Scientists prove that the ‘dead cone effect’ is turning particle physics upside down

  • The researchers observed for the first time the “dead cone effect”.
  • The dead cone effect is a fundamental element of the strong nuclear force, responsible for binding quarks and gluons.
  • This work, published last month in the journal Natureproves that the charm quark has mass.

    The ALICE collaboration at the Large Hadron Collider (LHC) in Geneva, Switzerland, recently made the first observation of an important aspect of particle physics called the “dead cone effect”.

    The effect is a fundamental part of the strong nuclear force – one of the four fundamental forces in nature – responsible for binding quarks and gluons. They are the fundamental particles that make up hadrons, such as protons and neutrons, which in turn make up all of the atomic nuclei, which are never seen alone under normal circumstances, only at the high energy levels generated at the LHC.

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    “We made a direct observation of an effect in the strong force theory called the dead cone effect,” said Nima Zardoshti, an experimental high-energy physicist at CERN. Popular mechanics. “This is a part of the theory that had been predicted for some time but had not been directly observed until now.

    The dead cone effect was predicted three decades ago as part of the strong force theory and it has already been indirectly observed in particle accelerators. Still, directly observing the effect has remained a challenge for physicists. Fortunately, the ALICE (A Large Ion Collider Experiment) detector – part of an experiment at the LHC which, unlike other experiments which collide protons and slam the nuclei of heavy atoms, particularly lead – was the ideal equipment to do this.

    “At ALICE, we can make measurements at quite low energies by LHC standards, which is important because the dead cone angle is only important for low-energy heavy quarks,” Zardoshti explained. “We also have detectors that work a bit like cameras and are really good at finding hadrons containing heavy quarks – a crucial step in reconstructing the isolated heavy quark.”

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      Zardoshti is lead author of a new paper on recent findings from the ALICE collaboration, published last month in the journal Nature. The team conducted experiments for this work between 2019 and spring 2021. In the paper, Zardoshti and his team explain that the observation of the dead cone effect led to another important experimental breakthrough in particle physics. .

      “In addition to observing and confirming [the dead cone] Indeed, which is important in itself, our result also shows us experimentally that the charm quark has mass — because massless particles don’t have a dead cone,” he explained.

      What are quarks?

      The Higgs field is a quantum field which, according to the standard model of particle physics, pervades all of space. When a particle (spheres) interacts with the Higgs field, it gains mass. Some particles, like the photon (yellow), do not interact with the Higgs field and are therefore massless.


      There are three generations of quarks that vary in mass, with charmed quarks being part of the second generation of quarks. The dead cone effect explains to physicists why second- and third-generation heavy quarks, such as charm and beauty quarks, evolve differently when they emerge from collisions at the LHC than lighter quarks and gluons, which do not have no mass.

      Particle collisions at the LHC release quarks and gluons – particles collectively called partons – which are usually confined in hadrons, like protons and neutrons, and only free at high energy levels. The collision of particles results in a cascade of events, called a parton shower, which emits energy in the form of gluons.

      “As these particles are created in the collisions and move outward, they will start emitting more quarks and gluons,” Zardoshti said. “The pattern of these emissions is quite important because they are closely related to the strong force and help us learn more about its properties. One of the ways these patterns are affected is through the mass of the emitting quark [in this case the charm quark] via the dead cone effect.

      How does the Dead Cone effect work?

      The dead cone is an angle around the emitting quark, the size of this angle depending on the weight of the quark. Within this cone, it is much less likely that gluons will be emitted. This means that by observing where gluons are not being emitted and measuring this dead cone, scientists can reveal the mass of the particle being studied.

      “For charm, beauty and top quarks, which are quite heavy, the angle is quite large and has a big impact on the pattern of gluons that the heavy quark can emit,” Zardoshti continued.

      dead cone effect


      “The charm quark has a large mass – along with the beauty and top quarks – which means it should have a large dead cone,” Zardoshti said. “So our technique was to isolate the charmed quark and reconstruct the gluon emissions from it and observe the dead cone region around the quark, where gluon emissions were rare.”

      The ALICE collaboration’s technique moved the parton shower back in time from its final product particles when the rarer particles created in the parton shower decayed. The team then looked for traces of the charm quark and traced its history of gluon emissions.

      Comparison of this emission pattern with emissions from lighter quarks and gluons revealed the dead cone in charm quark emissions. “Our technique has found a way to not only isolate the charm quark, but also to measure an effect that is directly sensitive to the mass it has before it binds to a hadron,” Zardoshti explains.

      The ALICE collaboration now plans to further investigate the dead cone effect with data that will be collected this summer as part of Run 3 at the LHC.

      “We then want to measure the dead cone of the beauty quark which should be even larger than that of the charm quark because the beauty quark is much heavier,” Zardoshti concluded. “We then want to extend this technique of isolating heavy quark emissions to try to characterize more information about the emission pattern in the strong force.”

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