Species-dependent viscous corrections at particlization: A novel relaxation time approximation approach

Abstract

We assess the effects of a recently proposed generalized relaxation time approximation (RTA) for multi-species relativistic gases within a realistic numerical hybrid framework and study its phenomenological consequences in p-Pb and Pb-Pb collisions. The novel approximation introduces counter-terms to the collision kernel, allowing for momentum-dependent relaxation times $\tau_i(p)$ while preserving local energy-momentum conservation. As a consequence, the resulting first-order viscous corrections $\delta f_i$ to the phase-space distribution functions depend explicitly on the particle species mass $m_i$. We systematically investigate the impact of these species-dependent corrections on particle production at particlization, focusing on identified hadron yields and transverse momentum ($p_T$) spectra obtained from Cooper-Frye sampling. We find that the yields and spectra of light hadrons ($\pi, K, p$) are significantly affected, leading to modifications of relative particle yields such as the $K/\pi$ and $p/\pi$ ratios. We show that these effects persist, albeit with reduced magnitude, after the inclusion of the hadronic cascade stage. In contrast, the impact on inclusive charged-particle observables is strongly reduced due to compensating enhancements and suppressions among different species. This controlled deformation of identified hadron observables, which selectively modifies flavor-sensitive quantities, makes the new prescription particularly well suited for Bayesian inference, as it introduces new sensitivity directions without spoiling existing constraints. Overall, our results demonstrate that species-dependent viscous corrections arising from the generalized RTA can leave significant and observable imprints on identified hadron production and relative yields, while remaining fully consistent with the successful description of bulk collective flow observables.