Experimental aspects of top quark physics Lecture #1 Regina Demina University of Rochester Topical Seminar on Frontier of Particle Physics Beijing, China 08/14/05

Regina Demina, Lecture #12 Novosibirsk Rochester Regina Demina, University of Rochester

08/14/05Regina Demina, Lecture #13 Rochester, Ny

08/14/05Regina Demina, Lecture #14Outline Introduction Colliders Parton density functions Top quark production –Meaning of luminosity Top quark decay Particle identification Top pair production cross section measurement Control questions

08/14/05Regina Demina, Lecture #15 Energy and matter Einstein taught us that matter and energy are equivalent E=mc 2 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/c 2

08/14/05Regina Demina, Lecture #16 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

08/14/05Regina Demina, Lecture #17 Accelerators: LHC Next collider – LHC - is being built in Europe 27 km; 14 Tev - LHC will discover Higgs if it exists. Two high P T experiments _CMS and Atlas

08/14/05Regina Demina, Lecture #18 Parton 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 p T interaction u u d

08/14/05Regina Demina, Lecture #19 Top 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

08/14/05Regina Demina, Lecture #110 Top 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

08/14/05Regina Demina, Lecture #111 integrated luminosityLuminosity This is delivered luminosity Recorded or good for physics is lower 1/3 used in the analyses presented here cm -2 sec fb -1 instantaneous luminosity

08/14/05Regina Demina, Lecture #112 Top Lifetime and Decay Since the top lifetime  top ~ 1/ M 3 top ~ sec  qcd ~  -1 ~ sec the top quark does not hadronize. It decays as a free quark!

08/14/05Regina Demina, Lecture #113 Top identification Need to reconstruct: Electrons, muons, jets, b-jets and 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

08/14/05Regina Demina, Lecture #114 Particle 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

08/14/05Regina Demina, Lecture #115 CDF and D0 in Run II New Silicon Detector New Central Drift Chamber New End Plug Calorimetry Extended muon coverage New electronics Silicon Detector 2 T solenoid and central fiber tracker Substantially upgraded muon system New electronics Driven by physics goals detectors are becoming “similar”: silicon, central magnetic field, hermetic calorimetry and muon systems CDF ØDØØDØ

08/14/05Regina Demina, Lecture #116 Parton 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

08/14/05Regina Demina, Lecture #117 Tagging b-jets Very precise measurements provided by silicon detectors tell if the particle has a significant impact parameter (d 0 ) wrt the primary vertex. PV b-quark d0d0  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.

08/14/05Regina Demina, Lecture #118 D0 Silicon system Total number of channels 792,576 Charge deposited by ionizing particle 1 MIP  4 fC  25 ADC counts Barrels + disks Barrels only

08/14/05Regina Demina, Lecture #119 Clusters of ionization Dot-like hits  MIP  Pulse Particle crossing silicon sensor

08/14/05Regina Demina, Lecture #120 Tracking: connecting the dots

08/14/05Regina Demina, Lecture #121 B-quarks ID DCA resolution ~ 50  m (using as built + surveyed alignment) beam spot ~  m DCA: Distance of Closest Approach track x y We correctly identify 44 out of a 100 b-jets with <1% mistake rate p T >3 GeV 48  m

08/14/05Regina Demina, Lecture #122 Top identification in lepton+jets channel t  bW, W  l, lepton (electron or muon) is identified in the detector, neutrino escapes, we infer its presence from transverse energy misbalance t  bW, W  qq’, 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(  l  jets Only a small fraction of these jets are b-jets

08/14/05Regina Demina, Lecture #123 l+MET +Njets W+ Njets QCD Before taggingAfter tagging W+ Light jets Wc Wcc Wb Wbb Matrix method ALPGEN fractions W+ Light jets Wc Wcc Wb Wbb Ptag light Ptag 1c jet Ptag 2c jets Ptag 2b jets Ptag 1b jet Ptag QCD N Bckg tag

08/14/05Regina Demina, Lecture #124 L+jets sample composition

08/14/05Regina Demina, Lecture #125 Cross 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. Z  ee

08/14/05Regina Demina, Lecture #126 ttbar cross section in l+jets with b-tag Isolated lepton –p T >20 GeV/c, |  e |<1.1, |   |<2.0 Missing E T >20GeV Four or more jets –p T >15 GeV/c, |   = (stat+syst)±0.5(lumi) pb DØ RunII Preliminary, 363pb -1 ≥4j, 1t≥4j, 2t Expect bkg21.8±3.01.9±0.5 Observe8821