Transverse flow of nuclear matter in collisions of heavy nuclei at intermediate energies

Abstract

The Quantum Molecular Dynamics Model (IQMD) is used to investigate the origin of the collective transverse velocity observed in heavy ion experiments. We find that there are three contributions to this effect: initial-final state correlations, potential interactions and collisions. For a given nuclear equation of state (eos) the increase of the transverse velocity with increasing beam energy is caused by the potential part. For a given beam energy the collective transverse velocity is independent of the nuclear eos but the relative contributions of potential and collisions differ. In view of the importance of the potential interactions between the nucleons it is not evident that the similarity of the radial velocities measured for fragments at beam energies below 1 AGeV and that for mesons at beam energies above 2 AGeV is more than accidental.A major goal of heavy ion collision studies is to extend our knowledge of the properties of dense and hot nuclear matter. In the past these studies were concentrated on the production of secondary particles like pions or kaons and on anisotropies in the momentum distribution like the directed in-plane flow (bounce-off) or the out of plane flow (squeeze-out) [1,2] of nuclear matter. For recent reviews of the different collective flows we refer to [3,4] Recently also the isotropic radial flow has gained large interest because at AGS and at CERN energies the existing data at mid-rapidity of most of the mesons and baryons can be well described by the assumption that the system is thermalized and expands with a collective velocity [5,6]. Later a similar observation has been made at energies between 150A MeV and 1A GeV (see eg. [7] - [10]). There the fragments, observed at mid-rapidity, show an energy which increases linearly with the fragment mass. At these low energies most of the pions come from the decay of nuclear resonances and therefore cannot be used to determine a transverse velocity and kaons are too rare to be of use for a such an analysis.The radial flow velocity β t determined with help of the fragment kinetic energies up to 1A GeV smoothly joints that determined with help of mesons and baryons starting from 11A GeV. This raised the question whether both transverse collective flows have a common origin and whether their functional dependence on the beam energy allows for conclusions about a possible transition between hadronic matter and a quark gluon plasma.A common origin of the transverse flow of fragments and mesons is not at all evident. Nucleons which are bound asymptotically in fragments do usually not suffer collisions with a large momentum transfer, otherwise they are not bound anymore. On the contrary, the creation of mesons requires a large momentum transfer.At beam energies below 1AGeV the radial flow seems to be a good candidate for measuring the nuclear equation of state. Increasing compression leads to an increasing pressure which is released isotropically. The directed transverse flow in the reaction plane, which in principle also depends on the press...

Nuclear modification factor in intermediate-energy heavy-ion collisions

“…We set the parameter α free in Blast-Wave fit, which is different from some previous works keeping a fixed value equal to 1 or 2 [29,31,32] or some assuming a common flow (i.e. α = 0) [11,12,33,34]. The α makes a little difference to the fitting value of β S while the choice of R value have no influence on the fitting which chosen R = 40 fm in our work.…”

Systematics of central heavy ion collisions in the regime

“…34, reveals that the strong dependence of the data on system size is not correctly reproduced. In [60], using IQMD, the authors conclude that radial flow is not sensitive to the EOS: they find that the sum of two contributions to this kind of flow, namely the mean field EOS and the nucleon-nucleon scatterings, tends to be the same for a stiff, resp. soft EOS.…”

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

“…1. This in-plane flow is considerably stronger for a hard EoS as compared to a soft one [25][26][27]. In y-direction the surplus in density of the hard EoS is concentrated at around z=0, being less extended but stronger at higher energies ( Fig.…”