Measurements of top quark spin observables in t t ¯ $$ t\overline{t} $$ events using dilepton final states in s = 8 $$ \sqrt{s}=8 $$ TeV pp collisions with the ATLAS detector

Abstract

Measurements of top quark spin observables in tt events are presented based on 20.2 fb −1 of √ s = 8 TeV proton-proton collisions recorded with the ATLAS detector at the LHC. The analysis is performed in the dilepton final state, characterised by the presence of two isolated leptons (electrons or muons). There are 15 observables, each sensitive to a different coefficient of the spin density matrix of tt production, which are measured independently. Ten of these observables are measured for the first time. All of them are corrected for detector resolution and acceptance effects back to the parton and stable-particle levels.The measured values of the observables at parton level are compared to Standard Model predictions at next-to-leading order in QCD. The corrected distributions at stable-particle level are presented and the means of the distributions are compared to Monte Carlo predictions. No significant deviation from the Standard Model is observed for any observable.

Indirect signs of the Peccei-Quinn mechanism

“…To this end, we follow Refs. [106][107][108][109][110] and consider CP-odd triple product observables in dileptonic tt production at hadron colliders (see also Ref. [111] for a study of related observables).…”

“…For comparison, the LHC collaborations are able to obtain up to ∼ 90% reconstruction efficiencies using more sophisticated techniques (see e.g. [110]). Thus, we expect that our sensitivity projections will be conservative from this standpoint.…”

Probing anomalous Wtb couplings at the LHC in single t -channel top quark production

“…Nevertheless, several measurements of the charged lepton angle in the helicity basis exist, e.g. in the tt system [107]. Hence, it can possibly be measured in the t-channel process in the future.…”

Top quark polarization as a probe of charged Higgs bosons

“…wherein α ± is the so-called spin analyzing power of the charged lepton and θ a = (ˆ ± ,Ŝ a ), withˆ ± being the direction of flight of the charged lepton in the top quark rest frame andŜ a the spin quantization axis in the basis a. There are three such bases relevant for top quark physics at the LHC (see, e.g., [34] for details about the sensitivity and [35] for a corresponding measurement in tt production): the helicity basis, the transverse basis and the r-basis. In the helicity basis,Ŝ a is the direction of motion of the top quark in the so-called (tt) Zero Momentum Frame (ZMF).…”