Top physics Peter Uwer Humboldt-Universität Berlin

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 2 Why are we interested in top-quarks ? 1) Top-quark = heaviest elementary particle discovered so far  Is the top-quark point-like ?  Why is the top-quark so heavy ?  How is the mass generated ? Questions: Important testground for theoretical developments Many interesting phenomena/aspects Interesting per seRequired for precision

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 3 Why are we interested in top-quarks ? Precise measurements of its properties, search for possible deviations i.e. anomalous couplings 2) Top-quarks ─ a sensitive tool to explore the electroweak symmetry breaking  Top-quark plays special role in many extensions of the Standard Model, ideal tool to search for new physics Important: precise predictions possible, only “two” input parameters: CKM matrix + top-quark mass 1) + 2)

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 4 Why are we interested in top-quarks ? 3) Top-quark mass is an important input parameter of the SM Fundamental parameter, should be known as precise as possible ! [Heinemeyer, Hollik, Stockinger, Weiglein, Zeune '12]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 5 Important measurements  Cross section for pair production  Top quark mass measurement  W-Polarisation in top decay  ttH cross section  ttZ cross section  Single top production  Spin correlations  tt+Jet(s) production  tt  cross section  b-quark distribution in decay  Top polarisation  Charge asymmetry Measurement of the electric charge Search for anomalous couplings, important background Weak decay of a `free’ quark, bound on the top width and V tb, search for anomalous couplings Direct measurement of the CKM matrix element Vtb, top polarization, search for anomalous Wtb couplings Measurement of the Yukawa coupling Consistency checks with theo. predictions, new physics in the tt invariant mass spectrum Test of the V-A structure in top decay Measurement of the Z couplings Sensitive to new physics t  bH + Sensitive to new physics ? new physics ? Consistency Standard Model See talks on Saturday: German Rodrigo and Aurelio Juste

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 6 Cross section for top-quark pair production

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 7 Hadronic top-quark pair production Partonic cross sections Tevatron, LHC Tevatron, LHC

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 8 Theory status: Total cross section  NLO QCD:  Beyond NLO QCD:  Soft gluon resummation  Threshold corrections  Full scale NNLO (in)dependence  High energy behaviour [Ahrens, Baernreuther, Beneke, Bonciani, Cacciari, Catani, Czakon, Ferroglia, Kidonakis, Laenen, Mangano, Mitov, Moch, Nason, Neubert, Pecjak, Ridolfi, Schwinn, Sterman, PU, Vogt, Yang…] feasible [Dawson, Ellis, Nason ’89, Beenakker et al ’89,’91,Bernreuther, Brandenburg, Si, PU ’04, Czakon,Mitov 08] [Moch, PU 08, Cacciari, Frixone, Mangano, Nason Ridolfi 08, Kidonakis Vogt 08] [Baernreuther, Czakon, Mitov ‘12] NNLO approx  NNLO QCD for qq  tt

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 9 Recent progress: qq  NNLO/NNLL Tevatron: [Baernreuther, Czakon, Mitov arXiv: ] gg  NNLO is underway ~3%

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 10 LHC cross section measurements [Ignacio Aracena, Moriond 2012] Consistent picture Most precise measurement: Lepton + jets  6.6% rel. uncertainty (diff. channels / diff. experiments !)

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 11 Combination of measurements ATLAS: CMS: All results consistent with SM  6.2 %  8 %

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 12 Aiming for precision: Beyond NNLO QCD “Resonance structure” from would be bound state ~1 % shift of total cross section at LHC [Kiyo,Kühn,Moch,Steinhauser,P.U. 08] [Hagiwara, Sumino, Yokoya 08] [Kühn, Scharf, P.U 06,07] [Beenakker et al 94, Bernreuther, Fücker, Si 06’, 07]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 13 Cross section measurements  Production mechanism seems well understood Possible applications:  Gluon PDF / Gluon Luminosity  Top-quark mass  Experimental goalseems feasible Use cross section to constrain `parameters´  Severe constraint for new physics scenarios Top-quark physics = precision physics

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 14 The top-quark mass

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 15 Top-quark mass measurements [Stijn Blyweert, Moriond 2012] Competitive with Tevatron

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 16 Some basic facts about theory parameters …and their determination. Top-quarks don’t appear as asymptotic states (no free quarks due to confinement) Top-quark mass is “just” a parameter like  s, only defined in a specific theory/model i.e. SM  renormalisation scheme dependent, only indirect determination possible through comparison (fit): theory   experiment Parameter determination relies on theory, scheme dependence encoded in theor. predictions

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 17 Different mass definitions  Pole mass scheme  MS mass Chose constants minimal to cancel 1/  poles in Schemes defined in perturbation theory  conversion possible Common schemes: Other schemes possible: 1S mass, PS mass,…

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 18 Conversion between schemes Pole mass   MS mass: Important:  Difference can be numerically significant [Chetyrkin,Steinhauser 99] Example:  Difference is formally of higher order in coupling constant ~10GeV NLO predictions are required for meaningful measurements

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 19 Bad choices — Good choices  Scheme might be ill defined beyond perturbation theory Example: Renormalon ambiguity in pole mass Pole mass has intrinsic uncertainty of order  QCD “There is no pole in full QCD” ! ! [Bigi, Shifman, Uraltsev, Vainshtein 94 Beneke, Braun,94 Smith, Willenbrock 97]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 20 Template method & kinematic reconstruction Present measurements:  Distribution: invariant masse of top quark decay products  Rely mostly on parton shower predictions  No NLO so far available (?) Main issues:  Corrections due to color reconnection / non perturbative physics (  momentum reconstruction of color triplet…)  Precise mass definition ? How important ?

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 21 Impact on current measurements Different channels and different experiments give consistent results Large effects unlikely  Study additional distributions / observables  Compare with NLO templates Possible improvements of current measurements: Template method:  Matrix element method at NLO Matrix element method Alternative measurements ?

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 22 Top quark mass from cross section Drawback: Limited sensitivity to m t Mass scheme well defined, higher orders can be included

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 23 Alternative observables ? Compare b-quark mass measurement at LEP using 3-jet rates [Bilenky, Fuster, Rodrigo, Santarmaria] For details, see Adrian Irles presentation Use tt+1-jet events First measurement of the Running b-quark mass at high scale

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 24 Spin correlations in top-quark pair production

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 25 Top-quark spin correlations Quantum mechanics: close to threshold:  Spins are parallel (qq) or anti-parallel (gg) close to threshold Parity invariance of QCD: But: Spins of top quark and antiquark are correlated [Bernreuther,Brandenburg 93, Mahlon, Parke 96, Stelzer,Willenbrock 96, Bernreuther, Brandenburg, Si, P.U. 04] [Dharmaratna, Goldstein,’90, Bernreuther, Brandenburg,PU. 95] Top’s produced in qq  tt and gg  tt are essentially unpolarized

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 26 Why are spin correlations interesting ?  You also measured the charge asymmetry….  Sensitive test of production and decay, may put severe constrains on new physics scenarios  LHC can improve a lot compared to Tevatron

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 27 Spin correlations: How to measure it Basic ingredients:  Top quark decays before hadronization  Parity violating decay t  Wb Polarisation can be studied through the angular distribution of the decay products! f

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 28 Spin correlations Study (azimuthal) opening angle distribution of leptons in dilepton events gg dominates [Parke, Mahlon ‘10] LHC: Ansatz:

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 29 LHC measurement Observation of spin-correlations (5.1  [arXiv: ]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 30 Constraining new physics [Fujfer, Kamenik, Melic, arXiv ] NLO corrections are known and found to be small

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 31 Summary  Tremendous progress in the recent past  Top-quark physics is now precision physics  Already after one year: LHC is competitive or even better than Tevatron  Ideal laboratory to search for new physics

Thank you for your attention !

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 33 Forward-Backward Charge Asymmetry in tt+1Jet [Dittmaier, PU, Weinzierl PRL 98:262002, ’07]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 34 Charge Asymmetry: Dependence on P t (tt) [Kühn, Top-quark workshop, Berlin 2012]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 35 Non-perturbative corrections Top-quark is a colour triplet  non-perturbative effects in the reconstruction of the top momentum from colour singlet's [Skands,Wicke ‘08] blue: pt-ordered PS green: virtuality ordered PS offset from generated mass different modeling of non- perturbative physics / colour reconnection Non-perturbative effects could result in uncertainty of the order of 500 MeV

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 36 Top-quark charge asymmetry Compare + + – + – + – + – + – + – - + – + – + – + – + – + – ─ Similar effect: Charge asymmetry SM: [Kühn, Rodrigo ´98,´07,´12, Almeida, Sterman, Vogelsang 08, Bernreuther, Si ´10, Hollik, Pagani ´11 Ahrens, Ferroglia,Neubert,Pecjak, Yang ´11] [Berends, Gaemers, Gastmans ´73, Berends, Kleiss, Jadach, Was ´83] [Kühn]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 37 Charge asymmetry: Theory predictions [Kühn, Rodrigo ´11]  Coherent picture of theoretical predictions, Theoretical uncertainties based on scale variations, possibly underestimates higher order effects (ratios!) Soft gluon resummation QCD+EW QCD

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 38 Tevatron results [Bernreuther, Si ’12] [1] CDF, arXiv: , [2] D0, arXiv: , [7] CDF note At most 2.4  deviation

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 39 Charge asymmetry at LHC  No forward-backward asymmetry since pp is P symmetric However:  t tend to follow initial q, while tb tend to follow initial qb  initial state is not symmetric with respect to q,qb  q tend to be more energetic should be broader w.r.t

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 40 Charge asymmetry at LHC y top anti-top Effect expected to be small since qq makes only a small fraction, more important for larger m tt (Additional cuts may enhance asymmetry)

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 41 CMS results [CMS-PAS-Top ]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 42 ATLAS results Inclusive: Theory [arXiv ]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 43 New physics scenarios [arXiv ] inclusive “Z´, W’ disfavoured, some tension”

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, | page 44 Final remarks on asymmetry  Discrepancy has reduced with new CDF measurement  Theory is only LO, in ttj where also NLO is known, large higher-order corrections observed  Charge asymmetry very sensitive to P t (tt)  LHC uncertainties are still large No conclusive picture yet Future: Look into observables which can be measured at LHC and Tevatron [Aguilar Saavedra, Juste ‘12] Improve current measurements