Rare process observed at LHC with most massive particles known

International scientific collaborations operating the ATLAS and CMS experiments at the European Laboratory for Particle Physics (CERN) have just announced the first observation of the simultaneous production of four top quarks. It is the most massive elementary particle known, so it requires a lot of energy to produce.

The Great Hadron Collider (LHC) at CERN, the largest and most powerful particle accelerator in the world, is the only one capable of producing four top quarks at a time, the rarest process observed until closing and producing the final state heaviest.

For the first time, the simultaneous production of four top quarks was observed.

The top quark, one of the building blocks of everything we see in the universe, could be the key to the mechanism that generates mass, as 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, since new particles or forces can change the probability of producing them from the predictions of the standard model. It is a kind of ‘holy grail’ of the quest for ‘new physics’.

The scientific collaboration ATLAS, identified as one of the two major experiments at the LHC and in which more than 5,000 researchers and technicians from all over the world participate, had already found evidence of the simultaneous production of four top quarks in data obtained between 2015 and 2018 (during the called operating period or Run 2).

Statistical confidence in the data

After four years of data collection and five years of analysis, this scientific team has revised the research taking advantage of improvements in detector performance, new analysis techniques (including machine learning called Graph Neural Network) and a better understanding of key background processes. All this means that the result, presented last week at the Moriond Conference (France), reach six sigma, a statistical confidence value that confirms the finding.

It is a great satisfaction to finally witness this discovery of the standard model.

Marcel Vos (IFIC)

“We are delighted 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”, comments Marcel Vos, researcher at the Institute of Corpuscular Physics (IFIC). This mixed center of the CSIC and the University of Valencia has participated in an outstanding way in the observation.

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This study is important because new particles or forces can change the probability of producing four top quarks from the Standard Model’s predictions.

The excitement for discovery in the particle physics scientific community stems from the spectacular end state. With four top quarks, the remaining masses alone add up to 700 gigaelectronvolts (GeV), close to the maximum collision energy achieved in the previous most powerful particle accelerator, the Tevatron at Fermilab (USA). The fact that the LHC could discover this process is a testament to the sheer power of this complex machine.

Relationship with the Higgs boson

During this investigation, the team also looked for signs of new physical phenomena related to the Higgs boson. The new analysis led them to narrow down the interaction between the top quark and this boson, setting a limit of 1.8 times the standard model prediction. Finally, a slight excess in the rate was also observed compared to the prediction of the reference model for physicists, making the result even more intriguing.

Over time it will be possible to confirm whether this is the first sign of an unexpected contribution from physics to this process beyond the standard model.

Marcel Vos

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

The ATLAS collaboration will continue to increase the accuracy of this measurement during Run 3 of the LHC, which has been ongoing since 2022. Future studies will provide additional information about the observed signal, helping to determine if it really matches the standard model or if there are suggestions for new physical phenomena that lead to a deeper understanding of the fundamental nature of the universe.

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