Impact of the Coulomb field on charged-pion spectra in few-GeV heavy-ion collisions

Abstract

In nuclear collisions the incident protons generate a Coulomb field which acts on produced charged particles. The impact of these interactions on charged-pion transversemass and rapidity spectra, as well as on pion-pion momentum correlations is investigated in Au+Au collisions at √ s NN = 2.4 GeV. We show that the low-m t part of the data (m t < 0.2 GeV/c 2 ) can be well described with a Coulomb-modified Boltzmann distribution that also takes changes of the Coulomb field during the expansion of the fireball into account. The observed centrality dependence of the fitted mean Coulomb potential deviates strongly from a A 2/3 part scaling, indicating that, next to the fireball, the non-interacting charged spectators have to be taken into account. For the most central collisions, the Coulomb modifications of the HBT source radii are found to be consistent with the potential extracted from the single-pion transverse-mass distributions. This finding suggests that the region of homogeneity obtained from two-pion correlations coincides with the region in which the pions freeze-out. Using the inferred mean-square radius of the charge distribution at freeze-out, we have deduced a baryon density, in fair agreement with values obtained from statistical hadronization model fits to the particle yields.

Charged pion production in few-GeV heavy-ion collisions measured with HADES

“…Charged 𝜋 mesons, being two particles of the same multiplet, are well suited probes of the Coulomb potential of the collision zone. This effect can be seen in the low-𝑚 𝑇 part of their emission spectra (as already observed by HADES in Au+Au collisons [3]). Comparing them with theoretical models allows an estimation of the size of the collision zone.…”