
Top properties scorecard

• We still know little about the top quark, limited
  by Tevatron statistics
• Mass precision lt2
• Electric charge ? -4/3 excluded _at_ 94 C.L.
  (preliminary)
• Spin ½ not really tested spin correlations
• Isospin ½ not really tested
• BR to b quark 100 at 20 level in 3
  generations case
• V A decay at 20 level
• FCNC probed at the 10 level
• Top width ?? First observe single top !
• Yukawa coupling ??
• This leaves plenty of room for new physics in
  top production and decay
• Tevatron run II starts to incisely probe the top
  quark sector
• The LHC will open a new opportunity for
  precision measurements

4
Top production and decay at LHC
Strong Interaction tt
Weak Interaction single top
W
Tevatron s 7 pb 85 qq, 15 gg
LHC s 850 pb 10 qq, 90 gg
Tevatron s 3 pb 65Wg, 30Wt
LHC s 300 pb 75Wg, 20Wt
W-g fusion
W t
not observed yet !
BR (t?Wb) 100 in SM and no top hadronisation
W?en, mn
W?en, mn, qq
tt final states (LHC,10 fb-1)
Single top final states (LHC, 10 fb-1)

• Full hadronic (3.7 M) 6 jets
• Semileptonic (2.5 M) l n 4jets
• Dileptonic (0.4 M) 2l 2n 2jets

• W-g (0.5 M) l n 2jets
• Wt (0.2 M) l n 3jets
• W (0.02 M) l n 2jets

5
Early studies (lt1 fb-1)

• Remarkable topology t and t central and
  back-to-back in the transverse plane
• Easy to trigger and select

L100 pb-1 (1 day _at_ 1033 cm-2s-1)
3 jets with highest ? pT
4 jets pTgt 40 GeV
NO b-TAG !!
Full simulation

Signal (MC_at_NLO)
Isolated lepton pTgt 20 GeV ? trigger
Wn jets (Alpgen) combinatorial
pTmiss gt 20 GeV
Mjjj (GeV)

• Observation of clean top sample should be very
  fast
• Initial measurement of cross-section and mass
• Feedback on detector performance (JES,
  b-tagging, ) and on MC description

6
Precision studies (1-10 fb-1)

• When performance improve, such as b-tagging
  (?b?60, ruds?100, rc?10) ? non tt background
  (Wjets, bb, ...) negligible

L10 fb-1
Selection
Full reconstruction

• 1 isolated lepton pTgt20 GeV
• pTmissgt20 GeV
• 4 jets (cone DR0.4) pTgt40 GeV
• 2 b-tagged jets

• Use W?jj to calibrate light jet energy
• b with max. pT(jjb) for hadronic top
• pTmiss for pTn and MW constraint for pZ
• Other b for leptonic top s 12 GeV

s11 GeV
combinatorial
esel 3, 80k evts/10 fb-1 S/B12 (tt?tX)

• High statistics with a few fb-1, measurements
  limited by systematics
• Dileptonic channel also interesting ? 6
  equations (SpT0, Mlv MW, Mlvb Mt) with 6
  unknowns (pn)

? Apply this selection-recons. for ?-section,
mass, polarization studies,
7
Top mass (1)

• Measurement method (semileptonic)

• Kinematic fit event by event using t and t sides
• Mjj Mlv MW and Mjjb Mlvb Mtfit
• ? (Mtfit, c2) by slices of c2
• ? top mass estimator mtMtfit(c20)
• This selects well reconstructed b-jets (low
  effect due to final state radiation or leptonic
  b-decay)

• Results (semileptonic)

• mt linear with generated top mass
• Statistical error with 10 fb-1 0.1 GeV

hep-ex/0403021
8
Top mass (2)

• Systematic errors on mt (GeV) in semileptonic
  channel

• Systematics from b-jet scale (full simulation)

Source Error 10 fb-1
b-jet scale (1) 0.7
Final State Radiation 0.5
Light jet scale (1) 0.2
b-quark fragmentation 0.1
Initial State Radiation 0.1
Combinatorial bkg 0.1
TOTAL Stat ? Syst 0.9
184
slope0.7 GeV /
180
176
Rec. Top mass (GeV)
172
168
0.9 0.95 1.
1.05 1.1
b-jet miscalibration factor

• Other methods (invariant 3 jet jjb mass, large pT
  events, ...) give higher systematics but will
  allow reliable cross-checks

hep-ex/0403021

• A 1 GeV accuracy on Mt seems achievable with 10
  fb-1 at ATLAS/CMS

9
Top mass (3)

• Dileptonic (10 fb-1)

Input top mass175 GeV

• Need to reconstruct full tt event to assess the 2
  n momenta ? 6 equations (SpT0, Mlv MW, Mlvb
  Mt)
• Assume mt and compute solution probability event
  by event using MC kinematic distributions
• Choose mt with highest mean probability on all
  events
• Systematic uncertainty 2 GeV (PDF b-frag.)

mean probability
hep-ex/0403021
Mass hypthesis (GeV)

• Final states with J/? (100 fb-1)

• Correlation between MlJ/? and mt
• Low statistics 1000 evts/100 fb-1
• No systematics on b-jet scale !
• Systematic uncertainty 1 GeV (b-frag.)

Charge identification
MlJ/y
hep-ph/9912320
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W polarization in top decay (1)

• Test the top decay (in fully reconstructed tt)
  with W polarization ...

Standard Model (Mtop175 GeV) 0.703 0.297 0.000
Longitudinal W (F0)
Right-handed W (FR)
Left-handed W (FL)
NLO
0.695
0.304
0.001
Sensitive to EWSB
Test of V-A structure

• ...measured through angular distribution of
  charged lepton in W rest frame

1/N dN/dcos?
n
b
W

• Angle between
• lepton in W rest frame and
• W in top rest frame

t
?
1/2
1
1/2
spin
l
cos?
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W polarization in top decay (2)
10 fb-1
Semilep.
SM Error (stat syst)
F0 0.703 ? 0.004 ? 0.015
FL 0.297 ? 0.003 ? 0.024
FR 0.000 ? 0.003 ? 0.012
1/N dN/dcos?
(Mt175 GeV)
Combined results of semilepdilep
2 parameter fit with F0FLFR1
hep-ex/0508061
cos?

• Systematics dominated by b-jet scale, top mass
  and final state radiation (FSR)
• With 10 fb-1, can measure F0 with a 2 accuracy
  and FR with a precision 1
• Tevatron expectations (2 fb-1) dF0stat/F012
  and dFRstat/FR3

12
W polarization in top decay (3)

• From W polarization, deduce sensitivity to tWb
  anomalous couplings ? model independent
  approach, i.e. effective Lagrangian

)
and 4 couplings (in SM LO
F0

• 2s limit (stat?syst) on 0.04
• 3 times better than indirect limits
  (B-factories, LEP)
• Less sensitive to and already
  severely constrained by B-factories

1s
Anomalous coupling
13
tt spin correlation

• Test the top production
• t and t are not polarized in tt pairs, but
  their spins are correlated

PL B374 (1996)169
Mttlt550 GeV
A0.42
0.33
LHC
s (a.u.)
-0.24
AD-0.29
Tevatron
top spin ? 1/2, anomalous couplings, t?Hb
Mass of tt system, Mtt (GeV)

• by measuring angular distribution of daughter
  particles in top rest frame

Semilep. dilep. (10 fb-1)
SM Error (stat syst)
A 0.42 ? 0.014 ? 0.023
AD -0.29 ? 0.008 ? 0.010

• Syst. dominated by b-JES, top mass and FSR
• 4 precision on spin correlation parameters
• Tevatron expectations (2 fb-1) dAstat/A40

hep-ex/0508061
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Direct search for new particles

• In top production
• Example of resonances decaying to tt, as
  predicted by various models
• Generic analysis for a resonance X with s?, G?
  and BR(??tt)
• In top decay
• Example of t?Hb with subsequent H?tn
  (2lttanßlt40)
• Search for excess of t-events or deficit of
  dilepton events
• H discovery for MHlt160 GeV with 30 fb-1

J.Phys.G28 (2002) 2443
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Flavor Changing Neutral Currents

• Standard Model FCNC are highly suppressed (BR lt
  10-13-10-10)
• Some models beyond SM can give HUGE enhancements
  (BR up to 10-3)
• FCNC could be detected directly through top decay
  (tt, single top) or anomalous
  single top production
• Any observation would be sign of new physics
• ATLAS/CMS 5s sensitivity / 95 CL to FCNC
  branching ratio in tt events

Process 95 CL (today) LHC 95 CL (10 fb-1) LHC 5s (10 fb-1)
t?Zq 0.1 (LEP) 310-4 510-4
t?gq 0.01 (HERA) 710-5 110-4
t?gq 0.2 (TEV.) 110-3 510-3
Reconstruct t?Zq ?(ll-)j
Huge QCD background
? improve current limits by 102-103 in 1 year
starts to probe models
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Conclusions

• LHC will be a top factory 107 events already
  with 10 fb-1
• First steps towards precision measurements driven
  by systematics
• Challenge to get top mass 1 GeV ? SM MH
  constrained to lt30
• Test top production and decay e.g. by measuring
  W polarization 1-2
• and top spin correlation 4 ? anomalous
  tWb/gtt couplings, t?Hb, FCNC,
• New era of precision measurements in top sector
  in 3 years from now
• Powerful probes in the search for new physics
• Prior to precision measurements, a huge effort is
  needed (2007-2008)
• Complete study using full simulations and NLO
  generators
• Understand the detectors and control systematics
• Early top signals will help !!

17
Conclusions
Rendez-vous in Moriond 2008 for first top events
at LHC
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SPARES
19
LHC statistics

• LHC pp collisions at vs14 TeV every 25 ns in
  2007
• 2 phases 1033cm-2s-1 (initial, 2008-2009),
  1034cm-2s-1 (design, gt2009)

• High statistics at low luminosity
• Hard cuts to select clean events
• Few pile-up events

SM Process s (nb) Evts / 10 fb-1
Minimum bias 108 1015
bb 5 105 1012
W ? e? 15 108
Z ? e e- 1.5 107
t t 0.8 107
Dibosons 0.2 106

• SM parameter measurements will be dominated by
  systematic errors

• From Monte Carlo (MC) ISR/FSR, PDF, ...
• From detector and machine

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Utilizing tt events

• Light jet energy scale (aim 1)
• Extrapolation from testbeam data (1998-2004)
  5-10
• Improve with in situ calibration (Zjet, W?jj in
  tt events)
• In situ calibration with tt events
• A clean W?jj sample (up to 80) can be extracted
• Shift of W mass peak related to absolute energy
  scale
• extract absolute jet energy scale ?(Ejet) from
  data

before
after
? 2-3 reachable on absolute scale with 300 pb-1
only
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Utilizing tt events

• b-tagging studies simple demonstration
• An enriched (gt80) sample of b-jets can be
  extracted
• Cut on m(Whad) and m(tophad) masses
• Look at b-jet probability for 4th jet
• (must be b-jet if all assignments are
  correct)

b-jet probability
b-jet probability
B-JET CANDIDATE
ttbar (signal) always b jet if all jet
assignments are OK b enrichment expected
Wjets (background) random jet no b
enhancement expected
? check/calibrate b-tagging performance with data
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b-tagging
b-tagging algorithms a weight is given to each
jet combining signed impact parameters
(2D1D) and secondary vertex
reconstruction (mass, number of vertices, )
b-jets
Light jets
2D
2D1D
3DSVX
eb60 R230
Jet weight
23
Dileptonic channel

• Clean channel, easy to trigger on
• 2 neutrinos in final state ? full reconstruction
  however possible

Selection
Full reconstruction

• 2 isolated leptons with opposite charge, pTgt20
  GeV
• pTmissgt40 GeV
• 2 b-tagged jets pTgt20 GeV

• Assume top mass is known
• 6 equations (SpT0, Mlv MW, Mlvb Mt) with 6
  unknowns (pn)
• If gt 1 solution (98), solutions probability
  based on MC kinematic distributions

esel 6, 20k evts/10 fb-1 S/B6 (tt?tX)

• High statistics with a few fb-1, measurements
  limited by systematics
• Complementary to semileptonic channel

24
W polarization full simulation

• Good agreement Full sim / Fast sim on W and top
  kinematics
• compute a unique function (from Fast sim.) to
  correct for cuts and rec. effects
• apply it on Fast and Full sim. samples

Preliminary
TopReX Fast sim.
TopReX Full sim.
MC_at_NLO Full sim.
10 fb-1
0.7 fb-1
0.5 fb-1
1/N dN/dcos?
F00.699 0.005 FL0.299 0.003 FR0.002 0.003
F00.70 0.03 FL0.29 0.02 FR0.01 0.02
F00.69 0.03 FL0.30 0.02 FR0.01 0.02
cos?
cos?
cos?

• Very good agreement Full sim / Fast sim

25
tt spin correlation
In top rest frame, polarisation (S) is measured
with angular distributions of daughter
Degree to which its direction is correlated with
top spin (spin analyzing power)
W b l,d,s v,u,c lej
? (NLO) 0.40 -0.40 1. -0.31 0.47
angle between daughter and top spin axis s
lej least energetic jet in top rest frame

• Measurement of tt spin correlation (NP B690
  (2004) 81)

angle btwn spin analyzers direction in the t(t)
rest frame
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Top charge

• Qtop-4/3 (t?W-b instead of t?Wb) ?
• Method 1 Measurement of radiative top
  production and/or decay
• s(pp?ttg) is proportional to Qtop2
• After selectionreconstruction (10 fb-1)
• s (Q-4/3) gt s (Q2/3)
• Method 2 Measurement of daughter particle
  charge
• Associate b-lepton pair from the same top
• Compute the charge of b on a statistical basis
• Separate the 2 Qtop hypothesis needs less data
  than Method 1 (1 fb-1)
• Tevatron (Method 2)
• D0 (360 pb-1) excludes Q-4/3 _at_ 94 CL (10/2005,
  not yet published)

Q2/3 Q-4/3
pp?tt? 80 250
Background 70 70
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Yukawa coupling

• gt v2 Mt / v 1 intriguing !!
• Most difficult top quark property to measure!
• Measurement from associated Higgs production ttH
  (? bb, WW)

• s a gt2 Br(H?bb, WW)
• Need separate measurements
  
  of Higgs decay branching ratios
• Statistical uncertainty on gt 20 for MHlt200 GeV
  with 30 fb-1
• Systematics have to be carefully determined

28
ATLAS/CMS
29
LHC planning
2007
ltLgt3 1030
2008
ltLgt5 1032
L1 1033
2009
L2 1033
2010
ltLgt5 1033
2011
L1 1034