Peering into Protons
How are the constituents of a proton—quarks and gluons—affected by their surroundings when the proton is bound in a nucleus? Jackson Pybus at MIT and his colleagues have tackled this fundamental and open question by firing beams of high-energy photons at various nuclei and analyzing the emitted particles [1]. The team’s new results provide the strongest evidence so far that the distribution of gluons in bound protons differs from that in free protons.
Pybus and his colleagues focused on a process in which a photon hits a proton in a nucleus with enough energy to eject the proton. The interaction also generates a J/ particle—a bound state of a charm quark and a charm antiquark—that quickly decays into an electron and a positron. For a free proton, the corresponding interaction would require the energy of the incoming photon to exceed 8.2 giga-electron-volts (GeV). But for a bound proton, the process can sometimes occur at energies below this threshold. Such subthreshold interactions are particularly sensitive to the gluon distribution in bound protons.
The researchers fired relativistic electrons at a diamond target to produce photons with energies between 7 and 10.8 GeV. These photons were aimed at nuclei of deuterium, helium, and carbon to generate J/ particles, whose decay products the researchers detected. Comparing the measurements with various nuclear-physics models, the team inferred that bound protons have a denser or more compact gluon structure. To analyze their data, the researchers introduced new methods, which they say will be useful for future measurements of J/ production at the upcoming Electron-Ion Collider at Brookhaven National Laboratory in New York and at other facilities.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- J. R. Pybus et al., “First measurement of near-threshold and subthreshold J/ photoproduction off nuclei,” Phys. Rev. Lett. 134, 201903 (2025).