Relative particle yield fluctuations in $\text{ Pb--Pb }$ collisions at $\sqrt{s_\mathrm{{NN}}} =2.76\hbox { TeV}$

Abstract

First results on $\hbox {K}/\pi$, $\hbox {p}/\pi$ and K/p fluctuations are obtained with the ALICE detector at the CERN LHC as a function of centrality in $\text{ Pb--Pb }$ collisions at $\sqrt{s_\mathrm{{NN}}} =2.76\hbox { TeV}$. The observable $\nu _{\mathrm{dyn}}$, which is defined in terms of the moments of particle multiplicity distributions, is used to quantify the magnitude of dynamical fluctuations of relative particle yields and also provides insight into the correlation between particle pairs. This study is based on a novel experimental technique, called the Identity Method, which allows one to measure the moments of multiplicity distributions in case of incomplete particle identification. The results for $\hbox {p}/\pi$ show a change of sign in $\nu _{\mathrm{dyn}}$ from positive to negative towards more peripheral collisions. For central collisions, the results follow the smooth trend of the data at lower energies and $\nu _{\mathrm{dyn}}$ exhibits a change in sign for $\hbox {p}/\pi$ and K/p.

1 Introduction

The theory of strong interactions, Quantum Chromodynamics (QCD), predicts that at sufficiently high energy density nuclear matter transforms into a deconfined state of quarks and gluons known as Quark–Gluon Plasma (QGP) [1, 2]. One of the possible signatures of a transition between the hadronic and partonic phases is the enhancement of fluctuations of the number of produced particles in the hadronic final state of relativistic heavy-ion collisions [3,4,5]. Event-by-event fluctuations and correlations may show critical behaviour near the phase boundary, including the crossover region where there is no thermal singularity, in a strict sense, associated with the transition from a QGP phase to a hadron-gas phase. A correlation analysis of event-by-event abundances of pions, kaons and protons produced in $\text{ Pb--Pb }$ collisions at LHC energies may provide a connection to fluctuations of globally conserved quantities such as electric charge, strangeness and baryon number, and therefore shed light on the phase structure of strongly interacting matter [6].

In view of the predicted criticality signals at crossover for vanishing net-baryon densities [7], event-by-event fluctuations of relative particle yields are studied using the fluctuation measure $\nu _\mathrm{dyn}[A,B]$ [8] defined in terms of moments of particle multiplicity distributions as

$$\begin{aligned} \nu _\mathrm{dyn}[A,B]= & {} \dfrac{\langle N_{A}(N_{A}-1) \rangle }{{\langle N_{A} \rangle }^{2}} + \dfrac{\langle N_{B}(N_{B}-1) \rangle }{{\langle N_{B} \rangle }^{2}} \nonumber \\&- 2\dfrac{\langle N_{A}N_{B} \rangle }{\langle N_{A} \rangle \langle N_{B} \rangle }, \end{aligned}$$

(1)

where $N_{A}$ and $N_{B}$ are the multiplicities of particles A and B measured event-by-event in a given kinematic range. The $\nu _\mathrm{dyn}[A,B]$¹ fluctuation measure contrasts the relative strength of fluctuations of species A and B to the relative strength of correlations between these two species. It vanishes when the particles A and B are produced in a statistically independent way [8, 9].

This study at LHC energies is of particular importance for establishing the energy and system size dependence of $\nu _{\mathrm{dyn}}$ in order to understand the trend observed at lower collision energies from the RHIC Beam Energy Scan (BES) results reported by the STAR collaboration [10]. Furthermore, the advantage of this fluctuation measurement is its robustness against non-dynamical contributions such as those stemming from participant nucleon fluctuations and finite particle detection efficiencies [8, 11]. Measurements of the $\nu _{\mathrm{dyn}}$ observable for net-charge fluctuations were already published by ALICE [12]. Moreover, for identified particles, it was measured at the Super Proton Synchrotron (SPS) [13] and at the Relativistic Heavy-Ion Collider (RHIC) [