Centrality dependence of charged-hadron pseudorapidity distributions in oxygen-oxygen collisions at $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV

Abstract: We report the first measurement of charged-hadron pseudorapidity ($η$) distributions in oxygen-oxygen (OO) collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV. The data were recorded by the CMS experiment at the LHC in 2025. Primary charged-hadron yields are measured in the range $|η|$ $\lt$ 2.4 as a function of centrality (the overlap of the two nuclei). The results are compared with previous measurements in lead-lead (PbPb) and xenon-xenon collisions at similar energies, as well as predictions from several Monte Carlo event generators and a hydrodynamic model. The charged-hadron pseudorapidity density in the midrapidity region ($|η|$ $\lt$ 0.5) is $\langle$dN$_{\text{ch}}$/d$η\rangle$ = 41.8 $\pm$ 1.1 (syst) integrated over centrality and 135.0 $\pm$ 4.0 (syst) for the most central (i.e., largest nuclear overlap) events. The hydrodynamic model TRAJECTUM provides the best overall description of the data, particularly in central collisions. The particle density at midrapidity divided by the number of nucleons participating in the interaction in central OO collisions is consistent with that observed in central PbPb collisions at similar collision energy. While the overall energy-scaling behavior observed in other nucleus-nucleus collisions is preserved, the data exhibit deviations from simple participant and system-size scaling, highlighting the role of collision geometry and finite-size effects in light ion collisions.