Centrality and pseudorapidity dependence of the charged-particle multiplicity density in Xe–Xe collisions at

In this Letter, the ALICE Collaboration presents the first measurements of the charged-particle multiplicity density, $d{N}_{\mathrm{ch}}/d\eta $, and total charged-particle multiplicity, ${N}_{\mathrm{ch}}^{\mathrm{tot}}$, in Xe–Xe collisions at a centre-of-mass energy per nucleon–nucleon pair of $\sqrt{{s}_{\mathrm{NN}}}=5.44\phantom{\rule{0.25em}{0ex}}\text{TeV}$. The measurements are performed as a function of collision centrality over a wide pseudorapidity range of $-3.5<\eta <5$. The values of $d{N}_{\mathrm{ch}}/d\eta $ at mid-rapidity and ${N}_{\mathrm{ch}}^{\mathrm{tot}}$ for central collisions, normalised to the number of nucleons participating in the collision (${N}_{\mathrm{part}}$) as a function of $\sqrt{{s}_{\mathrm{NN}}}$ follow the trends established in previous heavy-ion measurements. The same quantities are also found to increase as a function of ${N}_{\mathrm{part}}$, and up to the 5% most central collisions the trends are the same as the ones observed in Pb–Pb at a similar energy. For more central collisions, the Xe–Xe scaled multiplicities exceed those in Pb–Pb for a similar ${N}_{\mathrm{part}}$. The results are compared to phenomenological models and theoretical calculations based on different mechanisms for particle production in nuclear collisions. All considered models describe the data reasonably well within 15%.