Abstract
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 19.1pm 0.7 at |eta |<0.5. This quantity divided by langle N_{mathrm{part}} rangle / 2 is 4.73pm 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.
Highlights
This paper presents a measurement of the primary chargedparticle density in p–Pb collisions,√dNch/dηlab, at a nucleon– nucleon centre-of-mass energy of sNN = 8.16 TeV for pseudorapidities |ηlab| < 1.8 in the laboratory system
The result is compared with several models with different descriptions of particle production, all shifted by ηlab = 0.465 to take into account the shift to the laboratory system
In the improved HIJING 2.1 [12,13] version the Cronin effect is included, as well as a strong nuclear shadowing effect in order to explain the global properties of the final hadron system in p–Pb collisions [34]
Summary
This paper presents a measurement of the primary chargedparticle density in p–Pb collisions,√dNch/dηlab, at a nucleon– nucleon centre-of-mass energy of sNN = 8.16 TeV for pseudorapidities |ηlab| < 1.8 in the laboratory system. Diffractive events can be single-, double- or central-diffractive and results are presented for non-single-diffractive (NSD) events. Data are compared to other experimental measurements available in pp, p–Pb, d–Au and AA collisions. Results are compared with simulations (performed with HIJING 2.1 [12,13], EPOS 3 [14,15,16] and EPOS LHC [17]) and calculations incorporating the saturation of the gluon density in the colliding hadrons (MC-rcBK [18,19] and KLN [20,21])
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