Transcript Top Physics in ATLAS M. Cobal, University of Udine

Top Physics in ATLAS
M. Cobal, University of Udine
Bologna, 12 Feb 2007
What do we know about the top quark?

The top quark completes the three family structure of the SM

Its massive dm/m <2%

Spin=1/2,
Not directly

Charge=+2/3, -4/3 excluded @ 94%C.L.(D0)
Not directly
Isospin=+1/2

tbW

Large G=1.42GeV (mb,MW,as,EW corr.)

Short lifetime ct<52.5mm @95%C.L.(CDF)

~100%, FCNC: probed at the 10% level
thad = LQCD-1 >> tdecay
“t-quarks are produced and decay
as free particles”
NO top hadrons
The TEVATRON is probing better than ever the top sector…
The LHC will allow precision measurements of Top Quark Physics
The LHC tunnel at CERN
27 km diameter
14 TeV
pp collisions
Top quark production at the LHC
Production: σtt(LHC) ~ 830 ± 100 pb
 1 tt-event per second
Cross section LHC = 100 x Tevatron
Background LHC = 10 x Tevatron
90%
10%
t
Final states:
1) Fully-hadronic (4/9)
2) Semi-leptonic (4/9):
3) Fully-leptonic (1/9):
t
6 jets
1l + 1ν + 4 jets
2l + 2ν + 2 jets
Golden channel (l=e,μ) 2.5 million events/year
t  Wb ~ 1
W qq ~ 2/3
W lν ~ 1/3
ATLAS Experiment
• Em Calo
Pb-liquid argon
s/E ~ 10%/E
~1% uniform
• Hadronic Calo
Fe-scint Tile (10 l)
s/E ~ 50%/E  0.03
• Muon Detectors
s/pT < 10 % at 1 TeV
• Magnets
Toroide+Solenoid
(4 magnets)
in inner cavity
Calorimeters outside field
• Tracker
Si pixels + strips
TRD alows particle ID
Solenoid B=2T
s/pT ~ 5x10-4 pT  0.01
Top physics at the LHC
PRODUCTION
PROPERTIES
DECAY
Cross section
Spin-correlations
Resonances Xtt
Fourth generation t’
New physics (SUSY)
Flavour physics (FCNC)
Mass (matter vs. anti-matter)
Charge
Life-time and width
Spin
BOTTOM
Charged Higgs
W helicity
Anomalous couplings
CKM matrix elements
Calibration sample !!
jets
b-tagging
kinematic fit (mW)
W
n
TOP
missing energy
L
trigger
This data will extend the Tevatron precision reach and allow new possible topics.
Top quarks and search for new physics
First year at the LHC:
Process
#events
10 fb-1
A new detector AND a new energy regime
1 Understand ATLAS
using cosmics
2 Understand SM+ATLAS in
simple topologies
3
4
bb
1012
W  eν
10 8
2
Z  e  e - /μ  μ 
tt
10 7
10 7
3
Understand SM+ATLAS
in complex topologies
Min. bias
107
QCD jets PT  150 GeV
107
Look for new physics
in ATLAS at 14 TeV
h (mh  130 GeV)
g~g~ (m~  1 TeV)
10 5
g
4
10 4
2008 should look something like…
Hardware commissioning
to 7 TeV
Machine Checkout
 1 month
Commissioning with beam
 2 months
Pilot Physics
 1 month
Reach
1031
Running at 75 ns L~ 1032 cm-2s-1
~ 3 months of running
+some optimism ~ 1 fb-1
How many events at the beginning ?
Assumed selection efficiency:
W ln, Z ll : 20%
tt  ln+X :1.5%
(no b-tag, inside mass bin)
+ lots of minimum-bias and
jets (107 events in 2 weeks
of data taking if 20% of
trigger bandwidth allocated)
10 pb-1  1 month at
1030 and < 2 weeks
at 1031, =50%
100 pb-1  few days
at 1032 , =50%
1 fb-1
Similar statistics to D0/CDF
Which detector performance on day one ?
Based on detector construction quality, test-beam results, cosmics, simulation
Expected performance day 1
Physics samples to improve
ECAL uniformity
e/ scale
~ 1%
~2%
Minimum-bias, Z ee
Z  ee
HCAL uniformity
Jet scale
events
~3%
< 10%
Single pions, QCD jets
Z ( ll) +1j, W  jj in tt
Tracking alignment
20(100)-200 mm in R?
Generic tracks, isolated m , Z mm
Ultimate statistical precision achievable after few weeks of operation.
Then face systematics….
E.g. : tracker alignment :
100 mm (1 month)  20mm (4 months)  5 mm (1 year) ?
Top physics ‘easy’ at the LHC:
Selection: Lepton
Missing ET
4 (high-PT)-jets (2 b-jets)
 signal efficiency few %
 very small SM background
Number of Events
Top physics with b-tag information
S/B=O(100)
• ‘Standard’ Top physics at the LHC:
- b-tag is important in selection
- Most measurements limited by systematic uncertainties
• ‘Early’ top physics at the LHC:
- Cross-section measurement (~ 20%)
- Decay properties
Top signal
W+jets
background
Top mass (GeV)
Top physics without b-tag information
Still 1500 events/day
• Robust selection cuts:
Missing
ET > 20 GeV
1 lepton
PT > 20 GeV
Selection efficiency = 5.3%
4 jets(R=0.4) PT > 40 GeV
• Assign jets to W-boson and top-quark:
1) Hadronic top:
Three jets with highest vector-sum
pT as the decay products of the top
2) W boson:
Two jets in hadronic top with highest
momentum. in reconstructed jjj C.M.
frame.
W CANDIDATE
TOP
CANDIDATE
Results for a ‘no-b-tag’ analysis: 100 pb-1
100 pb-1 is a few days of nominal
low-luminosity LHC operation
3-jet invariant mass
Top-signal
Events / 4.15 GeV
Events / 4.15 GeV
We can easily see top peak
without b-tag requirement
3-jet invariant mass
electron+muon estimate
for L=100 pb-1
ATLAS preliminary
Cut on MW
Top-combinatorics
and W+jets background
Mjjj (GeV)
Mjjj (GeV)
What can you do with early tops?
 Calibrate light jet energy scale
- impose PDG value of the W mass (precision < 1%)
 Estimate/calibration b-tagging 
- From data (precision ~ 5%)
- Study b-tag (performance) in complex events
 Study lepton trigger
 Calibrate missing transverse energy
- use W mass constraint in the event
- range 50 GeV < p T < 200 GeV
 Estimate (accuracy ~20%) of mt and stt.
Events
Use W boson
mass to
enhance purity
Miscalibrated
detector or escaping
‘new’ particle
Perfect detector
Missing ET (GeV)
Top mass reconstruction
Selected 87000 signal events for L=10fb-1 (S/B~78)
In-situ jet energy calibration (W→jj)
Mass estimator via fit on spectrum
s =10.6GeV
ATLAS Eur.Phys.J C39 (2005) 63
Systematic Errors:
Comb.
(=3.5%)
Although errors are dominated by systematics
It seems possible to determine mt @ 1GeV level
(with L=10fb-1)
Single top @ LHC
Electroweak top production
Three different Processes (never observed yet)
Vtb
Vtb
Vtb
Vtb
t-channel
Wt-channel
W* (s-channel)
s ~ 250 pb
s ~70pb
s ~ 10 pb
Powerfull Probe of Vtb ( dVtb/Vtb~few% @ LHC )
Probe New Physics Differently: ex. FCNC affects more t-channel
[ PRD63 (2001) 014018] ex. W´ affects more s-channel
Single top and new physics
t-channel
T.Tait, C.-P.Yuan, Phys.Rev. D63 (2001) 0140018
FCNC
kZtc=1
4th generation,|Vts|=0.55, |Vtb|=0.835
(extreme values allowed w/o
the CKM unitarity assumption)
SM
Top-flavor
MZ’=1 TeV
sen2f=0.05
Top-pion
Mp±=450 GeV
tR-cR mixing ~ 20%
s-channel
Cross Sections
(Z.Sullivan, Phys.Rev. D70 (2004) 114012)
Theoretical errors at the LHC
Process
PDF
m-scale
(m/2-2m)
s-channel
4%
2%
2%
t-channel
<2%
3%
1%
Wt
?
<5%
1%
Should be similat to the
t-channel and to gg→tt
Dmtop
(at LHC)
Less than at Tevatron, since
the x-region for the gluon PDFs
is better known.
Single top production
Common feature:
(ATL-COM-PHYS-2006-002)
L=30fb-1
1 lepton, pT>25GeV/c
High Missing ET
2 jets (at least 1 b-jet)
Separate Channels by (Nj,Nb) in final state:
t-channel:
(Nj=2,Nb=1)
Stat: 7000 events (S/B=3)
Syst: dominated by Eb-jet and Lum. Error
Back: tt, Wbb and W+jets
( ds/s <1.5%)
Wt-channel:
(Nj=3,Nb=1)
s-channel:
(ATL-PHYS-PUB-2006-014)
(Nj=2,Nb=2)
Stat: 4700 events, ~1% (S/B=15%)
( ds/s ~ 4%)
Stat: 1200 events for tb (S/B=10%)
Syst: Eb-jet, Lum. Error, back X-section
Back_t-channel, tt
( ds/s ~7-8%)
Beyond the SM
 non-SM production (Xtt)
 resonances in the tt system
 MSSM production
~
~
~ t ~
 unique missing ET signatures from g
t , t  10, 2 t , b  1 t
 non-SM decay (tXb, Xq)
 charged Higgs
 change in the top BR, can be investigated via direct evidence or via
deviations of R(ℓℓ/ℓ)=BR(Wℓn) from 2/9 (H+tn,cs).
 FCNC t decays: tZq tq tgq
 highly suppressed in SM, less in MSSM, enhanced in some sector
of SEWSB and in theories with new exotic fermions
 non-SM loop correction
 precise measurement of the cross-section
 sttNLO-sttLO/ sttLO <10% (SUSY EW), <4% (SUSY QCD)
typical values, might be much bigger for certain regions of the
parameter space
 associated production of Higgs
 ttH
New physics: Resonances in Mtt
pp  X  tt
• Structure in Mtt
Gaemers, Hoogeveen (1984)
500 GeV
600 GeV
Z’, ZH, G(1), SUSY, ?
# events

t s< 10-23 s  no ttbar bound states within the SM
Many models include the existence of resonances decaying to ttbar
SM Higgs , MSSM Higgs, Technicolor Models, strong ElectroWeak Symmetry
Breaking, Topcolor
• Resonances in Mtt
Cross section (a.u.)

ATLAS
Resonance
at 1600 GeV
Dm
~ 6%
m
400 GeV
Mtt (GeV)
- Interference from MSSM Higgses
H,A tt (can be up to 6-7% effect)
Mtt (GeV)
Resonances in a tt system
Resolution m(tt)
Study the detector sensitivity in an
inclusive way:
Resonance
at 1600 GeV
 Study of a resonance Χ once known σΧ, ΓΧ
and BR(Χ→tt)
Δσ/σ ~ 6 %
 Assume detector resolution > ΓΧ
 Excellent experimental resolution in mass,
ranging from 3% to 6% !
Reconstruction efficiency for the
semileptonic channel:
 20% mtt=400 GeV
 15% mtt=2 TeV
mtt (GeV)
sxBR required
for a discovery
fast-sim
5s
 Shown sensitivity up to a few TeV
1 TeV
Top spin correlation
Although t and t are produced unpolarized their spins are correlated
s(tLtL) + s(tRtR) - s(tLtR) - s(tRtL)
A=
s(tLtL) + s(tRtR) + s(tLtR) + s(tRtL)
A(LO)
A(NLO)
SM:
New Physics affects A
q
t
t
Other angular distributions:
1 dN
lq
0.319
0.326
N dcos
=
1
2
( 1 – ADaXaX´cos )
l+,n
SM:
(Eur.Phys.J.C44S2 2005 13-33)
SM
Mtt<550 GeV
Error (±stat ±syst)
A
0.42
0.014 0.023
AD
-0.29
0.008 0.010
AD(LO)
AD(NLO)
aX=spin analysing
power of X
-0.217
-0.237
• Semileptonic + Dileptonic
• Syst (Eb-jet,mtop,FSR)
• ~4% precision
Probing the Wtb vertex
A) Test the tbW decay vertex
(1/G)dG/dcos(ql*)
Measure W polarization (F0, FL, FR) through
lepton angular distribution in W cm system:
L=10fb-1
Semilep.
+
Dileptonic
SM
(Mt=175 GeV)
Error
(±stat ±syst)
F0
0.703
 0.004  0.015
FL
0.297
 0.003  0.024
FR
0.000
(mb=0)
 0.003  0.012
• Syst ( Eb-jet,mtop,FSR )
• dF0/ F0 ~ 2% ; dFR ~ 0.01
(Eur.Phys.J.C44S2 2005 13-33)
Probing the Wtb vertex
B) Anomalous Couplings in the tbW decay
(PRD67 (2003) 014009, mb≠0)
Angular Asymmetries: AFB, A+ and A±
AFB [t=0] A± [t= (22/3-1)]
ASM(LO):
A+
AFB
cos(ql*)
Probing the Wtb vertex
B) Anomalous Couplings in the tbW decay
SM(LO):
rL=0.423
rR=0.0005 (mb≠0)
1s Results:
L=10fb-1
Top quark FCNC decay



GIM suppressed in the SM
Higher BR in some SM extensions (2-Higgs doublet, SUSY, exotic fermions)
BR in SM
2HDM
MSSM
R SUSY
QS
tqZ
~10-14
~10-7
~10-6
~10-5
~10-4
tq
~10-14
~10-6
~10-6
~10-6
~10-9
tqg
~10-12
~10-4
~10-5
~10-4
~10-7
3 channels studied:
Results



BR 5s sensitivity
tqZ
tq
tqg
L = 10 fb-1
5.1x10-4
1.2x10-4
4.6x10-3
L = 100 fb-1
1.6x10-4
3.8x10-5
1.4x10-3
Expected 95% CL limits on BR (no signal)
tqZ
tq
tqg
L = 10 fb-1
3.4x10-4
6.6x10-5
1.4x10-3
L = 100 fb-1
6.5x10-5
1.8x10-5
4.3x10-4
Dominant systematics: MT and tag < 20%
Present and future limits
SM bck
t → qZ
Topological likelihood for three channels
Resulting 95% CL limits
signal
t → q
t → qg
 With 10 fb-1 already 2 orders of magnitude better than LEP/HERA
Conclusions
1) Top quarks are produced by the millions at the LHC:
 Almost no background: measure top quark properties
2) Top quarks are THE calibration signal for complex topologies:
 Most complex SM candle at the LHC
 Vital inputs for detector operation and SUSY background
3) Top quarks pair-like events … window to new physics:
 FCNC, SUSY, MSSM Higgses, Resonances, …
DAY-2 top physics:
- Single top production
- Top charge, spin(-correlations), mass
Backup

B-jet identification efficiency:
Important in cross-section determination
and many new physics searches (like H, ttH)
# events
b-jet identification efficiency
CMS
• A clean sample of b-jets from top events
2 out of 4 jets in event are b-jets (a-priori)
Use W boson mass
to enhance purity
Combined b-tagging discriminator
B-jet sample from top quark pairs:
- Calibrate b-tagging efficiency from data (~ 5%)
Dominant systematic uncertainty: ISR/FSR jets
- Study b-tag (performance) in complex events
Note: Can also use di-lepton events
light jet energy scale
Light jet energy scale calibration (target ~1%)
Purity = 83%
Nevt ~ 2400 (1 fb-1)
Invariant mass of jets should add up
to well known W mass (80.4 GeV)
M jj  2 E j1 E j 2 (1  cos q j1 j 2 )  M W
Rescale jet energies:
Eparton = (1+ a) Ejet, with a=a(PT,η)
Pro:
- Complex topology, hadronic W
- Large statistics
Con:
- Only light quark jets
- Limited PT-range (50-200 GeV)
Precision:
# events

σ(Mjj)~ 8 GeV
MW (PDG) = 80.425 GeV
< 1% for 0.5 fb-1
Alternative: PT-balance in Z/γ+jet (6% b-jets)
Mjj (GeV)
t  W  jj to calibrate the light JES


Standard tt  lnb jjb selection cuts
Improve W  jj purity by requiring:


Etienvre,
Schwindling
2 light jets only
150 < mjjb < 200 GeV
 Purity ~ 83 %, ~ 1200 W selected for 500 pb-1
Number of jj for 491 pb-1:
(% purity as fraction of cases with 2 jets at DR < 0.25
from 2 W quarks)
PT cut = 40 GeV
All jj combinations
Only 2 light jets
Only 2 light jets +
150 < mjjb < 200
mjj (GeV)
all
60 < mjj < 100
Standard
selection
15833
16.1 ± 0.3 %
4001
56.7 ± 0.8 %
+ only 2
light jets
3558
41.0 ± 0.8 %
1903
69.0 ± 1.1 %
+ mtop
in 150 200
1401
73.5 ± 1.2 %
1205
82.6 ± 1.1 %
Jet energy scale (no b-tag analysis)

(1) Abundant source of W decays
into light jets


Events / 5.1 GeV
Determine Light-Jet
energy scale
Invariant mass of jets should add
up to well known W mass (80.4 GeV)
W-boson decays to light jets only
 Light jet energy scale calibration
(target precision 1%)
MW = 78.1±0.8 GeV
MW(had)
t
t
S/B = 0.5
Translate jet 4-vectors
to parton 4-vectors
Search strategies for H±tb

Resolving 3 b-jets: inclusive mode




Resolving 4 b-jets: exclusive mode




LO production through gb tH±
Large background from tt+jets
High combinatorics
LO production through gg tH±b
Smaller background (from ttbb and ttjj+ 2 mistags)
Even higher combinatorics
Both processes simulated with Pythia; same cross section if
calculated at all orders



gbtH±: massless b taken from b-pdf
gg tH±b: massive b from initial gluon
splitting
Cross sections for both processes as
the NLO gbtH±: cross section
Search for 4 b-jets

Signal properties



Exponential decrease with mA
Quadratic increase with tanb in interesting region tanb > 20
Final state: bbbbqq’ln




Isolated lepton to trigger on
Charged Higgs mass can be reconstructed
Only final state with muon investigated
Background simulation


ttbb
ttjj


(large mistag rates, large cross section)
b’s from gluon splitting passing theshold of ttbb generation)
Significance and Reach

Kinematic fit in top system




Both W mass constraints
Both top mass constraints
Neutrino taken from fit
Event selection and efficiencies
4

4
Significance and Reach



Significance as function of cut on signal-background
Due to low statistics interpolation of number of background
events as function of number of signal events
Optimization performed at each mass point
H±tb





Fast simulation
4 b-jets analysis
No systematics (apart uncertainty on background cross sec)
Runninng mb
B-tagging  static
L = 30 fb-1
ttH
The Yukawa coupling of top
to Higgs is the largest.
 It is a discovery mode of the Higgs boson for
masses less than 130 GeV
 Measuring the coupling of top to Higgs can test
the presence of new physics in the Higgs sector
 Very demanding selection in a high jet multiplicity final state
0.7 pb (NLO)
mH=120GeV
ttjj: 507 pb
ttZ: 0.7 pb
ttbb: 3.3 pb
Higgs boson reconstruction
 Reconstruct ttH(h)  WWbbbb  (ln)(jj)bbbb
 Isolated lepton selection using a likelihood method
 Jet reconstruction: 6 jets at least, 4 of which b-tagged
 Reconstruct missing ET from four-momentum conservation in the event (+W
mass constraint in z)
 Complete kinematic fit to associate the two bs to the Higgs
(can improve the pairing efficiency to 36%, under investigation)
results can be
extrapolated
to MSSM h
dgttH/gttH~16%
for mH=120 GeV
hep-ph/0003033
Probabilistic approach

Preselection

General criteria:







≥ 1 lepton (pT > 25 GeV and |h| < 2.5)
≥ 2 jets (pT > 20 GeV and |h| < 2.5)
Only 1 b-tagged jet
ETmiss > 20 GeV
Events classified into different channels (qZ, q or qg)
Specific criteria for each channel
After the preselection,
probabilistic analysis:
Ls  i 1 Pi signal
N
LB  i 1 Pi backnd
N
tqZ

Specific criteria:

≥ 3 leptons




PTl2,l3 > 10 GeV and |h|<2.5
2 leptons same flavour and
opposite charge
PTj1 > 30 GeV
453.8 backgnd evts,  x BR = 0.23%
Mjl+l-
L = 10 fb-1
Mlnb
tq

Specific criteria:

1 photon




PT > 75 GeV and |h|<2.5
20 GeV < mj < 270 GeV
< 3 leptons
290.7 backgnd evts,  x BR = 1,88%
Mj
L = 10 fb-1
PT
tqg

Specific criteria:







Only one lepton
No  with PT > 5 GeV
Evis > 300 GeV
3 jets (PT1 > 40 GeV, PT2,3 > 20 GeV and |h| < 2.5)
PTg > 75 GeV
125 < mgq < 200 GeV
L = 10 fb-1
8166.1 backgnd evts,  x BR = 0,39%
Mgq
Mlnb