Frédéric Derue Laboratoire de Physique Nucléaire et de Hautes Energies ( LPNHE) Paris on behalf of the ATLAS Collaboration

Transcription

1 Latest ATLAS measurements on top quark properties (excluding single top) Frédéric Derue Laboratoire de Physique Nucléaire et de Hautes Energies ( LPNHE) Paris on behalf of the ATLAS Collaboration 10th International Workshop on Top Quark Physics 17th-22nd September, Braga, Portugal 1

2 Motivations to study top quark properties Motivation heaviest fundamental particle in the Standard Model (SM) large Higgs coupling special role in ElectroWeak Symmetry Breaking mechanism? could be a place to see physics Beyond Standard Model (BSM) very short lifetime ~ s decays before hadronization ~ s possibility to study bare quark Why measuring top quark properties top quark mass is a fundamental parameter of SM precise measurement needed for checking consistency of the SM other properties of the top quark (electroweak coupling, production asymmetries,...) are predicted by SM precise measurements could reveal the SM breaking down 2

3 What can be measured? and what will be covered in this talk. Properties mass charge spin correlation charge asymmetry width Talks by E. Shabalina on ttv and N. Asbah and P. Vischia on tth Production Rate pair production cross section single (EWK) production, Vtb FCNC, anomalous couplings differential cross sections production mechanism (gg,qq) New Physics in production resonant production? heavy quark production?... Talk by R. Goldouzian on properties with single top Talk by B. Pearson on mass Decay branching ratios W helicity CP asymmetries rare decays, FCNC non-sm decays Talk by N. Castro on anomalous couplings Other colour flow Talk by F. Wilk on colour flow Posters by R. Torres and M. Ghneimat on ATLAS top properties 3

4 Spin measurement in top quark events Motivation ttt produced almost unpolarized in SM. Spins of t and tt are correlated top quark decays before any spin flip can take place Strategy dilepton ttt events, 2 jets, b-tag, reconstruction via neutrino weighting technique measure the angular distribution of the decay products JHEP03 (2017) 113 define 6 angles cos ab +- : 2 lepton directions, 3 spin quantization axes (a,b) helicity axis k : top quark direction in ttt rest frame transverse axis n : to production plane r-axis r : to k and n 15 top quark spin observables sensitive to a different coefficient of the ttt spin density matrix full spin matrix measured for the 1st time 4

5 Spin measurement in top quark events Results ab a b unfold distributions of cos cos cos ++ relate mean values of observables to polarisation and spin correlation top polarisation Bk+ = 3 < cos θk+ >,... value ~0 precision δb~3-4% Main systematic uncertainties : signal modelling spin correlation C(k, n) = 9 <cos θ k. cos θ n>,... + precision δc/c~20-30% C(n,n) along transverse axis at 5.1 from zero JHEP03 (2017) 113 results are in agreement with expectations from SM 5

6 Motivation Charge asymmetry at LHC ttt production has a small asymmetry under charge conjugation in SM BSM could lead to an enhancement in this effect definition of asymmetry observables Analysis of dilepton ttt events PRD94 (2016) inclusive measurements on parton/particle level in full/fiducial space differential measurements in pt, mass and z of ttt pairs in full/fiducial space results are in agreement with expectations from SM 6

7 Charge asymmetry Analysis of lepton+jets ttt events EPJ C76 (2016) 87 + EPJ C77 (2017) 564 inclusive measurements on parton/particle level in full/fiducial space differential measurements in pt, mass and z of ttt pairs in full/fiducial space ACtt = (stat.+syst.) can test BSM models ATLAS+CMS combination detailed in talk by A. Lister results are in agreement with expectations from SM 7

8 Top quark width Motivation arxiv: direct measurement, model independent Strategy select lepton+jets ttt events, jets, 1 b-tag reconstruction using Kinematic Likelihood Fitter KLFitter NIM A748 (2014) regions: (e/μ) x (=1b, 2b) x (4 jets with η 1, at least 1 jet with η >1) template fit leptonic : mlb hadronic : ΔRmin(b,q) (b-jet - closest light jet) Result top= (stat)+0.79 (syst) GeV assuming mtop=172.5 GeV consistent with top(sm)~1.32 GeV Main systematic uncertainties : Jet energy scale (JES), Jet energy resolution (JER), signal modelling (ME generator) first direct measurement in ATLAS (rel. prec. 50%) See poster by R. Torres 8

9 W helicity measurement in top quark events Motivation test V-A structure via angular observables : W helicity fractions W bosons from top quark decays longitudinal, left or right-handed polarisation Strategy NNLO QCD calculation F0 = ± FL = ± FR= ± PRD (2010) measure the angular distribution of the decay products θ angle between down-type fermion (charged lepton or down-type quark) from W and reversed direction of top quark, both in the W boson rest frame select lepton+jets ttt events, b-tag use leptonic and hadronic W decay modes leptonic: [e+μ] x [ 2b] most sensitive mode hadronic: [e+μ] x [1b + 2b] ttt reconstruction using KLFitter helicity fractions extracted using template fit Eur. Phys. J. C 77 (2017) 264 9

10 W helicity measurement in top quark events Result on helicity fractions Eur. Phys. J. C 77 (2017) 264 Main systematic uncertainties : JES, JER, signal modelling (PDF, ME generator, ISR/FSR...) public ATLAS page results in agreement with expectations from SM 10

11 W helicity measurement in top quark events Constraints on the Wtb vertex more details in talk by N. Castro on anomalous couplings interpret measurements by considering general expression for the Wtb vertex Eur. Phys. J. C 77 (2017)

12 CP violation in B hadron decays Motivation CPV expected in b-quark decays DØ single/dimuon asymmetry at tension with SM, not yet excluded b c PRD 89 (2014) A dir 0.3% A dir 1% Strategy select lepton+jets ttt event, 4 jets, 1 b-tag one jet being b-tagged by standard algo and by presence of a soft muon use the lepton (from W) the charge of this lepton gives the b-quark charge at production use of soft muon b-tagging the charge of the muon in the b-jet probes the decay chain reconstruction of events using KLFitter kinematic unfold data to a fiducial volume measure charge asymmetries JHEP02 (2017)

13 CP violation in B hadron decays Results from charge asymmetries deduce the CPV ones JHEP02 (2017) 071 competitive direct CP limits O(1%) precision reached (stat. limited), same order as the precision of the DØ anomaly effect first measurement of B properties in top quark decays 13

14 Colour flow in ttt events Motivation more details in talk by F. Wilk on colour flow colour connections between hadrons in jets affect the energy distributions inside and between jets if well understood, they can be exploited to aid SM measurements and BSM searches techniques to exploit colour connections can be tested using jets with an expected colour structure e.g. jets from a W boson in top quark decays Method the jet pull vector predicted to contain information about the colour representation of a dijet system measure quantities derived from the jet pull vector pt-weighted radial jet moment magnitude angle ATLAS-CONF

15 Colour flow in ttt events Strategy select lepton+jets ttt event, 4 jets, 2 b-tag define jets originating from hadronically decaying W boson and t Wb vertices NE W! jet constituents : (jet-associated) tracks, previous measurement was w/ calo clusters ATLAS-CONF

16 Colour flow in ttt events! Results for signal colour flow NE W angle : predictions suggest stronger effect than observation more sloped pull angle magnitude : data favours larger value «wider» jets ATLAS-CONF Main systematic uncertainties : signal modelling statistical uncertainty sub-dominant mostly poor modelling from MC Powheg+Herwig7 generally does a good job 16

17 Colour flow in ttt events Result for spurious colour flow as expected, very flat distribution Main systematic uncertainties : signal modelling statistical uncertainty sub-dominant NE W! ATLAS-CONF Powheg+Herwig7 models observation poorly Other predictions offer much better description 17

18 Colour flow in ttt events Results and comparison with other models ATLAS-CONF NE W replace Whad by ad-hoc colour-octet! comparison to maximally violating exotic ( colour-flipped ) colour flow model : Predicted colour flow shows significantly less sloped pull angle and prefers smaller pull magnitude Goodness-of-fit tests show data prefers SM-like distribution over exotic... but Powheg+Pythia8 does not provide a good description either 18

19 Conclusion All measurements of top quark properties are in good agreement with SM predictions most of the measurements: systematically limited (experimental uncertainty ~ modelling uncertainty) largest experimental uncertainties stem from the calibration of the jet and b-jet energy scales largest modelling uncertainties originate from the parton shower and hadronisation models SM measurements (rel. precisions achieved in ATLAS) spin observables (full spin matrix, 15 observables, 3-30%) charge asymmetries (~0.5 % in absolute) top quark width (first direct measurement ~50%) precise W-helicity measurement (2-5%) CP violation in B hadrons from top quark decays (~0.5 % in absolute) colour flow in ttt events All ATLAS results on top quark physics can be found in: 19

20 Backup slides 20

21 The ATLAS detector Muon spectrometer ( η <2.7) : air-cores toroids with gas-based chambers. Trigger and measurement. Momentum resolution <10% up to Eμ~1 TeV Inner Detector ( η <2.5) : Si pixel, SCT, TRT Tracking and vertexing. e/ separation (pt)/pt~0.038% pt (GeV) 1.5% EM calorimeter ( η <3.2) : Pb/LAr accordion Trigger and e/ reco and id (E)/E~10%/ E (GeV) 0.7% Computing : within WLCG HAD calorimeter ( η <5) : Fe/scintillator tiles (central), Cu/W LAr (fwd), Trigger, jets and Etmiss. (E)/E~50%/ E (GeV) 3% Trigger : L1 : hardware, L2-EF, ~1kHz in output 21

22 ttt decay modes General distinction according to the W± pairs decay modes high background All hadronic 45.7% e± νe + jets μ± νμ+ jets % % τ ντ + jets e± τ± ντνe μ± τ± ντνμ τ+ τ ντ ντ % 2.42 % 2.23 % 1.26 % e+ e ντνμ μ+ μ ντνμ e± μ± ντνμ 1.15 % 1.12 % 2.27 % ± Top pair decay channels } lepton+jets Plot from Angela Barbaro Galtieri et al Rep. Prog. Phys } leptonic or hadronic decay modes } Clean in terms of trigger and selection Presence of two 's. Transverse mass. dilepton 22

23 CP violation in B hadron decays Results from charge asymmetries deduce the CPV ones JHEP02 (2017) 071 competitive direct CP limits O(1%) precision reached (stat. limited), same order as the precision of the DØ anomaly effect first measurement of B properties in top quark decays 23