[Paper Review] How to Improve UrQMD Model to Describe NA61/SHINE Experimental Data

V. Uzhinsky|arXiv (Cornell University)|Jul 2, 2011

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper identifies that the UrQMD model overestimates low-momentum ($p < 1.5$ GeV/c) pion production in proton-carbon collisions due to incorrect treatment of low-mass string fragmentation, where two-body decays instead of proper fragmentation are simulated. By modifying the string decay subroutine in UrQMD 1.3 to restore transverse momentum in the final-state particles, the model's predictions are brought into strong agreement with NA61/SHINE experimental data, particularly in the low-$p$ region.

ABSTRACT

The NA61/SHINE collaboration measured inclusive cross sections of pi+ and pi- meson production in the interactions of 31 GeV/c proton with carbon nuclei at small emission angles (0 - 420 mrad). The collaboration presented also predictions of Monte Carlo models - FLUKA, VENUS and UrQMD, in a comparison with the data. The worst description of the data was observed for UrQMD model results. In the present paper it is shown that the drawback of the UrQMD model is connected with an inaccurate treatment of low mass string fragmentation. The strings appeared at a diffraction of target nucleons. A simple patch is proposed to overcome the problem.

Motivation & Objective

• To identify the origin of the UrQMD model's significant overestimation of low-momentum ($p < 1.5$ GeV/c) pion production in proton-carbon interactions at 31 GeV/c.
• To investigate whether the discrepancy arises from incorrect simulation of low-mass string fragmentation or from cascade effects in nuclear interactions.
• To propose and validate a minimal code-level fix to the UrQMD 1.3 model that restores proper transverse momentum in the decay of low-mass strings.
• To demonstrate that the corrected model accurately reproduces NA61/SHINE experimental data for inclusive meson cross sections at small angles (0–20 mrad).

Proposed method

• Identified that in the UrQMD 1.3 string fragmentation subroutine (CLUSTR), low-mass strings from single diffraction decay via isotropic two-body decays with zero transverse momentum.
• Modified the CLUSTR subroutine by reactivating transverse momentum assignment for baryons in the final state of low-mass string decays, restoring physical momentum distribution.
• Replaced the default zero-$P_T$ assignment with a non-zero $P_T$ based on the parton kick parameter $PA$, ensuring realistic momentum transfer in the decay.
• Preserved existing model parameters such as string tension ($XAP$), leading particle effect, and $CTParam$ values, isolating the string decay mechanism as the key issue.
• Validated the fix by re-running simulations and comparing the new UrQMD predictions with NA61/SHINE data for $\pi^+$ and $\pi^-$ at $\theta = 0$–20 mrad.
• Confirmed that changes to cascade parameters, $P_T$ transfer, or diffraction probability did not affect the low-$p$ excess, isolating the string decay error as the root cause.

Experimental results

Research questions

• RQ1 Why does the UrQMD model overpredict low-momentum pion production in proton-carbon collisions at 31 GeV/c?
• RQ2 Is the overprediction due to incorrect treatment of string fragmentation in low-mass states?
• RQ3 Can a minimal code modification to the UrQMD 1.3 model correct the low-$p$ discrepancy without altering other physical parameters?
• RQ4 Does restoring transverse momentum in low-mass string decays significantly improve agreement with NA61/SHINE experimental data?

Key findings

• The UrQMD model overestimates inclusive $\pi^+$ and $\pi^-$ cross sections at low momenta ($p < 1.5$ GeV/c) by a large margin, especially in the 0–20 mrad angular range.
• The root cause of the overprediction is the incorrect simulation of low-mass string decays, where transverse momentum is set to zero, leading to isotropic, low-$p$ meson production.
• The proposed fix—reactivating transverse momentum assignment in the CLUSTR subroutine for low-mass strings—results in a dramatic improvement in agreement with NA61/SHINE data.
• After the fix, the model's predictions for $\pi^+$ and $\pi^-$ production cross sections at $\theta = 0$–20 mrad match the experimental data across the full momentum spectrum.
• The correction is effective and minimal: only a few lines of code were changed, and no adjustments to other model parameters (e.g., string tension, $P_T$ transfer) were needed.
• The analysis confirmed that other model parameters, including $CTParam(31)$, $CTParam(2)$, and $\beta$, do not affect the low-$p$ excess, isolating the string decay mechanism as the sole issue.