A model of proton-oxygen collisions predicts a striking change in where charged-particle production is strongest. In the most central collision classes, the maximum of the modeled distribution lies on the oxygen-going side; in more peripheral classes, it moves to the proton-going side. The result is a prediction, not a measurement: the larger-pseudorapidity data needed to check the switch are not available, and the ALICE data used so far are preliminary.
How the comparison was built
The study examines proton-oxygen collisions at 9.618 TeV, comparing model calculations with preliminary ALICE measurements across eight centrality classes. It follows charged-hadron distributions in pseudorapidity, the coordinate in which the particle distribution is reported, and extends the calculation to larger pseudorapidities where data are unavailable. Before the comparison, the calculation applies a Jacobian transformation and accounts for the rapidity shift between the nucleon-nucleon center-of-mass frame and the laboratory frame.
At the heart of the calculation is a three-source construction. One source is a gluon-gluon central source, while two others are valence-quark soft-gluon fragmentation sources. Color-glass states initialize the fragmentation sources; partial thermalization is modeled with relativistic diffusion, and the central source uses kT factorization. The fragmentation-source evolution was computed with a C++ finite-element code originally written for proton-lead collisions and later modified for proton-oxygen. Transport parameters were taken from proton-lead chi-squared fits to ALICE data and scaled by centrality for different system sizes and timescales.
Centrality also enters through a Glauber calculation, which supplies the participant numbers used to scale the oxygen-going valence-quark distribution. The reported mean participant count falls from 6.9 in the 0-5% class to 2.3 in the 60-90% class, while the saturation-scale parameter falls from 0.031 to 0.010 GeV2.
What changes across centrality
One of the clearest gaps came in the overall scale. Preliminary ALICE data were reported as a factor of 1.72 higher than the initial prediction. The study then readjusted its model parameters, while the authors said they would avoid very precise quantitative conclusions because the measurements are preliminary rather than final.
Across the model's centrality sequence, the fitted transport parameter rose from 3.6 in the 0-5% class to 19.5 in the 60-90% class. At those endpoints, the central-to-fragmentation ratio fell from 4.98 to 1.66, the oxygen-going-to-proton-going ratio from 0.71 to 0.18, and total Nch from 468 to 81.
For central collisions, the model's proton-oxygen-to-proton-lead central-source ratio relative to fragmentation sources is approximately 5.5.
A prediction still waiting for data
The model's predicted switch is therefore still a model result, not an experimental confirmation. Existing data do not reach the larger pseudorapidities needed to test the inversion, so the movement of the maximum from the oxygen-going side to the proton-going side remains unconfirmed.
Final proton-oxygen data are needed to test the inversion, while measurements at larger pseudorapidities are needed to assess the predicted shape outside the currently compared region. That also leaves open why the preliminary measurements sit 1.72 times above the initial prediction and how final data would change the fitted parameters.
The work is an arXiv preprint, arXiv:2608.25501v1, dated 26 Aug 2026.
The paper says the preliminary ALICE data supporting the findings are available in the cited reference.
Paper data and sources
Original title: Particle production in $p$-O collisions at LHC energy
Authors: Chuan Li, Georg Wolschin
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text