First direct observation of dead cone effect in particle physics

First direct observation of dead cone effect in particle physics

Parton shower charm quarks (c) lose energy by emitting radiation in the form of gluons (g). The shower displays a dead cone of suppressed radiation around the quark at an angle smaller than the ratio of mass (m) to energy (E) of the quark. Energy is reduced at each stage of the shower. Credit: CERN

The first direct observation of the dead cone effect was made by ALICE’s collaborative research with the Large Hadron Collider (LHC). This is a fundamental feature of the theory of strong forces that bond quarks and gluons to protons, neutrons, and ultimately all atoms. Nuclear. In addition to confirming this effect, the observations reported in the paper published today are: NatureAllows direct experimental access to the mass of a single charm quark before being trapped within a hadron.

“It was very difficult to directly observe the dead cone,” says ALICE spokesman Luciano Musa. “But by using three years of data from the proton-proton collision at the LHC and advanced data analysis techniques, we were finally able to reveal that.”

Quarks and gluons, collectively called partons, are produced by particle collisions, such as those that occur at the LHC. After being created, a parton loses energy by emitting radiation in the form of a gluon through a series of events called a parton shower. This radiation also emits gluons. The radiation pattern of this shower depends on the mass of the parton that emits gluons and displays the area around the parton’s flight direction where the emission of gluons is suppressed, the dead cone.

The dead cone, predicted 30 years ago from the first principle of strong force theory, was indirectly observed by a particle collider. However, it is still difficult to observe it directly from the radiation pattern of the Parton shower. The main reason for this is that the partons emitted by the dead cone can be filled with deforming particles, and it is difficult to determine the changing direction of the partons throughout the shower process.

First direct observation of dead cone effect in particle physics

As the parton shower progresses, the gluons are emitted at a smaller angle and the quark energy is reduced, resulting in a larger dead cone with suppressed gluon emissions. Credit: CERN

The ALICE collaboration has overcome these challenges by applying state-of-the-art analytical techniques to large samples of proton-proton collisions at the LHC. These techniques allow the parton shower to be rolled back in time from the signal left in the ALICE detector by the final product, the spray of particles called jets. By looking for jets containing particles containing charm quarks, researchers were able to identify the jets produced by this type of quark and trace the entire history of quark gluon emissions. A comparison of the charm quark gluon emission pattern with that of the gluon and virtually massless quarks revealed a dead cone in the charm quark pattern.

The results also directly reveal the mass of the charm quark, as the theory predicts that massless particles do not have a corresponding dead cone.

“The mass of a quark is a basic quantity in particle physics, but with the exception of the top quark, the quark is confined within a composite particle and cannot be directly accessed and measured in an experiment,” ALICE said. Physical coordinator Andrea Dainese explains. “Our successful technique of directly observing the dead cone of a parton shower may provide a way to measure the mass of quarks.”


New insights into the internal structure of protons


For more information:
ALICE collaboration, direct observation of dead cone effect in quantum chromodynamics, Nature (2022). DOI: 10.1038 / s41586-022-04572-w

Quote: The first direct observation of the dead cone effect in particle physics (May 18, 2022) is from https://phys.org/news/2022-05-dead-cone-effect-particle-physics.html. Acquired on June 14, 2022.

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