Constraining the nucleon size with relativistic nuclear collisions

TL;DR

This paper uses hydrodynamic modeling of high-energy nuclear collisions to constrain the nucleon size, finding it to be around 0.5 fm or smaller, which impacts understanding of initial states in nuclear physics.

Contribution

It demonstrates that flow and transverse momentum correlations are highly sensitive to nucleon size, providing a new method to constrain this fundamental parameter from collision data.

Findings

Nucleon size is approximately 0.5 fm or smaller.

Flow-$p_t$ correlations are sensitive to nucleon size.

Results support a unified picture linking nuclear collisions and deep inelastic scattering.

Abstract

The notion of the "size" of nucleons and their constituents plays a pivotal role in the current paradigm of the formation and the fluctuations of the quark-gluon plasma produced in high-energy nuclear collision experiments. We report on state-of-the-art hydrodynamic results showing that the correlation between anisotropic flow, $v_n^2$, and the mean transverse momentum of hadrons, $[p_t]$, possesses a unique sensitivity to the nucleon size in off-central heavy-ion collisions. We argue that existing experimental measurements of this observable support a picture where the relevant length scale characterizing the colliding nucleons is of order 0.5 fm or smaller, and we discuss the broad implications of this finding for future global Bayesian analyses aimed at extracting initial-state and medium properties from nucleus-nucleus collision data, including $v_n^2$-$[p_t]$ correlations. Determinations of the nucleon size in heavy-ion collisions will provide a solid independent constraint on the initial state of small system collisions, and will establish a deep connection between collective flow data in nucleus-nucleus experiments and data on deep inelastic scattering on protons and nuclei.