Non-monotonic behavior of multiplicity fluctuations

We discuss recently measured event-by-event multiplicity fluctuations in relativistic heavy-ion collisions. It is shown that the non-monotonic behavior of the multiplicity fluctuations as a function of collision centrality can be fully explained by the correlations between produced particles.. 1. Introduction Event-by-event fluctuations in heavy-ion collisions have been recently measured both at CERN SPS and BNL RHIC (see a brief review [1]). The data (usually used F or Φ measures to quantify the fluctuations of transverse momentum) which show a non-trivial behavior as a function of collision centrality, have been theoretically discussed from very different points of view, including complete or partial equilibration, critical phenomena, string or cluster percolation, production of jets. In spite of these efforts a mechanism responsible for the fluctuations is far from being uniquely identified. In [2] we have established connection of the Φ-measure with the fluctuations of multiplicity and advocated that fluctuations of transverse momentum are practically irrelevant for the considered problem, i.e., for the apparent structure seen in collision centrality dependence. We argue [3] that the non-trivial behavior of the so called transverse momentum fluctuations (quantified by F or Φ-measure) can be fully explained by the multiplicity fluctuations. Recently, the NA49 Collaboration published very first data on multiplicity fluctuations as a function of collisions centrality [4], see also [5] in this volume. Unexpectedly, the ratio V ar(N)/N , where V ar(N) is the variance and N is the average multiplicity of negative particles, changes non-monotonically when number of wounded nucleons grows. It is close to unity at fully peripheral (N w ≤ 10) and completely central (N w ≥ 250) collisions but it manifests a prominent peak at N w ≈ 70. The measurement has been performed at the collision energy 158 AGeV in the transverse momentum and pion rapidity intervals (0.005, 1.5) GeV and (4.0, 5.5), respectively. The azimuthal acceptance has been also limited, and about 20% of all produced negative particles have been used in the analysis. The aim of this paper is to show that the observed effect (non-monotonic dependence of the ratio V ar(N)/ < N > on the number of projectile participants N P , i.e., on the production volume V) stem from the correlations between produced particles and the correlations are of nuclear and electromagnetic interactions origin.