Top hadron colliders. Zofia Czyczula Particle Physics II - FYS4560

Transcription

1 Top hadron colliders Zofia Czyczula Particle Physics II - FYS4560 1

2 Outline of the lectures Part I Top quark in the Standard Model and Beyond Indirect evidences of top quark Top quark production at hadronic colliders - pair production cross section - single top production cross section Top properties: mass, charge, W helicity, FCNC, FB asymmetry, spin correlations Direct searches for new physics using top quarks Summary & quiz Part II Experimental aspects of top physics : how do we measure top LHC? - the LHC and ATLAS detector - principles of b tagging - ttbar cross section measurement - summary 2

3 Part I: Top quark in Standard Model and Beyond 3

4 What is a top quark? The heaviest quark in the SM: mt ~ 170 GeV - Mass near the electroweak symmetry breaking scale, has natural Yukawa coupling: λt= sqrt(2) mt/v ~1 Most recently discovered (1995) Charge of +2/3 and weak isospin of +1/2 Lifetime s: - ~20 times shorter then time scale of strong int. -> decays before hadronization the spin information is kept by its decay products - gives opportunity to study bare quark 4

5 Indirect evidences of top quark Existence of top anticipated since discovery of b-quark Evidences from processes involving b quark: - B meson mixing - Studies of decay width of Y->e+e- in DESY e+e- collisions favored b charge of 1/3 - Measurement of FB asymmetry in e+e- ->bb process indicated b-quark isospin of -1/2 EW precision tests at LEP constraint the top mass: - Z->bb decays: FB asymmetry, ratio of b-quark partial width of Z (wrt total hadronic) - W boson mass 5

6 Tevatron top discovery machine Proton-antiproton collision At 1.96 TeV Run I: operating Run II: operating fb-1 data - Precision measurement Of top quark properties 6

7 LHC top factory Proton-proton 7 TeV Operating since March pb-1 data in 2010 ~ few fb-1 foreseen for > soon more tops on tape than in Tevatron 7

8 Top pair-production perturbative QCD Top pair production proceeds via strong interactions ( dominant source of top quarks at hadron colliders ) Parton level cross section calculated using pqcd Leading order contributions: + + Gluon-gluon fusion LHC Quark annihilation Tevatron Next-to-leading order contributions: brem-like and flavor excitation 8

9 Top production at colliders the big picture Hadron-hadron scattering - constituent partons from each (anti)proton interact at short scales: QCD factorization - separate long from short distance dynamics: PDF s - μf - factorization scale, typically ~m t parton level x-sect 9

10 10 Slide from Yuriy Pylypchenko

11 Top decay mt> mw : has one dominant decay mode: - t->wb (~100%) : Γt=1.4 GeV Lifetime s: -> decays before hadronization In the SM: Br(t Wb) ~ 100% (other decays CKM suppressed) Decay channels classified by W decays Top pair decay channels: - Dilepton: lνlνbb (5%) - Lepton+jets : lνqqbb (30%) - All-hadronic: qqqqbb (45%) (experimentally challenging!) - Tau+X (20%) (not easy) 11

12 Measurement of top pair production cross section tt production rate provides precision tests of (N)NLO pqcd sensitive to the presence of new physical phenomena Measured with high accuracy Tevatron First results from the LHC -> good agreement between data and theory Next lecture: Exp. aspects of this measurement 12

13 Single top production Proceeds via weak interactions: - exchange of virtual W in s and t channel - associated production with real W Tevatron cross Good agreement between measurement and theory 13

14 Properties of top quark 14

15 Top quark Mass - top is the most massive fundamental particle dominant contribution to radiative corrections ~(mt)2 ~log(mh ) - Precision measurements of Mtop and MW provide a better constraint to the Higgs mass than the combination of many EW observable Different channels: -> consistency Combined results: -> accuracy 15

16 Top quark charge Measurement of top charge probes exotic models where the top charge differs from the standard model charge Charge conservation requires the top charge to be the sum of the W and b charges Measure charge of lepton and b-jet Define asymmetry: Charge of -4/3 95% C.L. Conf. Note

17 W boson helicity in top decays Examines the structure of Wtb vertex, probes Weak Interactions near the EW symmetry breaking scale Stringent test of V-A interactions in SM -> SM predicts only left handed twb couplings: hep-ph v1 Model independent test based on cosθ* Left-handed Λ = -1 longitudinal Λ=0 Right- handed Λ=1 Helicity Fractions: SM: ~ 30.3 % ~69.6 % ~0.1 % 17

18 W boson helicity in top decays: results from Tevatron Results compatible with SM predictions -note large uncertainties CDF 18

19 BSM top decays: Anomalous couplings CP conserving Lagrangian Wtb vertex Anomalous coupling would affect single top production as well as W helicity structure in top pair production - combine both to set limits DØ Note 5838-CONF At tree level Loop induced In SM Limits on form factors Consistent with SM Anomalous coupling 95% C.L. 19

20 BSM top decays: FCNC in top decays FCNC strongly suppressed in SM - undetectable at Tevatron ( nor LHC) - if observed would provide a clear sign of New Physics hep-ph/ v1 CDF Limit (Z+4 jests): 95% C.L. Searched for in D0 x x x x x x ec f Ef ng i l up o c e tiv Limits on effective coupling translated to B(t->gu)<2.0 * C.L. B(t->gc)<3.9 * C.L. 20

21 Forward-backward (charge) asymmetry forward-backward asymmetry compares number of top quarks mov- ing for or against a given direction. charge asymmetry compares number of top and anti-top quarks produced with momentum in a given direction. CP CP NLO: Strong interactions not sensitive to charge: - LO: production is charge symmetric - NLO: top repelled at high rapidities by soft Coulomb field of incoming light quark, anti-top is simultaneously attracted at low rapidity Asymmetry is small : A = / > sensitive variable to test the new physics contribution. 21

22 Forward-backward charge asymmetry: observables Fundamental qqbar frame not accessible due to initial state radiation (ISR) pp frame Use angle between hadr decaying top and the proton direction Use charge Q to mark leptonic decay -> -Q cos(θ) net top current in proton direction produces positive asymmetry tt frame Production angle related to difference In rapidity (Lorentz inv) - asymmetry in angle translates to asymmetry in rapidity difference, hence 22

23 Measured forward-backward charge asymmetry pp frame tt frame 23

24 Forward backward asymmetry in 3rd generation quarks The asymmetry at Z pole measured at LEP in bb final state differs by 3 sigma from SM value -> dynamics of 3rd generation quarks is not yet fully understood Given a good agreement of top pair production cross section (~10%) and invariant mass spectra with SM makes any interpretation in a light of (well motivated) New Physics very difficult Less motivated extensions which could explain these anomalies include: - Warped extra dimensions (Phys.Rev.D82:071702,2010, arxiv: v1 ) - axi-gluons, colored gauge bosons,.. 24

25 tt spin correlations FERMILAB-CONF T arxiv: v1 Recall: tt pair produced mainly via qq (Tevatron) and gg (LHC) - qq or OS gluon helicities yield OS helicity top pairs ppbar: OS helicity dominates - SS gluon helicities lead to SS helicity top pairs pp: SS helicity dominates [ low M(tt) ] 25 hep-ph/

26 tt spin correlations Model independent observables are angles θ1 and θ2 - measured relative to the beam direction in the ttbar rest frame. =0 (tops are not polarized) - Tevatron: LO C= 0.777@NLO LHC: M(tt)<550 GeV ATL-COM-PHYS

27 Measured ttbar spin correlations Measurement compatible with the SM - note large stat(sys) uncertainties 27

28 Direct search for new physics using top quarks 28

29 Searches for new resonances in tt final state Main observable: Invariant mass spectrum No evidence of resonant production of ttbar events 29

30 BSM direct searches for t /W t ->Wb W ->tb 4th generation heavy t predicted by - Little Higgs model 2 Higgs Double Scenarios, N=2 SUSY models,... Direct searches: - assume: t pair-produced strongly - use invariant mass for the search W - heavier copy of W - predicted in many models BSM: UED, LR symmetry,... - chiral structure depends on model - model independent parametrization: Limits - Left-handed: M(W ) > 863 GeV -Right-handed: M(W ) > 885 GeV Mt >335GeV 30

31 Summary Top quark exists at the interface between Electroweak and QCD physics Since its discovery in 1995 many measurements performed giving confidence that the particle is indeed the 3rd generation quark EW top Production and properties - sensitive to the existence of new physical phenomena and offer insights on the electroweak symmetry breaking mechanism. - > hot LHC...and not only:-) QCD Higgs & New Physics Material: x 31

32 Exercise: Which measurements would you perform to find answers and why? Question 1. What is Higgs boson mass? 2. Do we understand heavy flavor production in QCD? 3. More then 3 generation of quarks? 4. New heavy Resonances? 5. Does the top quark have expected couplings?? Measurement 1. tt cross section 2. W boson helicity 3. FB asymmetry 4. FCNC 5. top quark mass 6. single top cross section 7. Search for t, H+ 8. Anomalous coupling Wtb 9. Mt distribution

33 Part II: Experimental measurement of top quark 33

34 The Large Hadron Collider 34

35 Top LHC LHC is a top factory Many measurements planed and ongoing this lecture 35

36 The ATLAS detector Muon Spectrometer ( η <2.7): Measurement of muons Detector characteristic Width: 44m Diameter: 22 m Weight: 7000t EM calorimeter: Identification and measurement of electrons Inner detector ( η <2.5): Vertexing, tracking, e/π separation, Identification of b quarks HAD calorimetry ( η <5): Measurement of jets and missing ET (neutrinos) 36

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38 Top events Top quarks once produce immediately decay Only their decay products are visible in ATLAS - Due to large QCD backgrounds only channels involving at least one lepton are feasible l+jets channel Objects visible in ATLAS Electrons, muons, tau-leptons Jets ( quarks ) Missing ET (attributed to neutrinos) Identification of jets steaming from hadronization of b-quark Is key in reconstruction of final states involving top quarks 38

39 Particle Identification in ATLAS (1) ELECTRON - Cluster in EM calo + track in the ID ID: narrow shower in EM calo, no deposition in Had calo, one track MUON ID: Track in Muon Spectrometer (matched with Inner Detector track) JETS ( quarks/gluons) - narrow cone of of hadrons produced in hadronization ID: Cluster in EM+HAD calo associated with tracks 39

40 Particle Identification in ATLAS (2) MISSING TRANSVERSE ENERGY - Energy of non-interacting particles (neutrinos) reconstructed based on energy and momentum conservations - In hadronic colliders only transverse component can be determined ( initial momenta of partons not known) Ex(miss, calo)= - i=1ncell Ei sin i cos i Ey(miss, calo)= - i=1ncell Ei sin i sin i ET(miss, calo)=sqrt( (Ex(miss, calo))2 +(Ey(miss, calo))2) + Muon corrections 40

41 Identification of b-quarks ( b-tagging ) Exploits several properties of B hadrons: Retain ~70% of original b-quark momentum m > 5 GeV -> decay products have high pt wrt to jet axis Key property is long lifetime ( cτ ~ 450 μm ) B in jet with pt= 50 GeV travels in ~3mm in the tr. plane Selection algorithms the taggers - decide whether a jet contained a b-quark, given the reconstructed track and vertices (place where particles decay) 41 ATLAS-CONF

42 Key observable is an Impact Parameter (IP) IP significance: S= d0/σ(d0) Characterizes time of flight of the mother particle It is calculated for each track Tracks from b decays have large IP It is signed + -> α(jet, IP-line ) < > α(jet, IP-line ) 900 longitudinal IP (z0) - the z coordinate of the track at the point of the closest approach in xy plane transversal IP (d0) - distance of the track at the point of closest approach, in the xy plane (wrt PV) y 42 x

43 Other observables Secondary vertices taggers Decay length significance Invariant mass of particles associated to reconstructed secondary vertex Ratio of kinetic energies of particles associated to secondary vertex to kinetic energy of all particles in jet Number of track pairs which were used to reconstruct secondary vertex Other taggers: Soft lepton id of lepton from B decays JetFilter - reconstruction of tertiary and other vertices 43

44 b-tagging is probability based Decision, whether a jet contains b- quark, is probability based W= Σi ln( b(oi)/u(oi) ) - jet weight b(oi) / u(oi) are the probabilities that the i-th track in b-jet /light-jet has a given value of observable O (IP, SV,...) 44

45 tt Event Selection Aras Papadelis slide 45

46 Electron + 4 jets event 46

47 Di-lepton (emu) Event 47

48 48

49 Background are large.. Backgrounds come from QCD and EW (W,Z) processes Irreducible have the same final states as ttbar decays and can pass all event selection cuts Reducible ( instrumental ) - comes from experimental mis-reconstruction 49

50 Irreducible backgrounds Most of backgrounds are well modeled in Monte Carlo Aras Papadelis slide 50

51 Instrumental backgrounds Aras Papadelis slide 51

52 Measurement of tt cross-section Aras Papadelis slide 52 Combined x-sect measured by simultaneous fit of the product of the channel likelihoods

53 Cross-section results Significance obtained from the fit 53

54 Summary Top quark was anticipated since 1977 and discovered in 1995 TEVATRON Many measurements of its properties: - ttbar xsect, top mass and charge - xsect of single top production - Wtb vertex: anomalous couplings, W helicity, FCNC - spin correlations in agreement with the SM Measurement of FB asymmetry 3 sigma away from the value predicted by SM - no convincing explanation LHC: Is a top factory Pair production cross section has been measured with first LHC data Many more ongoing measurements with potential to discover New Physics 54