Bimodality emerges from transport model calculations of heavy ion collisions at intermediate energy

TL;DR

This paper demonstrates that bimodal distributions, indicative of a first order phase transition, emerge in transport model calculations of heavy ion collisions at intermediate energies, revealing potential new signatures of phase transitions.

Contribution

It shows that bimodality and phase transition signatures can be observed in standard BUU calculations without fluctuations, a novel insight into identifying phase transitions in heavy ion collisions.

Findings

Bimodality appears in both mass 40 and mass 120 collisions.

Signatures of phase transition, such as derivative breaks, are observed in BUU calculations.

Bimodality correlates with other phase transition indicators at specific energies.

Abstract

This work is a continuation of our effort [Phys. Rev. C 91, 034616 (2015)] to examine if signatures of a phase transition can be extracted from transport model calculations of heavy ion collisions at intermediate energy. A signature of first order phase transition is the appearance of a bimodal distribution in $P_m(k)$ in finite systems. Here P_m(k) is the probability that the maximum of the multiplicity distribution occurs at mass number k. Using a well-known model for event generation (BUU plus fluctuation), we do two cases of central collision: mass 40 on mass 40 and mass 120 on mass 120. Bimodality is seen in both the cases. The results are quite similar to those obtained in statistical model calculations. An intriguing feature is seen. We observe that at the energy where bimodality occurs. other phase transition like signatures appear. There are breaks in certain first order derivatives. We then examine if such breaks appear in standard Botzmann-Uehling-Uhlenbeck (BUU) calculations without fluctuations. They do. The implication is interesting. If first order phase transition occurs, it may be possible to recognise that from ordinary BUU calculation. Probably the reason this was not seen already is because this aspect was not investigated before.