System-size dependence of the charged-particle pseudorapidity density at $\sqrt{s_{\rm NN}} = 5.02$ TeV for pp, p-Pb, and Pb-Pb collisions

We present and compare the charged-particle pseudorapidity densities for pp, p-Pb, and Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV measured over a wide pseudorapidity range (${-3.5 <~\eta <~5}$), using ALICE at the Large Hadron Collider. The distributions for p-Pb and Pb-Pb collisions are determined as a function of the centrality of the collisions, while results from pp collisions are reported for inelastic events with at least one charged particle at midrapidity. The charged-particle pseudorapidity densities are, under simple and robust assumptions, transformed to charged-particle rapidity densities. This allows for the calculation and the presentation of the evolution of the width of the rapidity distributions and of a lower bound on the Bjorken energy density, as a function of the number of participants in all three collision systems. We find a decreasing width of the particle production, and roughly a ten fold increase in the energy density, as the system size grows.

 

Submitted to: PLB
e-Print: arXiv:2204.10210 | PDF | inSPIRE
CERN-EP-2022-055
Figure group

Figure 1

Charged-particle pseudorapidity density in Pb\==Pb~ and p\==Pb for the $5\%$ most central collisions, and for pp collisions with \INELGtZero{} trigger class. For symmetric collision systems (Pb\==Pb and pp) the data has been symmetrised around ${\eta=0}$ and points for ${\eta>3.5}$ have been reflected around ${\eta=0}$. The lines show fits of \EqRef{eq:f} (Pb\==Pb and pp) and \EqRef{eq:g} (p\==Pb) to the data (see text). Please note that the ordinate has been cut twice to accommodate for the very different ranges of the charged-particle pseudorapidity densities.

Figure 2

Charged-particle pseudorapidity density in p\==Pb collisions at ${\FiveTeV}$ in seven centrality classes based on the V0A and V0C estimators. The lines are obtained using a fit of a scaled, normal distribution in rapidity Eq.~\eqref{eq:g} to the data (see text for details).

Figure 3

Ratio ${r_X}$ of the charged-particle pseudorapidity density in Pb\==Pb (top) and p\==Pb (bottom) in different centrality classes to the charged-particle pseudorapidity density in pp in the \INELGtZero{} event class. Note, for Pb\==Pb ${\etalab}$ is the same as the centre-of-mass pseudorapidity.

Figure 4

The width (top) and effective ${\textpTm}$ (bottom) fit parameters as a function of the mean number of participants in pp, p\==Pb, and Pb\==Pb collisions at ${\FiveTeV}$ Vertical uncertainties are the standard error on the best-fit parameter values, while horizontal uncertainties reflect the uncertainty on $\langle N_{\mathrm{part}}\rangle$ from the Glauber calculations. Also shown are similar fit parameters from the same parameterisation of EPOS-LHC calculations as well as the relevant numbers extracted directly from those calculations.

Figure 5

The transverse area ${\ST}$ as calculated in a numerical Glauber model for two extreme cases: a) only the exclusive overlap of nucleons is considered (${\cap}$, open markers) and b) the inclusive area of participating nucleons contribute (${\cup}$, closed markers) in both p\==Pb and Pb\==Pb at ${\FiveTeV}$.

Figure 6

Estimate of the lower bound on the Bjorken transverse energy density in pp, p\==Pb, and Pb\==Pb collisions at ${\FiveTeV}$, considering the exclusive ($\cap$, open markers) and inclusive ($\cup$, full markers) overlap area ${\ST}$ of the nucleons. The expression ${C\Npart{}^p}$ is fitted to case ${\cup}$, and we find ${C=\unit[(0.8\pm0.3)]{\GeVfmc}}$ and ${p=0.44\pm0.08}$ Also shown is a similar estimate from Pb\==Pb collisions at ${\TwoTeV}$ (stars with uncertainty band)~.