Fragmentation of Two Repelling Lund Strings

Abstract

Motivated by recent discoveries of flow-like effects in pp collisions, and noting that multiple string systems can form and hadronize simultaneously in such collisions, we develop a simple model for the repulsive interaction between two QCD strings with a positive (colour-oriented) overlap in rapidity. The model is formulated in momentum space and is based on a postulate of a constant net transverse momentum being acquired per unit of overlap along a common rapidity direction. To conserve energy, the strings shrink in the longitudinal direction, essentially converting m 2 to p 2 ⊥ for constant m 2 ⊥ = m 2 + p 2 ⊥ for each string. The reduction in m 2 implies a reduced overall multiplicity of produced hadrons; the increase in p 2 ⊥ is local and only affects hadrons in the overlapping region. Starting from the simplest case of two symmetric and parallel strings with massless endpoints, we generalize to progressively more complicated configurations. We present an implementation of this model in the Pythia event generator and use it to illustrate the effects on hadron p ⊥ distributions and dihadron azimuthal correlations, contrasting it with the current version of the "shoving" model implemented in the same generator. Contents

A Framework for Hadronic Rescattering in pp Collisions

“…Finally, the model only considers the effect of hadrons colliding with hadrons, not those of strings colliding/overlapping with each other or with hadrons. The former is actively being studied within Pythia, as a shoving/repulsion of strings [15,43]. Both shove and rescattering act to correlate the spatial location of strings/hadrons with a net push outwards, giving rise to a radial flow.…”