Rapidity-dependent chemical potentials in a statistical approach

TL;DR

This paper develops a rapidity-dependent statistical model to describe particle spectra in heavy-ion collisions at RHIC, revealing how chemical potentials and fireball geometry vary with rapidity.

Contribution

It introduces a model with spatially dependent thermal and geometric parameters, providing new insights into the fireball's topography and particle production.

Findings

Chemical potentials increase with rapidity, reaching ~200 MeV at high rapidity.

The fireball's transverse size decreases with increasing rapidity.

The model qualitatively reproduces net proton spectra.

Abstract

We present a single-freeze-out model with thermal and geometric parameters dependent on the position within the fireball and use it to describe the rapidity and transverse-momentum spectra of pions, kaons, protons, and antiprotons measured at RHIC at 200 GeV} by BRAHMS. THERMINATOR is used to perform the necessary simulation, which includes all resonance decays. The result of the fit to the data is the expected growth of the baryon and strange chemical potentials with the spatial rapidity\alpha_\parallel. The value of the baryon chemical potential at \alpha_\parallel ~ 3 is about 200 MeV, i.e. lies in the range of the highest SPS energies. The chosen geometry of the fireball has a decreasing transverse size as the magnitude of \alpha_\parallel is increased, which also corresponds to decreasing transverse flow. The strange chemical potential obtained from the fit to the K+/K- ratio is such that the local strangeness density in the fireball is compatible with zero. The resulting rapidity spectra of net protons are described qualitatively within the statistical approach. As a result of our study, the knowledge of the ``topography'' of the fireball is acquired, allowing for other analyses and predictions.