A spatially constrained QCD colour reconnection in $$\textrm{pp}$$ pp , $$\textrm{p}A$$ p A , and $$AA$$ AA collisions in the Pythia8/Angantyr model

Abstract

Abstract 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 $$\textrm{pp}$$ 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 $$\textrm{p}A$$ p A 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 $$\textrm{pp}$$ pp fixed. With Angantyr we can then extrapolate to $$AA$$ AA collisions without further parameter tuning and retaining a reasonable description of the basic multiplicity distributions.