Assessing saturation physics explanations of collectivity in small collision systems with the IP-Jazma model

Abstract

Experimental measurements in collisions of small systems from p+p to p/d/3He+A at RHIC and the LHC reveal particle emission patterns that are strikingly similar to those observed in A+A collisions. One explanation of these patterns is the formation of small droplets of quark-gluon plasma followed by hydrodynamic evolution. A geometry engineering program was proposed [1] to investigate these emission patterns, and the experimental data from that program in p+Au, d+Au, 3He+Au collisions for elliptic and triangular anisotropy coefficients v2 and v3 follow the pattern predicted by hydrodynamic calculations [2]. One alternative approach, referred to as initial-state correlations, suggests that for small systems the patterns observed in the final-state hadrons are encoded at the earliest moments of the collision, and therefore require no final-state parton scattering or hydrodynamic evolution [3,4]. Recently, new calculations using only initial-state correlations, in the dilute-dense approximation of gluon saturation physics, reported striking agreement with the v2 patterns observed in p/d/3He+Au data at RHIC [5]. The reported results are counterintuitive and thus we aim here to reproduce some of the basic features of these calculations. In this first investigation, we provide a description of our model, IP-Jazma, and investigate its implications for saturation scales, multiplicity distributions and eccentricities, reserving for later work the analysis of momentum spectra and azimuthal anisotropies. We find that our implementation of the saturation physics model reproduces the results of the earlier calculation of the multiplicity distribution in d+Au collisions at RHIC. However, our investigations, together with existing data, call into question some of the essential elements reported in Ref. [5].