High-Order Collective-Flow Correlations in Heavy-Ion Collisions

Abstract

Using 47r data from the Bevalac streamer chamber, we study azimuthal correlations for fragment pairs and higher-order multiplets. We point out that previous studies of sideward flow have not ruled out the possibility that the effect is dominated by a small number of correlated fragments in each event, as opposed to being a collective motion to which most or all fragments contribute. Based on a simulation, we infer that more than 70% of the forward-going fragments from collisions of Ar on Pb at OAA GeV carry a component of the collective motion.PACS numbers: 25.75.+rIn the field of heavy-ion collisions at intermediate energies, much attention has been devoted to measurements of collective sideward deflection of the reaction products ("sideward flow") [1-10]. Fluid-dynamical models had predicted such an effect and had indicated that measurements of flow can provide information about the equation of state of compressed nuclear matter [11]. More recent developments in the area of microscopic transport models have resulted in progress towards interpretation of data in terms of nuclear state variables [12]. In this Letter, we argue that a quantitative description of "collectivity" (the percentage of fragments that participate in the ordered motion) is complementary to measurements of the strength of the flow, and, for the first time, we present a study of this aspect of the flow phemonenon [13].All of the flow analyses cited above [1-10] focus on the information contained in pair correlations, even though these studies are based on multifragment data from detectors approaching 47r solid-angle acceptance. For instance, in the transverse momentum analysis devised by Danielewicz and Odyniec [4], the orientation of the reaction plane for each event is estimated from the fragment transverse momenta p+ using the vector Q = J2 w jPf> where w is a rapidity-dependent weighting factor with the property w(y c . m .) = -w(-y c . m .); the magnitude of the flow is then characterized by (p x ), the average component of p-1 in the estimated reaction plane. This flow observable is equivalent to a particle-pair correlation of the form (p^ • pf), where it is required only that both fragments in each pair belong to the same event [14]; there is no requirement that each event contribute more than one pair. Of course, the statistical uncertainty associated with the vector Q will become large if there are few measured fragments in each event, but this is immaterial to the present discussion.The concept [15] of studying flow using a detector combination that is substantially simpler [9] than one which identifies fragments and measures momenta over a large fraction of 4n is based on the fact that flow observables such as (p x ) and various differential cross sections as a function of azimuth relative to the event reaction plane utilize only correlations between fragment pairs. As demonstrated below, it is not possible for a pair analysis to distinguish between a highly collective multifragment correlation and a flowlike effect that might, to ...

Flow study in relativistic nuclear collisions by Fourier expansion of azimuthal particle distributions

“…An azimuthal two-particle correlation function C(ψ) = P cor (ψ)/P uncor (ψ), where ψ is the angle between two particles, was proposed in Refs. [13,14] to study transverse flow as a method which is free from uncertainty in the reaction plane determination. Here P cor (ψ) is observed pair distribution and P uncor (ψ) is the distribution generated by "event mixing".…”

Flow analysis from multiparticle azimuthal correlations

“…Multiparticle azimuthal correlations were also used in [43]. These methods are now considered obsolete because they apply only to the "integrated" flow, as defined above, whereas most of the recent analyses concentrate on the differential flow of identified particles, in particular kaons [26,44], η mesons [21], Λ hyperons [27,45], and antiprotons [46].…”

Multivariate cumulants in flow analyses: The Next Generation

“…Measurement of anisotropic flow phenomenon amounts to the measurements of flow amplitudes v n and symmetry planes Ψ n . However, since neither v n nor Ψ n degrees of freedom can be measured directly in each heavy-ion collision, they are estimated indirectly by using the correlation techniques [6,7]. The cornerstone of this alternative approach is the following analytic result,…”