What can break the Wandzura--Wilczek relation?

TL;DR

This paper investigates the conditions under which the Wandzura-Wilczek relation for the g_2 structure function is violated, linking the breaking to twist-3 terms and transverse momentum effects, with implications for experimental measurements.

Contribution

It identifies two twist-3 contributions responsible for breaking the Wandzura-Wilczek relation and proposes methods to separate these effects in semi-inclusive deep inelastic scattering measurements.

Findings

Breaking can be as large as 15-30% of g_2

Two distinct twist-3 terms cause the breaking

Separation of effects is possible in double-spin asymmetry measurements

Abstract

We analyze the breaking of the Wandzura-Wilczek relation for the g_2 structure function, emphasizing its connection with transverse momentum dependent parton distribution functions. We find that the relation is broken by two distinct twist-3 terms, and clarify how these can be separated in measurements of double-spin asymmetries in semi-inclusive deep inelastic scattering. Through a quantitative analysis of available g_2 data we also show that the breaking of the Wandzura-Wilczek relation can be as large as 15-30% of the size of g_2.