The ALICE collaboration at the Large Hadron Collider (LHC) has made the
first direct observation of the dead-cone effect—a fundamental feature of
the theory of the strong force that binds quarks and gluons together into
protons, neutrons and, ultimately, all atomic nuclei. In addition to
confirming this effect, the observation, reported in a paper published today
in Nature, provides direct experimental access to the mass of a single charm
quark before it is confined inside hadrons.
"It has been very challenging to observe the dead cone directly," says ALICE
spokesperson Luciano Musa. "But, by using three years' worth of data from
proton–proton collisions at the LHC and sophisticated data-analysis
techniques, we have finally been able to uncover it."
Quarks and gluons, collectively called partons, are produced in particle
collisions such as those that take place at the LHC. After their creation,
partons undergo a cascade of events called a parton shower, whereby 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 displays a region around the direction of flight
of the parton where gluon emission is suppressed—the dead cone.
Predicted thirty years ago from the first principles of the theory of the
strong force, the dead cone has been indirectly observed at particle
colliders. However, it has remained challenging to observe it directly from
the parton shower's radiation pattern. The main reasons for this are that
the dead cone can be filled with the particles into which the emitting
parton transforms, and that it is difficult to determine the changing
direction of the parton throughout the shower process.
The ALICE collaboration overcame these challenges by applying
state-of-the-art analysis techniques to a large sample of proton–proton
collisions at the LHC. These techniques can roll the parton shower back in
time from its end-products—the signals left in the ALICE detector by a spray
of particles known as a jet. By looking for jets that included a particle
containing a charm quark, the researchers were able to identify a jet
created by this type of quark and trace back the quark's entire history of
gluon emissions. A comparison between the gluon-emission pattern of the
charm quark with that of gluons and practically massless quarks then
revealed a dead cone in the charm quark's pattern.
The result also directly exposes the mass of the charm quark, as theory
predicts that massless particles do not have corresponding dead cones.
"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 confined inside composite
particles," explains ALICE physics coordinator Andrea Dainese. "Our
successful technique to directly observe a parton shower's dead cone may
offer a way to measure quark masses."
Reference:
ALICE Collaboration, Direct observation of the dead-cone effect in quantum
chromodynamics, Nature (2022).
DOI: 10.1038/s41586-022-04572-w
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Physics