Direct observation of the dead-cone effect in quantum chromodynamics

Abstract

In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD)1. These partons subsequently emit further partons in a process that can be described as a parton shower2, which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass mQ and energy E, within a cone of angular size mQ/E around the emitter3. Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques4,5 to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.

Measurement of beauty production via non-prompt D0 mesons in Pb-Pb collisions at $$ \sqrt{{\textrm{s}}_{\textrm{NN}}} $$ = 5.02 TeV

“…In particular, beauty quarks are expected to lose less energy than charm quarks. At high p T , where energy loss is caused mainly by radiative processes, this difference is expected to derive from the "dead-cone" effect, which suppresses gluon radiation off massive quarks at angles smaller than m Q /E (with m Q and E being the quark mass JHEP12(2022)126 and energy, respectively) with respect to the quark direction [46][47][48][49], an effect directly observed in pp collisions at the LHC [50]. This expectation is supported by experimental data showing higher R AA for beauty than charm signals, qualitatively in line with theoretical predictions [29,39,42,44,45,[51][52][53][54][55].…”

Heavy quark energy loss through polarization and radiation in the hot QCD medium

“…in this equation represents the dead cone effect [62,63], which is a suppression factor that depends on the mass of the HQ. The invariant amplitude for charm quarks is generally greater than bottom quarks in the inelastic processes.…”

Snowmass 2021 Whitepaper: Proton Structure at the Precision Frontier

“…Perturbative c, b masses suppress very soft QCD radiation e.g. in the final-state dead-cone effect [623][624][625][626][627]. At the same time they introduce power-suppressed terms that modify soft factorization compared to the massless case [628,629].…”