Latest results on colour reconnection in ATLAS and CMS

Colour reconnection (CR) is a mechanism that describes the interactions that can occur between colour fields during the hadronisation transition. In the context of precise top-quark mass measurements, it plays a crucial role. The modelling of CR has become one of the dominant sources of systematic uncertainty in these measurements. Ongoing top-quark mass analyses use PYTHIA 8 MC event generator for parton showering and hadronisation. PYTHIA 8 comes with several alternative CR models which should be explored to estimate the CR modelling uncertainty. At the same time, the models should be confronted with LHC data to test their validity. Only models which are in agreement with data, in general, are suitable to define the corresponding modelling uncertainty. A measurement of charged-particle distributions sensitive to the different CR models in PYTHIA 8 in top-quark pair production is presented. The measurement is based on data collected using the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 13 TeV with an integrated luminosity of 139 fb-1. Distributions of charged-particle multiplicity, charged-particles scalar sum of transverse momentum, and the same scalar sum in bins of nch are measured in the region outside of the selected jets and compared with predictions resulting from the different CR models. Afterwards, two new CMS PYTHIA8 tunes, CP5-CR1 and CP5-CR2, obtained respectively for the quantum chromodynamics-inspired and gluon-move colour reconnection (CR) models in PYTHIA8, are introduced. In addition, the uncertainty on the top quark mass due to CR effects, calculated with the new tunes, is presented.