This paper presents the primary charged-particle multiplicity distributions in proton-lead collisions at a centre-of-mass energy per nucleon-nucleon collision of $\sqrt{s_{\rm NN}} = 5.02$ TeV. The distributions are reported for non-single diffractive collisions in different pseudorapidity ranges. The measurements are performed using the combined information from the Silicon Pixel Detector and the Forward Multiplicity Detector of ALICE. The multiplicity distributions are parametrised with a double negative binomial distribution function which provides satisfactory descriptions of the distributions for all the studied pseudorapidity intervals. The data are compared to models and analysed quantitatively, evaluating the first four moments (mean, standard deviation, skewness, and kurtosis). The shape evolution of the measured multiplicity distributions is studied in terms of KNO variables and it is found that none of the considered models reproduces the measurements. This paper also reports on the average charged-particle multiplicity, normalised by the average number of participating nucleon pairs, as a function of the collision energy. The multiplicity results are then compared to measurements made in proton-proton and nucleus-nucleus collisions across a wide range of collision energies.
Submitted to: EPJC
e-Print: arXiv:2502.18081 | PDF | inSPIRE
CERN-EP-2025-023
Figure group
Figure 2
Charged-particle multiplicity distributions for different pseudorapidity intervals measured in p-Pb collisions at $\sqrt{s_{\rm NN} }= 5.02$ TeVfor NSD collisions. The green lines show fits of a double NBD function to the data. The ratios of the data to the fits are shown in the bottom panels. |  |
Figure 3
The pseudorapidity dependence of the double NBD parameters: $\langle n \rangle_1$, $\langle n \rangle_2$, $k_1$, and $k_2$ in p-Pb collisions at $\sqrt{s_{\rm NN} }= 5.02$ TeV in comparison with pp measurements at $\sqrt{s} =$ 0.9, 7, and 8 TeV. For p-Pb collisions, the $\langle n \rangle_1$ and $\langle n \rangle_2$ values are scaled by the $\langle \Npart \rangle$/2. |  |
Figure 4
Four moments: $\langle N_{\rm ch}\rangle$, $\sigma$, $S$, and $\kappa$ of charged-particle multiplicity distributions for different pseudorapidity intervals in \pPb collisions at \fivenn. Both skewness and kurtosis are plotted on two different ordinate scales to better visualize their respective variations. Predictions from the HIJING, DPMJET, and \linebreak \angantyr event generators are superimposed. |  |
Figure 5
KNO-scaled multiplicity distribution versus the KNO variable $N_{\rm ch}/\langle N_{\rm ch}\rangle$ in NSD \pPb collisions at \fivenn for various pseudorapidity intervals. Comparison with predictions from HIJING, DPMJET, \angantyr, and the IP-Glasma model are shown. The ratios between models and data are calculated using a linear interpolation between adjacent points. |  |
Figure 6
Values of $\frac{2}{\langle N_{\rm part} \rangle} \langle N_{\rm ch} \rangle$ for minimum-bias \pp , \ppbar , \pPb and central AA collisions as a function of \snn are shown. The $s_{\mathrm{NN}}$-dependencies of INEL \pp (\ppbar) and NSD \pp (\ppbar) collisions are proportional to $s_{\mathrm{NN}}^{0.120}$ and $s_{\mathrm{NN}}^{0.127}$ respectively. The results from central AA collisions are proportional to $s_{\mathrm{NN}}^{0.192}$. The bands represent the uncertainties on the extracted power-law dependencies. |  |