They observe a rare process with the most massive elementary particle known

The top quark, one of the building blocks of everything we see in the universe, may hold the key to the mechanism that generates mass, since it is the heaviest elementary particle in the Standard Model, the theory that describes the visible universe. Being so massive, the top quark requires a lot of energy to produce. Now the simultaneous production of four top quarks has been observed for the first time.

The discovery has been made in the Large Hadron Collider (LHC) of the European Laboratory for Particle Physics (CERN). The LHC is the largest and most powerful particle accelerator in the world, and the only machine on Earth capable of producing four top quarks at the same time.

The detection has been made with the Atlas and CMS detectors of the LHC. And it has been recently announced by the international scientific teams that operate these detectors.

The Institute of High Energy Physics (depending on the Generalitat of Catalonia and the Autonomous University of Barcelona), as well as the Institute of Corpuscular Physics (IFIC), a joint center of the Higher Council for Scientific Research (CSIC) and the University of Valencia ( UV), in Spain, have had a notable involvement in the finding.

The study of the production of four top quarks is particularly important, since new particles or forces could alter the probability of producing four top quarks from the Standard Model predictions. It is a kind of Holy Grail of the search for new physics.

The Atlas collaboration, one of the two large LHC experiments involving more than 5,000 scientists and technicians from all over the world, had already found indications of the simultaneous production of four top quarks in data obtained between 2015 and 2018 (Run 2). . Now, after four years of data collection and five of analysis, the Atlas scientific team has revised the search taking advantage of improvements in the detector’s performance, new analysis techniques (including one of machine learning (a form of artificial intelligence) called the Graph Neural Network) and a better understanding of the main background processes. All this means that the result, recently presented at the Moriond conference, reaches six sigmas of statistical confidence, which confirms the finding.

The research groups of the Institute of Corpuscular Physics (IFIC) and the Institute of High Energy Physics (IFAE) are involved in the search for rare processes with top quarks. Marcel Vos, a CSIC researcher at the IFIC, is the coordinator of the top quark physics group of the Atlas experiment, while Aurelio Juste, an IFAE researcher, is the president of the editorial board that has reviewed the publication.

“We are very pleased that this process has finally been discovered. Throughout my career I have been able to work on related phenomenological studies before the start of the LHC. It is a great satisfaction to finally witness this discovery of the Standard Model after all these years”, says Vos.

The excitement among particle-particle physicists about the find stems from the spectacular final state. With four top quarks, the remaining masses alone add up to 700 gigaelectronvolts (GeV), close to the maximum collision energy achieved at the previous most powerful particle accelerator, the Tevatron at Fermilab in the United States. The fact that the LHC can discover this process is a testament to the great power of this complex machine.

Image of one of the collisions of the four top quark signal in the Atlas detector. Three charged leptons appear (two muons, indicated with red lines, and one electron, indicated in blue) and seven jets (yellow and blue cones). (Image: ATLAS/CERN)

Relation to the Higgs boson

During this investigation, the team has also searched for signs of new physical phenomena in relation to the Higgs boson. This new analysis has led scientists to narrow down the interaction between the top quark and the Higgs boson, putting a limit of 1.8 times the Standard Model prediction. Finally, a slight excess in the rate has also been observed compared to the Standard Model prediction, making the result even more intriguing.

In many of the proposed extensions of the Standard Model, the rate of production of events with four top quarks increases. “In time it will be possible to confirm if this is the first sign of an unexpected contribution from physics to this process beyond the Standard Model, or if more precise measurements in the future will match the model. For now, CMS, the other major LHC experiment, has also confirmed the observation”, points out the CSIC scientist.

The Atlas collaboration will continue to push the precision of this measurement during Run 3 of the LHC, which is ongoing from 2022. Future studies will provide additional information about the observed signal, helping to determine whether it truly matches the Standard Model or whether, on the contrary, there are indications of new physical phenomena that lead to a deeper understanding of the fundamental nature of the universe. (Source: CSIC)