Charged-particle Pseudorapidity Density at Mid-Rapidity in P–pb Collisions at $$\pmb {\sqrt{s_{\scriptscriptstyle {\mathrm{nn}}}}}$$ S 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.