Charged-particle pseudorapidity density at mid-rapidity in p–Pb collisions at √sNN = 8.16 TeV

The pseudorapidity density of charged particles, d Nch /dη, in p–Pb collisions has been measured at a centre- of-mass energy per nucleon–nucleon pair of √sNN = 8.16 TeV at mid-pseudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, |η| < 1.8. The dNch/dη value is 19.1 ± 0.7 at |η| < 0.5. This quantity divided by ⟨Npart⟩/2 is 4.73±0.20, where ⟨Npart⟩is the aver- age number of participating nucleons, is 9.5% higher than the corresponding value for p–Pb collisions at √sNN = 5.02 TeV. Measurements are compared with models based on dif- ferent 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 d Nch /dη dis- tribution. Saturation-based models reproduce the distribu- tions well for η > −1.3. The dNch/dη 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 partic- ipants for systems like p–Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrat- ing the advantages of determining the centrality with energy deposited near beam rapidity.