Charged-pion production in $$\mathbf {Au+Au}$$ collisions at $$\sqrt{\mathbf {s}_{\mathbf {NN}}} = 2.4~{\mathbf {GeV}}$$

Abstract

We present high-statistic data on charged-pion emission from Au + Au collisions at $$\sqrt{s_{\mathrm{NN}}} = 2.4~\hbox {GeV}$$ s NN = 2.4 GeV (corresponding to $$E_{beam} = 1.23~\hbox {A GeV}$$ E beam = 1.23 A GeV ) in four centrality classes in the range 0–40% of the most central collisions. The data are analyzed as a function of transverse momentum, transverse mass, rapidity, and polar angle. Pion multiplicity per participating nucleon decreases moderately with increasing centrality. The polar angular distributions are found to be non-isotropic even for the most central event class. Our results on pion multiplicity fit well into the general trend of the available world data, but undershoot by $$2.5~\sigma $$ 2.5 σ data from the FOPI experiment measured at slightly lower beam energy. We compare our data to state-of-the-art transport model calculations (PHSD, IQMD, PHQMD, GiBUU and SMASH) and find substantial differences between the measurement and the results of these calculations.

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

“…In Ref. [30], we had shown that a proper description of the transverse-mass spectrum requires in fact the sum of two Boltzmann distributions, expressed as…”

“…( 4) -( 6) in the energy sampling routine. In the simulation, we have set the parameters to T 1 = 50 MeV, T 2 = 90 MeV, f = 0.95, V c = 15 MeV, and T p = 130 MeV which are typical values observed for the energy regime where HADES operates [30,32]. The left panel of Fig.…”

“…We have applied the fit procedure presented in the previous section to the charged pion spectra measured with HADES in Au+Au collisions at √ s NN = 2.4 GeV [30]. In total, 10 8 events were selected for this analysis; details of the event reconstruction and centrality selection are given in [35].…”

“…The High Acceptance Di-electron Spectrometer (HADES) experiment [29] at the SIS18 accelerator has extensively studied particle production in 1.23 A GeV Au+Au collisions, equivalent to a center-of-mass energy of √ s NN = 2.4 GeV. In particular, charged-pion spectra were measured with very high statistics and the pion phase-space distribution could be reconstructed with good accuracy [30], limited mostly by the systematic uncertainties of the detector efficiency and acceptance of order 5-10%. Especially, the extrapolation of the measured differential particle yields into the low-m t region, not fully covered by HADES, is a potential source of systematic errors.…”

“…One motivation for the analysis presented here was to provide a more accurate parameterization of pion distributions at low m t (or low p t ), allowing to reduce systematic uncertainties due to the extrapolation of measured yields into this region. This is particularly important owing to the fact that the geometry of the HADES detector and its toroidal magnetic field lead to somewhat different acceptances for lowmomentum π − , bent to large polar angles, and π + , bent towards the beam axis [29,30]. This paper is organized as follows: In section 2 we introduce the formalism used to describe the Coulomb effect on charged-particle spectra and we validate our fit procedure by simulations.…”