Charged-particle pseudorapidity density at mid-rapidity in p-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 8.16 TeV

The pseudorapidity density of charged particles, $\mathrm {d}N_{\mathrm{ch}}/\mathrm {d}\eta $ , in p–Pb collisions has been measured at a centre-of-mass energy per nucleon–nucleon pair of $\sqrt{s_{\scriptscriptstyle {\mathrm{NN}}}}$ = 8.16 TeV at mid-pseudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, $|\eta |<1.8$ . The $\mathrm {d}N_{\mathrm{ch}}/\mathrm {d}\eta $ value is $19.1\pm 0.7$ at $|\eta |<0.5$ . This quantity divided by $\langle N_{\mathrm{part}} \rangle $ / 2 is $4.73\pm 0.20$ , where $\langle N_{\mathrm{part}} \rangle $ is the average number of participating nucleons, is 9.5% higher than the corresponding value for p–Pb collisions at $\sqrt{s_{\scriptscriptstyle {\mathrm{NN}}}}$ = 5.02 TeV. Measurements are compared with models based on different mechanisms for particle production. All models agree within uncertainties with data in the Pb-going side, while HIJING overestimates, showing a symmetric behaviour, and EPOS underestimates the p-going side of the $\mathrm {d}N_{\mathrm{ch}}/\mathrm {d}\eta $ distribution. Saturation-based models reproduce the distributions well for $\eta >-1.3$ . The $\mathrm {d}N_{\mathrm{ch}}/\mathrm {d}\eta $ is also measured for different centrality estimators, based both on the charged-particle multiplicity and on the energy deposited in the Zero-Degree Calorimeters. A study of the implications of the large multiplicity fluctuations due to the small number of participants for systems like p–Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrating the advantages of determining the centrality with energy deposited near beam rapidity.