In particle collider experiments, interactions of elementary particles with large momentum transfer produce quarks and gluons (called partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics . These partons then emit other partons in a process that can be described as a shower of partons, which results in the formation of detectable hadrons. The study of the parton shower model is one of the main experimental tools for testing quantum chromodynamics. This model is expected to depend on the mass of the initiating parton, through a phenomenon known as dead cone effect. Now the physicists of ALICE Collaboration at CERN’s Large Hadron Collider made the first direct observation of this fundamental phenomenon. In addition to confirming this effect, observation provides direct experimental access to the mass of a single charm quark before it is confined inside hadrons.
Quarks and gluons, collectively called partons, are produced in particle collisions such as those that take place at the Large Hadron Collider.
After their creation, partons undergo a cascade of events called parton rain, during which they lose energy by emitting radiation in the form of gluons, which also emit gluons.
The radiation pattern of this shower depends on the mass of the gluon-emitting parton and shows a region around the direction of flight of the parton where gluon emission is suppressed – the dead cone.
Predicted 30 years ago from the first principles of strong force theory, the dead cone has been indirectly observed at particle colliders.
However, it remained difficult to observe it directly from the parton shower radiation pattern.
The main reasons for this are that the dead cone may be filled with particles into which the emitting parton transforms, and it is difficult to determine the change in direction of the parton throughout the shower process.
ALICE physicists overcame these challenges by applying state-of-the-art analysis techniques to a large sample of proton-proton collisions at the Large Hadron Collider.
These techniques can cause the parton shower to roll back in time from its end products – the signals left in the ALICE detector by a jet of particles known as the jet.
By searching for jets containing a particle containing a charm quark, the researchers were able to identify a jet created by this type of quark and trace the entire history of the quark’s gluon emissions.
A comparison of the charm quark’s gluon emission pattern with that of nearly massless gluons and quarks then revealed a dead cone in the charm quark pattern.
The result also directly exposes the mass of the charm quark, as the theory predicts that massless particles have no corresponding dead cones.
“The dead cone is a fundamental phenomenon of quantum chromodynamics, dictated by the non-zero masses of quarks, whose direct experimental observation had until now remained elusive,” the researchers said.
“This measurement provides insight into the influence of mass effects on jet properties and provides constraints for Monte Carlo models.”
“These results pave the way for a study of the mass dependence of the dead cone effect, by measuring the dead cone of beauty jets labeled with a reconstructed beauty hadron.”
“Future study of the dead cone effect in heavy ion collisions, in which the partons interact strongly with the hot quantum chromodynamic medium that forms and undergoes energy loss by radiation induced by the medium, is also considered,” they added.
“If a dead cone were observed for these medium-induced emissions, it would be a confirmation of the theoretical understanding of quantum chromodynamic radiation in the medium, which is a primary tool used to characterize the high-temperature phase of quantum chromodynamic matter. “
“Quark masses are fundamental constants of the Standard Model of particle physics and necessary for all numerical calculations within its framework,” they said.
“Due to confinement, their values are usually inferred from their influence on hadronic observables. An exception is the top quark, which decays before it can hadronize, because its mass can be constrained experimentally from the direct reconstruction of the decay final states.
“By accessing the kinematics of the charmed quark in rain, before hadronization, and directly uncovering the dead cone effect, our measurement provides direct sensitivity to the mass of quasi-free charmed quarks, before they bind. to hadrons.
“Furthermore, future high-precision measurements using this technique on labeled charm and beauty jets, potentially in conjunction with machine learning tools to separate quark and gluon emissions, could experimentally limit the magnitude of masses of quarks.
the results were published in the May 19, 2022 issue of the journal Nature.
ALICE Collaborative. 2022. Direct observation of the dead cone effect in quantum chromodynamics. Nature 605, 440-446; doi:10.1038/s41586-022-04572-w
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