Transcript Top Physics
Top physics
Peter Uwer Humboldt Universität Berlin
Why are we interested in top-quarks ?
1) Top-quark = heaviest elementary particle discovered so far
Questions:
Is the top-quark point-like ?
Why is the top-quark so heavy ?
How is the mass generated ? Important testground for theoretical developments Many interesting phenomena/aspects Interesting per se Required for precision
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 2
Why are we interested in top-quarks ?
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 1) + 2) Precise measurements of its properties, search for possible deviations i.e. anomalous couplings Important: precise predictions possible, only “two” input parameters: CKM matrix + top-quark mass
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 3
Why are we interested in top-quarks ?
3) Top-quark mass is an important input parameter of the SM [Heinemeyer, Hollik, Stockinger, Weiglein, Zeune '12] Fundamental parameter, should be known as precise as possible !
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 4
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 g cross section b-quark distribution in decay Top polarisation Charge asymmetry Consistency checks with theo. predictions, new physics in the tt invariant mass spectrum Consistency Standard Model Test of the V-A structure in top decay Measurement of the Yukawa coupling Measurement of the Z couplings Direct measurement of the CKM matrix element Vtb, top polarization, search for anomalous Wtb couplings Weak decay of a `free’ quark, bound on the top width and V tb , search for anomalous couplings Search for anomalous couplings, important background Measurement of the electric charge See talks on Saturday: German Rodrigo and Aurelio Juste Sensitive to new physics ? new physics ? +
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 5
Cross section for top-quark pair production
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 6
Hadronic top-quark pair production
~90% @ Tevatron, 10% @ LHC Partonic cross sections ~10% @ Tevatron, 90% @ LHC
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 7
Theory status: Total cross section
NLO Q C D: [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] Beyond NLO Q C D : Soft gluon resummation Threshold corrections Full scale NNLO (in)dependence [Ahrens, Baernreuther, Beneke, Bonciani, Cacciari, Catani, Czakon, Ferroglia, Kidonakis, Laenen, Mangano, Mitov, Moch, Nason, Neubert, Pecjak, Ridolfi, Schwinn, Sterman, PU, Vogt, Yang…] High energy behaviour NNLO Q C D for qq tt [Baernreuther, Czakon, Mitov ‘12] feasible
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 8
Recent progress: qq
tt @ NNLO/NNLL
[Baernreuther, Czakon, Mitov arXiv:1204.5201] Tevatron: gg tt @ NNLO is underway ~3%
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 9
LHC cross section measurements
[Ignacio Aracena, Moriond 2012] Consistent picture (diff. channels / diff. experiments !) Most precise measurement: Lepton + jets 6.6% rel. uncertainty
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 10
Combination of measurements
All results consistent with SM ATLAS: CMS: 6.2 % 8 %
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 11
Aiming for precision: Beyond NNLO QCD
[Hagiwara, Sumino, Yokoya 08] [Kiyo,Kühn,Moch,Steinhauser,P.U. 08] [Beenakker et al 94, Bernreuther, Fücker, Si 06’, 07] [Kühn, Scharf, P.U 06,07] “Resonance structure” from would be bound state ~1 % shift of total cross section at LHC
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 12
Cross section measurements
Production mechanism seems well understood Experimental goal seems feasible Severe constraint for new physics scenarios Top-quark physics = precision physics Possible applications: Use cross section to constrain `parameters ´ Gluon PDF / Gluon Luminosity Top-quark mass
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 13
The top-quark mass
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 14
Top-quark mass measurements
[Stijn Blyweert, Moriond 2012] Competitive with Tevatron
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 15
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 a 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, 24.05-03.06.2012 | page 16
Different mass definitions
Common schemes: Pole mass scheme MS mass Chose constants minimal to cancel 1/ e poles in Other schemes possible: 1S mass, PS mass,… Schemes defined in perturbation theory conversion possible
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 17
Example:
Conversion between schemes
Pole mass MS mass: Important: Difference can be numerically significant [Chetyrkin,Steinhauser 99] ~10GeV Difference is formally of higher order in coupling constant NLO predictions are required for meaningful measurements
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 18
Bad choices — Good choices
Scheme might be ill defined beyond perturbation theory Example: Renormalon ambiguity in pole mass [Bigi, Shifman, Uraltsev, Vainshtein 94 Beneke, Braun,94 Smith, Willenbrock 97]
“There is no pole in full
Q C
D”
!
L
Q C D
Pole mass has intrinsic uncertainty of order
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 19
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, 24.05-03.06.2012 | page 20
Impact on current measurements
Different channels and different experiments give consistent results Large effects unlikely Possible improvements of current measurements: Template method: Study additional distributions / observables Compare with NLO templates Matrix element method Matrix element method at NLO Alternative measurements ?
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 21
Top quark mass from cross section
Mass scheme well defined, higher orders can be included Drawback: Limited sensitivity to m t
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 22
Alternative observables ?
Compare b-quark mass measurement at LEP using 3-jet rates [Bilenky, Fuster, Rodrigo, Santarmaria] Use tt+1-jet events For details, see Adrian Irles presentation
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 23
Spin correlations in top-quark pair production
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 24
Top-quark spin correlations
Parity invariance of Q C D : [Dharmaratna, Goldstein,’90, Bernreuther, Brandenburg,PU. 95] Top’s produced in qq tt and gg tt are essentially unpolarized But: Spins of top quark and antiquark are correlated [Bernreuther,Brandenburg 93, Mahlon, Parke 96, Stelzer,Willenbrock 96, Bernreuther, Brandenburg, Si, P.U. 04] Quantum mechanics: close to threshold: Spins are parallel (qq) or anti-parallel (gg) close to threshold
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 25
Why are spin correlations interesting ?
You also measured the charge asymmetry….
LHC can improve a lot compared to Tevatron Sensitive test of production and decay, may put severe constrains on new physics scenarios
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 26
Spin correlations: How to measure it
Basic ingredients: Top quark decays before hadronization Parity violating decay
t
Wb
f
Polarisation can be studied through the angular distribution of the decay products!
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 27
Spin correlations
[Parke, Mahlon ‘10] Study (azimuthal) opening angle distribution of leptons in dilepton events LHC: gg dominates Ansatz:
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 28
LHC measurement
[arXiv:1203.4081] Observation of spin-correlations (5.1
s)
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 29
Constraining new physics
[Fujfer, Kamenik, Melic, arXiv1205.0264] NLO corrections are known and found to be small
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 30
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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 31
Thank you for your attention !
Forward-Backward Charge Asymmetry in tt+1Jet
[Dittmaier, PU, Weinzierl PRL 98:262002, ’07]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 33
Charge Asymmetry: Dependence on P
t
(tt)
[Kühn, Top-quark workshop, Berlin 2012]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 34
Non-perturbative corrections
Top-quark is a colour triplet [Skands,Wicke ‘08] non-perturbative effects in the reconstruction of the top momentum from colour singlet's different modeling of non perturbative physics / colour reconnection Non-perturbative effects could result in uncertainty of the order of 500 MeV offset from generated mass blue: pt-ordered PS green: virtuality ordered PS
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 35
Compare
Top-quark charge asymmetry
[Berends, Gaemers, Gastmans ´73, Berends, Kleiss, Jadach, Was ´83] – + + – + – + ─ + – [Kühn] 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]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 36
Charge asymmetry: Theory predictions
[Kühn, Rodrigo ´11] QCD+EW QCD QCD+EW Soft gluon resummation Coherent picture of theoretical predictions, Theoretical uncertainties based on scale variations, possibly underestimates higher order effects (ratios!)
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 37
Tevatron results
[Bernreuther, Si ’12] At most 2.4 s deviation [1] CDF, arXiv:1101.0034, [2] D0, arXiv:1107.4995, [7] CDF note 10807
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 38
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, 24.05-03.06.2012 | page 39
Charge asymmetry at LHC
top anti-top
y
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, 24.05-03.06.2012 | page 40
CMS results
[CMS-PAS-Top-11-030]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 41
Inclusive: Theory (MC@NLO):
ATLAS results
[arXiv 1203.4211]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 42
New physics scenarios
inclusive [arXiv 1203.4211] “Z´, W’ disfavoured, some tension”
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 43
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: Improve current measurements Look into observables which can be measured at LHC and Tevatron [Aguilar Saavedra, Juste ‘12]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 44