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Finally, Scientists Prove the ‘Dead Cone Effect,’ Shaking Up Particle Physics

Operators of the ALICE detector have observed the first direct evidence of the “dead cone effect,” allowing them to assess the mass of the elusive charm quark.

a cascade of particles and gluons initiated by a decelerating charm quark
CERN
  • Researchers have observed the “dead cone effect” for the first time ever.
  • The dead cone effect is a fundamental element of the strong nuclear force, which is responsible for binding quarks and gluons.
  • This work, published last month in the journal Nature, proves 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 element of the strong nuclear force — one of the four fundamental forces of nature — responsible for binding quarks and gluons. These are the fundamental particles that comprise hadrons, such as protons and neutrons, that in turn make up all atomic nuclei, which are never seen on their own under normal circumstances, only at the kind of high energy levels generated at the LHC.

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

    The dead cone effect was predicted three decades ago as part of the theory of the strong force and it has previously been indirectly observed at particle accelerators. Yet, 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 that, unlike other experiments which collide protons and slam together the nuclei of heavy atoms, particularly lead—was the ideal piece of equipment to do this.

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


    ✅ Get the Facts: The Large Hadron Collider


      Zardoshti is the lead author of a new paper discussing the ALICE collaboration’s recent findings, 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 observation of the dead cone effect led to another important experimental breakthrough in particle physics.

      “In addition to observing and confirming [the dead cone] effect, which is important in itself, our result is also showing us experimentally that the charm quark has mass — because particles without a mass don’t have a dead cone,” he explained.

      What Are Quarks?

      higgs field, conceptual illustration
      The Higgs field is a quantum field that according to the Standard Model of particle physics permeates all space. When a particle (spheres) interacts with the Higgs field it gains mass. Some particles, such as the photon (yellow), do not interact with the Higgs field and so are massless.
      MARK GARLICK/SCIENCE PHOTO LIBRARYGetty Images

      There are three generations of quarks varying in mass, with charm quarks being part of the second generation of quarks. The dead cone effect tells physicists why heavy quarks from the second and third generations, such as charm and beauty quarks, evolve differently when they emerge from collisions at the LHC when compared to the lighter quarks and gluons, which have no mass.

      Particle collisions at the LHC free up quarks and gluons — particles that are collectively known as partons — which are usually confined within hadrons, like protons and neutrons, and are only free at high-energy levels. The collision of particles leads to a cascade of events, called a parton shower, which emits energy in the form of gluons.

      “When these particles are created in the collisions and travel outwards, they will start emitting more quarks and gluons,” Zardoshti said. “The pattern of these emissions is quite important because they are closely linked to the strong force and help us learn 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 with the size of this angle depending on how heavy the quark is. Within this cone, it is much less likely that gluons are emitted. That means by observing where gluons are not 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
      CERN

      “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 charm quark and reconstruct the gluon emissions from it and observe the dead cone region around the quark, where gluons emissions were rare.”

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

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

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

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

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