Dilepton production and resonance properties within a new hadronic transport approach in the context of the GSI-HADES experimental data

The dilepton emission in heavy-ion reactions at low beam energies is examined within a hadronic transport approach. In this article, the production of electron-positron pairs from a new approach named SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) is introduced. The dilepton emission below the hadronic invariant mass threshold is taken into account for all direct vector meson decays. The calculations are systematically confronted with HADES data in the kinetic-energy range of $1\text{–}3.5A\mathrm{GeV}$ for elementary, proton-nucleus and nucleus-nucleus reactions. The present approach employing a resonance treatment based on vacuum properties is validated by an excellent agreement with experimental data up to system sizes of carbon-carbon collisions. After establishing this well-understood baseline in elementary and small systems, medium effects are investigated with a coarse-graining approach based on the same hadronic evolution. The effect of in-medium modifications to the vector meson spectral functions is important for dilepton invariant mass spectra in ArKCl and larger systems, even though the transport approach with vacuum properties reveals similar features due to the coupling to baryonic resonances and the intrinsically included collisional broadening. This article provides a comprehensive comparison of our calculations with published dielectron results from the HADES collaboration. In addition, the emission of dileptons in gold-gold and pion-beam experiments, for which results are expected, is predicted.

Figure 1

Spectral function of the vector mesons with dielectron decay mode.

Figure 2

Production cross section for $pp\to pp\eta$. Experimental data from [47, 48, 49].

Figure 3

Production cross section for $pp\to pp\omega$. Experimental data from [49].

Figure 4

Exclusive production cross section for $pp\to pp\rho$ (empty data points from [49, 50]) and inclusive $pp\to \rho +X$ cross section (full data point from [20]).

Figure 5

Cross section for $pn\to pn\eta$. Experimental data from [52].

Figure 6

Invariant mass spectrum of dielectrons produced in pp collisions at $E_{\mathrm{Kin}}=1.25\mathrm{GeV}$. Experimental data from [18].

Figure 7

Invariant mass spectrum of dielectrons produced in pp collisions at $E_{\mathrm{Kin}}=2.2\mathrm{GeV}$. Experimental data from [19].

Figure 8

Invariant mass spectrum of dielectrons produced by $pp$ collisions at $E_{\mathrm{Kin}}=3.5\mathrm{GeV}$. Experimental data from [20].

Figure 9

Different contributions to the invariant mass spectrum of dielectrons produced by decays of $\rho$ (top) and $\omega$ (bottom) mesons for $pp$ collisions at $E_{\mathrm{Kin}}=3.5\mathrm{GeV}$. Heavy $N^{*}$ states include contributions from $N^{*}\left(2080\right),N^{*}\left(2190\right),N^{*}\left(2220\right),N^{*}\left(2250\right)$, all of the form $N^{*}\to \rho N\to e^{+}{e}^{-}N$.

Figure 10

Invariant mass spectrum of dielectrons produced by quasifree $np$ reactions at $E_{\mathrm{Kin}}=1.25\mathrm{GeV}$. Experimental data from [18].

Figure 11

Invariant mass spectrum of dielectrons produced by $\pi p$ reactions at $E_{\mathrm{Kin}}=0.56\mathrm{GeV}$.

Figure 12

Invariant mass spectrum of dielectrons produced by $p\mathrm{Nb}$ reactions at $E_{\mathrm{Kin}}=3.5\mathrm{GeV}$. Experimental data from [21].

Figure 13

Invariant mass spectrum of dielectrons produced by $p\mathrm{Nb}$ reactions at $E_{\mathrm{Kin}}=3.5\mathrm{GeV}$ in different dielectron momentum ($p_{ee}$) windows. Experimental data from [21].

Figure 14

Invariant mass spectrum of dielectrons produced by CC reactions at $E_{\mathrm{Kin}}=1.0A\mathrm{GeV}$. Data from [22].

Figure 15

Invariant mass spectrum of dielectrons produced by CC reactions at $E_{\mathrm{Kin}}=2.0A\mathrm{GeV}$. Data from [80].

Figure 16

Invariant mass spectrum of dielectrons produced by ArKCl collisions at $E_{\mathrm{Kin}}=1.76A\mathrm{GeV}$. Experimental data from [24].

Figure 17

Invariant mass spectrum of dielectrons produced by AuAu collisions at $E_{\mathrm{Kin}}=1.23A\mathrm{GeV}$.

Figure 18

Evolution of the energy and baryon density in units of the ground-state density in the most central cell over time for ArKCl collisions at $E_{\mathrm{Kin}}=1.76\text{−}A\mathrm{GeV}$ and AuAu collisions at $E_{\mathrm{Kin}}=1.23A\mathrm{GeV}$.

Figure 19

Evolution of the temperature $T$ and baryochemical potential ${\mu }_b$ in the most central cell over time for ArKCl collisions at $E_{\mathrm{Kin}}=1.76A\mathrm{GeV}$ and AuAu collisions at $E_{\mathrm{Kin}}=1.23A\mathrm{GeV}$.

Figure 20

Invariant mass spectrum of dielectrons produced by ArKCl collisions at $E_{\mathrm{Kin}}=1.76A\mathrm{GeV}$ within the coarse-graining approach. Dashed lines from coarse graining and solid lines from SMASH dilepton production (as in Fig. 16). Experimental data from [24].

Figure 21

Invariant mass spectrum of dielectrons produced by AuAu collisions at $E_{\mathrm{Kin}}=1.23A\mathrm{GeV}$ within the coarse-graining approach. Dashed lines from coarse graining and solid lines from SMASH dilepton production (as in Fig. 17).

Figure 22

Comparison of invariant mass spectra of dielectrons produced by $\rho$ and $\omega$ in ArKCl collisions at $E_{\mathrm{Kin}}=1.76A\mathrm{GeV}$ within the coarse-graining approach vs the default SMASH dilepton production.

Figure 23

Comparison of invariant mass spectra of dielectrons produced by $\rho$ and $\omega$ in AuAu collisions at $E_{\mathrm{Kin}}=1.23A\mathrm{GeV}$ within the coarse-graining approach vs the default SMASH dilepton production.

Figure 24

Transverse momentum spectra of dielectrons produced by $pp$ collisions at $E_{\mathrm{Kin}}=3.5\mathrm{GeV}$ in different invariant mass windows. Experimental data from [20].

Figure 25

Rapidity spectra of dielectrons produced by $pp$ collisions at $E_{\mathrm{Kin}}=3.5\mathrm{GeV}$ in different invariant mass windows. Experimental data from [20].