First observation at the LHC of the ‘dead cone’, a key phenomenon in particle physics

Among elementary particles, quarks and the gluonsalso collectively called ‘parts‘, are produced in particle collisions such as those occurring inside the Large Hadron Collider (LHC) at the European Laboratory for Particle Physics (CERN), near the Swiss city of Geneva.

After their creation, the Partons undergo a cascade of events called ‘parton waterfall‘, whereby they lose energy by emitting radiation in the form of gluons, which also later emit gluons.

The radiation pattern of this cascade depends on the mass of the gluon-emitting parton and shows a region, around the flight direction of the parton, where gluons cannot be emitted. This area is known as dead coneThe dead cone in Spanish.

Now, the ALICE scientific collaboration at the LHC has made the first direct observation of the dead cone effect, a key feature of the theory of fstrong nuclear force, which joins quarks and gluons together to form protons, neutrons, and ultimately all atomic nuclei. The results are published in the journal Nature.

In addition to confirming this effect, the ALICE observation provides direct experimental access to the mass of the quark charm (quark charm), before being confined within the hadrons.

“It has been a great challenge to directly observe the effect dead cone”, says ALICE’s spokesperson, Luciano Musa“Using data recorded over three years of proton-proton collisions at the LHC and sophisticated data analysis techniques, we were finally able to find out.”

The dead cone effect was predicted 30 years ago from the first principles of strong force theory and is often observed indirectly in particle colliders. However, its direct observation from the radiation pattern produced by the parton cascade represents a great challenge for the research community.

Hard to follow the partons

The difficulty in observing the phenomenon dead cone mainly because this region may be full of particles into which the emitting parton has been transformed, producing noise in the observationand because it is difficult to determine the direction of the parton’s movement, since its position changes along the cascade phenomenon.

The ALICE collaboration overcame these challenges by applying advanced analysis techniques to a large sample of proton-proton collisions recorded at the LHC. These techniques allow us to reconstruct the parton cascade pattern from its final products: the signals left by a jet of particles known as jet on the ALICE detector.

Searching jets including a particle containing a charm quark, the researchers were able to identify a jet created by this type of quark and trace its entire history of gluon emission. A comparison between the charm quark’s gluon emission pattern and the virtually massless gluon and quark pattern revealed an emission-free region for the charm quark: the sought after. dead cone.

The result also shows a non-negligible value for the mass of the charm quark, since the theory predicts that massless particles do not have corresponding dead cone regions.

“The masses of quarks are fundamental quantities in particle physics, but they cannot be accessed and measured directly in experiments because, with the exception of the top quark, quarks are confined within composite particles”, explains the coordinator of alice Physics, Andrea Dainese., which highlights: “Our successful technique for directly observing the dead cone of a parton cascade may offer us a way to measure the masses of quarks.”

Spanish participation in ALICE

For several years, researchers from the Galician Institute of High Energy Physics (IGFAE) and the Center for Energy, Environmental and Technological Research (CIEMAT) participated in the analysis of the physics detected by ALICE and collaborated in the design of the computer system needed to analyze the data it records.

Today, the ALICE collaboration continues to count with the participation of Spanish scientists, although they are currently contracted by foreign institutions or international projects.

Letícia Cunqueiro, a researcher who completed her PhD at the IGFAE and who currently teaches at the University of La Sapienza (Rome) and is a member of the CMS, was one of the main authors of this analysis, work she led at the Oak Ridge National Laboratory when she was a member of ALICE. “The measurement allowed us to show, in a creative way, the direct effects of the charm quark mass on the angle of emission of QCD radiation”, says Cunqueiro, underlining the importance of this result.