# Fragmentation of Two Repelling Lund Strings

### Submission summary

 As Contributors: Cody B Duncan Preprint link: scipost_202001_00039v3 Date accepted: 2020-05-18 Date submitted: 2020-04-22 Submitted by: Duncan, Cody B Submitted to: SciPost Physics Discipline: Physics Subject area: High-Energy Physics - Phenomenology Approach: Phenomenological

### 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 Lund 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 a portion of the string invariant mass $m^2$ into $p_\perp^2$ for constant $m_\perp^2 = m^2 + p_\perp^2$ for each string. The reduction in string invariant mass implies a reduced overall multiplicity of produced hadrons; the increase in $p_\perp^2$ 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 $p_\perp$ distributions and dihadron azimuthal correlations, contrasting it with the current version of the "shoving" model implemented in the same generator.

Published as SciPost Phys. 8, 080 (2020)

### List of changes

1. Replaced all instances of the phrase "QCD strings" with the more appropriate term "Lund strings", since we are working with the Lund string model
2. Added an introductory paragraph with references to a number of different research approaches.

### Submission & Refereeing History

Resubmission scipost_202001_00039v3 on 22 April 2020
Submission scipost_202001_00039v1 on 4 January 2020

## Reports on this Submission

### Anonymous Report 2 on 2020-5-7 Invited Report

• Cite as: Anonymous, Report on arXiv:scipost_202001_00039v3, delivered 2020-05-07, doi: 10.21468/SciPost.Report.1667

### Strengths

The paper deals with the implementation of two point cumulant in the Lund model. The authors show that the two point cumulant is naturally implemented if one assumes the string repulsion proportional to rapidity overlap between strings. They show that there exists the value interval of their parameter c_R where the
cumulant has a right order of magnitude, although it is not clear how much physical this is since only the two string configurations are accounted for
In a new version the paper became much more understandable
and easy to read

### Weaknesses

the weakness of the paper is ad-hoc ansats for repulsion
which does not seem to follow from anywhere. the major check of such ansats may be the study of multi particle correlators and check
for their collectivity (they change signs)-this was not achieved in
any of CGC based model, although derived in ref. [51].
I do not think this weakness should prevent however the publication of the article, since Lund model is itself rather phenomenological model, and all its successful modification should be welcomed.

### Report

I believe the author took into account all my remarks and the paper should be published.

• validity: top
• significance: high
• originality: top
• clarity: high
• formatting: perfect
• grammar: perfect

### Report

The authors have been addressing my main criticism, so I am happy to recommend the manuscript for publication.

• validity: -
• significance: -
• originality: -
• clarity: -
• formatting: -
• grammar: -