Bimodality - a general feature of heavy ion reactions

TL;DR

This paper demonstrates that bimodality in heavy ion reactions, characterized by two distinct reaction outcomes at the same transverse energy, is a general phenomenon caused by impact parameter fluctuations, challenging the phase coexistence interpretation.

Contribution

The study shows that bimodality is a universal feature in heavy ion collisions across various systems and energies, driven by impact parameter fluctuations rather than phase coexistence.

Findings

Bimodality appears in all symmetric systems larger than Ca between 50 and 600 A.MeV.

Neither fragment momenta nor energies align with statistical equilibrium expectations.

The process responsible for bimodality is rapid, not indicative of phase coexistence.

Abstract

Recently, is has been observed that events with the {\it same} total transverse energy of light charged particles (LCP) in the quasi target region, $E_{\perp 12}^{QT}$, show two quite distinct reaction scenarios in the projectile domain: multifragmentation and residue production. This phenomenon has been dubbed "bimodality". Using Quantum Molecular Dynamics calculations we demonstrate that this observation is very general. It appears in collisions of all symmetric systems larger than Ca and at beam energies between 50 A.MeV and 600 A.MeV and is due to large fluctuations of the impact parameter for a given $E_{\perp 12}^{QT}$. Investigating in detail the $E_{\perp 12}^{QT}$ bin in which both scenarios are present, we find that neither the average fragment momenta nor the average transverse and longitudinal energies of fragments show the behavior expected from a system in statistical equilibrium, in experiment as well as in QMD simulations. On the contrary, the experimental as well as the theoretical results point towards a fast process. This observation questions the conjecture that the observed bimodality is due to the coexistence of 2 phases at a given temperature in finite systems.