A New Method is Being Used to Study the Internal Structure of Atoms at CERN
The CMS experiment is one of the experiments conducted at the Large Hadron Collider (LHC), which investigates high-energy collisions of protons and atomic nuclei. In high-energy heavy-ion collisions, atomic nuclei generate a strong electromagnetic field around them, enabling the detection of collisions in which the two nuclei, in a classical sense, merely pass by each other, as the distance between them is greater than twice the radius of the nucleus. These are referred to as ultra-peripheral collisions. Protons (and neutrons) are traditionally modeled as being composed of three quarks, but in reality, in addition to these three quarks, there are numerous quark-antiquark pairs and gluons present, which contribute significantly to the nucleons’ energy. Ultra-peripheral collisions provide a unique opportunity to map the behavior of gluons within atomic nuclei.
The CMS researchers studied ultra-peripheral lead-lead collisions at the highest energy levels achieved so far, where a (virtual) photon from the electromagnetic field of one nucleus interacted with the other nucleus. As a result of the interaction, the participating nucleus broke apart into fragments, producing numerous particles, while the other nucleus (from whose field the photon originated) continued on its path unharmed.
A key component of the measurement was the so-called Zero Degree Calorimeters, located 140 meters on either side of the collision point. These are positioned to detect neutral particles traveling in the beam direction after the collision. Using these detectors, it is possible to measure whether a nucleus has broken apart, releasing neutrons, and to identify events where only one of the two nuclei disintegrated. Another critical requirement during the measurements was ensuring that no high-energy particles were present in the beam-aligned angular range in the direction of the intact nucleus. With these two criteria, the researchers effectively isolated ultra-peripheral collisions, as these typically produce significantly fewer particles compared to central collisions, where the two nuclei collide directly.
The latest results focus on measuring the production cross-section of D0 mesons in ultra-peripheral lead-lead collisions. D0 mesons and their antiparticles are produced when the interaction between a photon and a nucleus creates a charm quark-antiquark pair. Balázs Csaba Kovács, a BSc student at ELTE’s Institute of Physics and Astronomy, actively participated in the analysis under the supervision of Gábor Veres. They collaborated with researchers from the Massachusetts Institute of Technology (MIT) to evaluate the data, calibrate the Zero Degree Calorimeters used in the measurements, and conduct recent beam test experiments. This work was carried out partly at ELTE, and during study visits, at MIT and CERN, within the CMS Quantum Chromodynamics Research Group under the Astro- and Particle Physics program of the Higher Education Institutional Excellence Program (FIKP).
The D0 mesons themselves cannot be directly detected by the CMS detector because they decay within an extremely short time, close to the beamline. Instead, researchers identified D0 mesons by detecting their decay products. They specifically searched for D0 mesons that decayed into a kaon and a pion, as these particles can be detected using the calorimeters and tracking detectors. To achieve this, the researchers paired tracks from kaons and pions in the recorded events and analyzed whether the two particles could have originated from the decay of a single D0 meson. The new results determine the production rates of D0 mesons and their antiparticles in ultra-peripheral lead-lead collisions, marking the world’s first experimental data for this process. These findings are significant because they can be used to refine the understanding of the behavior (distribution functions) of quarks and gluons within atomic nuclei. Further refinement of the measurement techniques could advance the exploration of the fundamental building blocks of our world, the atomic nuclei. The results were highlighted by CERN in an exceptional press release.
Source: ELTE Faculty of Science