Collective Expansion in High Energy Heavy Ion Collisions

Abstract

Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200A GeV S 1 S and 158A GeV Pb 1 Pb, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions. The purpose of studying ultrarelativistic heavy-ion collisions is to understand the nature of hadronic matter under extreme conditions. Specifically, we are interested in a new form of matter, quark-gluon plasma, which may be produced in such collisions. Transverse momentum distributions are one of the most common tools used in studying high energy collisions. This is because the transverse motion is generated during the collision and hence is sensitive to the dynamics. More than 45 years ago, Fermi proposed a statistical method [1] to understand the results of high energy hadron-hadron collisions. Because of saturation of the phase space, the multiparticle production resulting from the high energy elementary collisions is consistent with a thermal description [1][2][3]. In heavy-ion collisions hydrodynamical behavior, that is, local thermal equilibrium and collective motion, may be expected because of the large number of secondary scatterings.It is now possible to identify and quantitatively measure the collective motion by systematic studies of results from different collision systems, using light (Si at BNL and S at CERN) and heavy (Au at BNL and Pb at CERN) ion beams [4][5][6]. A high degree of nuclear stopping and a strong Coulomb effect (also due to the high stopping) have already been reported in Pb 1 Pb central collisions [7,8]. In this Letter, we present transverse momentum distributions of pions, kaons, and protons, measured in the NA44 spectrometer, from Pb 1 Pb and S 1 S collisions. Results of calculations from a hydrodynamical model [5] will be used to aid in this analysis.The NA44 magnetic focusing spectrometer consists of two room-temperature dipoles and three superconducting quadruples. Particles originating from the target are focused at a plane about ten meters downstream and detected by a tracking system consisting of a pad chamberstrip chamber-scintillator hodoscope complex. Particle identification is done with two threshold Cherenkov counters and two highly segmented TOF hodoscopes. The phase-space coverage (transverse momentum p T vs rapidity y) is determined by the combination of the spectrometer angle (relative to the beam direction) and the nominal momentum setting of the magnets. The momentum resolution is typically s p ͞p # 0.2% and the TOF counters have an average time resolution of 100 ps. More details of the spectrometer can be found elsewhere [9].The spectrometer momentum range is 620% around the nominal values of 4 and 8 GeV͞c. For kaons and protons, the 8 GeV͞c setting was used and the rapidity coverage is (2.5-3.4) and (2.4-2.8) for kaons and protons, respectively. Two angular settings (44 and 130 mrad) were utilized in order ...

Hydrodynamic Expansion of a Strongly Interacting Fermi-Fermi Mixture

“…The second analogy becomes evident in the transverse energy spectra of the collision products. Here it is found that heavier particles carry larger energies than the lighter ones [24]. Such a mass-dependence is interpreted as a result of "collective flow" (see e.g.…”

Scaling Properties of Spectra in New Exact Solutions of Rotating, Multi-Component Fireball Hydrodynamics

“…Table IV of [2]. Dashed lines are drawn to guide the eye, corresponding to Equation (1) and also to earlier theoretical results for a single-component hadron gas in non-central heavy ion collisions as detailed in References [7,8]. Green symbols indicate our new result, based on our new exact solutions of fireball hydrodynamics for a collectively expanding and rotating, multi-component hadronic matter, as indicated by symbols with "MC theory" in the legends.…”

“…Interestingly, the NA44 Collaboration observed signals of collective expansion in symmetric heavy-ion collisions in 200 AGeV S+S and 158 AGeV Pb+Pb reactions at CERN SPS: the transverse mass spectra of pions, kaons and protons was found to be nearly exponential in these reactions, with a slope parameter that increased nearly linearly with the mass of the observed hadrons [1]. This linear mass dependence of the slope parameters was interpreted as an evidence of collective transverse flow in these heavy ion induced central collisions.…”

Simple analytic solution of fireball hydrodynamics

“…Such a solution of the equations of non-relativistic (NR) hydrodynamics is presented herewith. Physical realization of the solution is possible both by heavy ion collisions in the 30 MeV A -2 GeV A energy domain [3,4,5] and in a more limited sense by the NR transversal flows that seem to develop in high energy hadron-hadron and nucleus-nucleus collisions [1,6,7].…”