Threshold Factorization Redux

TL;DR

This paper revisits the factorization theorems for Drell-Yan and deep inelastic scattering near threshold within SCET, introducing a collinear-soft mode to improve the theoretical consistency and divergence handling.

Contribution

It introduces a new perspective on factorization near threshold by including collinear-soft modes, ensuring soft functions are infrared finite and free of rapidity divergences.

Findings

Soft functions become infrared finite.

All factorized parts are free of rapidity divergence.

Separation of scales in factorization is clarified.

Abstract

We reanalyze the factorization theorems for Drell-Yan process and for deep inelastic scattering near threshold, as constructed in the framework of the soft-collinear effective theory (SCET), from a new, consistent perspective. In order to formulate the factorization near threshold in SCET, we should include an additional degree of freedom with small energy, collinear to the beam direction. The corresponding collinear-soft mode is included to describe the parton distribution function (PDF) near threshold. The soft function is modified by subtracting the contribution of the collinear-soft modes in order to avoid double counting on the overlap region. As a result, the proper soft function becomes infrared finite, and all the factorized parts are free of rapidity divergence. Furthermore, the separation of the relevant scales in each factorized part becomes manifest. We apply the same idea to the dihadron production in $e^+ e^-$ annihilation near threshold, and show that the resultant soft function is also free of infrared and rapidity divergences.