Production and evolution path of dileptons at energies accessible to the HADES detector

Abstract

Dilepton production in intermediate energy nucleus-nucleus collisions as well as in elementary proton-proton reactions is analysed within the UrQMD transport model. For C+C collisions at 1 AGeV and 2 AGeV the resulting invariant mass spectra are compared to recent HADES data. We find that the experimental spectrum for C+C at 2 AGeV is slightly overestimated by the theoretical calculations in the region around the vector meson peak, but fairly described in the low mass region, where the data is satisfactorily saturated by the Dalitz decay of the η meson and the ∆ resonance. At 1 AGeV an underestimation of the experimental data is found, pointing that at lower energies the low mass region is not fully saturated by standardly parametrized ∆ Dalitz decays alone. Furthermore, predictions for dilepton spectra for pp reactions at 1.25 GeV, 2.2 GeV and 3.5 GeV and Ar+KCl reactions at 1.75 AGeV are presented. The study is complemented by a detailed investigation of the role of absorption of the parent particles on the corresponding dilepton yields in the regime which has so far been probed by HADES.

Inclusive dielectron production in proton-proton collisions at 2.2 GeV beam energy

“…[24]. Under the assumption and as suggested indeed by various calculations [3][4][5] that dielectron production is dominated in the mass range 0.15-0.45 GeV/c 2 by the and η contributions, the total yield measured for these masses can be used to constrain the η contribution for any assumed cross section. In other words, the η and contributions are complementary.…”

“…Inclusive e + e − production in p + p and p + d reactions has formerly been studied in the range of [1][2][3][4][5] GeV by the dilepton spectrometer (DLS) experiment at the Bevalac facility [13] and has more recently been studied by the HADES experiment at 1.25 [7] and 3.5 GeV [8]. In particular, the comparison of the latter data sets with various model calculations demonstrated a need for improved theoretical descriptions.…”

“…For that reason they are used to probe medium modifications of hadrons intensively searched for in photon-and protoninduced reactions on nuclei as well as in heavy-ion collisions [2]. Transport models are commonly employed to describe particle production and propagation through the medium, in particular, when dealing with the complex dynamics of nucleus-nucleus reactions [3][4][5]. The proper modeling of lepton pair production mechanisms requires a solid understanding of the underlying elementary processes, be it in terms of resonance excitations or in terms of a string fragmentation picture [6].…”

Resonance studies with the UrQMD model

“…This permits to explore the emission patterns of resonances in detail and to gain insight into their origins and decay channels. For previous studies of resonances within this model see [14][15][16][17][18][19][20][21][22][23] the UrQMD model the reader is referred to [26,27]. Recent development concerning the inclusion of a hydrodynamic phase are described in [28].…”

“…[5,6]) and the transition from a possibly decon ned phase to a hadronic phase is quite poorly understood. Resonances provide an unique approach to learn about the hot and dense phase which is produced in heavy ion collisions [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. Since most hadronic resonances decay on time scales of several fm/c, plenty of short-lived resonances will decay within the created medium and will carry the information from that stage of the collision.…”