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Scientists of the European Organization for Nuclear Research CERN, working as part of the ATLAS experiment at the Large Hadron Collider (LHC), have experimentally verified one of the fundamental principles of the Standard Model of particle physics — the so-called principle of the universality of the lepton flavor. And, the results obtained by scientists contradict earlier results obtained at the time at the Large Electron-Positron Collider (LEP) and indicate that there may be a fairly "wide crack"in the Standard Model.

We remind our readers that our entire understanding of elementary particles, the tiny building blocks that make up everything in the universe, plus the forces of electromagnetism, strong and weak nuclear interactions, is now reduced to one theory, called the Standard Model of Elementary Particle Physics. According to this theory, electrons, muons and tau-leptons are three varieties (flavors) of electrically charged elementary particles of the lepton family. The theory also assumes that the coupling force between leptons and particles that are "carriers" of the forces of weak interactions, 'W' and 'Z' bosons, does not depend on the flavor of the lepton particle. This principle is known as the principle of universality of the lepton flavor, but this principle was questioned by experiments conducted at the time at the LEP collider.

Scientists of the ATLAS experiment analyzed approximately half a million proton collisions, testing the validity of the principle of universality of the lepton flavor with respect to muons and tau-leptons. More specifically, the scientists studied the processes of decay of W-bosons into tau-leptons and muons. Having measured the quantitative ratio of different types of decay, the scientists came to the conclusion that the carriers of weak forces interact with both types of leptons in exactly the same way.

The measurements mentioned above are the most accurate such measurements to date. Their accuracy is more than twice as high as the accuracy of the data obtained at the time at the Large Electron-Positron Collider, which is the predecessor of the Large Hadron Collider.

"Measurements made at the time at the LEP collider showed that there may be differences between the decays of particles to different types of leptons. This intriguing hint of the possibility of inconsistency of the Standard Model has not been confirmed experimentally for 20 years, " the researchers write — "The measurements we made do not confirm the results of 20 years ago, and this is a clear demonstration of the power of the Large Hadron Collider, which allows making such high-precision measurements."

All of the above is more than convincing proof that the principle of universality of the lepton flavor and the Standard Model itself are still "out of danger". However, there is still a possibility that leptons may behave completely differently in other processes that were not covered by CERN experiments, and this will be tested in future experiments at the collider and other scientific facilities.

Image © ATLAS Collaboration/CERN

The article was published in the journal Nature Physics
Source: dailytechinfo.org, sci-dig.ru