# Off-shell initial state effects and gauge invariance in the Drell-Yan process

## TL;DR

This paper explores the impact of off-shell initial state effects on the Drell-Yan process within a gauge-invariant framework that incorporates transverse momentum, clarifying its relation to traditional TMD parton models.

## Contribution

It introduces a gauge-invariant approach using Parton Reggeization to analyze initial state effects in the Drell-Yan process, connecting it with existing TMD formalism.

## Key findings

- Helicity structure functions are formulated with off-shell initial partons.
- The approach maintains QED gauge invariance.
- Relationships with conventional TMD models are clarified.

## Abstract

The Helicity Structure Functions in the Drell-Yan process are discussed in a framework of Parton Reggeization Approach, which includes the transverse momentum of initial-state partons in a way compatible with QED gauge-invariance. Relationships with conventional Transverse Momentum Dependent Parton Model formalism are clarified.