Importantly, observations by both ATLAS and CMS exceed the 5σ statistical confidence level at which detection is possible. In the case of ATLAS, this level was 6.1 degrees, and in the case of CMS – 5.5 degrees.
The up quark is the most massive particle in the Standard Model, which means it is most closely related to the Higgs boson. This makes quarks the best particle for studying physics outside the Standard Model.
Most often, t quarks are observed in pairs with their corresponding antiquark. Sometimes they arise on their own. According to the Standard Model, four up quarks, or two pairs consisting of a quark and an antiquark, can be created at one time. However, the probability of such an event occurring is 70 thousand. times smaller than the probability of forming a quark-antiquark pair. So capturing four t quarks is very difficult.
ATLAS has already recorded some signals in 2020 and 2021 indicating that four quarks formed simultaneously, and CMS detected such a signal in 2022, but so far, apart from some clues, no certainty has ever been obtained. No such event has been logged.
Not only is this a rare occurrence, but it is also difficult to record. Physicists, looking for specific particles, look for their signatures, that is, decay products. The t quark decays into a W boson and a down quark (the b quark), and then the W boson decays into a charged lepton and a neutrino or a quark-antiquark pair. This means that the signature of an event in which four quarks formed simultaneously could contain from 0 to 4 charged leptons and up to 12 jets formed as a result of the formation of quarks. So it is difficult to find such a signature.
For the purposes of the research, scientists from ATLAS and CMS used innovative machine learning techniques, thanks to which the algorithm extracted from the huge amount of data information that could be the signatures of the four quarks. And after the success, scientists hope that during the current search campaign – Run 3 – more events of this kind will be recorded. Round 3 will run intermittently until the end of 2025. In December 2025, the Large Hadron Collider will shut down, and the hiatus will last until February 2029.
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