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Top quarks at the LHC The heavy weights among the elementary particles

Top quarks at the LHC The heavy weights among the elementary particles. Markus Cristinziani IVICFA Easter Workshop Valencia, March 25 th -26 th , 2013. Top quark: motivation. Elementary particle with the largest mass ( y t ~ 1). no bound states role in SM loop diagrams

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Top quarks at the LHC The heavy weights among the elementary particles

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  1. Top quarks at the LHCThe heavy weights among the elementary particles Markus Cristinziani IVICFA Easter Workshop Valencia, March 25th-26th, 2013

  2. Top quark: motivation • Elementary particle with the largest mass (yt ~ 1) • no bound states • role in SM loop diagrams • decays from and to BSM • important background to SM & BSM searches Mass in GeV/c2 discovered 1995 at Tevatron: PRL 74, 2632 (1995), PRL 74, 2626 (1995) M. Cristinziani - Top Quarks at the LHC

  3. Top quark: production • tt̄: strong interaction • single t: weak interaction M. Cristinziani - Top Quarks at the LHC

  4. Top quark: physics program overview BR(tWb)/BR(tWq) W helicity new decays spin correlations charge asymmetry mass (difference) charge lifetime, width polarisation prod. cross-section prod. kinematics resonances new particles M. Cristinziani - Top Quarks at the LHC

  5. Top quark pair signatures • Classified according to W boson decay • t→Wb at ~100% • W decays to (τ→)e/µ+ν, qq̄ or τhad+ν dilepton lepton+jets all hadronic τhad+jets lepton+jets all hadronic W+ → ud, csτ+ντ e+νe,µ+νµ τhad+jets τhad+lept. lepton+jets dilepton τhad+lept. W- → du,scτ-ντ e-νe,µ-νµ M. Cristinziani - Top Quarks at the LHC

  6. Plan fortoday • Top quarkproduction • inclusiveand differential tt̄ cross-section • associatedproduction: tt̄g, tt̄0, tt̄bb, tt̄γ, tt̄Z, tt̄H, tt̄ETmiss • single top • Properties • intrinsic: mass, charge • decay: Rb, FCNC • Angular variables • top and W polarisation, CP violationtest • top pairs: spincorrelations, chargeasymmetry • Search for tt̄ resonances M. Cristinziani - Top Quarks at the LHC

  7. Pair production cross-section σtt̄ • Best precision in lepton+jets channel • here: without explicit identification of b-jets • using discriminating kinematic variables • tt̄ are more central, spherical and energetic δσ/σ = 6.6% CONF-2011-121 σtt̄ = 179 ± 4stat ± 9syst ± 7lumi pb systematicsconstrained in-situ M. Cristinziani - Top Quarks at the LHC

  8. σtt̄ in the dilepton channel JHEP 11 (2012) 67 • Opposite sign ee, eμ, μμ • Z+jets bckgnd: mll≠ mZ, ETmiss> 40GeV • At least two jets • consider number of b-tags (0,1,≥2) • Results • 7 TeV profile likelihood ratio (#jets, #b-tags) • 8 TeV counting experiment ≥1 b-tag → most precise channel at LHC 4.2% PAS TOP-12-007 6.7% M. Cristinziani - Top Quarks at the LHC

  9. More determinations of σtt̄ all hadronic arXiv:1302.0508 subm. to JHEP τhad+jets lepton+jets all hadronic W+ → ud, csτ+ντ e+νe,µ+νµ τhad+e/µ τhad+jets PLB 717 (2012) 89 τhad+jets τhad+lept. lepton+jets dilepton τhad+lept. W- → du,scτ-ντ e-νe,µ-νµ EPJ C73 (2013) 2328 M. Cristinziani - Top Quarks at the LHC

  10. Summary σtt̄ and theory comparison • inclusivemeasurementsconsistentacross all channels • experimental uncertainties <5 – 15%, challengingtheory M. Cristinziani - Top Quarks at the LHC

  11. Differential tt̄ cross-section • Test ofpQCD in dσ/dx ℓ+jets, dilepton @ 7 and 8 TeV • check dependence on QCD scales, ME-PS matching, generators • enhancesensitivitytonewphysics • Analysis ingredients • kinematicreconstruction • unfoldtoparticlelevel • Differential in pT, η(and m) for ℓ, ℓℓ, b, bℓ, t, tt̄ Kidonakis PR D82 (2010) 114030 EPJ C73 (2013) 2261 Gooddescription in general aNNLOdescribes softer pT(top) CMS databest arXiv:1211.2220 subm. to EPJC PAS TOP-12-028 PAS TOP-12-027 M. Cristinziani - Top Quarks at the LHC

  12. tt̄ + extra jets • Allowsto check modelingat high #jetsat top quarkscale • importantfor top, Higgs, BSM • unfoldspectrum in visible experimental phasespace • High #jetsis not wellmodeledby MC@NLO CONF-2012-155 PAS TOP-12-018 PAS TOP-12-023 M. Cristinziani - Top Quarks at the LHC

  13. tt̄ + no extra jets • Analysis on tt̄ → ℓℓbbνν with veto on jets (beyond 2b) • Gap fraction • fraction of events without an additional jet above threshold • Result • reasonable description of data, except for MC@NLO in central region; helps reducing allowed radiation variation EPJ C72 (2012) 2043 PAS TOP-12-023 |y| 2.4 M. Cristinziani - Top Quarks at the LHC

  14. tt̄ + bb̄ • Importantbackgroundto tt̄H(→bb̄) channel • Interested in fractionofeventswith b-flavour • in σ(tt̄bb̄)/σ(tt̄jj) many systematics cancel • Analysis • dilepton selection + ≥4 jets • signal extracted by fitting #b-jets • corrected to particle level • dominant syst. uncertainty is ε(b-tag) • Theory • Madgraph 1.2%, Powheg 1.3% • NLO calculations predict 4.7% (parton level, can’t be compared) PAS TOP-12-024 Bevilacqua, Czakon, Papadopoulos et al., PR D84 (2011) 114017 M. Cristinziani - Top Quarks at the LHC

  15. tt̄ + photon • Sensitive to tt̄γvertex • Eventually probe vector and axial vector couplings • Photons • Radiative production & decay (and interference) • Require pT(γ) > 8 GeV, SM σtt̄γ = 2.1±0.4 pb • Isolation variable used for bckgnd determination CONF-2011-153 52 events 70 events Significance 2.7σ Expected 3.0σ BR*σttγ = 2.0±0.5stat±0.7syst±0.1lumipb M. Cristinziani - Top Quarks at the LHC

  16. tt̄ + Z • Access tocouplingof top tovectorboson • also importantbackgroundto SUSY and BSM searches • analysis also designedtomeasurett̄W (not coupling) • Analysis • same-sign dilepton (tt̄V) ortrileptonevents(tt̄Z) • nowwithupdatedgeneratorunc. (Powheg-BOX, +50% syst.) arXiv:1303.3239 submittedto PRL 3.3σ NLO: 3.0σ NLO: Garzelli et al., JHEP 11 (2012) 056 Campbell, Ellis, JHEP 07 (2012) 052 M. Cristinziani - Top Quarks at the LHC

  17. CONF-2012-126 Candidate event (eµµ) ETmiss= 78GeV mll= 91 GeV mT(l,ETmiss) = 67 GeV 4 jets (2 b-tagged) σtt̄Z < 0.71 pb at 95% C.L. M. Cristinziani - Top Quarks at the LHC

  18. tt̄ + Higgs • Can access top-Higgs Yukawa coupling • givenenoughluminosity • In 2011 lookedat tt̄H (H→bb) • divide sample in categories #jets #b-jets • constructlikelihood (ATLAS) orneuralnetwork (CMS) • Not yet sensitive • analysing 2012 data • Limits formH=125GeV • CMS 4.6 x SM (3.8) • ATLAS 13.1 x SM (10.5) CONF-2012-135 PAS HIG-12-025 M. Cristinziani - Top Quarks at the LHC

  19. tt̄ + ETmiss • Search for T→tA0 • A0scalardark matter candidate • searchforexcessETmissℓ+jets • Verificationof top productionmodel in ℓ+jets • backgroundfor BSM searches • In eachETmiss bin subtractbckgndbased on ηℓ-distribution • correctfor bin migration PAS TOP-12-019 PRL 108 (2012) 041805 Alwall, Feng, Kumar et al. (2010), Berger, Cao (2009) M. Cristinziani - Top Quarks at the LHC

  20. Single top • Alternative production via weakinteraction • Sensitive tonewphysicsmodels • Can measure |Vtb| withoutassumptions on #generations • Can beusedtomeasure b-quark PDF t-channel s-channel Wtchannel M. Cristinziani - Top Quarks at the LHC

  21. EvidenceforWtproduction • First evidenceat ATLAS • use dilepton channel 7 TeV 2.05/fbdata • fit BDT discriminatoroutputfor 1, 2 and >=3 jetbins • σWt= 16.8 ± 2.9 ± 4.9 pb (ATLAS) 3.3σ • σWt= 16 +5-4pb (CMS) 4.0σ Phys. Lett. B 716 (2012) 142-159 Phys. Rev. Lett. 110 (2013) 022003 prediction: 15.7 ± 1.1 pb (Kidonakis 2010) M. Cristinziani - Top Quarks at the LHC

  22. t-channelsingle top • Final state: 1 lepton, ETmiss, 1 b-jet, 1 recoiljet • Signal extraction: cut-based, NN, BDT CONF-2012-132 Phys. Lett. B 717 (2012) 330 JHEP 12 (2012) 035 PAS TOP-12-011 M. Cristinziani - Top Quarks at the LHC

  23. Single top t vs t̄ production • Proton composedofuudvalencequarks • single top productionfollowsincomingquark type • ug dtb̄anddg  ut̄b • experimental handle: leptoncharge • expectRt = σ(t)/σ(t̄) ~ 2 (1.84, Kidonakis arXiv:1205.3453 8TeV) PAS TOP-12-038 CONF-2012-056 M. Cristinziani - Top Quarks at the LHC

  24. Plan fortoday • Top quarkproduction • inclusiveand differential tt̄ cross-section • associatedproduction: tt̄g, tt̄0, tt̄bb, tt̄γ, tt̄Z, tt̄H, tt̄ETmiss • single top • Properties • intrinsic: mass, charge • decay: Rb, FCNC • Angular variables • top and W polarisation, CP violationtest • top pairs: spincorrelations, chargeasymmetry • Search for tt̄ resonances M. Cristinziani - Top Quarks at the LHC

  25. Relation Higgs-W-top mass radiative corrections t W W b ATLAS direct search ΔmW ~ mt2 H W W ΔmW ~ ln (mH) M. Cristinziani - Top Quarks at the LHC

  26. Determination of the top quark mass • Direct reconstruction of top quark pairs • template method • compare data to templates from simulation with different masses • ideogram method • event likelihood from Breit-Wigner (signal) convoluted with resolution • matrix element method • event probability as a function of LO matrix element → uncertainties: jet energy scale and QCD radiation • Indirect methods (some new at LHC) • dependence of top mass on • cross section • kin. observables: B hadron decay length, lepton pT, J/ψ+lepton from W → can profit from large data samples t t t t b-tag b-tag b-tag t t b-tag t t M. Cristinziani - Top Quarks at the LHC

  27. Top massmeasurement in ℓ+jets • CMS: ideogrammethod • useskin fit togetmtandrecofor mW • JES correctionevaluatedfromthemethod • methodvalidated/calibratedusing MC • mt = 173.49 ±0.43(stat+JES)±0.98 (syst) GeV • ATLAS: template fit • usesrecomtand mWasinputto fit • JSF impactevaluated in situ from mW • methodvalidatedusing MC • mt = 174.5±0.6(stat)±2.3(syst) GeV M. Cristinziani - Top Quarks at the LHC

  28. Top mass in the dilepton channel • Dilepton channel is kinematicallyunderconstrained • missing transverse momentum due to 2 neutrinos • employing variable • kinematic endpoint is mtop Summary: top mass at ATLAS M. Cristinziani - Top Quarks at the LHC

  29. Top mass summary • First combination of mtop at LHC (TOPLHCWG) • detailed studies of uncertainty mapping and correlation are necessary CONF-2012-095 PAS TOP-12-001 PAS TOP-11-018 • mt (Tevatron) = 173.18 ± 0.56 ± 0.75 GeV • mt(LHC) = 173.3 ± 0.5 ± 1.3 GeV • mt(CMS, new) = 173.36± 0.38 ± 0.91 GeV M. Cristinziani - Top Quarks at the LHC

  30. Which top quark mass? • Issue • measured mass parameter mexp vs. pole mass mtop • top quark is a colored object, final state is color neutral Γt >1 GeV: additional antiquarks contribute to mexp If Γt < 1 GeV: T-hadrons • Trying to address the problem • extract the pole mass from cross-section, but • study top mass kinematic dependence (CMS) • use differential tt+1j distribution Aioli, Fuster, Irles, Moch, Uwer, Vos 2012 M. Cristinziani - Top Quarks at the LHC

  31. Dependenceof top mass on eventkinematics • top massmeasurementbinned in kinematic observables • withincurrentprecisionnomismodelingeffect due to • colorreconnection, ISR/FSR, b-quark kinematics PAS TOP-12-029 color reconnection ISR/FSR b-quark M. Cristinziani - Top Quarks at the LHC

  32. Test of CPT invariance in top mass PAS TOP-12-031 • CPT invariancepredictsm(particle) = m(anti-particle) • welltested so far • couldbeviolatedfor top quarks (e.g. Cembranos et al.) • Analysis • lepton+jetschannel, 8 TeV (19/fb) updatedfrom 7 TeV 5/fb • useideogrammethod M. Cristinziani - Top Quarks at the LHC

  33. Intrinsicproperties: top quarkcharge CONF-2011-141 • top isospinpartnerof b-quark • in SM expecttohaveq = +2/3e • can check in dataagainstexotichypothesisofq = -4/3e • Analysis • identifychargeof W (lepton) • chargeof b • semileptonicdecays (soft lepton, usepTrel) • trackchargeweighting • correctlymatch W and b • Result • canexcludeexotic hypothesisat >5σ jis b-jet axis k = 0.5 optimal M. Cristinziani - Top Quarks at the LHC

  34. Branchingfractions: t → Wb PAS TOP-12-035 • EvaluateRb=BF(t→Wb)/BF(t→Wq) • SM expectation: Rb = |Vtb|2 = 0.99829±0.00009 • can probe newphysics (such as 4th gen., H+) • Measured in dilepton channel • modeldependenceofmeasured #b-tags on Rbfromdata • fractionof tt̄ and t in sample, correctjetassignment, ε(b),ε(q) M. Cristinziani - Top Quarks at the LHC

  35. Flavor-changing neutral currents (FCNC) Top quark decays in the standard model GIM mechanism BR ~100% in SM BR ~10-14 in SM FCNC appears in BSM models M. Cristinziani - Top Quarks at the LHC

  36. Search for FCNC in ATLAS • Search for FCNC t→uZ and t→cZ • use tt̄ decays tt̄ →qZbW→ qllblν • identifythreeleptons • backgroundestimation • based on numberoffakeleptons • BF(t→qZ) < 7.3 · 10-3 • Search for FCNC t→ugand t→cg • in single-top quark production qg→t→blν • train neural network • BF(t→ug) < 5.7 · 10-5 • BF(t→cg) < 2.7 · 10-4 M. Cristinziani - Top Quarks at the LHC

  37. Plan fortoday • Top quarkproduction • inclusiveand differential tt̄ cross-section • associatedproduction: tt̄g, tt̄0, tt̄bb, tt̄γ, tt̄Z, tt̄H, tt̄ETmiss • single top • Properties • intrinsic: mass, charge • decay: Rb, FCNC • Angular variables • top and W polarisation, CP violationtest • top pairs: spincorrelations, chargeasymmetry • Search for tt̄ resonances M. Cristinziani - Top Quarks at the LHC

  38. Top polarisation • Top quarks predicted unpolarised in SM • Short lifetime → reflected in lepton angular distribution • Analysis • Lepton+jets channel, kinematic fit to reconstruct rest frame • Template likelihood fit to cosθlepton • background • positive and negative polarisation fractions • Result Top quark fraction with positive polarisation Spin analysing power αl = 1 muons CONF-2012-133 M. Cristinziani - Top Quarks at the LHC

  39. W helicity in top quarkdecays • Preciselypredicted in SM • righthandedhelicity FRsuppressed (V-A) • measurements based on angle ϑ*(l,–b) in W rest frame BSM contributionstoWtbvertexcanmodifyhelicityfractions In effectiveoperatorframework JHEP 06 (2012) 088 PAS TOP-12-020 Measuredin tt̄ (singleand dilepton) and also in single top M. Cristinziani - Top Quarks at the LHC

  40. Search for CP violation in single top decays W direction • Probe Wtbusingcomplementaryangles • ϑ★not sensitive tocomplexphase • top quarkpolarised, use cos ϑN • defineasymmetry • Single top selection • subtractbckgnd, unfoldtopartonlevel top polarisation • Aguilar-Saavedra, Bernabéu 2010 lepton CONF-2013-032 for P=0.9 → -0.20 < Im (gR) < 0.30 at 95%CL M. Cristinziani - Top Quarks at the LHC

  41. Spin correlations • Decay of top quark pairs • top quarks not polarised • spins are correlated • τ(top) small→ polarisation can be probed spin-asymmetry • tt̄ production and decay at LHC • produced mainly in gg fusion • helicity axis as quantisation axis → ASM = 0.32 • new physics can change this value M. Cristinziani - Top Quarks at the LHC

  42. Spin correlations at LHC • Δϕ between leptons in the lab • reconstruction not needed • sensitive to correlations • Mahlon, Parke, PRD 81, 074024 (2010) • Likelihood fit • fraction with correlation (fSM) • fraction w/o correlation (1–fSM) • Simultaneous fit to ee, µµ, eµ • fSM = 1.30 ± 0.14 (stat.)+ 0.27 (syst.) • Null-hypothesis excluded at 5.1σ • Aexp = 0.40 ± 0.09 (ASM = 0.32) - 0.22 MC@NLO √s = 7 TeV no cut on mtt Phys. Rev. Lett. 108 (2012) 212001 • First observation of tt̄ spin correlations at LHC M. Cristinziani - Top Quarks at the LHC

  43. Forward-backward asymmetry AFB • NLO: Asymmetry in pp̄→tt̄ production • through interference • QCD NLO AFB = 5% • with EW corrections AFB = 9% Born Box ISR FSR Exchange of new particles AFB up to 30% forward-backward asymmetry Tevatron results: AFB = (17±4)% AFB = (30±7)% for m(tt̄)>450GeV (CDF) M. Cristinziani - Top Quarks at the LHC

  44. New physics to explain AFB • O(200) papers since 2011 on AFB • LHC predictions possible • classification of models axigluons J.Aguilar-Saavedra et al., JHEP 1109 (2011) 097 M. Cristinziani - Top Quarks at the LHC

  45. Asymmetry at the LHC • Initial state is symmetric • small qq̄ contribution • only valence quarks center-forward asymmetry forward-backward asymmetry? AC(lept) = (2.3±1.2stat±0.8syst) % SM prediction: (0.4±0.1)% AC(tt̄) = (5.7±2.4stat±1.5syst) % SM prediction: (0.6±0.2)% lepton+jets dilepton M. Cristinziani - Top Quarks at the LHC

  46. Plan fortoday • Top quarkproduction • inclusiveand differential tt̄ cross-section • associatedproduction: tt̄g, tt̄0, tt̄bb, tt̄γ, tt̄Z, tt̄H, tt̄ETmiss • single top • Properties • intrinsic: mass, charge • decay: Rb, FCNC • Angular variables • top and W polarisation, CP violationtest • top pairs: spincorrelations, chargeasymmetry • Search for tt̄ resonances M. Cristinziani - Top Quarks at the LHC

  47. Search for tt̄ resonances • Motivation • top couples strongly with new massive particles in many BSM scenarios→ possible resonance m(tt̄) distribution • Benchmarks • narrow resonance: top-color Z’ with Γ(Z’)/m(Z’)=1.2% • wide resonance: Kaluza-Klein gluon (gKK) • Boosted top quarks • for large m(tt̄) the top quarks get a forward boost • decay products (jets, leptons) overlap • new reconstruction techniques required • jet sub-structure • less well-isolated leptons m(tt̄) = 2.5 TeV M. Cristinziani - Top Quarks at the LHC

  48. Search for tt̄ resonances • Measured in channels with 0,1 or 2 leptons • best limits in lepton+jets channel • Combined measurement • using boosted reconstruction improves analysis m(Z’) > 1.7 TeV @ 95%C.L. m(gKK) > 1.9 TeV @ 95%C.L. M. Cristinziani - Top Quarks at the LHC

  49. Conclusion • Top quarks • the heavy weights among the quarks • Large Hadron Collider: the top quark factory • comprehensive set of measurements on Run 1 data • precision tests possible • sensitivity in searches is extended into the TeV region • Outlook • most of the data not yet analysed • try to constrain systematics • continue to challenge the Standard Model from every corner! M. Cristinziani - Top Quarks at the LHC

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