Charged Particle Ratio Fluctuation as a Signal for Quark-Gluon Plasma

Abstract

In this letter we argue that the event-by-event fluctuations of the ratio of the positively charged and the negatively charged pions provides a signal of quark-gluon plasma. The fact that quarks carry fractional charges is ultimately responsible for this distinct signal.It is of great importance that we have a clear signal of the long-sought quark-gluon plasma (QGP) not only for the experiments at RHIC but also for theoretical reasons. At stake is our fundamental understanding of strong interactions as well as understanding of the state of matter in the very early universe [1]. Proposed signals of this new state of matter abound in literature [2] one of the most studied being the J/ψ suppression [3,4].In this paper, we propose the event-by-event h + /h − fluctuations as a distinct signal of QGP formation. We would also like to stress that this observable is something that can be and already has been calculated on a lattice. The idea is very simple and is reminiscent of the original detection of color in e + e − experiment where one measuresHere Q q is the charge of each flavor and N c is the number of colors. Note that if the fundamental degrees of freedom were hadrons, R e + e − would be very different from this simple counting. We would like to establish that the event-by-event h + /h − fluctuations can similarly determine whether the underlying degrees of freedom are quarks and gluons or hadrons.The point is that in the QGP phase, the unit of charge is 1/3 while in the hadronic phase, the unit of charge is 1. The net charge, of course does not depend on such subtleties. However, the fluctuation in the net charge depends on the squares of the charges and hence strongly depend on which phase it originates from. Measuring the charge fluctuation itself, however, is plagued by systematic uncertainties such as volume fluctuations due to the impact parameter variation. In a previous letter [5], we showed that the multiplicity ratio fluctuation is only sensitive to the density fluctuations and not to the volume fluctuations. The task for us is then to find a suitable ratio whose fluctuation is easy to measure and simply related to the net charge fluctuation.The obvious candidate is the ratio F = Q/N ch whereis the net charge andis the charge multiplicity. Here N ± denote the positive and negative multiplicities. Instead of using F , however, in this paper we propose to use the charge ratio R = N + /N − . The advantages of using R over F are that although trivially related, R is more fundamental to experiments and the signal is about 4 times amplified in R as we show below.To relate R with F , we first rewrite the charge ratio asWhen N ch ≫ Q we can safely say |F | ≪ 1. Expanding in terms of F yieldsDefining δx = x − x for any fluctuating quantity x, it is easy to showwhere · · · denotes the average over all events. Let us now consider δF 2 more closely. In a previous letter [5] (see also [6] and the upcoming paper [7]), we showed that a ratio fluctuation can be expressed asWe then showed that when the average ratio i...

Event-by-event fluctuations at 40, 80 and 158AGeV/cin Pb+Au collisions

“…The observed fluctuations are lower than the charge conservation limit. Since the fluctuations are much above the QGP model predictions of ∼ −3.5 [2,3], no indication for the formation of a deconfined phase is found. Decreasing tendencies of N ν dyn with N part are observed at the three beam energies.…”

“…On the other hand, the fluctuations can be used to evaluate the extent of thermal equilibrium by comparison with the extrapolated fluctuations from nucleon-nucleon collisions. Suppressed net charge fluctuations have been proposed as a possible signal for the formation of a QGP, due to the smaller charge units of deconfined (anti-) quarks compared to those of confined hadrons [2,3]. Comparisons with cascade models may clarify whether the observed fluctuations are described by resonance gas models.…”

Search for deconfinement in NA49 at the CERN SPS

“…Recently it was proposed that event-to-event fluctuations of the charge ratio R=N + /N − or the net charge Q=N + -N − may be sensitive to deconfinement in the early stage of nucleus-nucleus collisions [18,19]. The smaller charge quanta in a partonic phase are expected to result in a reduction of such fluctuations.…”

Fluctuations and correlations in STAR

“…The study of correlations and fluctuations can provide evidence for the production of the quark gluon plasma (QGP) in relativistic heavy ion collisions. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Various theories predict that the production of a QGP phase in relativistic heavy ion collisions could produce significant event-by-event correlations and fluctuations in temperature, transverse momentum, multiplicity, and conserved quantities such as net charge. Several recent experimental studies at the SPS [17][18][19] and at RHIC [20][21][22][23][24] have focused on the study of fluctuations in relativistic heavy ion collisions.…”