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^2m2 into p_\perp^2p⊥2 for constant m_\perp^2 = m^2 + p_\perp^2m⊥2=m2+p⊥2 for each string. The reduction in string invariant mass implies a reduced overall multiplicity of produced hadrons; the increase in p_\perp^2p⊥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_\perpp⊥ distributions and dihadron azimuthal correlations, contrasting it with the current version of the “shoving” model implemented in the same generator.
Highlights
Invariant mass implies a reduced overall multiplicity of produced hadrons; the increase in p2 is local and only affects hadrons in the overlapping region
In the rest of this section, we study the consequences of our model for an explicit example configuration defined by: p+1 = p+2 = 400 1, 0, 0⊥ GeV, p−1 = p−2 = 400 0, 1, 0⊥ GeV
The red dashed histogram shows the results of using the ordinary Lund model, which — since the Gaussian transverse momentum generation in the baseline Lund model is independent of the rapidity span — is a flat distribution modulo endpoint effects, The two blue histograms illustrate the effects of our compression and fragmentation repulsion model, for a representative value of cR = 0.2 GeV
Summary
Hadronization models play an essential role in the description of hadronic events in highenergy collisions, connecting the short-distance physics of quarks and gluons with the observable world of colourless (long-lived) hadrons via a dynamical process that enforces confinement. A mechanism for microscopic string-string interactions which generates transverse momentum pressure between overlapping strings, was proposed in [37, 38] and showed long-range azimuthal correlations Both the Rope and shoving model have been implemented in Pythia, Dipsy [39], and Angantyr [40]. (The space-time structure of hadronization in the Lund model was recently further explored in [61].) the effect of the interaction is in our model represented via a global rescaling of the 4-momenta of the string endpoints combined with a local addition of p⊥ to hadrons formed in regions of string overlap, while the shoving model imparts transverse momentum by adding a number of low-energy slightly massive gluons to each string.
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