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Measurements of the fluctuations of identified particles in ALICE at the LHC
Abstract
The event-by-event fluctuations of identified particles in ultrarelativistic nucleus-nucleus collisions give information about the state of matter created in these collisions as well as the phase diagram of nuclear matter. In this proceedings, we present the latest results from ALICE on the centrality and pseudorapidity dependence of net-proton fluctuations, which are closely related to net-baryon fluctuations, as well as net-kaon and net-pion fluctuations. The effects of volume fluctuations and global baryon conservation on these observables are discussed. Furthermore, the correlated fluctuations between different particle species, quantified by the observable $\nu_{dyn}$, are also shown as functions of multiplicity and collision energy and are compared with Monte Carlo models. These measurements are performed in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV using the novel Identity Method and take advantage of the excellent particle identification capabilities of ALICE.
... A large variety of susceptibilities are currently determined by lattice QCD methods up to the fourth order [11]. On the experimental side, moments of the net proton number distribution are also measured up to the fourth order [12][13][14]. Unfortunately, baryon number cannot be measured, as neutrons are not detected in many current experimental set-ups. ...
... Nevertheless, there are arguments that claim that the protons are a good proxy for the baryon number [15,16]. In addition to net baryon number, the fluctuations of the number of strange particles are also measured [13,25]. ...
... Their equilibrium values can all be calculated from proper combinations of the factorial moments and using the equilibrium values derived in eqs. (13). ...
Measured moments of the multiplicity distribution for a given sort of particles are used in the literature for the determination of the phase transition parameters of hot QCD matter in ultrarelativistic heavy-ion collisions. We argue that the subsequent cooling in the hadronic phase, however, may drive the multiplicity distribution out of equilibrium. We use a master equation for the description of the evolution of the multiplicity distribution to demonstrate how the different moments depart away from their equilibrium values. If such moments were measured and interpreted as if they were equilibrated, one would obtain different apparent temperatures from different moments.
... Fluctuations of net numbers of 's, kaons and pions are also being performed, and preliminary results were reported in Refs. [96][97][98]. Here we would like to discuss the behavior of these quantities within our approach. ...
... The BQS-canonical calculation is depicted by the dash-dotted magenta line, showing a further suppression of the variance-over-Skellam ratio when strangeness and electric charge conservation is implemented. The resulting η acc dependence of net kaon fluctuations agrees with the preliminary data of the ALICE Collaboration [96,98], shown in Fig. 8 by the gray bands, although the experimental uncertainties are quite large. ...
We revisit the problem of particlization of a QCD fluid into hadrons and resonances at the end of the fluid dynamical stage in relativistic heavy-ion collisions in a context of fluctuation measurements. The existing methods sample an ideal hadron resonance gas, and therefore, they do not capture the non-Poissonian nature of the grand-canonical fluctuations, expected due to QCD dynamics such as the chiral transition or QCD critical point. We address the issue by partitioning the particlization hypersurface into locally grand-canonical fireballs populating the space-time rapidity axis that are constrained by global conservation laws. The procedure allows to quantify the effect of global conservation laws, volume fluctuations, thermal smearing, and resonance decays on fluctuation measurements in various rapidity acceptances and can be used in fluid dynamical simulations of heavy-ion collisions. As a first application, we study event-by-event fluctuations in heavy-ion collisions at the Large Hadron Collider (LHC) using an excluded volume hadron resonance gas model matched to lattice QCD susceptibilities, with a focus on (pseudo)rapidity acceptance dependence of net baryon, net proton, and net charge cumulants. We point out large differences between net proton and net baryon cumulant ratios that make direct comparisons between the two unjustified. We observe that the existing experimental data on net-charge fluctuations at the LHC shows a strong suppression relative to a hadronic description.
We study the influence of the centrality definitionCentrality definition and detector efficiency on the net-proton kurtosis for minimum bias AuAu collisions at a beam energy of GeV by using the UrQMD model. We find that different ways of defining the centrality lead to different cumulant ratiosCumulant ratios. Moreover, we demonstrate that the kurtosis is suppressed for central collisions when a wider transverse momentum acceptance is used. Finally, the influence of a detector efficiency on the measured cumulant ratiosCumulant ratios is estimated.
Measured moments of the multiplicity distribution for a given sort of particles are used in the literature for the determination of the phase transition parameters of hot QCD matter in ultrarelativistic heavy-ion collisions. We argue that the subsequent cooling in the hadronic phase, however, may drive the multiplicity distribution out of equilibrium. We use a master equation for the description of the evolution of the multiplicity distribution to demonstrate how the different moments depart away from their equilibrium values. If such moments were measured and interpreted as if they were equilibrated, one would obtain different apparent temperatures from different moments.
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