Spin correlations in top physics at ATLAS and CMS in Run 2

TL;DR

This paper reports measurements of top quark pair spin correlations at the LHC using ATLAS and CMS data, testing the Standard Model and searching for new physics such as top squarks and anomalous dipole moments.

Contribution

It provides the first combined measurement of top quark spin correlations at 13 TeV with detailed unfolding to parton and particle levels, and sets constraints on new physics models.

Findings

Measured spin correlations are consistent with Standard Model predictions.

Set limits on light top squark production and top quark chromo-magnetic dipole moments.

Enhanced understanding of top quark spin dynamics at high energies.

Abstract

Measurements of $\mathrm{t\bar{t}}$ spin correlations are presented in events with top quarks produced in $\mathrm{pp}$ collisions at the LHC. The data correspond to an integrated luminosity of $36\:\mathrm{fb^{-1}}$ at $\sqrt{s}=13\:\mathrm{TeV}$ collected at both the ATLAS and CMS detectors. The spin correlations are measured using the angular distributions of the leptons in dilepton channel $\mathrm{t\bar{t}}$ events. The spin correlations are probed both directly, using distributions measured in the top quark rest frames that depend only on the top quark spin, and indirectly, using distributions measured in the laboratory frame. The distributions are unfolded to the parton level and extrapolated to the full phase space. Some of the laboratory frame distributions are additionally unfolded to the particle level in the fiducial phase space of the ATLAS detector. The spin correlation measurements are used to search for new physics in the form of a light top squark or an anomalous top quark chromo-magnetic dipole moment, and stringent constraints are placed in both cases.