Top quark charge asymmetry measurements with ATLAS detector
SSNSN-323-63April 8, 2019
Top quark charge asymmetry measurements withATLAS detector
Umberto De Sanctis, on behalf of the ATLAS CollaborationDepartment of Physics and AstronomyUniversity of Sussex, Brighton, Sussex, BN1 9QH, United Kingdom
PRESENTED AT
International Workshop on the CKM Unitarity Triangle(CKM2014)Wien, Austria, September 8–12, 2014 a r X i v : . [ h e p - e x ] N ov Introduction
Since its discovery in 1995, the top quark is playing a key role in the understandingof Quantum Chromodynamics (QCD) processes at high energies. The top quarkpair( tt ) production cross-section at the LHC allows to deeply explore the productionmechanisms and search for signals of New Physics processes beyond the StandardModel (SM). In this article, the top quark charge asymmetry measurements performedby the ATLAS [1] experiment are presented. Results in single-lepton and dilepton topdecay channels for pp collisions at 7 TeV center-of-mass energy using data collectedin 2011 are shown. At the LHC collider, tt pairs are produced mainly through gluon-gluon ( gg ) fusionprocess. Only around 20% of the events are produced from quark-antiquark ( qq )hard collisions, while the fraction coming from quark-gluon ( qg ) partonic processesis almost negligible. The charge asymmetry A C is a manifestation of the forward-backward asymmetry when the CP invariance holds. It is a tiny NLO QCD effect( A SMC = 0.0123 ± qq and qg .In the tt -system center-of-mass frame, the effect of the charge asymmetry is that tops(antitops) are produced preferentially in the incoming quark (antiquark) direction. Athadron colliders it is difficult to determine the quark/antiquark direction, so anotherquantity in the laboratory frame is needed to measure this asymmetry. The variable∆ y = y t − y t , where y represents the rapidity of the top/antitop quark, measuredin the laboratory frame, is Lorentz invariant. It has the same value as the forward-backward asymmetry in the tt center-of-mass frame, computed as a function of thecos θ ∗ angle between the top and the incoming quark. TeVatron experiments used∆ y variable to measure this asymmetry, counting the number of events where ∆ y ispositive or negative.At the LHC, due to the symmetry of the incoming beams, an asymmetry based onthe ∆ y variable would vanish. Hence the variable ∆ | y | = | y t | − | y t | has been chosen,based on the fact that quarks are more boosted than antiquarks, due to the differentmean momenta carried by valence quarks and sea antiquarks. The asymmetry A C obtained counting the number of events where ∆ | y | is positive or negative, is calledtop quark charge asymmetry. The top quark charge asymmetry A C has been measured by the ATLAS experimentwith data collected at 7 TeV center-of-mass energy corresponding to an integrated1uminosity of 4.7 fb − [3]. The charge asymmetry has been measured inclusively anddifferentially as a function of the mass ( m tt ), the transverse momentum ( p T,tt ) and theabsolute rapidity ( | y tt | ) of the tt -system. In addition, an inclusive and a differentialmeasurement as a function of m tt has been performed with the additional requirementof a minimum velocity β z,tt > . tt -system velocity along the beam axis toenhance the sensitivity to new physics processes beyond the SM (BSM).Events are selected requiring the presence of exactly one reconstructed isolated elec-tron (muon) with p T >
25 (20) GeV, at least four jets (reconstructed with the anti- k T algorithm with a 0.4 radius parameter in the η − φ plane) with p T >
25 GeV of whichat least one tagged as a b -jet. Additional cuts are applied on the missing transversemomentum E missT >
30 GeV and the transverse W mass m WT >
30 GeV in the electronchannel, while a cut on their sum ( E missT + m WT ) >
60 GeV is applied on the muonchannel. In both cases the aim is to reduce the multijet background.The main backgrounds for this analysis, which are multijet and W +jets, are esti-mated using data-driven techniques, while sub-dominant backgrounds like Z +jets,single top and diboson ( W W , W Z , ZZ ) production are estimated using Monte Carlosimulated samples.The tt system is reconstructed using a kinematic likelihood fit that assesses the com-patibility of the observed events with the topology of a simulated tt decays. Thismethod identifies the correct decay topology in 75% of the cases.The reconstructed ∆ | y | distributions are distorted by acceptance and detector res-olution effects. An unfolding procedure is used in order to correct for these effectsand to pass from the reconstructed asymmetries to the partonic relative quantities.Simulated tt events are used to build a response matrix M relating true T and recon-structed R observed quantities. Its elements M tr represent the probability and theefficiency of an event produced in the true bin t of the distribution of interest to bereconstructed in any bin r . After the subtraction of the backgrounds to the data inthe signal region, this matrix is inverted using the FBU (Fully Bayesian Unfolding)technique based on the application of Bayes’ theorem to the unfolding problem [4].Given an observed spectrum D in data for ∆ | y | and the response matrix M ( T, R ),the posterior probability density p of the true T spectrum can be computed using theBayes’ theorem: p ( T | D, M ) ∝ L ( D | T, M ) × π ( T ) (1)where L ( D | T, M ) is the conditional likelihood for the data D assuming the truespectrum T and the response matrix M , and π is the prior probability density for T . Assuming that data follows a Poisson distribution, the likelihood L ( D | T, M )can be computed using the information of the response matrix M ( T, R ) obtainedin simulated tt events. Conversely, the prior distribution π ( T ) represents our priorknowledge about T before the measurement is performed. In this context, the choiceof π ( T ), which is arbitrary, can be interpreted as the choice of a regularisation functionin other unfolding techniques. Both a flat prior and a prior distribution based on the2 C Data TheoryUnfolded 0.006 ± ± m tt >
600 GeV 0.018 ± . +0 . − . Unfolded with β z,tt > . ± . +0 . − . Table 1: Measured values of the inclusive charge asymmetry, A C , for the electronand muon channels combined after unfolding without and with the β z,tt > . A C measurement with a cut on m tt >
600 GeV is also shown.SM predictions, as described in the text, are also reported. The quoted uncertaintiesinclude statistical and systematic components after the marginalisation [3].curvature of the true ∆ | y | spectrum have been used for the various measurementsperformed. In both cases it has been checked that any bias in the A C measurementsand their uncertainties was introduced.The inclusive and the differential A C measurements have been performed combiningthe electron and muon channels. The inclusive charge asymmetry, together with themeasurements and predictions for m tt >
600 GeV and β z,tt > . | y | distributions after the unfolding procedureand the differential A C measurements as a function of m tt , p T,tt and | y tt | are shown.A combination with the inclusive A C measurement at the same centre-of-mass energyperformed by CMS experiment [5] has been also performed. Using the BLUE (BestUnbiased Linear Estimator) method [7], the combination is performed taking intoaccount the central values, the statistical and the systematic uncertainties of the twomeasurements and their correlations. The combined value for the top quark chargeasymmetry was found to be: A C = 0 . ± . stat. ) ± . syst. ), compatiblewith the SM predictions [6]. A measurement of the charge asymmetry has been done by the ATLAS experimentalso in the dileptonic tt decay channel with an integrated luminosity of 4.7 fb − [8].The events are selected requiring exactly two oppositely charged leptons with thesame flavor (i.e. ee , eµ and µµ ) and p T > p T >
25 GeV are also required. In the ee and µµ channels ad-ditional cuts are applied on the missing transverse momentum E missT >
60 GeV andthe invariant mass of the lepton pair ( m ( ll )) within 10 GeV from the Z boson massto remove Z +jets background. In the eµ channel a cut on H T , that is the scalar sumof the lepton and jets transverse momentum, is also applied: H T >
130 GeV.Having two neutrinos in the final state, the kinematics of the tt decays is under-3 y| ∆ -3 -2 -1 0 1 2 3 E ve n t s / . DatattW+jetsZ+jetsDibosonSingle topMulti-jetsUncertainty ℓ + ≥ jets ( ≥ b − tag ) = 7 TeVs -1 L dt = 4.7 fb ∫ ATLAS [GeV] tt m C A -0.1-0.0500.050.10.150.20.25 DataSM Axigluon m=300 GeV Axigluon m=7000 GeV
ATLAS = 7 TeVs -1 L dt = 4.7 fb ∫ [GeV] tT,t p C A -0.1-0.0500.050.10.150.20.25 DataSM
ATLAS = 7 TeVs -1 L dt = 4.7 fb ∫ | tt |y C A -0.1-0.0500.050.10.150.20.25 DataSM Axigluon m=300 GeV Axigluon m=7000 GeV
ATLAS = 7 TeVs -1 L dt = 4.7 fb ∫ Figure 1: ∆ | y | distribution (top left) and the unfolded A C asymmetry distributionsas a function of m tt (top right), p T,tt (bottom left) and | y tt | are shown. The A C valuesafter the unfolding (points) are compared with the SM predictions (green lines) andthe predictions for a colour–octet axigluon with a mass of 300 GeV (red lines) and7000 GeV (blue lines) respectively. The SM predictions include factorisation andrenormalisation scale uncertainties. The values plotted are the average A C in eachbin. The error bars include both the statistical and the systematic uncertainties on A C values [3].constrained. Hence several combinations of the physical objects in the final state areadmissible for each event. In each event, each solution has been weighted accordingto a likelihood estimator derived from matrix elements for the LO process gg → tt .The combination with the highest weight is finally chosen.To measure the asymmetry at the partonic level, a calibration procedure is usedin this measurement. After the subtraction of the background, the measured rawasymmetry is calibrated using calibration curves that relate reconstructed and trueasymmetries. The curves have been derived from simulated tt events where trueasymmetries ranging from −