Direct Measurement of \DeltaG/G at Compass

TL;DR

The COMPASS experiment measures the gluon polarization G/G in the nucleon using photon-gluon fusion channels, finding it to be small, with the high p_T quasi-real photoproduction channel providing the most precise results.

Contribution

This paper presents the first direct measurement of G/G at COMPASS, utilizing multiple channels and theoretical modeling to improve understanding of nucleon spin structure.

Findings

Gluon polarization G/G is small.

High p_T quasi-real photoproduction channel yields the most precise measurement.

Preliminary results from other channels are reported.

Abstract

The gluon polarization $\Delta{G}/G$ is the key to a further clarification of the spin structure of the nucleon. The COMPASS collaboration at CERN has set out to undertake the direct determination of this quantity. It accesses the gluon distribution via the photon-gluon fusion process (PGF) in scattering polarized muons off a polarized deuteron target. And it explores three different channels to tag the PGF: open charm production and high transverse momentum (high p_T), in either electroproduction (Q^2 > 1 GeV^2) or quasi-real photoproduction (Q^2 < 1 GeV^2). The high p_T quasi-real photoproduction channel yields the most precise measurement. The result indicates that the gluon polarization is small. I describe its experimental aspects and its theoretical framework, based on PYTHIA. And I report on the preliminary results obtained in the other two channels, and on the prospects for future analysis and data taking.