Extended Gibbs ensembles with flow

Abstract

A statistical treatment of finite unbound systems in the presence of collective motions is presented and applied to a classical Lennard-Jones Hamiltonian, numerically simulated through molecular dynamics. In the ideal gas limit, the flow dynamics can be exactly re-casted into effective time-dependent Lagrange parameters acting on a standard Gibbs ensemble with an extra total energy conservation constraint. Using this same ansatz for the low density freeze-out configurations of an interacting expanding system, we show that the presence of flow can have a sizeable effect on the microstate distribution.Comment: 7 pages, 4 figure

Investigation of collective radial expansion and stopping in heavy ion collisions at Fermi energies

“…Observed first by the FOPI collaboration [5][6][7][8] and interpreted as an "extra push" with respect to the thermal pressure of an equilibrated composite system, a collective expansion energy has been extracted from experimental data over a wide range of beam energy [9]. Several studies of the multifragmentation process, and of the corresponding role of the collective expansion, have been undertaken in the framework of transport theories [4, 10-12] and some analyses [13][14][15] have pointed out the importance of this effect on fragment formation, looking at, for example, the balance between the amount of radial collective flow and recombination probability [13].…”

“…In this case two-body correlations and fluctuations are expected to provide the seeds for fragment formation leading to the occurrence of multifragmentation phenomena which can be described in the framework of a liquid-gas-type phase transition [4].Observed first by the FOPI collaboration [5][6][7][8] and interpreted as an "extra push" with respect to the thermal pressure of an equilibrated composite system, a collective expansion energy has been extracted from experimental data over a wide range of beam energy [9]. Several studies of the multifragmentation process, and of the corresponding role of the collective expansion, have been undertaken in the framework of transport theories [4, 10-12] and some analyses [13][14][15] have pointed out the importance of this effect on fragment formation, looking at, for example, the balance between the amount of radial collective flow and recombination probability [13].However, to our knowledge, estimates of the radial collective energy present in experimental multifragmentation data have been mostly obtained by employing statistical models [16][17][18] which treat separately fragment production and collective expansion effects [19][20][21][22][23][24]. The main justification is the small contribution of the collective expansion energy [25] with respect to the total excitation energy characterizing the Fermi energy domain (around 20-30%).…”

Relevance of equilibrium in multifragmentation

“…It is an interesting question whether the equivalence between the multifragmentation reaction and the equilibrium system still holds under such circumstances. It is also interesting to compare observables such as the momentum distribution of fragments and the system size of multifragmentation reactions with those of the corresponding equilibrium systems in the explicit presence of expansion and flow effects [57,58].…”

Dynamical and statistical bimodality in nuclear fragmentation

“…Such extended ensembles can be coherently modelled by accounting for the experimental constraints including time-odd observables and collective flows [8,9], and lead to predictions that can interpolate between the standard canonical, microcanonical and grandcanonical ensembles of macroscopic (N, V, T ) systems [10,11].…”