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arXiv · 2509.05495

Knudsen number and universal behavior of collective flows in conformal and non-conformal systems

Vincenzo Nugara·Nicolas Borghini·Vincenzo Greco·Salvatore Plumari
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Abstract

We investigate the role of the Knudsen number (Kn) as a scaling parameter governing the emergence of collective behavior in relativistic heavy-ion collisions. Using the Relativistic Boltzmann Transport approach, we explore different initial conditions for both conformal (massless) and non-conformal (massive) systems with a constant specific shear viscosity $η/s$. Observables such as the time evolution of anisotropic flow coefficients collapse onto universal curves for fixed classes of Knudsen number, when using a scaled time variable accounting for the system size and the speed of sound $c_s$. More differential quantities, such as $v_n(p_T/\langle E_T\rangle)$, show a larger sensitivity to $c_s$. We also study events with fluctuating initial profiles from the \trento\ model, simulating collision systems from O-O to Pb-Pb at RHIC and LHC energies. Universal scaling at a given Kn value also holds in these event-by-event simulations, suggesting that the Knudsen number provides a unified criterion for classifying collectivity across different systems, including small systems where thermalisation may not be fully realised.

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Vincenzo Nugara, Nicolas Borghini, Vincenzo Greco, Salvatore Plumari. 2025-09-05. Knudsen number and universal behavior of collective flows in conformal and non-conformal systems. https://arxiv.org/abs/2509.05495

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