Neutron Spectra of Proton-Oxygen Collisions at the Large Hadron Collider (CERN)

The 2025 proton–oxygen (p–O) collision run at the LHC provides a bridge between small systems (p–p, p–Pb) and heavy-ion (Pb–Pb) collisions, creating an opportunity to explore the onset of collective phenomena and nuclear effects in an intermediate-size system. Forward neutron production is of particular interest, as it offers direct sensitivity to nuclear breakup, spectator dynamics, and initial geometry. These observables are also relevant for astrophysics, since they improve hadronic interaction models used to describe cosmic-ray collisions with the oxygen-rich atmosphere.

This study focuses on measuring and characterizing the neutron spectra in p–O collisions at √sₙₙ = 13.6 TeV using the CMS Zero Degree Calorimeter (ZDC). Neutron yields and multiplicities are reconstructed in the hadronic section of the ZDC, with calibration anchored to the one-neutron peak. Timing information from the ZDC’s 25 ns time slices is employed to suppress background and select in-time signals. Multiplicity classes (0n, 1n, 2n, …) are extracted via template fits to the ZDC spectra, while correlations with forward energy in the Hadronic Forward (HF) calorimeter are used to study centrality-like features in p–O events.

Preliminary results show distinct neutron peaks, consistent with expectations from oxygen nuclear breakup, and indicate that forward neutron multiplicity can serve as a tag for collision geometry. These spectra provide a crucial baseline for comparisons with p–p and Pb–Pb results, enabling tests of the minimum system size required for collective effects and parton energy loss. Beyond collider physics, the results deliver valuable input to cosmic-ray air-shower models. Overall, this work highlights the capability of the CMS ZDC to extend its physics reach from heavy-ion to light-ion collisions, opening new avenues for studying nuclear structure and small-system QCD phenomena at the LHC.