How Sensitive are Di-Leptons from Rho Mesons to the High Baryon Density Region?

TL;DR

This paper investigates how di-lepton signals from rho mesons are affected by high baryon densities in heavy ion collisions, highlighting the limitations of current measurement methods and the impact of resonance absorption.

Contribution

It demonstrates that di-lepton measurements may underestimate dense phase effects due to resonance absorption and compares different extraction approaches, revealing method sensitivities.

Findings

Di-lepton signals are strongly affected by resonance absorption at high baryon densities.

Different extraction methods yield significantly different di-lepton yields.

Restricting rho meson production to final baryon resonance decays biases results towards lower densities.

Abstract

We show that the measurement of di-leptons might provide only a restricted view into the most dense stages of heavy ion reactions. Thus, possible studies of meson and baryon properties at high baryon densities, as e.g. done at GSI-HADES and envisioned for FAIR-CBM, might observe weaker effects than currently expected in certain approaches. We argue that the strong absorption of resonances in the high baryon density region of the heavy ion collision masks information from the early hot and dense phase due to a strong increase of the total decay width because of collisional broadening. To obtain additional information, we also compare the currently used approaches to extract di-leptons from transport simulations - i.e. shining, only vector mesons from final baryon resonance decays and instant emission of di-leptons and find a strong sensitivity on the method employed in particular at FAIR and SPS energies. It is shown explicitly that a restriction to rho meson (and therefore di-lepton) production only in final state baryon resonance decays provide a strong bias towards rather low baryon densities. The results presented are obtained from UrQMD v2.3 calculations using the standard set-up.