Rapidity-dependent chemical potentials in a statistical approach

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 distribution and transverse-momentum spectra of pions, kaons, protons and antiprotons measured at RHIC at root s(NN) = 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 to). The value of the baryon chemical potential at alpha(parallel to) similar to 3 is about 200 MeV, i. e. it 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 to) 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 distribution 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.