Scientists prove ‘dead cone effect’, shaking up particle physics

  • Researchers have observed the “dead cone effect” for the first time.
  • The dead cone effect is a fundamental element of the strong nuclear forcewho is responsible for binding quarks and gluons
  • This work, published last month in the magazine 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 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 make up hadrons, such as protons and neutrons, which in turn make up all atomic nuclei, which under normal conditions are never seen alone, only at the kind of high energy levels generated by the LHC.

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

    The dead cone effect was predicted three decades ago as part of the strong force theory and has previously been indirectly observed on 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 that, unlike other experiments that collide protons and slam the nuclei of heavy atoms, especially lead — was the ideal device. to do this.

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

    ✅Get the Facts: The Large Hadron Collider

      Zardoshti is the lead author of a new article in which the ALICE the recent findings of the 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 observation of the dead cone effect led to another major experimental breakthrough in particle physics.

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

      What are quarks?

      higgs field, conceptual image

      The Higgs field is a quantum field that permeates all space according to the Standard Model of particle physics. 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 are therefore massless.


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

      Particle collisions at the LHC release quarks and gluons — particles collectively known as partons — which are usually locked in hadrons, such as 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 that emits energy in the form of gluons.

      “When these particles arise in the collisions and travel outwards, they will start to emit 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 by the mass of the emitting quark. [in this case the charm quark] through the dead cone effect.”

      How does the Dead Cone effect work?

      The dead cone is an angle around the emitting quark where the magnitude of this angle depends on how heavy the quark is. Within this cone, gluons are much less likely to be emitted. That means that by observing where gluons are not being emitted and measuring this dead cone, scientists can reveal the mass of the particle To be 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 — meaning it should have a large dead cone,” Zardoshti said. “So our technique was to isolate the charm quark and reconstruct its gluon emission and observe the dead cone region around the quark, where gluon emissions were rare.”

      The technique of the ALICE collaboration rolled the parton shower back in timeof its final product particles when the rarer particles created in the parton shower have decayed. The team then looked for traces of the charm quark and tracked the history of the gluon emissions.

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

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

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

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