Sivers effect at Hermes, Compass and Clas12

TL;DR

This paper reviews the extraction of the Sivers function from HERMES and COMPASS data, highlighting significant sea-quark contributions and proposing a new fit, while emphasizing the need for future measurements at CLAS12 and COMPASS.

Contribution

It provides a comprehensive extraction of the Sivers function including sea-quark effects and introduces a new fit that describes the data more accurately.

Findings

Sea-quark contributions are significant at x ≈ 0.15.

The new fit improves data description but struggles with K+ data.

Future measurements at CLAS12 and COMPASS are essential for clarification.

Abstract

Single spin asymmetries in semi-inclusive deep-inelastic scattering off transversely polarized targets give information on, among other fascinating effects, a pseudo time-reversal odd parton distribution function, the 'Sivers function'. In this proceeding we review the extractions of this function from HERMES and COMPASS data. In particular, the HERMES pion and kaon data suggest significant sea-quarks contributions at $x \simeq 0.15$ to the Sivers effect. We present a new fit that includes all relevant sea quark distributions and gives a statistically satisfactory overall description of the data, but does not describe ideally the $K^+$ data from HERMES. We argue that measurements of the pion- and kaon Sivers effect at CLAS12, and COMPASS, will clarify the situation.