Transverse Extension of Partons in the Proton probed by Deeply Virtual Compton Scattering

TL;DR

This paper presents the first measurement of the transverse spatial distribution of partons in the proton using deeply virtual Compton scattering data from the COMPASS experiment, revealing the proton's internal structure.

Contribution

It provides the first experimental determination of the transverse extension of partons in the proton through exclusive muoproduction measurements at COMPASS.

Findings

Measured the $t$-dependence of DVCS cross sections.

Extracted the average transverse size of partons as approximately 0.58 fm.

Quantified the $t$-slope parameter with statistical and systematic uncertainties.

Abstract

We report on the first measurement of exclusive single-photon muoproduction on the proton by COMPASS using 160 GeV/$c$ polarized $\mu^+$ and $\mu^-$ beams of the CERN SPS impinging on a liquid hydrogen target. We determine the dependence of the average of the measured $\mu^+$ and $\mu^-$ cross sections for deeply virtual Compton scattering on the squared four-momentum transfer $t$ from the initial to the final final proton. The slope $B$ of the $t$-dependence is fitted with a single exponential function, which yields $B=(4.3 \ \pm \ 0.6_{\text{stat}}\_{- \ 0.3}^{+ \ 0.1}\big\rvert_{\text{sys}}) (\text{GeV}/c)^{-2}$. This result can be converted into an average transverse extension of partons in the proton, $\sqrt{\langle r_{\perp}^2 \rangle} = (0.58 \ \pm \ 0.04_{\text{stat}}\_{- \ 0.02}^{+ \ 0.01}\big\rvert_{\text{sys}})\text{fm}$. For this measurement, the average virtuality of the photon mediating the interaction is $\langle Q^2 \rangle = 1.8\,(\text{GeV/}c)^2$ and the average value of the Bjorken variable is $\langle x_{\text{Bj}} \rangle = 0.056$.