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Top Physics in ATLAS
M. Cobal, University of UdineM. Cobal, University of Udine
Bologna, 12 Feb 2007Bologna, 12 Feb 2007
What do we know about the top quark?
The top quark completes the three family structure of the SM
Its massive
Spin=1/2,
Charge=+2/3, Isospin=+1/2
tbW
Large =1.42GeV (mb,MW,s,EW corr.)
Short lifetime
had = QCD-1 >> decay
“t-quarks are produced and decay as free particles”
NO top hadrons
m/m <2%
-4/3 excluded @ 94%C.L.(D0)
c<52.5mm @95%C.L.(CDF)
Not directly
The TEVATRON is probing better than ever the top sector…The LHC will allow precision measurements of Top Quark Physics
~100%, FCNC: probed at the 10% level
Not directly
The LHC tunnel at CERN
27 km diameter14 TeVpp collisions
10%
90%
Production: σtt(LHC) ~ 830 ± 100 pb
1 tt-event per second
Top quark production at the LHC
Cross section LHC = 100 x TevatronBackground LHC = 10 x Tevatron
tt
Final states:
t Wb ~ 1 W qq ~ 2/3W lν ~ 1/3
t Wb ~ 1 W qq ~ 2/3W lν ~ 1/3
1) Fully-hadronic (4/9) 6 jets
2) Semi-leptonic (4/9): 1l + 1ν + 4 jets
3) Fully-leptonic (1/9): 2l + 2ν + 2 jetsGolden channel (l=e,μ) 2.5 million events/year
ATLAS Experiment
Toroide+Solenoid (4 magnets) in inner cavityCalorimeters outside field
• Magnets • TrackerSi pixels + stripsTRD alows particle IDSolenoid B=2T/pT ~ 5x10-4 pT 0.01
• Em Calo
Pb-liquid argon/E ~ 10%/E ~1% uniform
• Hadronic Calo
Fe-scint Tile (10 ) /E ~ 50%/E 0.03
• Muon Detectors
/pT < 10 % at 1 TeV
Top physics at the LHC
PRODUCTIONCross sectionSpin-correlationsResonances XttFourth generation t’New physics (SUSY)Flavour physics (FCNC)
PROPERTIESMass (matter vs. anti-matter)ChargeLife-time and widthSpin
DECAYCharged HiggsW helicityAnomalous couplingsCKM matrix elementsCalibration sample !!
kinematic fit (mW)
missing energy
TOP
≈ 0.4 10-24 s
BOTTOM
W
L
jetsb-tagging
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:
A new detector AND a new energy regime
2 Understand SM+ATLAS in simple topologies
Understand SM+ATLASin complex topologies
3
Look for new physicsin ATLAS at 14 TeV
4
Process #events 10 fb-1
4 TeV) 1g(m
5GeV) 130h(m
7T
7
-
10 gg
10 h
10 GeV 150P jets QCD
10 bias Min.
10 tt
10 μ/μeeZ
10 eνW
10 bb
~
7
7
8
12
~~
2
3
4
1 Understand ATLASusing cosmics
2008 should look something like…
Hardware commissioning to 7 TeV
Machine Checkout 1 month
Commissioning with beam 2 months
Pilot Physics 1 month
Provisional 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 ?
10 pb-1 1 month at1030 and < 2 weeks
at 1031, =50%
100 pb-1 few days
at 1032 , =50%
Assumed selection efficiency:W l, Z ll : 20%tt l+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)
1 fb-1
Similar statistics to D0/CDF
Which detector performance on day one ?
Based on detector construction quality, test-beam results, cosmics, simulation
Ultimate statistical precision achievable after few weeks of operation. Then face systematics…. E.g. : tracker alignment : 100 m (1 month) 20m (4 months) 5 m (1 year) ?
Expected performance day 1 Physics samples to improve
ECAL uniformity ~ 1% Minimum-bias, Z eee/ scale ~ 2 % Z ee
HCAL uniformity ~ 3 % Single pions, QCD jetsJet scale < 10% Z ( ll) +1j, W jj in tt events
Tracking alignment 20(100)-200 m in R? Generic tracks, isolated , Z m
Top physics ‘easy’ at the LHC:
Top physics with b-tag information
Selection: Lepton Missing ET 4 (high-PT)-jets (2 b-jets) signal efficiency few % very small SM background
• ‘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
S/B=O(100)Top signal
W+jets background
Top mass (GeV)
Nu
mb
er
of
Even
ts
Top physics without b-tag information
Selection efficiency = 5.3%1 lepton PT > 20 GeV
Missing ET > 20 GeV
4 jets(R=0.4) PT > 40 GeV
TOP CANDIDATE
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
• Assign jets to W-boson and top-quark:
• Robust selection cuts: Still 1500 events/day
Cut on MW
Results for a ‘no-b-tag’ analysis: 100 pb-1
Mjjj (GeV)
electron+muon estimate for L=100 pb-1
Even
ts /
4.1
5 G
eV
ATLAS preliminary
3-jet invariant mass3-jet invariant mass
Mjjj (GeV)
Even
ts /
4.1
5 G
eV
Top-combinatoricsand W+jets background
Top-signal
We can easily see top peak without b-tag requirementWe can easily see top peak without b-tag requirement
100 pb-1 is a few days of nominal low-luminosity LHC operation
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 tt.
Use W boson mass to enhance purity
Missing ET (GeV)
Even
ts
Perfect detector
Miscalibrated detector or
escaping ‘new’ particle
Selected 87000 signal events for L=10fb-1 (S/B~78)
In-situ jet energy calibration (W→jj)
Mass estimator via fit on spectrum
Although errors are dominated by systematics It seems possible to determine mt @ 1GeV level
(with L=10fb-1)
(=3.5%)
Comb.
=10.6GeV
Systematic Errors:Systematic Errors:
ATLAS Eur.Phys.J C39 (2005) 63
Top mass reconstruction
Single top @ LHC
Electroweak top productionThree different Processes (never observed yet)
Powerfull Probe of Vtb ( dVtb/Vtb~few% @ LHC )
t-channel Wt-channel W* (s-channel)
~ 250 pb ~70pb ~ 10 pb
Vtb
Vtb Vtb
Vtb
Probe New Physics Differently: ex. FCNC affects more t-channel ex. W´ affects more s-channel[ PRD63 (2001) 014018]
Single top and new physics
T.Tait, C.-P.Yuan, Phys.Rev. D63 (2001) 0140018
FCNCkZtc=1
4th generation,|Vts|=0.55, |Vtb|=0.835(extreme values allowed w/o the CKM unitarity assumption)
SMTop-flavorMZ’=1 TeVsen2f=0.05
Top-pionMp±=450 GeVtR-cR mixing ~ 20%
s-channel
t-ch
annel
Cross Sections
Theoretical errors at the LHC
Process PDF-scale
(/2-2)
mtop
(at LHC)
s-channel 4% 2% 2%
t-channel <2% 3% 1%
Wt ? <5% 1%
(Z.S
ulli
van
, Phys.
Rev. D
70
(2
00
4)
11
40
12
)
Should be similat to thet-channel and to gg→tt
Less than at Tevatron, since the x-region for the gluon PDFs
is better known.
Single top production
Common feature: 1 lepton, pT>25GeV/cHigh Missing ET
2 jets (at least 1 b-jet)(ATL-COM-PHYS-2006-002)
(ATL-PHYS-PUB-2006-014)
L=30fb-1Separate Channels by (Nj,Nb) in final state:
( <1.5%)
t-channel: Stat: 7000 events (S/B=3)Syst: dominated by Eb-jet and Lum. ErrorBack: tt, Wbb and W+jets
(Nj=2,Nb=1)
Wt-channel: Stat: 4700 events, ~1% (S/B=15%)( / ~ 4%)
(Nj=3,Nb=1)
s-channel:Stat: 1200 events for tb (S/B=10%)Syst: Eb-jet, Lum. Error, back X-sectionBack_t-channel, tt
7-8%)(Nj=2,Nb=2)
Beyond the SM
non-SM production (Xtt) resonances in the tt system MSSM production
unique missing ET signatures from non-SM decay (tXb, Xq)
charged Higgs change in the top BR, can be investigated via direct evidence or via
deviations of R(ℓℓ/ℓ)=BR(Wℓ) from 2/9 (H+,cs). FCNC t decays: tZq tq tgq
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
ttNLO-tt
LO/ ttLO <10% (SUSY EW), <4% (SUSY QCD)
typical values, might be much bigger for certain regions of the parameter space
associated production of Higgs ttH
tbttttg ~
, ~ ,~ ~1
02,1
New physics: Resonances in Mtt
• Structure in Mtt
- Interference from MSSM Higgses H,A tt (can be up to 6-7% effect)
Cro
ss s
ecti
on
(a.u
.)
Mtt (GeV)
• Resonances in Mtt
Resonanceat 1600 GeV
# e
ven
ts
ttXpp Z’, ZH, G(1), SUSY, ?
Mtt (GeV)
400 GeV
500 GeV
600 GeV
Gaemers, Hoogeveen (1984)
ATLAS
%6~m
m
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
Study of a resonance Χ once known σΧ, ΓΧ and BR(Χ→tt)
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
xBR required for a discovery
1 TeV1 TeV
Shown sensitivity up to a few TeV
Resonances in a tt system
mtt (GeV)
Resonanceat 1600 GeVΔσ/σ ~ 6 %
Resolution m(tt)Study the detector sensitivity in an
inclusive way:
55
fast-sim
(tLtL) + (tRtR) - (tLtR) - (tRtL)
(tLtL) + (tRtR) + (tLtR) + (tRtL)A=
l+,t
lqq
t
Other angular distributions:
AD(LO)
AD(NLO)
-0.217
-0.237
SM:
A(LO)
A(NLO)
0.319
0.326
Although t and t are produced unpolarized their spins are correlated
New Physics affects A
aX=spin analysing power of X
SM:
1 dN 1
N dcos 2( 1 – ADaXaX´cos ) =
Top spin correlation
(Eur.Phys.J.C44S2 2005 13-33)
• Semileptonic + Dileptonic• Syst (Eb-jet,mtop,FSR)• ~4% precision-0.29
0.42
SM
Mtt<550 GeV
0.008 0.010AD
0.014 0.023A
Error (±stat ±syst)
A) Test the tbW decay vertex
Measure W polarization (F0, FL, FR) through
lepton angular distribution in W cm system:
Semilep. +
Dileptonic
• Syst ( Eb-jet,mtop,FSR )• F0/ F0 ~ 2% ; FR ~ 0.01
0.000(mb=0)
0.297
0.703
SM
0.003 0.024FL
0.003 0.012FR
0.004 0.015F0
Error (±stat ±syst) (Mt=175 GeV)
(Eur.Phys.J.C44S2 2005 13-33)
L=10fb-1
Probing the Wtb vertex(1
/)d
/d
cos(
l*)
B) Anomalous Couplings in the tbW decay
Angular Asymmetries: AFB, A+ and A-
AFBA+
A-
cos(cos(ll*)*)
AFB [t=0] A± [t= (22/3-1)]
±
SM(LO):SM(LO):
(PRD67 (2003) 014009, mb≠0)
Probing the Wtb vertex
1 Results:
B) Anomalous Couplings in the tbW decay
SM(LO):
L=0.423R=0.0005 (mb≠0)
L=10fb-1
Probing the Wtb vertex
Top quark FCNC decay
GIM suppressed in the SM Higher BR in some SM extensions (2-Higgs doublet, SUSY, exotic fermions)
3 channels studied:
BR in SM 2HDM MSSM R SUSY QS
tqZ ~10-14 ~10-7 ~10-6 ~10-5 ~10-4
tq ~10-14 ~10-6 ~10-6 ~10-6 ~10-9
tqg ~10-12 ~10-4 ~10-5 ~10-4 ~10-7
Results
BR 5 sensitivity
Expected 95% CL limits on BR (no signal)
Dominant systematics: MT and tag < 20%
tqZ tq tqg
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
tqZ tq tqg
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
Present and future limitsTopological likelihood for three channels
Resulting 95% CL limitst → qZSM bck
signal
t → q
t → qg
With 10 fb-1 already 2 orders of magnitude better than LEP/HERA
Conclusions
DAY-2 top physics: - Single top production - Top charge, spin(-correlations), mass
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, …
Backup
b-jet identification efficiency
CMS
Combined b-tagging discriminator
# e
ven
tsNote: Can also use di-lepton events
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
• 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
B-jet identification efficiency: Important in cross-section determination and many new physics searches (like H, ttH)
light jet energy scale Light jet energy scale calibration (target ~1%)
Precision: < 1% for 0.5 fb-1 Alternative: PT-balance in Z/γ+jet (6% b-jets)
Pro: - Complex topology, hadronic W - Large statisticsCon: - Only light quark jets - Limited PT-range (50-200 GeV)
Pro: - Complex topology, hadronic W - Large statisticsCon: - Only light quark jets - Limited PT-range (50-200 GeV)
Rescale jet energies:Eparton = (1+ ) Ejet, with =(PT,η)
Wjjjjjj MEEM )cos1(2 2121
Invariant mass of jets should add upto well known W mass (80.4 GeV)
Mjj (GeV)#
even
ts σ(Mjj)~ 8 GeV
Purity = 83%Nevt ~ 2400 (1 fb-1)
MW (PDG) = 80.425 GeV
t W jj to calibrate the light JES
all 60 < mjj < 100
Standard selection
1583316.1 ± 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 %
Number of jj for 491 pb-1:(% purity as fraction of cases with 2 jets at R < 0.25
from 2 W quarks)PT cut = 40 GeV
All jj combinations
Only 2 light jets + 150 < mjjb < 200
Only 2 light jets
mjj (GeV)
Etienvre, Schwindling
Standard tt lb jjb selection cuts Improve W jj purity by requiring:
2 light jets only 150 < mjjb < 200 GeV
Purity ~ 83 %, ~ 1200 W selected for 500 pb-1
(1) Abundant source of W decays into light jets 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%)
t
t
Jet energy scale (no b-tag analysis) Determine Light-Jet
energy scale
Translate jet 4-vectors to parton 4-vectors
MW = 78.1±0.8 GeV
S/B = 0.5
MW(had)
Even
ts /
5.1
GeV
Search strategies for H±tb
Resolving 3 b-jets: inclusive mode LO production through gb tH±
Large background from tt+jets High combinatorics
Resolving 4 b-jets: exclusive mode 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 gbtH±: massless b taken from b-pdf gg tH±b: massive b from initial gluon splitting Cross sections for both processes as the NLO gbtH±: cross section
Search for 4 b-jets
Signal properties Exponential decrease with mA
Quadratic increase with tan in interesting region tan > 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
44
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
0.7 pb (NLO)mH=120GeV
Very demanding selection in a high jet multiplicity final state
ttjj: 507 pb ttZ: 0.7 pb ttbb: 3.3 pb
Higgs boson reconstruction Reconstruct ttH(h) WWbbbb (l)(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)
gttH/gttH~16%for mH=120 GeV hep-ph/0003033
results can be
extrapolated
to MSSM h
Probabilistic approach
Preselection General criteria:
≥ 1 lepton (pT > 25 GeV and || < 2.5) ≥ 2 jets (pT > 20 GeV and || < 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:
N
i
backndiB
N
i
signalis
PL
PL
1
1
tqZ
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% L = 10 fb-1
Mjl+l- Mlb
tq Specific criteria:
1 photon PT > 75 GeV and ||<2.5
20 GeV < mj < 270 GeV < 3 leptons
290.7 backgnd evts, x BR = 1,88%
Mj
L = 10 fb-1
PT
tqg 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
8166.1 backgnd evts, x BR = 0,39%
L = 10 fb-1
MlbMgq