The GPD program at COMPASS

TL;DR

The COMPASS experiment at CERN has conducted and planned extensive studies on Generalized Parton Distributions (GPDs) using polarized muon beams, with initial results from vector meson production and upcoming data on DVCS and $A0^0$ channels.

Contribution

This paper summarizes the initial results and the development of the GPD program at COMPASS, including new detector setups and planned measurements for 2016-2017.

Findings

Initial azimuthal asymmetries in vector meson production observed

First results on DVCS and $A0^0$ channels demonstrated setup performance

Preparation for extensive GPD data collection in 2016-2017

Abstract

The 160 GeV polarised muon beam available at CERN, with positive or negative charge, makes COMPASS a unique place for GPD studies. The first GPD related COMPASS results come from exclusive vector meson production on transversely polarised protons and deuterons. The data were taken in 2003-2010 with large solid-state polarised targets, although without detection of recoil particles. Results on various transverse target spin dependent azimuthal asymmetries are presented and their relations to GPDs are discussed. The dedicated COMPASS GPD program started in 2012 with commissioning of a new long liquid hydrogen target and new detectors such as the large recoil proton detector and the large-angle electromagnetic calorimeter. It was followed by a short pilot 'DVCS run'. The performance of the setup and first results on DVCS and exclusive $\pi ^0$ channels have been demonstrated. The full data taking for the GPD program approved within COMPASS-II proposal is planned for 2016 and 2017.