Experimental Aspects of New Physics at TeV Scale & Precision Electroweak
Experimental aspects of top quark physics Lecture #1
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Transcript of Experimental aspects of top quark physics Lecture #1
Experimental aspects of top Experimental aspects of top quark physics quark physics
Lecture #1Lecture #1
Regina Demina
University of Rochester
Topical Seminar on Frontier of Particle Physics
Beijing, China08/14/05
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Novosibirsk
Rochester
Regina Demina, University of Rochester
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Rochester, NyRochester, Ny
www.pas.rochester.edu
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OutlineOutline• Introduction• Colliders• Parton density functions• Top quark production
– Meaning of luminosity
• Top quark decay• Particle identification• Top pair production cross section measurement• Control questions
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Energy and matterEnergy and matter
• Einstein taught us that matter and energy are equivalent
E=mc2
• We can use energy to create matter:– Protons and antiprotons are
accelerated to high energies – They are then collided producing
new more massive particles (matter), e.g. top quarks
That is why a convenient unit for mass is eV/c2
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Accelerators: Tevatron Accelerators: Tevatron • Fermilab
• 40 miles west of Chicago
• Tevatron – at the moment world’s highest energy collider – 2 teraelectronvolts in CM
– 6.28 km circumference
• Two instrumented interaction points –CDF and D0
• Top quark discovery - 1995
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Accelerators: LHCAccelerators: LHC
• Next collider – LHC - is being built in Europe
• 27 km;• 14 Tev - LHC will discover Higgs if it
exists.• Two high PT experiments _CMS and
Atlas
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Parton density functionsParton density functions• Proton (q=+1e) is not an elementary particle• It consists of three valence quarks uud (q=+2/3e +2/3e -1/3e) • Valence quarks interact with each other via gluons• Gluons can split into a pair of virtual quarks• Thus, in addition to valence quarks we have a Sea of quarks and gluons • Same is true for antiprotons• Quarks and gluons inside proton are called partons• Probability for a parton to carry a certain fraction of momentum of proton
x=p(parton)/p(proton) is called parton density function (pdf)• When proton and antiproton interact with each other only one parton from
each participate in high pT interaction
u
u
d
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Top production at TevatronTop production at Tevatron
At √s=1.96 TeV top is produced in pairs via quark-antiquark annihilation 85% of the time, gluon fusion accounts for 15% of ttbar production
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Top Quark ProductionTop Quark Production
Top quarks at hadron colliders are (mainly) produced in pairs via strong interaction
top
Quark-antiquark annihilation:TeV:85%LHC:~0%
Gluon fusion:TeV:15%LHC:~100%
Top pair cross section at 1.96 TeV is 6.7 pb
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integrated luminosity
LuminosityLuminosity
This is deliveredluminosity
Recorded orgood forphysics is lower
1/3 used in theanalyses presentedhere
1.1032 cm-2sec-1
0.75 fb-1
instantaneous luminosity
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Top Lifetime and DecayTop Lifetime and Decay
• Since the top lifetime
top ~ 1/ M3top~10 -24 sec
qcd ~ -1 ~10 -23 sec e-e (1/81)
mu-mu (1/81)
tau-tau (1/81)
e -mu (2/81)
e -tau (2/81)
mu-tau (2/81)
e+jets (12/81)
mu+jets(12/81)
tau+jets(12/81)
jets (36/81)
the top quark does not hadronize. It decays as a free quark!
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Top identificationTop identification
Need to reconstruct:Electrons, muons, jets, b-jetsand missing transverse energy
All jet:high BR, high BG
Lepton + jet: BR and BG are OK
di-lepton:BR low, BG low
t->Wb in 99.8%Always two b-jets in the final state
the top is produced almost at rest and the decay products are much lighter: they have good angular separation in the lab frame and high transverse momentum
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Particle identificationParticle identification
• Electrons are identified as clusters of energy in EM section of the calorimeter with tracks pointing to them
• Muons are identified as particles passing through entire detector volume and leaving track stubs in muon chambers. Track in the central tracking system (silicon+SciFi) is matched to track in muon system
• Jets are reconstructed as clusters of energy in calorimeter using cone algorithm DR<0.5
Charged particles curve in B-field, which enables their momentum measurement
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CDF and D0 in Run IICDF and D0 in Run II
New Silicon DetectorNew Central Drift ChamberNew End Plug CalorimetryExtended muon coverageNew electronics
Silicon Detector2 T solenoid and central fiber trackerSubstantially upgraded muon systemNew electronics
Driven by physics goals detectors are becoming “similar”: silicon, central magnetic field, hermetic calorimetry and muon systems
CDF
DØØ
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Parton and jetsParton and jets• Partons (quarks produced as a result of
hard collision) realize themselves as jets seen by detectors– Due to strong interaction partons turn into
parton jets– Each quark hardonizes into particles (mostly
and K’s)– Energy of these particles is absorbed by
calorimeter – Clustered into calorimeter jet using cone
algorithm• Jet energy is not exactly equal to parton
energy– Particles can get out of cone– Some energy due to underlying event (and
detector noise) can get added– Detector response has its resolution
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Tagging b-jetsTagging b-jets
• Very precise measurements provided by silicon detectors tell if the particle has a significant impact parameter (d0) wrt the primary vertex.PV
b-quarkd0
After traveling ~1mm from the primary vertex (PV) b-quarks decay into a jet of lighter particles.
Charged products from b-quark decay ionize silicon sensors, leaving dot-like hits.
Dots are connected and form a track corresponding to a particle’s path.
Jet is tagged as a b-jet if it contains several tracks not coming from the primary vertex.
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D0 Silicon systemD0 Silicon system
Total number of channels 792,576
Charge deposited by ionizing particle
1 MIP 4 fC 25 ADC counts
Barrels+ disks
Barrelsonly
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Clusters of ionizationClusters of ionizationDot-like hitsDot-like hits
cos
1ADC
MIP
MIP
Pulse
Particle crossing silicon sensor
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Tracking: connecting the dotsTracking: connecting the dots
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B-quarks IDB-quarks ID
DCA resolution ~ 50 m (using as built + surveyed
alignment)beam spot ~ 30-40 m
DCA: Distance ofClosest Approach
track
x
y
We correctly identify 44 out of a 100 b-jets with <1% mistake rate
pT>3 GeV
48 m
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Top identification in lepton+jets Top identification in lepton+jets channelchannel
• tbW, Wl, lepton (electron or muon) is identified in the detector, neutrino escapes, we infer its presence from transverse energy misbalance
• tbW, Wqq’, two light jets from W-boson decay
•Top pair production signature:•high pT lepton, •missing transverse energy,• two b-jets
•identified by b-tagging algorithm•two light jets
Main background (process that can mimic your signal): W(ljetsOnly a small fraction of these jets are b-jets
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l+MET+Njets
W+Njets
QCD
Before tagging After tagging
W+Light jets
Wc
Wcc
Wb
Wbb
Mat
rix
met
hod
AL
PG
EN
fra
ctio
ns
W+Light jets
Wc
Wcc
Wb
Wbb
Ptag light
Ptag 1c jet
Ptag 2c jets
Ptag 2b jets
Ptag 1b jet
Ptag QCD
QCD
N Bckgtag
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L+jets sample composition L+jets sample composition
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Cross section calculationCross section calculation
• Number of observed events is the sum of the number of signal and background events:
• Number of observed signal events is proportional to the process cross section, total integrated luminosity, efficiency to detect a certain signature
• Efficiency is calculated using Monte Carlo simulation and verified on data samples with known efficiency, e.g. Zee
backgroundsignalobs NNN
LdtN signalsignal
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ttbar cross section in ttbar cross section in l+jetsl+jets with b-tag with b-tag
• Isolated lepton – pT>20 GeV/c, |e|<1.1, ||<2.0
• Missing ET>20GeV• Four or more jets
– pT>15 GeV/c, |
=8.1+1.3-1.2(stat+syst)±0.5(lumi) pb
DØ RunII Preliminary, 363pb-1
≥4j, 1t ≥4j, 2t
Expect bkg 21.8±3.0 1.9±0.5
Observe 88 21