Positivity bounds on gluon TMDs for hadrons of spin ≤ 1

Abstract

We consider the transverse momentum dependent gluon distribution functions (called gluon TMDs) by studying the light-front gluon-gluon correlator, extending the results for unpolarized and vector polarized targets to also include tensor polarized targets — the latter type of polarization is relevant for targets of spin ≥ 1. The light-front correlator includes process-dependent gauge links to guarantee color gauge invariance. As from the experimental side the gluon TMDs are largely unknown, we present positivity bounds for combinations of leading-twist gluon distributions that may be used to estimate their maximal contribution to observables. Since the gluonic content of hadrons is particularly relevant in the small-x kinematic region, we also study these bounds in the small-x limit for the dipole-type gauge link structure using matrix elements of a single Wilson loop.