Since its inception in 2008, the LHC (large hadron collider) at CERN has been a key player in pushing the boundaries of particle physics research. Consisting of four main experiments, the circular particle accelerator is built to push the frontiers of particle physics. The CMS collaboration, one of the four key experiments at the LHC, has announced the possible discovery of the tiniest hadron ever seen. Known as the toponium, this is a composite particle consisting of quark-antiquark pairs of the top quark. A type of fundamental particle, quarks make up protons and neutrons—the components of atomic nuclei. Toponium is an example of quarkonium, an unstable quark-antiquark state formed from pairings of heavy quarks, such as the top quark. The toponium marks the last quarkonium for the heavy quarks to be discovered. Its detection has eluded scientists as it is extremely short-lived, leading scientists to believe that it wasn’t observable at the LHC. However, the findings indicate a high likelihood that the particle discovered is the toponium. It all began with the search for new types of the Higgs boson particle, which was first detected in 2012 at the LHC. The Higgs boson is key to understanding how fundamental particles in the universe acquire mass.
The Standard Model of particle physics is our best understanding of the fundamental particles and forces in the universe. However, it is not a complete theory as it does not account for several phenomena, including dark matter, dark energy, and the absence of gravity from the model. These theoretical additional Higgs particles are expected to interact most strongly with the top quark. This has led researchers to pursue additional theories that could fill in the gaps. A few of these additional theories suggest the existence of new types of Higgs boson particles, which the CMS collaboration at the LHC has been looking for. While conducting this search, the CMS collaboration detected more top quark-antiquark pairs than expected at the minimum energy required for their creation. This led the researchers to consider the possibility that the observation seen could be a sign of the short-lived toponium. The CMS collaboration analyzed two years of data collected between 2016 and 2018 of proton-proton collisions at energies of 13 Tera electronvolts, which is the standard energy of operations at the LHC today. To grasp the nature of the collisions, the researchers examined how the collision decayed and how the particles dispersed in space. This gives information about the quantum state of the particles before the collision. They additionally used a simplified toponium model to compare the experimental results with those from the model to explain the excess and hint at the particle actually being toponium.
The researchers estimated the possibility of toponium being produced in high-energy collisions such as the ones conducted at CERN. They found that the collisions would occur 8.8 times for every trillion collisions, also known as 8.8 picobarns. This result has a 15% uncertainty associated with it, which is strong enough to meet the five sigma threshold in particle physics, the gold standard to claim a real observation. While the results favor the hypothesis that the observed particle is indeed the toponium, the researchers are cautious about claiming it since it could also be an additional Higgs particle. To confirm their hypothesis, researchers at the CMS collaboration plan to build a more accurate toponium model and also use CMS’s sister experiment, ATLAS, to corroborate their data. If confirmed, the toponium would be the smallest hadron ever discovered. The toponium also stands out from other quarkonia because of its decay, which happens due to the disintegration of the quarks instead of the matter-antimatter annihilation seen in others.
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