Top quarks at the LHC The heavy weights

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 (yt ~ 1) • no bound states Mass in Ge. V/c 2 • role in SM loop diagrams • decays from and to BSM • important background to SM & BSM searches discovered 1995 at Tevatron: PRL 74, 2632 (1995), PRL 74, 2626 (1995) M. Cristinziani - Top Quarks at the LHC 2/50

Top quark: production tt : strong interaction single t: weak interaction M. Cristinziani - Top Quarks at the LHC 3/50

Top quark: physics program overview mass (difference) charge lifetime, width polarisation BR(t. Wb)/BR(t. Wq) W helicity new decays spin correlations charge asymmetry prod. cross-section prod. kinematics resonances new particles M. Cristinziani - Top Quarks at the LHC 4/50

Top quark pair signatures Classified according to W boson decay • t→Wb at ~100% • W decays to (τ→)e/µ+ν, qq or τhad +ν W+ → ud, cs ro ni c lepton+jets to n lepton+jets al lh ad ro ni c lepton+jets ad τhad+lept. τ-ντ e-νe, µ-νµ τhad+jets lh lep τ+ντ e+νe, µ+νµ τhad+jets W- → du, sc al di di lep to n M. Cristinziani - Top Quarks at the LHC 5/50

Plan for today Top quark production • inclusive and differential tt cross-section • associated production: tt g, tt 0, tt bb, tt γ, tt Z, tt H, tt E Tmiss • single top Properties • intrinsic: mass, charge • decay: Rb, FCNC Angular variables • top and W polarisation, CP violation test • top pairs: spin correlations, charge asymmetry Search for tt resonances M. Cristinziani - Top Quarks at the LHC 6/50

Pair production cross-section σtt Best precision in lepton+jets channel • here: without explicit identification of b-jets • using discriminating kinematic variables • tt are more central, spherical and energetic δσ/σ = 6. 6% CONF-2011 -121 systematics constrained in-situ σtt = 179 ± 4 stat ± 9 syst ± 7 lumi pb M. Cristinziani - Top Quarks at the LHC 7/50

σtt in the dilepton channel Opposite sign ee, eμ, μμ JHEP 11 (2012) 67 • Z+jets bckgnd: mll ≠ m. Z, ETmiss > 40 Ge. V At least two jets • consider number of b-tags (0, 1, ≥ 2) Results • 7 Te. V profile likelihood ratio (#jets, #b-tags) 4. 2% PAS TOP-12 -007 • 8 Te. V counting experiment ≥ 1 b-tag 6. 7% → most precise channel at LHC M. Cristinziani - Top Quarks at the LHC 8/50

More determinations of σtt W+ → ud, cs ad ro ni c lepton+jets ar. Xiv: 1302. 0508 subm. to JHEP τhad+lept. τ-ντ e-νe, µ-νµ τhad+jets all hadronic lepton+jets lh τhad+jets W- → du, sc al τ+ντ e+νe, µ+νµ di lep to n τhad+jets τhad+e/µ PLB 717 (2012) 89 EPJ C 73 (2013) 2328 M. Cristinziani - Top Quarks at the LHC 9/50

Summary σtt and theory comparison • inclusive measurements consistent across all channels • experimental uncertainties <5 – 15%, challenging theory M. Cristinziani - Top Quarks at the LHC 10/50

Differential tt cross-section Test of p. QCD in dσ/dx ℓ+jets, dilepton @ 7 and 8 Te. V • check dependence on QCD scales, ME-PS matching, generators • enhance sensitivity to new physics Analysis ingredients • kinematic reconstruction • unfold to particle level Kidonakis PR D 82 (2010) 114030 Differential in p. T, η (and m) for ℓ, ℓℓ, b, bℓ, t, tt EPJ C 73 (2013) 2261 Good description in general a. NNLO describes softer p. T(top) CMS data best ar. Xiv: 1211. 2220 subm. to EPJC PAS TOP-12 -028 PAS TOP-12 -027 M. Cristinziani - Top Quarks at the LHC 11/50

tt + extra jets Allows to check modeling at high #jets at top quark scale • important for top, Higgs, BSM • unfold spectrum in visible experimental phase space CONF-2012 -155 PAS TOP-12 -018 PAS TOP-12 -023 • High #jets is not well modeled by MC@NLO M. Cristinziani - Top Quarks at the LHC 12/50

tt + no extra jets Analysis on tt → ℓℓbbνν with veto on jets (beyond 2 b) 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 C 72 (2012) 2043 PAS TOP-12 -023 |y|� 2. 4 M. Cristinziani - Top Quarks at the LHC 13/50

tt + bb Important background to tt H(→bb ) channel Interested in fraction of events with b-flavour • in σ(tt bb )/σ(tt jj) many systematics cancel PAS TOP-12 -024 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) Bevilacqua, Czakon, Papadopoulos et al. , PR D 84 (2011) 114017 M. Cristinziani - Top Quarks at the LHC 14/50

tt + photon Sensitive to tt γ vertex • Eventually probe vector and axial vector couplings Photons • Radiative production & decay (and interference) • Require p. T(γ) > 8 Ge. V, SM σtt γ = 2. 1± 0. 4 pb • Isolation variable used for bckgnd determination 52 events CONF-2011 -153 BR*σttγ = 2. 0± 0. 5 stat± 0. 7 syst± 0. 1 lumi pb 70 events Significance 2. 7σ Expected 3. 0σ M. Cristinziani - Top Quarks at the LHC 15/50

tt + Z Access to coupling of top to vector boson • also important background to SUSY and BSM searches • analysis also designed to measure tt W (not coupling) ar. Xiv: 1303. 3239 submitted to PRL Analysis • same-sign dilepton (tt V) or trilepton events (tt Z) • now with updated generator unc. (Powheg-BOX, +50% syst. ) 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 16/50

CONF-2012 -126 Candidate event (eµµ) ETmiss = 78 Ge. V mll= 91 Ge. V m. T(l, ETmiss) = 67 Ge. V 4 jets (2 b-tagged) σtt Z < 0. 71 pb at 95% C. L. M. Cristinziani - Top Quarks at the LHC 17/50

tt + Higgs Can access top-Higgs Yukawa coupling • given enough luminosity In 2011 looked at tt H (H→bb) • divide sample in categories #jets #b-jets • construct likelihood (ATLAS) or neural network (CMS) Not yet sensitive PAS HIG-12 -025 CONF-2012 -135 • analysing 2012 data Limits for m. H=125 Ge. V • CMS 4. 6 x SM (3. 8) • ATLAS 13. 1 x SM (10. 5) M. Cristinziani - Top Quarks at the LHC 18/50

tt + ETmiss Search for T→t. A 0 • A 0 scalar dark matter candidate • search for excess ETmiss ℓ+jets Verification of top production model in ℓ+jets • background for BSM searches • In each ETmiss bin subtract bckgnd based on ηℓ-distribution • correct for 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 19/50

Single top t-channel s-channel Wt channel • Alternative production via weak interaction • Sensitive to new physics models • Can measure |Vtb| without assumptions on #generations • Can be used to measure b-quark PDF M. Cristinziani - Top Quarks at the LHC 20/50

Evidence for Wt production First evidence at ATLAS • use dilepton channel 7 Te. V 2. 05/fb data • fit BDT discriminator output for 1, 2 and >=3 jet bins Phys. Lett. B 716 (2012) 142 -159 Phys. Rev. Lett. 110 (2013) 022003 σWt = 16. 8 ± 2. 9 ± 4. 9 pb (ATLAS) 3. 3σ σWt = 16 +5 -4 pb (CMS) 4. 0σ M. Cristinziani - Top Quarks at the LHC prediction: 15. 7 ± 1. 1 pb (Kidonakis 2010) 21/50

t-channel single top Final state: 1 lepton, ETmiss, 1 b-jet, 1 recoil jet Signal extraction: cut-based, NN, BDT Phys. Lett. B 717 (2012) 330 CONF-2012 -132 JHEP 12 (2012) 035 PAS TOP-12 -011 M. Cristinziani - Top Quarks at the LHC 22/50

Single top t vs t production Proton composed of uud valence quarks • single top production follows incoming quark type - ug dtb and dg ut b - experimental handle: lepton charge • expect Rt = σ(t)/σ(t ) ~ 2 (1. 84, Kidonakis ar. Xiv: 1205. 3453 8 Te. V) PAS TOP-12 -038 CONF-2012 -056 M. Cristinziani - Top Quarks at the LHC 23/50

Plan for today Top quark production • inclusive and differential tt cross-section • associated production: tt g, tt 0, tt bb, tt γ, tt Z, tt H, tt E Tmiss • single top Properties • intrinsic: mass, charge • decay: Rb, FCNC Angular variables • top and W polarisation, CP violation test • top pairs: spin correlations, charge asymmetry Search for tt resonances M. Cristinziani - Top Quarks at the LHC 24/50

Relation Higgs-W-top mass radiative corrections t W W ATLAS direct search b Δm. W ~ mt 2 H W W Δm. W ~ ln (m. H) M. Cristinziani - Top Quarks at the LHC 25/50

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 t t (signal) convoluted with resolution • matrix element method t t - event probability as a function of LO matrix element b-tag → uncertainties: jet energy scale and QCD radiation b-tag t Indirect methods (some new tat LHC) • dependence of top mass on b-tag t - cross section t - kin. observables: B hadron decay length, lepton p. T, J/ψ+lepton from W b-tag → can profit from large data samples M. Cristinziani - Top Quarks at the LHC 26/50

Top mass measurement in ℓ+jets CMS: ideogram method • • uses kin fit to get mt and reco for m. W JES correction evaluated from the method validated/calibrated using MC mt = 173. 49 ± 0. 43(stat+JES)± 0. 98 (syst) Ge. V ATLAS: template fit • • uses reco mt and m. W as input to fit JSF impact evaluated in situ from m. W method validated using MC mt = 174. 5± 0. 6(stat)± 2. 3(syst) Ge. V M. Cristinziani - Top Quarks at the LHC 27/50

Top mass in the dilepton channel Dilepton channel is kinematically underconstrained • 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 28/50

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 mt (Tevatron) = 173. 18 ± 0. 56 ± 0. 75 Ge. V mt (LHC) = 173. 3 ± 0. 5 ± 1. 3 Ge. V mt (CMS, new) = 173. 36± 0. 38 ± 0. 91 Ge. V M. Cristinziani - Top Quarks at the LHC PAS TOP-11 -018 29/50

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 Ge. V: additional antiquarks contribute to mexp If Γt < 1 Ge. V: T-hadrons Trying to address the problem • extract the pole mass from cross-section, but • study top mass kinematic dependence (CMS) • use differential tt+1 j distribution Aioli, Fuster, Irles, Moch, Uwer, Vos 2012 M. Cristinziani - Top Quarks at the LHC 30/50

Dependence of top mass on event kinematics PAS TOP-12 -029 color reconnection ISR/FSR b-quark • top mass measurement binned in kinematic observables • within current precision no mismodeling effect due to - color reconnection, ISR/FSR, b-quark kinematics M. Cristinziani - Top Quarks at the LHC 31/50

Test of CPT invariance in top mass PAS TOP-12 -031 CPT invariance predicts m(particle) = m(anti-particle) • well tested so far • could be violated for top quarks (e. g. Cembranos et al. ) Analysis • lepton+jets channel, 8 Te. V (19/fb) updated from 7 Te. V 5/fb • use ideogram method M. Cristinziani - Top Quarks at the LHC 32/50

Intrinsic properties: top quark charge CONF-2011 -141 top isospin partner of b-quark • in SM expect to have q = +2/3 e • can check in data against exotic hypothesis of q = -4/3 e Analysis • identify charge of W (lepton) • charge of b - semileptonic decays (soft lepton, use p. Trel) - track charge weighting • correctly match W and b j is b-jet axis k = 0. 5 optimal Result • can exclude exotic hypothesis at >5σ M. Cristinziani - Top Quarks at the LHC 33/50

Branching fractions: t → Wb PAS TOP-12 -035 Evaluate Rb =BF(t→Wb)/BF(t→Wq) • SM expectation: Rb = |Vtb|2 = 0. 99829± 0. 00009 • can probe new physics (such as 4 th gen. , H+) Measured in dilepton channel • model dependence of measured #b-tags on Rb from data - fraction of tt and t in sample, correct jet assignment, ε(b), ε(q) M. Cristinziani - Top Quarks at the LHC 34/50

Flavor-changing neutral currents (FCNC) Top quark decays in the standard model BR ~100% in SM GIM mechanism BR ~10 -14 in SM FCNC appears in BSM models M. Cristinziani - Top Quarks at the LHC 35/50

Search for FCNC in ATLAS Search for FCNC t→u. Z and t→c. Z • use tt decays tt → q. Z b. W → qll blν • identify three leptons • background estimation - based on number of fake leptons • BF(t→q. Z) < 7. 3 · 10 -3 Search for FCNC t→ug and 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 36/50

Plan for today Top quark production • inclusive and differential tt cross-section • associated production: tt g, tt 0, tt bb, tt γ, tt Z, tt H, tt E Tmiss • single top Properties • intrinsic: mass, charge • decay: Rb, FCNC Angular variables • top and W polarisation, CP violation test • top pairs: spin correlations, charge asymmetry Search for tt resonances M. Cristinziani - Top Quarks at the LHC 37/50

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 cos θlepton Spin analysing power αl = 1 - background - positive and negative polarisation fractions Result Top quark fraction with positive polarisation muons M. Cristinziani - Top Quarks at the LHC CONF-2012 -133 38/50

W helicity in top quark decays Precisely predicted in SM • right handed helicity FR suppressed (V-A) • measurements based on angle ϑ*(l, –b) in W rest frame BSM contributions to Wtb vertex can modify helicity fractions In effective operator framework JHEP 06 (2012) 088 PAS TOP-12 -020 Measured in tt (single and dilepton) and also in single top M. Cristinziani - Top Quarks at the LHC 39/50

Search for CP violation in single top decays Probe Wtb using complementary angles • ϑ★ not sensitive to complex phase • top quark polarised, use cos ϑN • define asymmetry W direction top polarisation Aguilar-Saavedra, Bernabéu 2010 Single top selection • subtract bckgnd, unfold to parton level lepton CONF-2013 -032 for P=0. 9 → -0. 20 < Im (g. R) < 0. 30 at 95%CL M. Cristinziani - Top Quarks at the LHC 40/50

Spin correlations Decay of top quark pairs • top quarks not polarised • spins are correlated • τ(top) small→ polarisation can be probed spin-asymmetry tt 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 41/50

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 (f. SM) • fraction w/o correlation (1–f. SM) Simultaneous fit to ee, µµ, eµ 0. 27 (syst. ) • f. SM = 1. 30 ± 0. 14 (stat. ) +- 0. 22 • Null-hypothesis excluded at 5. 1σ • Aexp = 0. 40 ± 0. 09 (ASM = 0. 32) MC@NLO √s = 7 Te. V no cut on mtt Phys. Rev. Lett. 108 (2012) 212001 First observation of tt spin correlations at LHC M. Cristinziani - Top Quarks at the LHC 42/50

Forward-backward asymmetry AFB NLO: Asymmetry in pp →tt production • through interference • QCD NLO AFB = 5% • with EW corrections AFB = 9% Born ISR forward-backward asymmetry Box FSR Exchange of new particles AFB up to 30% Tevatron results: AFB = (17± 4)% AFB = (30± 7)% for m(tt )>450 Ge. V (CDF) M. Cristinziani - Top Quarks at the LHC 43/50

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 44/50

Asymmetry at the LHC Initial state is symmetric • small qq contribution • only valence quarks forward-backward center-forward asymmetry? asymmetry AC(lept) = (2. 3± 1. 2 stat± 0. 8 syst) % SM prediction: (0. 4± 0. 1)% lepton+jets AC(tt ) = (5. 7± 2. 4 stat± 1. 5 syst) % SM prediction: (0. 6± 0. 2)% dilepton M. Cristinziani - Top Quarks at the LHC 45/50

Plan for today Top quark production • inclusive and differential tt cross-section • associated production: tt g, tt 0, tt bb, tt γ, tt Z, tt H, tt E Tmiss • single top Properties • intrinsic: mass, charge • decay: Rb, FCNC Angular variables • top and W polarisation, CP violation test • top pairs: spin correlations, charge asymmetry Search for tt resonances M. Cristinziani - Top Quarks at the LHC 46/50

Search for tt resonances Motivation • top couples strongly with new massive particles in many BSM scenarios→ possible resonance m(tt ) distribution Benchmarks • narrow resonance: top-color Z’ with Γ(Z’)/m(Z’)=1. 2% • wide resonance: Kaluza-Klein gluon (g. KK) Boosted top quarks • for large m(tt ) the top quarks get a forward boost • decay products (jets, leptons) overlap • new reconstruction techniques required - jet sub-structure - less well-isolated leptons m(tt ) = 2. 5 Te. V M. Cristinziani - Top Quarks at the LHC 47/50

Search for tt 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 Te. V @ 95%C. L. m(g. KK) > 1. 9 Te. V @ 95%C. L. M. Cristinziani - Top Quarks at the LHC 48/50

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 Te. V 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 49/50