The QCD dead cone, an angular region of suppressed gluon bremsstrahlung surrounding an emitting heavy quark, is directly revealed for the first time in pp collisions using D$^{0}$-meson tagged jets. The dead-cone effect is responsible for mass related differences in the shower of light and heavy quarks in both vacuum and the quark-gluon plasma environments.
Jets initiated by a scattered charm quark are tagged via the presence of a fully reconstructed D$^{0}$ meson amongst their constituents. Jet declustering tools allow for the sequential reconstruction of the charm quark shower, giving access to the dynamically evolving emission kinematics. In this way, the angular distribution of gluon emissions can be compared for charm-initiated and inclusive jets, revealing a suppression of small angle emissions in the charm sector. This suppression increases with decreasing charm quark energy, which is probed down to 5 GeV with the ALICE detector. The measured mass and energy dependence of the dead-cone effect follow parametric expectations from theory and are well described by MC generators.
Sketch of the reconstruction of the showering charm quark, using iterative declustering.
Further reading
ALICE Collab. arXiv:2106.0573 [nucl-ex]