Transverse-momentum-dependent parton distribution functions up to twist 4 for spin-1 hadrons

TL;DR

This paper systematically derives all transverse-momentum-dependent parton distribution functions up to twist 4 for spin-1 hadrons, revealing new functions, sum rules, and the role of gauge links in their structure.

Contribution

It provides a complete classification of TMDs for spin-1 hadrons up to twist 4, including previously missing terms and new sum rules, expanding the theoretical understanding of hadron structure.

Findings

Identified 40 TMDs for tensor-polarized spin-1 hadrons up to twist 4.

Discovered 30 new structure functions in twists 3 and 4.

Derived sum rules indicating certain TMD integrals vanish due to time-reversal symmetry.

Abstract

We show possible transverse-momentum-dependent parton distribution functions (TMDs) for spin-1 hadrons including twist-3 and 4 functions in addition to the leading twist-2 ones by investigating all the possible decomposition of a quark correlation function in the Lorentz-invariant way. The Hermiticity and parity invariance are imposed in the decomposition; however, the time-reversal invariance is not used due to an active role of gauge links in the TMDs. Therefore, there exist time-reversal-odd functions in addition to the time-reversal even ones in the TMDs. We list all the functions up to twist-4 level because there were missing terms associated with the lightcone vector $n$ in previous works on the twist-2 part and there was no correlation-function study in the twist-3 and 4 parts for spin-1 hadrons. We show that 40 TMDs exist in the tensor-polarized spin-1 hadron in twists 2, 3, and 4. Some expressions of twist-2 structure functions are modified from previous derivations due to the new terms with $n$, and we find 30 new structure functions in twists 3 and 4 in this work. Since time-reversal-odd terms of the collinear correlation function should vanish after integrals over the partonic transverse momentum, we obtain new sum rules for the time-reversal-odd structure functions, $ \int d^2 k_T h_{LT}= \int d^2 k_T h_{LL} = \int d^2 k_T h_{3LL} =0$. In addition, we indicate that new transverse-momentum-dependent fragmentation functions exist in tensor-polarized spin-1 hadrons. The TMDs are rare observables to find explicit color degrees of freedom in terms of color flow, which cannot be usually measured because the color is confined in hadrons.