Observation of top quark pairs produced in association with a vector boson in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

Abstract

Measurements of the cross sections for top quark pairs produced in association with a W or Z boson are presented, using 8 TeV pp collision data corresponding to an integrated luminosity of 19.5 fb −1 , collected by the CMS experiment at the LHC. Final states are selected in which the associated W boson decays to a charged lepton and a neutrino or the Z boson decays to two charged leptons. Signal events are identified by matching reconstructed objects in the detector to specific final state particles from ttW or ttZ decays. The ttW cross section is measured to be 382 +117 −102 fb with a significance of 4.8 standard deviations from the background-only hypothesis. The ttZ cross section is measured to be 242 +65 −55 fb with a significance of 6.4 standard deviations from the background-only hypothesis. These measurements are used to set bounds on five anomalous dimension-six operators that would affect the ttW and ttZ cross sections. 11.1 Constraints on the axial and vector components of the tZ coupling 25 Constraints on dimension-six operators 26 Summary 27A Input variables to linear discriminant for event reconstruction 30 B Input variables to final discriminants (BDTs) 31The CMS collaboration 36-1 - JHEP01(2016)096 1 IntroductionSince the LHC at CERN achieved proton-proton collisions at center-of-mass energies of 7 and 8 TeV, it has become possible to study signatures at significantly higher mass scales than ever before. The two heaviest sets of particles produced in standard model (SM) processes that could be observed using the data already collected are top quark pairs produced in association with a W or Z boson (ttW and ttZ), which have expected cross sections of σ(ttW) = 203 +20 −22 fb and σ(ttZ) = 206 +19 −24 fb in the SM in 8 TeV collisions [1]. The dominant production mechanisms for ttW and ttZ in pp collisions are shown in figure 1. The ttZ production cross section provides the most accessible direct measurement of the top quark coupling to the Z boson. Both σ(ttW) and σ(ttZ) would be altered in a variety of new physics models that can be parameterized by dimension-six operators added to the SM Lagrangian.The ttZ cross section was first measured by the CMS experiment in 7 TeV collisions, with a precision of about 50% [2]. Measurements in events containing three or four leptons in 8 TeV collisions at CMS [3] have constrained σ(ttZ) to within 45% of its SM value, and yielded evidence of ttZ production at 3.1 standard deviations from the backgroundonly hypothesis. The CMS collaboration also used same-sign dilepton events to constrain σ(ttW) to within 70% of the SM prediction, with a significance of 1.6 standard deviations from the background-only hypothesis. Most recently, the ATLAS experiment used events containing two to four leptons to measure σ(ttW) = 369 +100 −91 fb at 5.0 standard deviations from the background-only hypothesis, and σ(ttZ) = 176 +58 −52 fb with a significance of 4.2 standard deviations from the background-only hypothesis [4].We present the first observation of ttZ production ...

Probing top quark neutral couplings in the Standard Model Effective Field Theory at NLO in QCD

“…Experimental results from [2,5,6,54] are used. For the ttZ measurement by ATLAS [6] and the ttγ measurement by CMS [2], a direct comparison is difficult, because of the way in which the measured cross sections are defined.…”

“…The range for the Wilson coefficients is determined by the current constraints as discussed in section 2. The experimental measurements used in this plot are taken from [2] and [5] for ttγ and ttZ respectively. The squared contribution of the O tB operator is very large, and therefore we employ a separate smaller interval to obtain cross sections within the boundaries of this plot.…”

“…Measurements of ttγ have been performed at the Tevatron by CDF [1], and at the LHC by CMS [2] and by ATLAS [3]. Results for ttZ and ttW by CMS appear in [4,5] and by ATLAS in [6].…”

Pinning down electroweak dipole operators of the top quark

“…a e-mail: markus.schulze@cern.ch b e-mail: soreqy@mit.edu of the tt + X processes, small branching fractions and large backgrounds. First measurements and searches at 7 and 8 TeV LHC have established some of these SM processes [5][6][7][8][9][10][11][12][13], but detailed studies of electroweak couplings will only be possible with a larger data set at higher energies during Run-II. These studies will certainly improve our understanding of top interactions with the electroweak sector and potentially probe physics beyond the SM.…”

“…A hard photon was never present in the event sample and the fundamental ttγ interaction was not probed. Similarly, current LHC data only allows constraints on the vector and axial parts of the tt Z vertex with O(100 %) uncertainties [13], while the respective dipole couplings are unconstrained from hadron collider experiments. We note that low-energy observables, such as rare K and B decays [22][23][24], together with electroweak precision data [25][26][27][28] can provide strong constraints on modified tt Z interactions.…”