1 Introduction

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atlas-document.bib \AtlasTitleMeasurement of the charge asymmetry in highly boosted top-quark pair production in collision data collected by the ATLAS experiment \AtlasRefCodeTOPQ-2015-10 \PreprintIdNumberCERN-PH-EP-2015-303 \AtlasJournalRef\PLB756 (2016) 52-71 \AtlasDOI10.1016/j.physletb.2016.02.055 \AtlasAbstractIn the process the angular distributions of top and anti-top quarks are expected to present a subtle difference, which could be enhanced by processes not included in the Standard Model. This Letter presents a measurement of the charge asymmetry in events where the top-quark pair is produced with a large invariant mass. The analysis is performed on 20.3 fb of collision data at collected by the ATLAS experiment at the LHC, using reconstruction techniques specifically designed for the decay topology of highly boosted top quarks. The charge asymmetry in a fiducial region with large invariant mass of the top-quark pair ( 0.75 ) and an absolute rapidity difference of the top and anti-top quark candidates within 2 2 is measured to be 4.2 3.2%, in agreement with the Standard Model prediction at next-to-leading order. A differential measurement in three mass bins is also presented.

1 Introduction

The charge asymmetry [Kuhn:1998kw, Kuhn:1998jr] in top-quark pair production at hadron colliders constitutes one of the more interesting developments in the last decade of top-quark physics. In the Standard Model (SM), a forward–backward asymmetry (), of order , is expected at a proton–antiproton () collider such as the Tevatron, with a much enhanced asymmetry in certain kinematical regions. Early measurements [Abazov:2007ab, Aaltonen:2008hc] found a larger than predicted by the SM. Later determinations confirmed this deviation and measurements in intervals of the invariant mass, , of the system formed by the top-quark pair [Abazov:2015fna, Abazov:2014cca, Aaltonen:2012it, Abazov:2011rq, Aaltonen:2011kc] found a stronger dependence on than anticipated. Recent calculations of electroweak effects [Hollik:2011ps] and the full next-to-next-to-leading-order (NNLO) corrections [Czakon:2014xsa] to the asymmetry have brought the difference between the observed asymmetry at the Tevatron and the SM prediction down to the 1.5 level and reduced the tension with the differential measurements in  [Aguilar-Saavedra:2014kpa, Kuhn:2011ri].

At the Large Hadron Collider (LHC), the forward–backward asymmetry is not present due to the symmetric initial state, but a related charge asymmetry, , is expected in the distribution of the difference of absolute rapidities of the top and anti-top quarks,

(1)

where and denotes the rapidity of the top and anti-top quarks. 1 For quark–antiquark () initial states, the difference in the average momentum carried by valence and sea quarks leads to a positive asymmetry. These quark-initiated processes are strongly diluted by the charge-symmetric gluon-initiated processes, yielding a SM expectation for the charge asymmetry of less than 1%. Many beyond-the-Standard-Model (BSM) scenarios predict an alteration to this asymmetry. Previous measurements at 7 \tev [Chatrchyan:2014yta, Aad:2013cea, Chatrchyan:2012cxa, ATLAS:2012an] and 8 \tev [Khachatryan:2015oga, Khachatryan:2015mna, ATLAS-TOPQ-2014-16] by ATLAS and CMS are consistent with the SM prediction.

With a centre-of-mass energy of 8 \tev and a top-quark pair sample of millions of events, the LHC experiments can access the charge asymmetry in a kinematic regime not probed by previous experiments. The development of new techniques involving Lorentz-boosted objects and jet substructure [Adams:2015hiv, Altheimer:2013yza, Altheimer:2012mn, Abdesselam:2010pt] and their use in the analysis of LHC data [Aad:2013gja, ATLAS:2012am] have enabled an efficient selection of highly boosted objects and an accurate reconstruction of their momentum.

This Letter presents a measurement of the rapidity-dependent charge asymmetry in top-quark pair production that is based on techniques specifically designed to deal with the collimated decay topology of boosted top quarks. Specifically, it is based on the techniques described in Refs. [Aad:2015fna, Aad:2013nca, Aad:2012dpa, ATL-PHYS-PUB-2010-008]. The analysis focuses on the lepton+jets (+jets) final state, where the hadronic top-quark decay is reconstructed as a single large-radius (large-) jet and tagged as such using jet substructure variables. The leptonic top-quark decay is reconstructed from a single small-radius (small-) jet, a single charged lepton (muon or electron), and missing transverse momentum, corresponding to the neutrino from the boson decay. The event selection and reconstruction follow the prescriptions of Ref. [Aad:2015fna], where a detailed description and discussion of their performance can be found.

Compared to previous analyses [Khachatryan:2015oga, ATLAS-TOPQ-2014-16] based on the classical, resolved top-quark selection criteria and reconstruction schemes, this approach offers a more precise reconstruction of the \ttbar invariant mass and top-quark direction for highly boosted top quarks. It is therefore possible to perform accurate measurements of the charge asymmetry in events with a \ttbar invariant mass in the \tev range. This kinematic regime has a higher sensitivity for the SM asymmetry due to a higher fraction of quark-initiated processes, as well as for BSM models that introduce massive new states.

This Letter is structured as follows. The data sample analysed is presented in Section 2, along with a description of the Monte Carlo (MC) simulation samples in Section LABEL:sec:mc. A brief overview of the reconstructed object definitions and of the event selection and reconstruction is given in Sections LABEL:sec:obj and LABEL:sec:evtselreco. The observed yields and several kinematic distributions are compared to the SM expectations in Section LABEL:sec:yield. The unfolding technique used to correct the reconstructed \dy spectrum to the parton level is discussed in Section LABEL:sec:unfolding. The estimates of the systematic uncertainties that affect the measurement are described and estimated in Section LABEL:sec:syst. The results are presented in Section LABEL:sec:results, and their impact on several BSM theories is discussed in Section LABEL:sec:bsm. Finally, the conclusions are presented in Section LABEL:sec:conclusions.

2 Data sample

The data for this analysis were collected by the ATLAS [Aad:2008zzm] experiment in the 8 \tev proton–proton () collisions at the CERN LHC in 2012. Collision events are selected using isolated or non-isolated single-lepton triggers, where the isolated triggers have a threshold of 24 \gev on the transverse momentum (\pt) of muons or on the transverse energy of electrons. The non-isolated triggers have higher thresholds: 60 \gev for electrons and 36 \gev for muons. The contribution from events with leptons passing only the non-isolated triggers but having below these higher thresholds is negligible. The collected data set is limited to periods with stable beam conditions when all sub-systems were operational. The sample corresponds to an integrated luminosity of 20.3 0.6 

Footnotes

  1. ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the -axis coinciding with the axis of the beam pipe. The -axis points from the IP towards the centre of the LHC ring, and the -axis points upward. Polar coordinates (, ) are used in the transverse plane, being the azimuthal angle around the -axis. The rapidity is given as , while the pseudorapidity is defined in terms of the polar angle as . The distance in (,) coordinates, , is used to define cone sizes and the distance between reconstructed objects. Transverse momentum and energy are defined as and , respectively.
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