The "Ridge" in Proton-Proton Scattering at 7 TeV

TL;DR

This paper demonstrates that hydrodynamical expansion with flux tube initial conditions naturally explains the ridge structure observed in proton-proton collisions at 7 TeV, indicating fluid-like behavior at very small scales.

Contribution

It introduces an event-by-event hydrodynamical model based on flux tube initial conditions to explain the ridge in $pp$ scattering, highlighting collective behavior at small scales.

Findings

Hydrodynamical expansion reproduces the ridge structure.

Statistical fluctuations of initial conditions are crucial.

Fluid-like behavior observed at 0.1 fm scales.

Abstract

One of the most important experimental results for proton-proton scattering at the LHC is the observation of a so-called "ridge" structure in the two particle correlation function versus the pseudorapidity difference $\Delta\eta$ and the azimuthal angle difference $\Delta\phi$. One finds a strong correlation around $\Delta\phi=0$, extended over many units in $\Delta\eta$. We show that a hydrodynamical expansion based on flux tube initial conditions leads in a natural way to the observed structure. To get this result, we have to perform an event-by-event calculation, because the effect is due to statistical fluctuations of the initial conditions, together with a subsequent collective expansion. This is a strong point in favour of a fluid-like behavior even in $pp$ scattering, where we have to deal with length scales of the order of 0.1 fm.