Measurement of the splitting function in pp and PbPb collisions at $\sqrt{s_{_{\mathrm{NN}}}} =$ 5.02 TeV

TL;DR

This paper measures the splitting function in proton-proton and lead-lead collisions at 5.02 TeV to understand how the dense medium in heavy ion collisions modifies parton shower evolution, revealing more unbalanced splittings in central PbPb collisions.

Contribution

It provides the first measurement of the splitting function in PbPb collisions at this energy, comparing it to pp collisions and theoretical models.

Findings

More unbalanced momentum ratios in central PbPb collisions.

Splitting function modifications are observed compared to pp collisions.

Results are consistent with some theoretical predictions.

Abstract

Data from heavy ion collisions suggest that the evolution of a parton shower is modified by interactions with the color charges in the dense partonic medium created in these collisions, but it is not known where in the shower evolution the modifications occur. The momentum ratio of the two leading partons, resolved as subjets, provides information about the parton shower evolution. This substructure observable, known as the splitting function, reflects the process of a parton splitting into two other partons and has been measured for jets with transverse momentum between 140 and 500 GeV, in pp and PbPb collisions at a center-of-mass energy of 5.02 GeV per nucleon pair. In central PbPb collisions, the splitting function indicates a more unbalanced momentum ratio, compared to peripheral PbPb and pp collisions. The measurements are compared to various predictions from event generators and analytical calculations.