Imaging baryon number density within the proton

TL;DR

This study maps the baryon number density within the proton, revealing it is concentrated in the core with a transverse radius of 0.33-0.53 fm, smaller than the charge and mass distributions.

Contribution

It introduces a novel method to probe the baryon number distribution inside the proton using exclusive meson production and Fourier-Bessel analysis.

Findings

Baryon number is confined to a transverse radius of 0.33-0.53 fm.

Baryon number distribution is more concentrated than charge and mass distributions.

Baryon number is localized at the proton's center.

Abstract

The spatial extent of the proton is a key factor in nuclear physics. Different measurement techniques probe different aspects of the proton, yielding different radii. The mass and charge radii depend on the parton and quark distributions respectively, while the mechanical radius depends on the mass/energy distribution. Here, we probe the spatial distribution of a new proton characteristic, studying the distribution of baryon number within the proton. We investigate the baryon number distribution by studying four exclusive meson production channels arising from photon-proton collisions ($\gamma p \rightarrow p \rho^0$, $\gamma p \rightarrow p \omega$, $\gamma p \rightarrow n \pi^+$, and $\gamma p \rightarrow p \pi^0$). The two-dimensional transverse sizes of the interacting systems are extracted by analyzing the transverse momentum, $p_T$, dependence of the meson production cross section, using Fourier-Bessel transformations. We find that baryon number is confined to a transverse radius of $0.33 - 0.53$~fm. In comparison, the transverse radius of the proton charge and mass distributions are considerably larger, at least 0.67~fm. The baryon number is concentrated in the center of the proton.