Measurement of the correlations between the polar angles of leptons from top quark decays in the helicity basis at $\sqrt{s}=7$TeV using the ATLAS detector

TL;DR

This paper measures the spin correlations between leptons from top quark decays at 7 TeV using ATLAS data, comparing the results to theoretical predictions and finding good agreement.

Contribution

First measurement of top quark spin correlations in dilepton channel at 7 TeV using ATLAS data, with results unfolded to parton level.

Findings

Distribution of cosθ₁·cosθ₂ agrees with NLO predictions

Good agreement between measured and predicted spin correlations

Analysis demonstrates the feasibility of spin correlation measurements at LHC

Abstract

A measurement of the correlations between the polar angles of leptons from the decay of pair-produced $t$ and $\bar{t}$ quarks in the helicity basis is reported, using proton-proton collision data collected by the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 4.6fb$^{-1}$ at a center-of-mass energy of $\sqrt{s}=7$TeV collected during 2011. Candidate events are selected in the dilepton topology with large missing transverse momentum and at least two jets. The angles $\theta_1$ and $\theta_2$ between the charged leptons and the direction of motion of the parent quarks in the $t\bar{t}$ rest frame are sensitive to the spin information, and the distribution of {\mbox{$\cos\theta_1\cdot\cos\theta_2$}} is sensitive to the spin correlation between the $t$ and $\bar{t}$ quarks. The distribution is unfolded to parton level and compared to the next-to-leading order prediction. A good agreement is observed.