Do p+p Collisions Flow at RHIC? Understanding One-Particle Distributions, Multiplicity Evolution, and Conservation Laws

TL;DR

This paper investigates whether collective flow occurs in proton-proton collisions at RHIC by analyzing one-particle distributions and conservation laws, suggesting that conservation effects may mask true collective behavior.

Contribution

It highlights the importance of phase-space restrictions due to conservation laws in interpreting p+p collision data, challenging the assumption that p+p collisions lack collective flow.

Findings

Conservation laws significantly influence particle spectra in p+p collisions.

Phase-space restrictions are evident in two-particle correlations at RHIC.

Energy and momentum conservation effects may make p+p collisions more similar to heavy ion collisions.

Abstract

Collective, explosive flow in central heavy ion collisions manifests itself in the mass dependence of $p_T$ distributions and femtoscopic length scales, measured in the soft sector ($p_T\lesssim 1$ GeV/c). Measured $p_T$ distributions from proton-proton collisions differ significantly from those from heavy ion collisions. This has been taken as evidence that p+p collisions generate little collective flow, a conclusion in line with naive expectations. We point out possible hazards of ignoring phase-space restrictions due to conservation laws when comparing high- and low-multiplicity final states. Already in two-particle correlation functions, we see clear signals of such phase-space restrictions in low-multiplicity collisions at RHIC. We discuss how these same effects, then, {\it must} appear in the single particle spectra. We argue that the effects of energy and momentum conservation actually dominate the observed systematics, and that $p+p$ collisions may be much more similar to heavy ion collisions than generally thought.