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

08/14/05 Regina Demina, Lecture #1 2

Novosibirsk

Rochester

Regina Demina, University of Rochester


Rochester, NyRochester, Ny

www.pas.rochester.edu


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


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


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


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


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


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


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


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


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!


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


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


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


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


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.


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


Clusters of ionizationClusters of ionizationDot-like hitsDot-like hits

cos

1ADC

MIP

MIP

Pulse

Particle crossing silicon sensor


Tracking: connecting the dotsTracking: connecting the dots


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


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


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


L+jets sample composition L+jets sample composition


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


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

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Experimental aspects of top quark physics Lecture #1