The Large Hadron Collider’s ALICE experiment has, for the first time, directly measured a phenomenon known as a “deadcone”, which allowed physicists to directly measure the mass of a fundamental particle known as a “quark charm”.
Many particles that make up the visible universe around us are actually composite particles built from less massive fundamental particles called quarks. Protons and neutrons, for example, contain three quarks each. There are six different “the flavours“ quark – high, low, high, low, weird and charming – which each have different masses, spins and other quantum properties. Different combinations of quarks also form different particles. The quarks are held together in these composite particles by the a mighty force, which is transmitted via a massless particle called a gluon. Collectively, quarks and gluons are known as “partons”.
At Large Hadron Collider (LHC) at CERN near Geneva, Switzerland, protons are accelerated by powerful magnetic fields through a 16.8 mile (27 kilometer) tunnel to energies up to 6.8 TeV, before crashing into each other. The collisions produce a cascade of other particles, which themselves emit or decay into yet 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 a parton shower, where the quarks emit gluons, and the gluons themselves can emit other lower energy gluons.
Scientists working on ALICE (its name is short for A Large Ion Collider Experiment) combed through three years of proton-proton collisions to find evidence of the dead cone. According to the theory of quantum chromodynamics, or QCD, which describes how the strong force works, the dead cone is a region where partons of a certain mass and energy are prevented from emitting gluons.
“It was very difficult to directly observe the dead cone,” ALICE spokesman Luciano Musa said in a statement. Press statement.
Part of the difficulty is that the dead zone can be filled with other subatomic particles created in proton-proton collisions, while tracking the motion of a parton through the shower as it constantly changes direction is also delicate.
To solve this problem, scientists collaborating with ALICE have developed a technique that allows them to rewind recordings of parton rains in time, allowing them to determine where and when the byproducts of the rain were emitted. In particular, they looked for showers involving a charm quark. By deconstructing these showers, the scientists discovered a region in the pattern of gluon radiation emitted during parton showers where gluon emission was suppressed. It is the dead cone.
The discovery is important not only because it verifies a QCD prediction, but also because it now offers a direct opportunity to directly measure the mass of the charm quark, which theory and indirect measurements place at 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 produce a dead cone.
“Quark masses are fundamental quantities in particle physics, but they cannot be consulted or measured directly in experiments because, with the exception of the top quark, quarks are confined inside composite particles”, said Andrea Dainese, who is ALICE’s physical coordinator.
Therefore, the discovery of the dead cone could usher in a new era of quark physics.
The results were published on May 18 in the journal Nature.
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