Understanding the role of parton mass and Casimir color factors in the quantum chromodynamics parton shower represents an important step in characterizing the emission properties of heavy quarks. Recent experimental advances in jet substructure techniques have provided the opportunity to isolate and characterize gluon emissions from heavy quarks. In this Letter, the first direct experimental constraint on the charm-quark splitting function is presented, obtained via the measurement of the groomed shared momentum fraction of the first splitting in charm jets, tagged by a reconstructed ${\mathit{D}}^0$ meson. The measurement is made in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$, in the low jet transverse-momentum interval of $15\le p_T^{\text{jet ch}}<30\mathrm{GeV}/c$ where the emission properties are sensitive to parton mass effects. In addition, the opening angle of the first perturbative emission of the charm quark, as well as the number of perturbative emissions it undergoes, is reported. Comparisons to measurements of an inclusive-jet sample show a steeper splitting function for charm quarks compared with gluons and light quarks. Charm quarks also undergo fewer perturbative emissions in the parton shower, with a reduced probability of large-angle emissions.

• Received 25 August 2022
• Revised 13 January 2023
• Accepted 19 July 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.192301

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

© 2023 CERN, for the ALICE Collaboration