Ridge correlation structure in high multiplicity pp collisions with CMS

TL;DR

This paper reports the observation of a ridge-like correlation structure in high multiplicity proton-proton collisions at 7 TeV, similar to heavy ion collision phenomena, which challenges existing models and varies with event characteristics.

Contribution

First observation of a ridge structure in high multiplicity pp collisions at 7 TeV, expanding understanding of collective effects in small systems.

Findings

Ridge structure appears in high multiplicity events

Not predicted by standard Monte Carlo models

Structure varies with particle momentum and event features

Abstract

Results on two-particle angular correlations are presented in proton-proton collisions at center of mass energies of 7 TeV, over a broad range of pseudorapidity and azimuthal angle. In very high multiplicity events at 7 TeV, a pronounced structure emerges in the two-dimensional correlation function for particle pairs with intermediate $p_{\mathrm{T}}$ of 1--3 GeV/c, in the kinematic region $2.0 < |\Delta\eta| < 4.8$ and small $\Delta\phi$. This structure, which has not been observed in pp collisions before, is similar to what is known as the "ridge" in heavy ion collisions. It is not predicted by commonly used proton-proton Monte Carlo models and is not seen in lower multiplicity pp collisions. Updated studies of this new effect as a function of particle transverse momentum, rapidity and event characteristics are shown.