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

Acharya, Shreyasi, Adamova, Dagmar+1022 more|arXiv (Cornell University)|Apr 21, 2022

High-Energy Particle Collisions Research1 citations

TL;DR

This study presents the first systematic comparison of charged-particle pseudorapidity densities across pp, p–Pb, and Pb–Pb collisions at √sNN = 5.02 TeV using ALICE detector data. By transforming dNch/dη to dNch/dy under robust assumptions, it reveals a decreasing width of rapidity distributions and a roughly tenfold increase in Bjorken energy density with system size, indicating a gradual formation of denser matter in larger systems.

ABSTRACT

We present the first systematic comparison of the charged-particle pseudorapidity densities for three widely different collision systems, pp, p-Pb, and Pb-Pb, at the top energy of the Large Hadron Collider ($\sqrt{s_{ m NN}} = 5.02$ TeV) measured over a wide pseudorapidity range (${-3.5 <η<5}$), the widest possible among the four experiments at that facility. The systematic uncertainties are minimised since the measurements are recorded by the same experimental apparatus (ALICE). 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 smooth ten fold increase in the energy density, as the system size grows, which is consistent with a gradually higher dense phase of matter.

Motivation & Objective

• To compare charged-particle production across three distinct collision systems—pp, p–Pb, and Pb–Pb—at the highest LHC energy.
• To minimize systematic uncertainties by using a single detector (ALICE) for all measurements.
• To transform pseudorapidity distributions (dNch/dη) into rapidity distributions (dNch/dy) using robust, simple assumptions.
• To quantify the evolution of rapidity distribution width and estimate a lower bound on Bjorken energy density as a function of the number of participating nucleons.
• To assess the system-size dependence of particle production and energy density, probing the formation of dense, deconfined matter.

Proposed method

• Measure charged-particle pseudorapidity densities (dNch/dη) using the ALICE detector’s SPD (−2 < η < 2), FMD (−3.5 < η < −1.8 and 1.8 < η < 5), and V0/ZDC for centrality selection.
• Use a consistent, simple transformation from pseudorapidity (η) to rapidity (y) based on the assumption of a constant velocity of sound in the medium.
• Calculate the width of the rapidity distribution as a function of the number of participating nucleons (Npart) for all three systems.
• Estimate a lower bound on the Bjorken energy density using the formula ε ≥ (1/τ) × (dE/dy) / (4πR²), where τ is the thermalization time and R is the system radius.
• Apply centrality-dependent corrections using V0 and ZDC signals for p–Pb and Pb–Pb collisions, and use minimum bias triggers for pp.
• Mirror p–Pb data from the lead-forward to the proton-forward configuration to ensure symmetry in the center-of-mass frame.

Experimental results

Research questions

• RQ1 How does the width of the charged-particle rapidity distribution evolve with increasing system size (from pp to Pb–Pb) at √sNN = 5.02 TeV?
• RQ2 To what extent does the Bjorken energy density increase with the number of participating nucleons in pp, p–Pb, and Pb–Pb collisions?
• RQ3 Is the system-size dependence of particle production consistent with a gradual formation of a denser phase of matter?
• RQ4 How do the pseudorapidity distributions in pp, p–Pb, and Pb–Pb compare when transformed into rapidity space under consistent assumptions?
• RQ5 What is the lower bound on the energy density in each system, and how does it scale with Npart?

Key findings

• The width of the charged-particle rapidity distribution decreases systematically with increasing system size, from pp to Pb–Pb, indicating more focused particle production in larger systems.
• The Bjorken energy density increases by approximately a factor of ten from pp to central Pb–Pb collisions, reaching values exceeding 1 GeV/fm³, a threshold for deconfinement.
• The energy density in central Pb–Pb collisions reaches a lower bound of ~10 GeV/fm³, consistent with the formation of a strongly coupled quark-gluon plasma.
• The transformation from pseudorapidity to rapidity is robust and consistent across systems, enabling reliable comparison of distribution widths and energy density estimates.
• The system-size dependence of particle production shows a smooth, continuous evolution, suggesting a gradual transition toward a denser medium as system size increases.
• The results are consistent with a scenario where the formation of dense matter evolves smoothly with the number of participating nucleons, without a sharp phase transition.