Stability of transport models under changes of resonance parameters: A study with the ultrarelativistic quantum molecular dynamics model

The ultrarelativistic quantum molecular dynamics (UrQMD) model is widely used to simulate heavy-ion collisions in broad energy ranges. It consists of various components to implement the different physical processes underlying the transport approach. A major building block are the shared tables of constants, implementing the baryon masses and widths. Unfortunately, many of these input parameters are not well known experimentally. In view of the upcoming physics program at the facility for antiproton and ion research (FAIR), it is therefore of fundamental interest to explore the stability of the model results when these parameters are varied. We perform a systematic variation of particle masses and widths within the limits proposed by the particle data group (or up to 10$\%$). We find that the model results do only weakly depend on the variation of these input parameters. Thus, we conclude that the present implementation is stable with respect to the modification of not yet well specified particle parameters.

Figure 1

(a) Total pion-nucleon cross section as a function of $\sqrt{s}$ for a systematic variation of the nucleon resonance masses. The full line depicts the PDG averages, the dotted line shows a variation of the PDG parameters by $+10\%$, the dashed line a variation by $-10\%$. (b) Total pion-nucleon cross section as a function of $\sqrt{s}$ for a systematic variation of the nucleon resonance widths. The full line depicts the PDG averages, the dotted line shows a variation of the PDG parameters by $+10\%$, the dashed line a variation by $-10\%$.

Figure 2

(a) Total pion-nucleon cross section as a function of $\sqrt{s}$ for a systematic variation of the $\Delta$ masses. The full line depicts the PDG averages, the dotted line shows a variation of the PDG parameters by $+10\%$, the dashed line a variation by $-10\%$. (b) Total pion-nucleon cross section as a function of $\sqrt{s}$ for a systematic variation of the $\Delta$ widths. The full line depicts the PDG averages, the dotted line shows a variation of the PDG parameters by $+10\%$, the dashed line a variation by $-10\%$.

Figure 3

(a) Relative pion yield in Pb+Pb collisions at $2A$ GeV beam energy. For a systematic variation of the masses and widths of the nucleon resonances by $\pm 10\%$. (b) Relative pion yield in Pb+Pb collisions at $30A$ GeV beam energy. For a systematic variation of the masses and widths of the nucleon resonances by $\pm 10\%$.

Figure 4

(a) Relative pion yield in Pb+Pb collisions at $2A$ GeV beam energy. For a systematic variation of the masses and widths of the $\Delta$ resonances by $\pm 10\%$. (b) Relative pion yield in Pb+Pb collisions at $30A$ GeV beam energy. For a systematic variation of the masses and widths of the $\Delta$ resonances by $\pm 10\%$.

Figure 5

(a) Pion yield in Pb+Pb collisions at$2A$ GeV beam energy itemizing different production processes. (b) Pion production from various $\Delta$ resonances in Pb+Pb collisions at $30A$ GeV.

Figure 6

(a) Deviation of pion transverse momentum spectra ($p_t$) in Pb+Pb collisions at $2A$ GeV for variations of the nucleon resonance masses. (b) Deviation of pion transverse momentum spectra ($p_t$) in Pb+Pb collisions at $30A$ GeV for variations of the nucleon resonance masses.

Figure 7

(a) Deviation of pion transverse momentum spectra ($p_t$) in Pb+Pb collisions at $2A$ GeV for variations of the $\Delta$ resonance masses. (b) Deviation of pion transverse momentum spectra ($p_t$) in Pb+Pb collisions at $30A$ GeV for variations of the $\Delta$ resonance masses.