The Standard Model predicts that two top quark-antiquark pairs can combine to form four top quarks at once. Four-top quark production is tricky because the production rate is believed to be 70,000 times lower than that of top-quark-antiquark pairs. ATLAS previously made observations of this phenomenon in 2020 and 2021 and CMS in 2022. Yet there had never been an observation until today.
Using data from collisions during Run 2 of the Large Hadron Collider (LHC), the ATLAS and CMS collaborations observed the simultaneous production of four top quarks.
Production of four-top quarks is not only unusual, but notoriously challenging to watch. To find a specific event, physicists hunt for the “signature,” or the characteristics of the last particles of the decay. They give them hints about the passing events they need to find. Each top quark breaks down into a bottom quark and a W particle.
The W particle can then decay into a quark-antiquark pair, a charged lepton and a neutrino. As a result, the signature of four-top quark events can contain between 0 and 4 charged leptons and up to 12 quark-produced jets. This makes it challenging to find the signature of the four-top quark manufacturer.
ATLAS and CMS used advanced machine learning approaches to create algorithms that choose four top quark candidate events to aid in the search for these events. The analyzes distinguish the four top quark events from the background caused by other Standard Model processes with higher production rates using the unique four top quark signature with many electrons, muons and (bottom quark labeled) jets. The event signatures containing two or more leptons were sought after by ATLAS and CMS.
The AtLAS observation has 6.1 sigma, higher than the expected significance of 4.3 sigma, and the CMS observation has 5.5 sigma, higher than the expected 4.9 sigma, making them the first observations of this process . The results of both experiments meet the necessary 5-sigma statistical significance to be considered an observation.
The first direct observation of four-top quark production is an exciting new step in learning more about this fascinating particle. Both experiments look forward to further studying this phenomenon during LHC Run 3.