Centrality dependence of the charged-particle multiplicity density at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

Abstract

The pseudorapidity density of charged particles ($\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta$) at mid-rapidity in Pb-Pb collisions has been measured at a center-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. It increases with centrality and reaches a value of $1943\pm 54$ in $|\eta |<0.5$ for the 5% most central collisions. A rise in $\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta$ as a function of $\sqrt{s_{\mathrm{NN}}}$ for the most central collisions is observed, steeper than that observed in proton-proton collisions and following the trend established by measurements at lower energy. The centrality dependence of $\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta$ as a function of the average number of participant nucleons, $⟨N_{\mathrm{part}}⟩$, calculated in a Glauber model, is compared with the previous measurement at lower energy. A constant factor of about 1.2 describes the increase in $\frac{2}{⟨N_{\mathrm{part}}⟩}⟨\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta ⟩$ from $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV to $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV for all centrality intervals, within the measured range of 0-80% centrality. The results are also compared to models based on different mechanisms for particle production in nuclear collisions.

17 pages, 3 captioned figures, 1 table, authors from page 12, published version, figures a http://aliceinfo.cern.ch/ArtSubmission/node/2215