1. Anne-Isabelle ETIENVRE
Top quark physics
Anne-Isabelle ETIENVRE

2. Outline Introduction Top quark discovery: a long search!
Top-antitop production at hadron colliders
Single top production
Sensitivity to physics beyond Standard Model
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3. Introduction (1/2) Identity card (a peculiar quark):
SU(2)L partner of the bottom
Q = 2/3, T3=1/2
Heaviest quark (gold atom!): 40 * m(bottom quark),
Mtop = ± 1.2 GeV/c2 (Tevatron)
Produced predominantly (in hadron-hadron collisions) by strong interaction
Width Gtop = 1-2 GeV/c2 (increases with top mass)
Corresponding lifetime short = 0.5 x s
Top decays before hadronisation keeping its properties
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4. Introduction (2/2) Identity card: For each measurement, we will see:
Yukawa coupling ytop = 1 (mtop = yt v2)
Decays almost exclusively through t Wb:
In the Standard Model, 99.9%
i.e. CKM matrix |Vtb|1
For each measurement, we will see:
Why it is interesting to be performed
Where do we stand
What we should learn with LHC
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5. Motivations for top quark physics studies
Top quark exists and will be produced abundantly
In Standard Model (SM): top- and W-mass constrain Higgs mass
through radiative corrections
Scrutinize SM by precise determination of the top quark 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
Commissioning (jet energy scale, b-tagging,..)
Beyond top quark:
Top quarks will be a major source of background for many searchs (Higgs, SUSY, exotics,…)
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6. Top quark discovery (1975  1995)
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7. Top quark discovery 1974 : 2 quarks and leptons families
A third family
1974 : 2 quarks and leptons families
1975: t discovery (SLAC)
a third family is needed!
1977 : b quark discovery (Y resonance )
μ+μ- spectrum
b quark should have an electroweak partner : the top quark
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8. Top quark discovery Direct searches:
Search for a toponium (bounded state t t) around 27 GeV/c2
top mass expected around 15 GeV/c2
(mass s/c/b : 0.5/1.5/4.5 -> mtop~15 GeV/c2)
Search at the e+e- colliders:
DESY-PETRA (1980) :  s = GeV  mtop > 30 GeV/c2
LEP 1 (1989), study of Z t t , with  s = 91 GeV  mtop > 45.8 GeV/c2
Search at the p p̅ colliders:
UA1 and UA2 (1981 -> 1990) ,  s = 640 GeV  mtop > 69 GeV/c2
« discovery » by UA1 in 1984 (mtop= 40 GeV/c2) : 9 signal evts / 0.2 background evts (background was underestimated)
Tevatron (1990  1992),  s = 1.8 TeV  mtop > 91 GeV/c2
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9. Top quark discovery Discovery : Tevatron (1995) Indirect searches:
mtop = 176 ± 8 (stat.) ± 10 (syst.) GeV/c2 (CDF)
mtop = 199 ± 19(stat.) ± 22 (syst.) GeV/c2 (D0)
Indirect searches:
Precise electroweak measurements correlated to the radiative corrections:
Dr = K mtop2 , Drrésiduel= f(ln(mH2))
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10. Top quark discovery Top mass evolution Tevatron discovery Current
measurement
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11. Colliders Tevatron (p p̅ ),  s = 1.96 TeV LHC (pp): s = 14 TeV
Luminosity: 2.7 fb-1 summer pb-1 december 2008 ( s = 10 TeV)
7-8 fb fb ( s = 14 TeV)
expected!
Tevatron (p p̅ ),  s = 1.96 TeV
LHC (pp): s = 14 TeV
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12. Colliders Cross section comparison Tevatron/LHC Process Tevatron (pb)
LHC (pb)
tt̅
6.8
833
bb̅
W+2jets
18 103 WW
12.0
117 WZ
3.68 23
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13. Colliders Comparison Tevatron/LHC ( s = 10 TeV) Luminosity
Number of inclusive tt
Number of
W (lv)+jets
(background)
Start-up LHC
( s = 10 TeV)
20 pb-1 ≈ ≈
Low-luminosity LHC
(2009)
1 fb-1 ≈ ≈
Tevatron
(summer 2009)
5.1 fb-1 ≈ ≈
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14. Total collected before start LHC
Top pair production
Opposite @ Tevatron
Production (strong interaction):
Cross section
LHC NLO(tt) = 834 ± 100 pb
Tevatron: 7 pb
Comparison to other production
LHC:
~90% gg ~10% qq
process
(pb)
Total collected before start LHC bb
5108
109
Zee
1.5103
107
Wℓ (ℓ=e,μ)
3104 tt
830
104
H(130 GeV/c2) 1 ?
LHC is (also) a top factory!
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15. Top quark decay Standard Model: Branching ratio / main backgrounds
Br(tWb)  100%
Studies % products of W decay
Branching ratio / main backgrounds
In tt events:
Background: QCD (bb̅)
Large combinatorial background
Clean but low BR
Many unknowns
Bkgd:Z+jets, W+ jets,
Z+jets, WW+jets, QCD (bb̅)
Golden channel:
Good B.R.
Clean sample (background = W+jets,
Z+jets, diboson, QCD)
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16. Systematic errors in top quark studies
Jet energy scale (light jets / b-jets)
LHC aim : jet energy knowledge better than 1 %
light jet energy scale contribution:
can be strongly reduced using an in-situ calibration based on the W mass constraint (see later on)
B jet energy scale:
Dominant jet energy scale
Tevatron : estimated from Monte Carlo (global rescaling factor)
LHC : could be estimated from data
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17. Systematic errors in top quark studies
Initial and final state radiations (ISR, FSR)
ISR
Enhancement of the combinatorial background
Bias on the top quark mass (over-estimation of the jet energies in the final state)
FSR :
Bias on the top quark mass (under-estimation of the jet energies in the final state)
Estimated on Monte Carlo at the Tevatron
Could be estimated on data at LHC
b-quark fragmentation
error estimated changing the Peterson parameter ( ) within
its theoretical uncertainty (0.0025)
combinatorial background
error estimated varying the background shape and size in the fitting procedure
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18. Top-antitop production
Mass measurement
top-antitop cross section measurement
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19. Top quark mass measurement
Why do we need a precise measurement of mtop?
The uncertainties on mtop, and mW are the dominating ones in the electroweak fit
Precise measurement of mtop, mW 
one can get information on the missing parameter mHiggs
one can test the validity of the Standard Model
Dr = K mtop2 , Drrésiduel= f(ln(mH2))
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20. Top quark mass measurement
Present measurement (Tevatron, July 2008):
Mtop= ± 0.7 (stat.) ± 1.0 (syst.)
Most accurate measurement
in the l+jets channel
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21. Top quark mass measurement
Electroweak fit:
The blue band plot
Incidence of precision:
On Da:
On mtop: mtop (2007) = ± 1.8 GeV (2007)
On mW: if with the same central values , 33 24 76 + - = H m
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22. Top quark mass measurement
Electroweak fit:
direct mtop and mw
indirect mtop and mw
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23. Top quark mass measurement
Leptonic side
Hadronic side
At LHC : Lepton+jets channel
Event selection:
Direct hadronic top reconstruction:
Pairing of the 2 light jets < hadronic W
Association hadronic W  b-jet
Typical selection efficiency: ~5-10%:
Isolated lepton PT>20 GeV
ETmiss>20 GeV
≥ 2 light jets and 2 b-jets with pT>40 GeV
S/B: 10-4  30
for a generated top mass = 175 GeV/c2 :
M(top) = ± 0.3 GeV/c2
s(top) = ± 0.3 GeV/c2
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24. Top quark mass measurement
Lepton + jets channel (cont.)
Systematic uncertainties:
Jet energy scale (JES):
light jet energy scale constrained by an in-situ rescaling based on the W mass
b jet energy scale: dominant source of uncertainty
Statistical uncertainty
will be quickly negligible;
Error on the top mass =
1 to 3.5 GeV for a
JES = 1 to 5 %
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25. Top quark mass measurement
In-situ light jet energy scale (LHC)
Light jet energy scale using W constraint:
Template histograms of the invariant mass mjj have been generated, from W  qq PYTHIA for several values of the energy scale a
c2 (template – data) minimum a
All jets are calibrated with a
a can be evaluated % energy,
and h
Ratio E(light jet) / E(b jets)
estimated on Monte Carlo
1% on JES is achievable with 1 fb-1
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26. Top quark mass measurement
Alternative measurements:
Di-leptons
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27. Top quark mass measurement
Top quark mass measurement expected for 10 fb-1
l +jets
di-lepton
All-jets
ATLAS
(stat.)
0.05
0.04
0.18
(syst.) 1
1.7 3
CMS
0.3
0.5
0.2
1.3
1.1
4.2
 Huge effort to be performed for JES in order to reach < 1 GeV
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28. Cross section measurement
Motivations for a precise measurement of s(tt):
Sensitivity to new physics:
Search for resonances
Beyond Standard Model top decay
Not seen up to now (Tevatron)
Indirect top mass measurement
First step = reconstruction of the tt final state (in common with top mass)
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29. Cross section measurement
Cross section estimation:
Counting method:
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30. Cross section measurement
Present tt cross section measurement (Tevatron):
Stat. syst. Lumi
Stat. syst. Lumi.
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31. Cross section measurement
Studies at LHC
Early measurement (100 pb-1)
Counting method
Di-lepton channel:(in % of the cross-section):
Systematic uncertainties dominated by JES, ISR and FSR, luminosity
L+jets channel (in % of the cross-section):
Systematic uncertainties dominated by JES and ISR/FSR, luminosity
ATLAS
ATLAS
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32. Top mass from tt cross section
Assuming that tt production is governed by Standard Model, mtop can be extracted from s(tt)
If Ds/s(theo.)  5%, and
Ds/s(exp.)  5 %,
Dmtop  2.6 GeV
Not so far from direct
measurements
at the beginning of LHC
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33. Single top production
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34. Single top production
Electroweak production of the top quark: 3 channels
s channel:
Cross section Tevatron = 0.88 ± 0.14 pb
Cross section LHC : 10 ± 0.8 pb
t channel :
Cross section Tevatron = 1.98 ± 0.30 pb
Cross section LHC: 245 ± 30 pb
Wt channel :
Cross sectionTevatron = 0.21 ± 0.03 pb
Not Tevatron
Cross section LHC : 60 ± 15 pb
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35. Single top production Interest of the measurement:
Cross section proportional to |Vtb|2 : single top is a way to measure |Vtb|
Irreducible background for many processes (Higgs, SUSY)
Sensitivity to new physics
Each of the processes have different systematic errors for Vtb and are sensitive to different new physics
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36. Single top production Tevatron study: Challenge Low Cross sections
Large background (W+ 2 jets)
S and t channels can be observed,
Wt not reachable
Evidence for single top process at
Tevatron in 2006 for the first time
(both s and t channels)
July results
D0 will update its results
with more statistic
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37. Single top production At LHC:
With 1 fb-1, an accurate measurement is foreseen in t and Wt channels;
s channel could be seen with 10 fb-1, but difficult (background)
The Wt channel should be observed for the first time
A limit on |Vtb| will be extracted from this measurement
Process
s/s
(1 fb-1)
Systematic
t channel
23 %
Luminosity, b-tagging, JES, ISR/FSR Wt
20.5% s
52%
Background, luminosity, b-tagging, JES, ISR/FSR
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38. Physics beyond Standard Model
In top – antitop production
In single top production
No evidence seen at Tevatron
At LHC?
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39. Physics beyond Standard Model
In top pair production
Search for heavy resonance in M(tt)
Non Standard Model distribution of M(tt) would be
a signal of heavy particle X  tt
Interference from non SM process
Would also appear as a deviation in ds(tt)/dm(tt)
Example: Z’ search
Z’ is an hypothetical massive
boson (spin 1) predicted
by several extensions of SM
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40. Physics beyond Standard Model
In top pair production
Z’ search
LHC:
For mZ’ = 700 GeV/c2,
Can be discovered if
s > 11 pb for an inclusive
decay -- >tt
Likelihood analysis:
No excess
M(Z’) > % C.L.
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41. Physics beyond Standard Model
Sensitivity to charged Higgs:
Context:
In Minimal extension of Standard Model (MSSM):
2 Higgs doublet  5 Higgs boson (h,H,A,H+,H-)
Charged Higgs could modify top decay (BR(t Wb)≠ 1)
Can be searched in single top or in tt production
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42. Physics beyond Standard Model
Sensitivity to charged Higgs
Example in tt production: mH+ < mtop
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43. Physics beyond Standard Model
Sensitivity to charged Higgs
search in single top production: for H+ mass > mtop
Leads to same final state as s-channel
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44. Physics beyond Standard Model
In single top production:
Modification of the coupling (Flavour Changing Neutral Current : « FCNC »):
tcZ, tcg, tcg,…
Strongly suppressed in SM (BR  – 10-11)
Less suppressed in MSSM (10-6 – 10-4)
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45. Physics beyond Standard Model
In single top production
BR(tqg) < 95% C.L.
BR(tqZ) < 95% C.L.
BR(tqg) < 95% C.L.
LHC (1 fb-1)
B.R.
t->Zq
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46. Top quark electric charge
Never measured: should be 2/3, but -4/3 exist in non SM
Tevatron (l+jets channel): measurement via b-jet charge measurement
LHC : measurement should be achieved with 1 fb-1
jet
e,m
b-jet n
B-jet
MET
Qt= - 4/3 e 94% C.L.
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47. W polarisation Goal: tt decay (l+jets) used for W polarisation study
SM: 2 states of helicity: F- = 0.297, F0 = (LO)
2 discriminant distributions:
pT(lepton)
angle(lepton, W) t b W
Left-handed W
(lW=-1 )
Longitudinal W
(lW=0 )
Right-handed W
(lW=+1 )
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48. W polarisation Results LHC: D0: CDF:
 no evidence for physics beyond SM
LHC:
Polarisation % with 10 fb-1
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49. Conclusion Tevatron and LHC are complementary Mass ≈ 1 % <1%
@2fb-1
LHC: goals for 10fb-1
Mass
≈ 1 %
<1%
Cross section
10%
<5%
Top properties
W polarisation
Spin correlations
Charge
FCNC
40% _
-4/3 excluded 94%
Limits 2% 4%
-4/3 excluded
Limits better x 100
Single Top
5?
Seems difficult
Precise measurement
(10%)
+ Wt channel
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50. Conclusion What could we do with the first data taken @LHC? This year:
s = 10 TeV  cross section top-antitop divided by 2
Many tops should be produced
And used as a tool for commissioning (JES, b-tagging)
But also first cross section measurement
Next year:
s = 14 TeV
With 1 fb-1, several measurements should be achieved
Their precision relies strongly on how well we will understand our detector
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