Transverse Polarization of Top Quark Pairs at the Tevatron and the Large Hadron Collider

TL;DR

This paper addresses whether QCD-induced transverse polarization of top quarks in $pp,p\bar{p}\to t\bar{t}X$ can be observed. It computes one-loop absorptive contributions to the production density matrix for the main parton processes ($q\bar{q}$ and $gg$), deriving explicit expressions for the transverse polarization and showing parton-level polarizations of a few percent that are damped at the hadronic level. The authors then construct $T_N$-odd momentum correlations in semileptonic $t\bar{t}$ decays as measurable proxies, and evaluate their sizes for Tevatron and LHC energies. They find ${\cal S}_1$ around 0.43% at the Tevatron and ${\cal S}_2$ around 0.03% at the LHC, with sizable statistical and systematic challenges to measurement despite high event rates. The results provide concrete SM predictions that can serve as sensors for new physics in top-quark final-state interactions.

Abstract

We investigate the prospects to observe effects of transverse polarization of top quarks in $pp,p\bar{p}\to t\bar{t}X$. QCD absorptive parts generate a polarization of top quarks and antiquarks transverse to the production plane in the partonic processes $q\bar{q}\to t\bar{t}$ and $gg\to t\bar{t}$, which reaches values of a few percent. These perturbative QCD effects are decreased at the hadronic level. A measurement through momentum correlations among the $t$ and $\bar{t}$ decay products will be difficult both at an upgraded Tevatron and at the LHC.