Direct observation of the dead-cone effect in QCD

At 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). The vacuum is not transparent to the partons and induces gluon radiation and quark pair production in a process that can be described as a parton shower. Studying the pattern of the parton shower is one of the key experimental tools in understanding the properties of 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 $m$ and energy $E$, within a cone of angular size $m$/$E$ around the emitter. A direct observation of the dead-cone effect in QCD has not been possible until now, due to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible bound hadronic states. We report the first direct observation of the QCD dead-cone by using new iterative declustering techniques to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD, which is derived more generally from its origin as a gauge quantum field theory. 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.

 

Submitted to: OTHERS
e-Print: arXiv:2106.05713 | PDF | inSPIRE

Figure 1

A sketch detailing the reconstruction of the showering charm quark, using iterative declustering, is presented. The top panels show the initial reclustering procedure with the C/A algorithm, where the particles separated by the smallest angles are brought together first. Once the reclustering is complete, the declustering procedure is carried out by unwinding the reclustering history. Each splitting node is numbered according to the declustering step in which it is reconstructed. With each splitting, the charm quark energy, $E_{\rm{Radiator,n}}$, is reduced and the gluon is emitted at a smaller angle, $\theta_{\rm{n}}$, with respect to previous emissions. At each splitting, gluon emissions are suppressed in the dead-cone region (shown by a red cone for the last splitting), which increases in angle as the quark energy decreases throughout the shower.

Figure 2

The ratios of the splitting-angle probability distributions for D$^{0}$-meson tagged jets to inclusive jets, $R(\theta)$, measured in pp collisions at $\sqrt{s}=13$ TeV, are shown for $5