Uncovering the Mystery of Proton Mass: Muon Channel Analysis for the J/Ψ-007 Experiment

The proton is an essential building block of our existence–found in every atom in the universe–but the origins of the proton's mass are not well understood. Although the proton is composed of quarks and gluons, the masses of those particles sum to only a small fraction of the proton's total mass. The majority of the proton's mass, then, must derive from something else: the energy contained in the strong force itself. The strong force, mediated by the gluons, binds quarks and gluons together into protons. However, little is known about the behavior of these gluons and their distribution within the proton. This experiment aimed to probe the gluonic structure of the proton by measuring the near-threshold photoproduction of the J/ψ particle, which can be used to determine the gluonic gravitational form factors of the proton. J/ψ particles have about equal probability to decay into an electron or a muon pair. We analyzed the muon channel of J/ψ for the first time, complementing the electron-channel results that were recently published in Nature. By measuring the differential cross-section of J/ψ that decay into muons, we doubled the available statistics to study the proton mass structure, compared to the published results. We determined the gravitational form factors of the proton, and we concluded that the proton's mass radius is significantly smaller than its electric charge radius.