The Large Hadron Collider has found a new way to measure the mass of a quark.

The ALICE experiment at the Large Hadron Collider for the first time directly measured a phenomenon known as the “dead cone”, allowing physicists to directly measure the mass of a fundamental particle known as the “charm quark”.

Many of the particles that make up the visible universe around us are actually composite particles built from less massive fundamental particles known as quarks. Protons and neutrons, for example, each contain three quarks. There are six different “flavors” of a quark – up, down, up, down, weird, and charming – each with different masses, spins, and other quantum properties. Different combinations of quarks also form different particles. Quarks are held together in these complex particles by a strong force that is transmitted through a massless particle called a gluon. Collectively, quarks and gluons are known as “partons”.

At the Large Hadron Collider (LHC) at CERN near Geneva, Switzerland, protons are accelerated by strong magnetic fields through a 16.8-mile (27 km) tunnel to energies up to 6.8 TeV before colliding with each other. Collisions produce a cascade of other particles that themselves emit or decay into even more particles, and so on in a cascade that can shed light on aspects of fundamental physics.

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In particular, quarks and gluons are produced and emitted in a cascade called parton flow, where quarks emit gluons, and gluons themselves can emit other lower-energy gluons.

Scientists working on ALICE (short for Large Ion Collider Experiment) have analyzed three years of proton-proton collisions to find evidence for the dead cone. According to the theory of quantum chromodynamics, or QCD, which describes how the strong force works, a dead cone is a region where partons of a certain mass and energy cannot emit gluons.

“It was very difficult to observe the dead cone directly,” ALICE spokesman Luciano Musa said in a press statement.

Diagram of dead zones around a charmed quark every time it emits a gluon in a stream of partons, which prevents more gluons from being emitted for a while. (Image credit: CERN)

Part of the difficulty is that the dead zone can be filled with other subatomic particles produced by proton-proton collisions, while tracing the movement of a parton through a shower, as it constantly changes direction, is also difficult.

To solve this problem, scientists collaborating with ALICE developed a method by which they were able to rewind records of parton streams back in time, allowing them to figure out where and when stream by-products were emitted. In particular, they were looking for flows in which the charmed quark participates. Analyzing these showers, the scientists found a region in the structure of gluon radiation emitted during parton showers where gluon emission was suppressed. This is a dead cone.

This discovery is important not only because it confirms the prediction of QCD, but also because it now makes it possible to directly measure the mass of the charmed quark, which according to theory and indirect measurements is 1.275 +/- 25 MeV/c^2. . According to QCD, the dead cone is directly related to the mass of the parton, and massless particles cannot form a dead cone.

“Quark masses 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 trapped inside compound particles,” said Andrea Dainese, ALICE physics coordinator.

Therefore, the discovery of the dead cone could pave the way for a new era in quark physics.

The results were published May 18 in the journal Nature.

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