LHC experiments observe rare process involving four top quarks, the most massive particles known

The international scientific collaborations operating the ATLAS and CMS experiments at the European Laboratory for Particle Physics (CERN) has just announced the first observation of the simultaneous production of four top quarks in the same week. The Institute of Corpuscular Physics (IFIC, UV-CSIC) has a prominent role in this finding, considered key to searching for new particles beyond the Standard Model.

The top quark is the most massive elementary particle known, so it requires a lot of energy to produce. CERN’s Large Hadron Collider (LHC), the largest and most powerful particle accelerator in the world, is the only one capable of producing four top quarks at the same time, the rarest process observed to date and which produces the heaviest end state. The Institute of Corpuscular Physics (IFIC), a joint centre of the University of Valencia (UV) and the Higher Council for Scientific Research (CSIC), participates in the discovery.

The top quark, one of the ’bricks’ that form 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. The study of the production of four top quarks is particularly important; 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 around the world, had found indications of the simultaneous production of four top quarks in data obtained between 2015 and 2018 (Run 2). Now, the ATLAS science team has revised the search, taking advantage of improvements in detector performance, new analysis techniques - including machine learning called the Graph Neural Network - and a better understanding of the main background processes. All this means that the result, presented last week at the Moriond conference , reaches six sigmas of statistical confidence and confirms the finding.

The research groups of the Institute of Corpuscular Physics and the Institute of High Energy Physics (IFAE) have been highly relevant in the search for rare processes with top quarks. Marcel Vos, IFIC researcher, is the coordinator of the top physics group of the ATLAS experiment, while Aurelio Juste, IFAE researcher, is the president of the editorial board that has reviewed the publication.

Excitement for the finding in the particle physics scientific community stems from the spectacular end state. With 4 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 (USA). The fact that the LHC can discover this process is a testament to the great power of this complex machine.

"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 this observation", says Marcel Vos.

The ATLAS Collaboration will continue to push the precision of this measurement during Run 3 of the LHC, which is ongoing since 2022. Future studies will provide additional information about the observed signal, helping to determine whether it truly matches the Standard Model or whether there is hints of new physical phenomena that lead to a deeper understanding of the fundamental nature of the universe.