First analysis of anisotropic flow with Lee–Yang zeros

Abstract

We report on the first analysis of directed and elliptic flow with the new method of Lee-Yang zeroes. Experimental data are presented for Ru+Ru reactions at 1.69A GeV measured with the FOPI detector at SIS/GSI. The results obtained with several methods, based on the event-plane reconstruction, on Lee-Yang zeroes, and on multi-particle cumulants (up to 5th order) applied for the first time at SIS energies, are compared. They show conclusive evidence that azimuthal correlations between nucleons and composite particles at this energy are largely dominated by anisotropic flow. The study of collective flow in relativistic heavy ion reactions is of great interest since it is expected to shed light on our knowledge about the properties of hot and dense nuclear matter and the underlying equation of state (EoS) [1]. As pointed out early on, nuclear collective flow is also influenced by the momentum-dependent interactions and the in-medium nucleon-nucleon cross section [2,3]. Both effects play a crucial role in the determination of the EoS and cannot be neglected at intermediate energies. In this regard both directed and elliptic flow are a field of intense experimental and theoretical researches (see [4] and references therein).Most flow analyses, based either on the reaction plane reconstruction (the so-called event-plane method) [5] or on two-particle azimuthal correlations [6] rely on the assumption that the only correlations are those stemming from the existence of the reaction plane. Other correlations (usually called non-flow), such as small-angle correlations due to final state interactions and quantum sta- * Electronic address: nicole.bastid@clermont.in2p3.fr tistical effects [7], correlations due to resonance decays [8] and mini-jet production [9] are neglected. In recent years, several alternative techniques were introduced, in which non-flow correlations can be unraveled. The cumulant method is based on a cumulant expansion of multiparticle (typically four particles) correlations [10], which eliminates most of non-flow correlations. It has been applied at ultra-relativistic energies, at RHIC and SPS for directed and elliptic flow studies and also for higher harmonic measurements [11,12,13,14,15]. More recently, a new method based on an analogy with the Lee-Yang theory of phase transitions [16], where flow is extracted directly from the genuine correlation between a large number of particles, has been proposed [17,18,19]. This method is expected to provide the cleanest separation between flow and non-flow effects.We present the first analysis of collective flow using the new method of Lee-Yang zeroes. The cumulant method is also applied, for the first time at SIS energies. A comparison with results obtained with the event-plane method is performed. We are thus able to check for the first time the validity of standard methods at SIS energies, by investigating possible contributions of correla-

Systematics of azimuthal asymmetries in heavy ion collisions in the regime

“…Nevertheless we believe that theoretical simulations that reproduce all of the presented results are likely to reproduce also the projections not shown here. Additional information can be found in some of our earlier publications on nucleonic flow at SIS energies [29,31,45,46,53,54,55,56,57,58,59].…”

Selected Experimental Results from Heavy-Ion Collisions at LHC

“…A preference for an in-plane emission versus out-ofplane ( "squeeze-out") pattern of particles as a function of beam energy is observed. The experimental data used are from FOPI [87,88], EOS, E895 [89], E877 [90], CERES [91], NA49 [92], STAR [93], PHO-BOS [94], PHENIX [95], ALICE [19], ATLAS [96] and CMS [17] experiments. Charged particles are used for LHC, RHIC, CERES and E877 experi-ments, pion data is used from NA49 experiment, protons results are from EOS and E895 experiment and FOPI results are for all particles with Z=1.…”

Origin of elliptic flow and its dependence on the equation of state in heavy ion reactions at intermediate energies

“…Contrary to findings at higher beam energies where fluctuations contribute to the elliptic flow (see e.g. [13,14]) there is no convincing experimental evidence at beam energies between 0.4 and 2A GeV that event-byevent fluctuations play a significant role in the elliptic flow pattern [15]. The interactions with the surrounding spectator matter and the much longer collision times might be responsible for this.…”