Ridge from Strings

TL;DR

This paper explains the origin of the ridge phenomenon in high-energy collisions using a string fusion model, showing it arises from superimposed events and matches experimental data across different collision types.

Contribution

It introduces a novel explanation for the ridge effect based on superposition of multiple string cluster events, linking it to flow harmonics and confirmed by Monte Carlo simulations.

Findings

Ridge arises from event superposition, not a single event.

Good agreement with experimental data for pp, pPb, and AA collisions.

Ridge appears only in high-multiplicity rare events in pp collisions.

Abstract

In the colour string picture with fusion and percolation it is shown that long range azimuthal-rapidity correlations (ridge) can arise from the superposition of many events with exchange of clusters of different number of strings and not from a single event. Relation of the ridge with the flow harmonics coefficients is derived. By direct Monte-Carlo simulations, in the technique previously used to calculate these coefficients, ridge correlations are calculated for AA, pA and pp collisions. The azimuthal anisotropy follows from the assumed quenching of the emitted particles in the strong colour fields inside string clusters. It is confirmed that in pp collisions the ridge structure only appears in rare events with abnormally high multiplicity. Comarison with the experimental data shows a good agreement. Also a good agreement is found for pPb collisions. For AA collisions a reasonable agreement is found for both near-side and away-side angular correlations although it worsens at intermediate angles.