Mass ordering of spectra from fragmentation of saturated gluon states in high multiplicity proton-proton collisions

TL;DR

This paper presents an initial state QCD-based model that reproduces the mass ordering of spectra in high multiplicity proton-proton collisions, challenging the hydrodynamic flow interpretation.

Contribution

It introduces a novel initial state approach using Yang-Mills equations and string fragmentation to explain mass ordering without hydrodynamics.

Findings

Reproduces mass dependence of <p_T> and v_2(p_T)

Supports initial state effects as alternative to hydrodynamics

Matches key features of bulk particle spectra

Abstract

The mass ordering of mean transverse momentum $\left<p_T\right>$ and of the Fourier harmonic coefficient $v_2 (p_T)$ of azimuthally anisotropic particle distributions in high energy hadron collisions is often interpreted as evidence for the hydrodynamic flow of the matter produced. We investigate an alternative initial state interpretation of this pattern in high multiplicity proton-proton collisions at the LHC. The QCD Yang-Mills equations describing the dynamics of saturated gluons are solved numerically with initial conditions obtained from the Color Glass Condensate based IP-Glasma model. The gluons are subsequently fragmented into various hadron species employing the well established Lund string fragmentation algorithm of the PYTHIA event generator. We find that this ab initio initial state approach reproduces characteristic features of bulk spectra, in particular the particle mass dependence of $\left<p_T\right>$ and $v_2 (p_T)$.