/ nucl-ex CMS-HIN-16-006

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

Pages: 30

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. However, the stage in the shower evolution at which the modifications occur is not known. By removing the soft wide-angle radiation inside a jet, the two leading partons can be resolved as subjets. The momentum ratio of these subjets provides information about the corresponding partons at an early stage of the shower. This substructure observable, known as the splitting function, relates to the process of a parton splitting into two other partons. The measurement is performed for jets with transverse momentum larger than 140 GeV and smaller than 500 GeV, using pp and PbPb collision samples collected by the CMS detector at a center-of-mass energy of 5.02 GeV per nucleon pair. In central PbPb collisions, the splitting function is observed to be modified toward a more unbalanced momentum ratio compared to peripheral PbPb and pp collisions. The measurements are compared to the predictions from event generators and analytical calculations using various models for the medium modification of the jet evolution.

Note: Submitted to Phys. Rev. Lett. All figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-16-006 (CMS Public Pages)'

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