Physicists with the ATLAS experiment at the Large Hadron Collider report evidence that the way quarks and gluons are distributed inside a lead nucleus changes depending on where the nucleus is struck. They studied photon‑induced collisions of lead nuclei at a center‑of‑mass energy of 5.02 TeV per nucleon pair. The result uses 2018 data with an integrated luminosity of 1.72 nb⁻¹ and shows a statistically significant difference between peripheral and less peripheral collisions.

The measurement used ultra‑peripheral collisions (UPCs), in which the two lead nuclei pass by each other far enough apart that they interact mainly by exchanging photons. One photon can hit a nucleon inside the other nucleus and produce pairs of jets — sprays of particles that come from fast quarks or gluons. The experiment reconstructs those jets with the anti‑kT algorithm (radius R = 0.4) and determines kinematic variables called z− (for the photon) and x+ (for the struck nuclear parton). These variables let the team probe the nuclear parton distribution functions, or nPDFs, which describe how quarks and gluons are shared inside a nucleus compared with a free proton.

To separate more peripheral hits from more central ones, the analysis uses the presence or absence of forward neutrons. Neutrons emitted close to the beam direction are measured in zero‑degree calorimeters (ZDCs). Events with no detected forward neutrons on either side (called 0n0n) tend to come from more peripheral collisions, where the photon hits near the edge of the nucleus. Events with at least one forward neutron from the struck nucleus (0nXn) are expected, on average, to come from smaller impact parameters, where the struck region is deeper inside the nucleus. The team also uses rapidity gap requirements to reduce backgrounds from diffractive processes in some kinematic regions.