A spatially constrained QCD colour reconnection in pp, pA, and AA collisions in the PYTHIA8/Angantyr model

We present an updated version of the QCD-based colour reconnection model in Pythia8, where we constrain the range in impact parameter for which reconnections are allowed. In this way, we can introduce more realistic colour reconnections in the Angantyr model for heavy ion collisions, where previously only reconnections within separate nucleon sub-collisions have been allowed. We investigate how the new impact parameter constraint influences final states in pp collisions, and retune parameters of the multi-parton interaction parameters in Pythia to compensate so that minimum bias data are reproduced. We also study multiplicity distributions in pA collisions and find that, in order to counteract the loss in multiplicity due to the introduction of global colour reconnections, we need to modify some parameters in the Angantyr model while keeping the parameters tuned to pp fixed. With Angantyr we can then extrapolate to AA collisions without further parameter tuning and retaining a reasonable description of the basic multiplicity distributions.

Automated summary

This article presents an updated version of the QCD-based colour reconnection model in Pythia8, which introduces more realistic colour reconnections in heavy ion collisions. The impact parameter constraint is investigated to understand its influence on final states in pp collisions. Additionally, modifications to parameters in the Angantyr model are needed to counteract the loss in multiplicity due to global colour reconnections in pA collisions.