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Experimental aspects of top quark physics Lecture #1

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, University of Rochester. Novosibirsk. Rochester. Rochester, Ny. www.pas.rochester.edu. Outline.

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Experimental aspects of top quark physics Lecture #1

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  1. 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

  2. Regina Demina, University of Rochester Novosibirsk Rochester Regina Demina, Lecture #1

  3. Rochester, Ny www.pas.rochester.edu Regina Demina, Lecture #1

  4. Outline • Introduction • Colliders • Parton density functions • Top quark production • Meaning of luminosity • Top quark decay • Particle identification • Top pair production cross section measurement • Control questions Regina Demina, Lecture #1

  5. Energy 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 Regina Demina, Lecture #1

  6. 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 Regina Demina, Lecture #1

  7. Accelerators: 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 Regina Demina, Lecture #1

  8. Parton density functions • Proton (q=+1e) is not an elementary particle • It consists of three valence quarksuud (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 d u u Regina Demina, Lecture #1

  9. 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 Regina Demina, Lecture #1

  10. Top Quark Production Top quarks at hadron colliders are (mainly) produced in pairs via strong interaction Quark-antiquark annihilation: TeV:85% LHC:~0% top Gluon fusion: TeV:15% LHC:~100% Top pair cross section at 1.96 TeV is 6.7 pb Regina Demina, Lecture #1

  11. Luminosity integrated luminosity 0.75 fb-1 This is delivered luminosity Recorded orgood forphysics is lower 1/3 used in the analyses presented here instantaneous luminosity 1.1032 cm-2sec-1 Regina Demina, Lecture #1

  12. Since the top lifetime top ~ 1/ M3top~10 -24 sec qcd ~ -1 ~10 -23 sec Top Lifetime and Decay the top quark does not hadronize. It decays as a free quark! Regina Demina, Lecture #1

  13. Top identification All jet: high BR, high BG t->Wb in 99.8% Always two b-jets in the final state Lepton + jet: BR and BG are OK di-lepton: BR low, BG low 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 Need to reconstruct:Electrons, muons, jets, b-jetsand missing transverse energy Regina Demina, Lecture #1

  14. Particle identification Charged particles curve inB-field, which enables their momentum measurement • 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 Regina Demina, Lecture #1

  15. CDF and D0 in Run II CDF DØ 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 Regina Demina, Lecture #1

  16. 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 p 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 Regina Demina, Lecture #1

  17. Tagging b-jets b-quark d0 PV • After traveling ~1mm from the primary vertex (PV) b-quarks decay into a jetof lighter particles. • Very precise measurements provided by silicon detectors tell if the particle has a significant impact parameter (d0) wrt the primary vertex. • 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 taggedas a b-jet if it contains several tracks not coming from the primary vertex. Regina Demina, Lecture #1

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

  19. Clusters of ionizationDot-like hits MIP MIP Pulse q Particle crossing silicon sensor q Regina Demina, Lecture #1

  20. Tracking: connecting the dots Regina Demina, Lecture #1

  21. B-quarks ID pT>3 GeV 48 m y track x DCA: Distance of Closest Approach We correctly identify 44 out of a 100 b-jets with <1% mistake rate DCA resolution ~ 50 m (using as built + surveyed alignment) beam spot ~ 30-40 m Regina Demina, Lecture #1

  22. Top identification in lepton+jets channel • tbW, Wln, 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(ln)+jets Only a small fraction of these jets are b-jets Regina Demina, Lecture #1

  23. Before tagging After tagging Ptag light W+ Light jets W+ Light jets W+ Njets Ptag 1c jet Wc Wc ALPGEN fractions Ptag 1b jet N Bckg tag l+MET +Njets Wb Wb Matrix method Ptag 2c jets Wcc Wcc Ptag 2b jets Wbb Wbb QCD Ptag QCD QCD Regina Demina, Lecture #1

  24. L+jets sample composition Regina Demina, Lecture #1

  25. 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 Regina Demina, Lecture #1

  26. ttbar cross section in l+jets with b-tag DØ RunII Preliminary, 363pb-1 • Isolated lepton • pT>20 GeV/c, |he|<1.1, |hm|<2.0 • Missing ET>20GeV • Four or more jets • pT>15 GeV/c, |h|<2.5 s=8.1+1.3-1.2(stat+syst)±0.5(lumi) pb Regina Demina, Lecture #1

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