Top quark properties in ATLAS Ruth Laura Sandbach X-SILAFAE-2014, Medellin, Colombia 27/11/2014 X-SILAFAE

Introduction Top quark: Heaviest known elementary particle Shortest lifetime of any quark ( ) x s Can be studied as bare quarks (before hadronisation) Study of top- and antitop-quarks provide opportunities both to test the standard model and probe new physics beyond the standard model (BSM) Various properties of the top quark have been measured since its discovery at the Tevatron in 1995: Most precise quantity measured is the top quark mass Spin information can be deduced from the angular distributions of decay particles Top-anti-top production charge asymmetry, an important test of QCD at high energies 27/11/2014 X-SILAFAE top publications from Run-I since 2011 We will focus on latest results from Top14 from ATLAS Frédéric Déliot

Top production at the LHC  LHC collides protons (pp) at √s = 7 TeV (2011) and √s = 8 TeV (2012), top quarks predominantly produced in pairs via gluon gluon fusion  Top decays almost entirely to a W boson and a b-jet ( ), so analyses are divided into final state-dependent categories: single lepton (lepton+jets), dilepton and fully hadronic Single lepton Dilepton 27/11/2014 X-SILAFAE Gluon-gluon fusion Typical event Selection:  Single lepton:  At least four jets, two b-tagged  Exactly one lepton  Large missing transverse momentum  Dilepton  At least two jets, two b-tagged  2 leptons  Large missing transverse momentum

Measurements of the Top-quark Mass 27/11/2014 X-SILAFAE Latest summary plots for m top ATLAS-CONF  Sensitive to the top-quark mass since gluon radiation depends on the mass of the quarks  The mass is extracted from a measurement of the normalized differential cross section of top pair production in association with one jet, as a function of the inverse mass of the system, using: Invariant mass of system  Reconstruction of the top-antitop system  Unfold the cross section distribution (using SVD)  Compare the distribution with the NLO+PS distribution (Powheg) at different mass using a χ 2

Measurements of the Top-quark Mass arXiv: /11/2014 X-SILAFAE  Template fit to the ratio of 3-jet mass (top candidates) to 2-jet mass (W candidates)  Top mass obtained from fit is less sensitive to uncertainty in the energy measurement of the jets  Binned likelihood top mass of Event selection Hadronic Final State

Measurements of the Top-quark Mass arXiv: Mass extraction from the comparison between the theoretical and measured σ tt  theoretical calculation using a well defined top mass scheme (pole mass)  almost no dependency on the top mass in the measured cross section: almost no dependency on the mass definition in the MC 27/11/2014 X-SILAFAE Determination from top-antitop cross section

Measurements of the Top-quark Mass  Fewer ambiguities in final state reconstruction and reduced combinatorial background if we only consider leptonic W decay, there is only one b-jet in the final state. Categorise events by a high-p T isolated lepton, missing transverse momentum and exactly two jets (b- and light)  Typical Q 2 energy scale much lower than top-antitop pair production  Less colour connection between top and protons  Statistically independent sample ATLAS-CONF /11/2014 X-SILAFAE t-channel Associated Wts-channel Motivation  MVA techniques separate signal from background (NN preprocesses input variables to enhance separation power)  Treat top pair production and single top as signal  Distributions of m(lb) are constructed, since this is sensitive to top quark mass, suing a range of discrete values for m top Method Systematics dominated by JES uncertainties Value of mt in good agreement with other tt measurements Single top

Measurements of the Spin Correlation between top-antitop quarks  Top-antitop pairs have been observed to be produced essentially unpolarized 1 in pp collisions, however the correlation of the spin orientation of the top and anti-top can be studied, and is predicted to have a non-zero value  Some BSM models can alter both the production mechanism altering the spin correlation:  E.g. heavy Higgs boson decay into a tt pair  … and decay of the tt pair, affecting how the spin information is accessed:  E.g. supersymmetric models in which a top decays to a charged Higgs boson, subsequently decaying to a lepton neutrino pair  Hence, measuring the spin correlation in tt events can simultaneously probe top production and decay effects due to new physics 1 ATLAS, Phys. Rev. Lett. 111, arXiv: v 1 Measurements of spin correlation using full 7 and 8 TeV data sample of 4.6fb -1 Both single and dilepton final states explored 27/11/2014 X-SILAFAE

Measurements of the Spin Correlation between top-antitop quarks arXiv: v1 1: Azimuthal angle Δφ: Both single and dilepton final states 2. “S-Ratio” of matrix elements for top production and decay from the fusion of like helicity gluons Top and antitop have to be fully reconstructed 3 & 4. Double differential distribution Top direction in tt rest frame is used as the spin quantization axis (“helicity” basis) Event-by-event quantization axis that maximises spin correlation (“maximal” basis) used as the top spin quantization axis Four observables: 27/11/2014 X-SILAFAE Spin correlation strength Correlation coefficient

Results: arXiv: v1 27/11/2014 X-SILAFAE Dilepton Single lepton Both in good agreement with SM predictions stop between the top quark mass and 191 GeV are excluded at 95% CL √s = 7 TeV √s = 8 TeV

Top-quark Charge Asymmetry At LO in SM, quark pair production is symmetric under charge conjugation However, at next-to-leading order (NLO) symmetry no longer valid due to interferences between the Born and 1-loop diagram, and similarly for due to interferences between initial and final state radiation processes Results in a charge asymmetry, and consequently a forward-backward asymmetry in events Measurement of asymmetry in production allows search for unknown top quark production mechanisms which are invisible in the invariant mass spectrum Stringent test on QCD at very high energies At the LHC.. Dominant production is gg fusion which is symmetric under charge conjugation Difficult to define the quark direction since the two incoming beams are symmetric Perform measurement in the laboratory frame using the ATLAS detector and data collected at an integrated luminosiy of 4.6 fb -1 in the dilepton channel 27/11/2014 X-SILAFAE

Top-quark Charge Asymmetry Final state requirements: exactly two charged leptons ≥ 2 jets Large missing transverse energy Final state contains two neutrinos: challenging to reconstruct final state Since charge asymmetry from is transmitted to leptons, can obtain a measurement of purely leptonic based asymmetry Doesn’t require full final state reconstruction Benefits from high precision lepton reconstruction LEPTONIC ASYMMETRY Lepton pseudorapidities After full reconstruction in the lab frame, the difference in the top(antitop) rapidities is computed and used to measure the asymmetry : SM predictions at the LHC: (ATLAS) (CMS) Recent measurements from LHC (before Sep 2014) 27/11/2014 X-SILAFAE

Top-quark Charge Asymmetry Update for 2014 dilepton measurement for 4.6 fb -1 : New analysis software framework Latest MC simulation samples Use a bin-by-bin correction method and unfolding (instead of calibration) for leptonic symmetry measurement Neutrino weighting technique to reconstruct kinematics Samples and background estimation Results: In agreement with standard model prediction 27/11/2014 X-SILAFAE

Top-quark Charge Asymmetry Update for 2014 dilepton measurement for 4.6 fb -1 : New analysis software framework Latest MC simulation samples Use a bin-by-bin correction method and unfolding (instead of calibration) for leptonic symmetry measurement Neutrino weighting technique to reconstruct kinematics Samples and background estimation Results: In agreement with standard model prediction 27/11/2014 X-SILAFAE Outlook for future measurements: Run-I data at √s = 8 TeV has L =20.3fb -1 Cross section for top-antitop at 8TeV = pb  6 times more signal events for asymmetry measurement: Will decrease stat. uncertainty Will be feasible to perform differential measurements of asymmetry as a function of rapidity, transverse momentum and invariant mass Differential measurements will increase sensitivity of asymmetry measurements to new BSM physics Differential measurements will enhance measurements of SM charge asymmetry during Run-II at √s = 14 TeV

Thanks! 27/11/2014 X-SILAFAE

Backup 27/11/2014 X-SILAFAE

Backup Spin correlation Electron channel DILEPTON arXiv: v1 27/11/2014 X-SILAFAE

Backup Spin correlation Muon channel DILEPTON arXiv: v1 27/11/2014 X-SILAFAE

Backup Charge Asymmetry 27/11/2014 X-SILAFAE