Transcript Top physics at LHC with tt events Fabrice Hubaut

Top physics at LHC
with tt events
Fabrice Hubaut
([email protected])
CPPM/IN2P3–Univ. de la Méditerranée (Marseille, FRANCE)
On Behalf of the ATLAS and CMS Collaborations
Rencontres de Moriond 2006, QCD session, March 18-25
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
1
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 (<tQCD): 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
5 orders of
magnitude
 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
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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Top properties scorecard
We still know little about the top quark, limited by Tevatron statistics
Mass
Electric charge ⅔
Spin ½
Isospin ½
BR to b quark ~ 100%
V – A decay
FCNC
Top width
Yukawa coupling
precision <2%
-4/3 excluded @ 94% C.L. (preliminary)
not really tested – spin correlations
not really tested
at 20% level in 3 generations case
at 20% level
probed at the 10% level
?? First observe single top !
??
 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
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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Top production and decay at LHC
Strong Interaction
tt
Weak Interaction
single top*
LHC
Tevatron
LHC
σ ~850 pb
σ ~7 pb
σ ~300 pb
σ ~3 pb
10% qq, 90% gg
85% qq, 15% gg
75%Wg, 20%Wt
65%Wg, 30%Wt
Wt
Tevatron
W*
W-g fusion
*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
• W-g (0.5 M) : l + n + 2jets
• Semileptonic (2.5 M) : l + n + 4jets
• Wt (0.2 M) : l + n + 3jets
• Dileptonic
• W* (0.02 M) : l + n + 2jets
(0.4 M) : 2l + 2n + 2jets
Golden channel (early physics, precision meas.)
Fabrice Hubaut (CPPM)
See talk by M. Najafabadi
Top physics at LHC with tt events
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Early studies (<1 fb-1)
 Remarkable topology: t and t central and back-to-back in the transverse plane
 Easy to trigger and select
3 jets with highest ∑ pT
L=100 pb-1
(1 day @ 1033 cm-2s-1)
4 jets pT> 40 GeV
NO b-TAG !!
Full simulation
Signal (MC@NLO)
Isolated lepton
pT> 20 GeV  trigger
W+n jets (Alpgen)
+ combinatorial
pTmiss > 20 GeV
 Observation of clean top sample should be very fast
Mjjj (GeV)
 Initial measurement of cross-section and mass
 Feedback on detector performance (JES, b-tagging, …) and on MC description
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
5
Precision studies (1-10 fb-1)
 When performance improve, such as b-tagging (b60%, ruds100, rc10)
 non tt background (W+jets, bb, ...) negligible
Selection
Full reconstruction
• 1 isolated lepton pT>20 GeV
• Use Wjj to calibrate light jet energy
• pTmiss>20 GeV
• b with max. pT(jjb) for hadronic top
• ≥4 jets
(cone DR=0.4)
pT>40 GeV
• 2 b-tagged jets
L=10 fb-1
s~11 GeV
• pTmiss for pTn and MW constraint for pZ
• Other b for leptonic top: s ~12 GeV
combinatorial
sel ~ 3%, 80k evts/10 fb-1
S/B~12 (ttt+X)
Purity of reconstructed tt ~ 70%
with rec ~ 30%
 High statistics with a few fb-1, measurements limited by systematics
 Dileptonic channel also interesting  6 equations (ΣpT=0, Mlv= MW, Mlvb= Mt) with 6 unknowns (pn)
 Apply this selection-recons. for Χ-section, mass, polarization studies, …
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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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: mt=Mtfit(c2=0)
 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
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
hep-ex/0403021
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Top mass (2)
 Systematic errors on mt (GeV) in semileptonic channel
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
 Systematics from b-jet scale (full simulation):
Rec. Top mass (GeV)
Source
184
slope=0.7 GeV / %
180
176
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
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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Top mass (3)
 Dileptonic (10 fb-1)
Input top mass=175 GeV
 Need to reconstruct full tt event to assess the 2 n
momenta  6 equations (ΣpT=0, 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
hep-ex/0403021
 Systematic uncertainty: ~2 GeV (PDF + b-frag.)
Mass hypthesis (GeV)
 Final states with J/ (100 fb-1)
 Correlation between MlJ/ and mt
MlJ/
 Low statistics: ~1000 evts/100
Charge identification
fb-1
 No systematics on b-jet scale !
 Systematic uncertainty: ~1 GeV (b-frag.)
Fabrice Hubaut (CPPM)
hep-ph/9912320
Top physics at LHC with tt events
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W polarization in top decay (1)
 Test the top decay (in fully reconstructed tt) with W polarization ...
Longitudinal W+ (F0)
Standard Model
(Mtop=175 GeV)
NLO
Left-handed W+ (FL)
Right-handed W+ (FR)
2
2




Mt
2
M
W






0.703 
2
2  0.297 
2
2 
M

2
M
t
W 

 M t  2M W 
0.695
0.304
Sensitive to EWSB
0.000
0.001
Test of V-A structure
2
2
2
1 dN
3   sin  
1

cos

1

cos




 
  F0  
  FL  
  FR  
 
N d cos  2   2 
2
2



 
n
b
1/2
t
1/2
spin
W+
1
Fabrice Hubaut (CPPM)

l+
Angle between:
•lepton in W rest frame and
•W in top rest frame
Top physics at LHC with tt events
1/N dN/dcos
 ...measured through angular distribution of charged lepton in W rest frame
cos
10
1/N dN/dcos
W polarization in top decay (2)
10 fb-1
SM
(Mt=175 GeV)
Error
(±stat ±syst)
F0
0.703
 0.004  0.015
FL
0.297
 0.003  0.024
FR
0.000
 0.003  0.012
Semilep.
Combined results
of semilep+dilep
2 parameter fit with
F0+FL+FR=1
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): δF0stat/F0~12% and δFRstat/FR~3%
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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W polarization in top decay (3)
 From W polarization, deduce sensitivity to tWb anomalous couplings
 model independent approach, i.e. effective Lagrangian
L
g
g
Wm b m ( f1L PL  f1R PR )t 
nWm bs mn ( f 2L PL  f 2R PR )t h.c.
2
2
1
(1   5 ) and 4 couplings (in SM LO f1L  Vtb  1, f1R  f 2L  f 2R  0)
2
F0
PR / L 
±1s
f1R
f 2L
f 2R
 2s limit (statsyst) on
f 2R = 0.04
 3 times better than indirect limits
(B-factories, LEP)
 Less sensitive to f1R and f 2L already
severely constrained by B-factories
Anomalous coupling
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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tt spin correlation
 Test the top production …
t and t are not polarized in tt pairs, but their spins are correlated
AD  AX  AY  AZ
=-0.24
PL B374 (1996)169
s (a.u.)
Mtt<550 GeV
s (t L t L )  s (t Rt R )  s (t Lt R )  s (t Rt L )
=0.33
A=0.42
A
s (t Lt L )  s (t Rt R )  s (t Lt R )  (t Rt L )
AD=-0.29
tRtR  tLtL
LHC
tRtL  tLtR
A=0.33
tRtL  tLtR
tRtR  tLtL
Tevatron
A=-0.35
top spin ≠ 1/2, anomalous couplings, tH+b
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/A~40%
hep-ex/0508061
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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Direct search for new particles
 In top production
 Example of resonances decaying to tt, as predicted by various models
1.6 TeV resonance
combinatorics
+ tt continuum
σxBR [fb]
 Generic analysis for a resonance X with σΧ, ΓΧ and BR(Χtt)
sxBR required for
a discovery
830 fb
30 fb-1
300 fb-1
1 TeV
mtt (GeV)
 In top decay
 Example of tH+b with subsequent H+tn (2<tanβ<40)
 Search for excess of t-events or deficit of dilepton events
 H+ discovery for MH+<160 GeV with 30 fb-1
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
J.Phys.G28 (2002) 2443
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Flavor Changing Neutral Currents
 Standard Model FCNC are highly suppressed (BR < 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)
3·10-4
5·10-4
tq
~ 0.01 (HERA)
7·10-5
1·10-4
tgq
~ 0.2 (TEV.)
1·10-3
5·10-3
Reconstruct tZq (l+l-)j
Huge QCD background
 improve current limits by ~102-103 in 1 year: starts to probe models
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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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 <30%
 Test top production and decay e.g. by measuring W polarization ~1-2%
and top spin correlation ~4%  anomalous tWb/gtt couplings, tH+b, 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 !!
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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Conclusions
Rendez-vous in Moriond 2008 for first top events at LHC
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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SPARES
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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LHC statistics
• LHC: pp collisions at √s=14 TeV every 25 ns in 2007
• 2 phases: 1033cm-2s-1 (initial, 2008-2009), 1034cm-2s-1 (design, >2009)
 High statistics at low luminosity
SM Process
σ (nb)
Evts /
10 fb-1
Minimum bias
108
~ 1015
bb
5 105
~ 1012
W → e
15
~ 108
Z → e+ e-
1.5
~ 107
tt
0.8
~ 107
Dibosons
0.2
~ 106
 Hard cuts to select clean events
 Few pile-up events
 SM parameter measurements will be
dominated by systematic errors
 From Monte Carlo (MC): ISR/FSR, PDF, ...
 From detector and machine
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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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 (Z+jet, Wjj in tt events)
 In situ calibration with tt events
before
 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
W
j2
M jj  2 E j1 E j 2 (1  cos  j1 j 2 )  M W
j1
b-jet
t
M WPDG  1 2 M W
part
E
with  i  i jet
Ei
after
 2-3% reachable on absolute scale with 300 pb-1 only
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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Utilizing tt events
 b-tagging studies: simple demonstration
W CANDIDATE
 An enriched (>80%) sample of b-jets can be extracted
TOP CANDIDATE
 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
ttbar (signal)
B-JET CANDIDATE
W+jets
(background)
‘always b jet if all jet
assignments are OK’
b enrichment expected
‘random jet’
no b enhancement
expected
 check/calibrate b-tagging performance with data
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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b-tagging
b-tagging algorithms: a weight is given to each jet combining signed impact
parameters (2D+1D) and secondary vertex
reconstruction (mass, number of vertices, …)
b-jets
Light jets
2D
2D+1D
3D+SVX
b=60%
R=230
Jet weight
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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Dileptonic channel
 Clean channel, easy to trigger on
 2 neutrinos in final state  full reconstruction however possible
Selection
• 2 isolated leptons with
opposite charge, pT>20 GeV
• pTmiss>40 GeV
• 2 b-tagged jets pT>20 GeV
sel ~ 6%, 20k evts/10 fb-1
S/B~6 (ttt+X)
Full reconstruction
• Assume top mass is known
• 6 equations (ΣpT=0, Mlv= MW, Mlvb= Mt)
with 6 unknowns (pn)
• If >= 1 solution (98%), solution’s probability
based on MC kinematic distributions
Purity of reconstructed tt ~ 65%
with rec ~ 80%
 High statistics with a few fb-1, measurements limited by systematics
 Complementary to semileptonic channel
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
23
W polarization: full simulation
Good agreement Full sim / Fast sim on W and top kinematics
Preliminary
 compute a unique function (from Fast sim.) to correct for cuts and rec. effects
 apply it on Fast and Full sim. samples
1/N dN/dcos
TopReX – Fast sim.
TopReX – Full sim.
10 fb-1
F0=0.699 ± 0.005
FL=0.299 ± 0.003
FR=0.002 ± 0.003
cos
MC@NLO – Full sim.
0.7 fb-1
0.5 fb-1
F0=0.70 ± 0.03
FL=0.29 ± 0.02
FR=0.01 ± 0.02
F0=0.69 ± 0.03
FL=0.30 ± 0.02
FR=0.01 ± 0.02
cos
cos
 Very good agreement Full sim / Fast sim
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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tt spin correlation
In top rest frame, polarisation (S) is measured
Degree to which its direction is correlated
with angular distributions of daughter:
with top spin (spin analyzing power)
1 dN
1
 (1  Si cosi )
N d cosi 2
W
b
 (NLO) 0.40 -0.40
angle between daughter and top spin axis s
l+,d,s
1.
v,u,c
lej*
-0.31 0.47
* lej = least energetic jet in top rest frame
 Measurement of tt spin correlation (NP B690 (2004) 81)
1
d 2N
1
 (1  C cos 1 cos 2)
N d (cos  1)d (cos  2) 4
1
dN
1
 (1  D cos  )
N d cos 
2
AD 1  2
Fabrice Hubaut (CPPM)
A 1  2
angle btwn spin analyzers
direction in the t(t) rest frame
Top physics at LHC with tt events
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Top charge
 Qtop=-4/3 (tW-b instead of tW+b) ?
 Method 1: Measurement of radiative top production and/or decay
 s(pptt) is proportional to Qtop2
 After selection+reconstruction (10 fb-1)
s (Q=-4/3) > s (Q=2/3)
Q=2/3
Q=-4/3
pptt
80
250
Background
70
70
 Method 2: Measurement of daughter particle charge
 Associate b-lepton pair from the same top
 Compute the charge of b on a statistical basis:
q bjet
  κ
q
i i j  p i ,   0.6

  κ
i j  p i
 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 @ 94% CL (10/2005, not yet published)
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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Yukawa coupling
 gt = √2 Mt / v ~ 1 : intriguing !!
 Most difficult top quark property to measure!
 Measurement from associated Higgs
production ttH ( bb, WW)
 σ α gt2 ·Br(Hbb, WW)
 Need separate measurements
of Higgs decay branching ratios
 Statistical uncertainty on gt ~20% for MH<200 GeV with 30 fb-1
Systematics have to be carefully determined
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
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ATLAS/CMS
ATLAS
CMS
Air-core toroids + solenoid in inner cavity
MAGNET (S) Calorimeters outside field
4 magnets
TRACKER
EM CALO
HAD CALO
MUON
Si pixels+ strips
TRD  particle identification
B=2T
s/pT ~ 5x10-4 pT  0.01
Pb-liquid argon
s/E ~ 10%/E
uniform
longitudinal segmentation
Fe-scint. + Cu-liquid argon (10 l)
s/E ~ 50%/E  0.03
Air  s/pT < 10 % at 1 TeV
standalone; larger acceptance
Fabrice Hubaut (CPPM)
Solenoid
Calorimeters inside field
1 magnet
Si pixels + strips
No particle identification
B=4T
s/pT ~ 1.5x10-4 pT  0.005
PbWO4 crystals
s/E ~ 2-5%/E
no longitudinal segmentation
Brass-scint. (> 5.8 l +catcher)
s/E ~ 100%/E  0.05
Fe  s/pT ~ 5% at 1 TeV
combining with tracker
Top physics at LHC with tt events
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LHC planning
<L>=3 1030
<L>=5 1032
2007
2008
L=1 1033
L=2 1033
<L>=5 1033
L=1 1034
Fabrice Hubaut (CPPM)
Top physics at LHC with tt events
2009
2010
2011
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