The $t$-dependence of the pure DVCS cross section at COMPASS

TL;DR

This paper reports the first model-independent measurement of the transverse size of the nucleon's partonic distribution using DVCS cross sections at COMPASS, covering an unexplored $x_B$ range with high precision.

Contribution

It provides a novel, model-independent determination of the nucleon's transverse size from $t$-dependence of DVCS cross sections at COMPASS.

Findings

First model-independent measurement of nucleon transverse size.

$x_B$ domain from 0.01 to 0.15 explored.

Transverse size of the nucleon: 0.578 fm at $x_B$=0.056.

Abstract

The key reactions to study the Generalised Parton Distributions are Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production (DVMP). At COMPASS, these processes are investigated using a high intensity muon beam with a momentum of 160\,GeV/c and a 2.5\,m-long liquid hydrogen target. In order to optimize the selection of exclusive reactions at these energies, the target is surrounded by a new barrel-shaped time-of-flight system to detect the recoiling particles. COMPASS-II covers the up to now unexplored $x_{B}$ domain ranging from 0.01 to 0.15. From the sum of cross sections measured with positive and negative beam polarities, the pure DVCS cross-section and its $t$-dependence have been extracted resulting in a first model-independent determination of the transverse size of the partonic distribution of the nucleon $\sqrt{<r_{\perp}^2>}= (0.578 \ \pm \ 0.042\ _{- \ 0.018}^{+ \ 0.006})\,\textsf{fm} $ at a mean $x_{B}$ value of 0.056.