Top quark at LHC. M. Villa. Bologna, oggi

Transcription

1 Top quark at LHC M. Villa Bologna, oggi 1

2 Outline Top quark characteristics Top production and decay Measurements 2

3 Outline Top quark characteristics Top production and decay Measurements 3

4 Top quark I.D. card Topness +1 Electric Charge Mass +2/3e 171.2±2.1 GeV I(J P ) 0(½ + ) Decays Wb ~100% Discovered

5 Outline Top quark characteristics Top production and decay Top pairs production Single top production Measurements 5

6 Production of top quarks At the LHC, top quarks will be produced in pairs ( t t ) and isolated (single top) t-tbar are mainly (~90%) produced via gluon-gluon fusion Production 7TeV: (8% theoretical error) tt 165 pb NLO 6

7 Single top production Hadronic production: t-tbar pairs σtt ~ 165 7TeV Electroweak production: single top σt ~ 60 7TeV 7

8 Decay channels Almost 99% of top quarks decay to Wb W l ± ν 33% ; W qq 66% 8

9 Backgrounds Channel Topology Backgrounds Di-leptonic 2 b-jets + 2l ± + ET miss WW+jets, Z+jets Semi-leptonic 2 jets + 2 b-jets + l ± + ET miss W + jets All-hadronic 4 jets + 2 b-jets QCD Multi-jet QCD W+jets WW+jets 9

10 Outline Top quark characteristics Top production and decay Measurements Reconstruction Production cross-section Mass Spin alignment; Wtb anomalous coupling Asymmetries FCNC 10

11 It s all about the Lumi Different physical processes have different probabilities (crosssection σ) The number of events for each process is given by: More luminosity, more events! We aim to measure: 11

12 LHC start-up programme 45 pb -1 Integrated luminosity 1 fb 1 3 pb pb Look for new physics in ATLAS at 14 TeV Understand SM+ATLAS in complex topologies Higgs/SUSY 1 Understand SM+ATLAS in simple topologies Top quark pairs 0 Understand ATLAS Testbeam/cosmics W/Z LHC startup Time 12

13 Reconstruction Goal: assign ( = sum 4-vectors) the correct final state objects to the decayed top quark b-tagging will be very useful to reduce combinatorial background and to improve S/N ratio. One can reconstruct first the W bosons and then sum the b-jet, or reconstruct the top quark and check if there is a W boson in the triplet 13

14 Top event selections 14

15 ATLAS t-tbar candidates 15

16 Simulation of HEP events W Emission of coloured partons is simulated by parton shower This is implemented in HERWIG, PYTHIA and other programs Jet = stream of particles + algorithm e/μ/τ ν jet jet +b tag jet jet Parton Shower 16

17 Hadronic W, Hadronic t - MC 2 hadronic jets + 1 b-jet 2 hadronic jets 17

18 single lepton channels 18

19 Outline Top quark characteristics Top production and decay Measurements Reconstruction Production cross-section Mass Spin alignment; Wtb anomalous coupling Asymmetries FCNC 19

20 Data invariant masses 37 ttbar candidates 20

21 t-tbar Cross section Results are in good agreement with the theory and fits nicely with previous data Currently, the uncertainties remain large to constrain the theory but : impressive results after few tens of pb-1 (the current LHC regime is ~10pb-1 / 3 days of run) methods are ready and work well there is still room for improvement (syst., luminosity uncert., combination,...) 21

22 Single LHC t channel Two analyses A 2D template fit to cos(θ lj) and η lj A multivariate technique (BDT) Both analyses: significance > 3 Combination Boosted Decision Tree Output top = 83.6 ±29.8 (stat+syst) ±3.3 (lum) pb 22

23 Wt-channel: cut-based Single LHC l+jets and dilepton combine channels, expect σ Wt < 94 pb new channel σ Wt < 158 pb at 95% 6 7 = 13% V tb = 7% 23

24 Outline Top quark characteristics Top production and decay Measurements Reconstruction Production cross-section Mass Spin alignment; Wtb anomalous coupling Asymmetries FCNC 24

25 Top quark Mass Along with reconstruction, one can fit the top mass CDF&D0 introduced several methods based on templates, neural networks, Fisher discriminants, etc... Plain vanilla technique: chi2 minimization Caveat: Jet energy scale In situ methods, Z+ 1 jet Curiosity: top mass has been measured by CDF before the 5σ discovery! 25

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29 Outline Top quark characteristics Top production and decay Measurements Reconstruction Production cross-section Mass Spin alignment; Wtb anomalous coupling Asymmetries FCNC 29

30 Spin correlations and W polarization As the top quark decays before it can form hadronic bound states, a consequence of its high mass, the spin information of the top quark is propagated to its decay products (W+b). The top quark spin can be reconstructed measuring angular distributions of the decay products W-boson polarization states can be measured through the longitudinal, left-handed and righthanded helicity fractions 30

31 W polarization Longitudinal F 0 +½ W t b 0 +½ Left-handed F L +½ -½ t b W +1 Right handed F R +½ W t b +1 -½ 31

32 W polarization Longitudinal F 0 +½ W t b 0 +½ Left-handed F L +½ -½ t b W +1 Right handed F R +½ W t b +1 -½ SM

33 Wtb Anomalous Couplings Within the Standard Model, the Wtb coupling is purely left-handed (CKW matrix element Vtb) g L b VtbPL tw cc 2 Departures from the Standard Model expectation and new radiative contributions to the Wtb vertex are possible in BSM models (4 quark generation) V ( 44) tb 33

34 W anomalous coupling F 0 F R F L 34

35 Outline Top quark characteristics Top production and decay Measurements Reconstruction Production cross-section Mass Spin alignment; Wtb anomalous coupling Asymmetries FCNC 35

36 Forward-backward asymmetry A FB In leading order QCD, top production symmetric NLO QCD predicts small asymmetry AFB about 5% New physics can give rise to a larger asymmetry (Z, W. axigluons, technicolours ) Tevatron 36

37 A Tevatron >3 AFB (MC) = 5.0± 1.5% 2.3 AFB(Data) = 42 ±16% 37

38 Charge asymmetry A LHC At Tevatron deviation > 3 s from SM predicted A FB 5% At LHC: Initial state symmetric -> charge asymmetry visible in h t - h tbar Expected asymmetry small A c = (11) [Ferrario et al.] L 1 fb -1 needed to compete with Tevatron A c =( N + N )/(N + + N ) N + = t tbar > 0 A C =0.060 ±0.134(stat.)±0.026(syst.) 38

39 Outline Top quark characteristics Top production and decay Measurements Reconstruction Production cross-section Mass Spin alignment; Wtb anomalous coupling Asymmetries FCNC 39

40 Flavour-Changing Neutral Currents Strongly suppressed in the Standard Model due to the Glashow-Iliopoulos-Maiani (GIM) mechanism Small FCNC contributions are expected at one-loop level, determined by the CKM mixing SUSY, multi-higgs and other models allow FCNC at tree level BR( t qz) BR(t qz) CDF t c(u)+z/ LEP ATLAS (1 fb -1 ) 95% C.L. EXCLUDED REGIONS ZEUS (q=u only) CDF (2 fb -1 ) ZEUS (q=u only) (630 pb -1 ) BR(t qγ) BR( t q ) 40

41 41 FCNC top decay Not enough luminosity yet

42 FCNC top production 42 Not enough luminosity yet

43 Conclusions LHC is the first top quark factory Top mass and production cross sections have been already measured at LHC. The rapid luminosity increase will correspond to higher accuracies every few monthes! Deviations from standard model are expected in several theories. LHC (ATLAS/CMS) has the discovery potential to find or disproove the several theories on the market. First upper limits already avaliable. More interesting limits, proving physics BSM are expected for next year. 43

44 Bibliography J.Kuhn, Theory of Top production and decay, hep-ph/ W.Wagner, Top quark physics in hadron collisions, hep-ph/ M.Beneke et al., Top Quark Physics, hep-ph/ ATLAS and CMS notes on CDS on top quark 44

45 Mauro Villa Dipartimento di Fisica

46 tt Resonances The top quark may play a privileged role in the electroweak symmetry breaking (EWSB) mechanism (mt ~ mew breaking ~ 100 GeV) Any new physics connected to the EWSB could be preferentially coupled to the top quark New resonances and gauge bosons strongly coupled to the top quark are expected in a large variety of models These new particles could reveal themselves in the top anti-top quark invariant mass distribution No resonance seen yet 46