Fifteen years ago, physicists from BrookhavenNational Laboratory discovered something amazing. Muons, a type of subatomic particle, moved in unexpected ways that did not match theoretical predictions. Since then, physicists have been trying to figure out why. Recently, a group of researchers from Fermilab took up the experimental side of the issue and published results on April 7, 2021, confirming the original measurement. A number of researchers, however, take a different approach, believing that there is no "new physics" on the horizon. So, a team of scientists in the framework of the Budapest-Marseille-Wuppertal Collaboration tried to find out if the old theoretical prediction was wrong. A new method was used to calculate the interaction of muons with magnetic fields. If the calculations of the researchers are correct, then there is no discrepancy between theory and experiment, just as there is no discovered force of nature.
Muon and the Standard Model
Although not obvious, muons are heavier.and the unstable sisters of the electron - surround us from all sides. These subatomic particles are created, for example, when cosmic rays collide with particles in the atmosphere of our planet. Interestingly, muons can pass through matter, and scientists use them to explore inaccessible internal structures - from giant volcanoes to Egyptian pyramids.
Muons, like electrons, have an electrical charge and generate tiny magnetic fields. The strength and orientation of this magnetic field is called magnetic moment.
Almost everything in the Universe, from the structure of atoms tothe work of computers and the movement of galaxies can be described using four interactions: gravity; electromagnetism; weak interaction responsible for radioactive decay; strong interaction, responsible for the confinement of protons and neutrons in the nucleus of an atom. Scientists call this structure Standard model particle physics.
Interestingly, all interactions of the Standardthe models contribute to the magnetic moment of the muon, but each of them does so in several different ways, which have proven incredibly difficult to determine.
“Most of the phenomena in nature can be explained withusing the Standard Model, says Zoltan Fodor, professor of physics at the University of Pennsylvania and research team leader. "We can predict the properties of particles extremely accurately based on this theory alone, so when theory and experiment do not match, we consider the likelihood that we have discovered something new, something outside the Standard Model."
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In the past, to calculate the magnetic moment of a muonphysicists used a mixed approach - they collected data on collisions between electrons and positrons - the opposite of electrons - and used it to calculate the contribution of the strong force to the magnetic moment of the muon. This approach has been used to further refine the estimate for decades. The latest results are for 2020 and are very accurate.
In a study published April 6 in the journalNature, physicists have taken a new approach that gives an estimate of the muon's magnetic field strength and closely matches its experimental value. It is noteworthy that the scientists used a fully tested theory that was completely independent of reliance on experimental measurements.
“We started with fairly simple equations andbuilt the entire assessment from scratch, ”the researchers write. The new computation required hundreds of millions of processor hours at several supercomputer centers in Europe and brought theory into line with measurements.
See also: Physicists have rethought the structure of the Universe. Is dark energy no longer needed?
The data collected significantly narrows the gapbetween theory and experimental measurements and, if true, confirm the primacy of the Standard Model, which has ruled particle physics for decades. But the story doesn't end there, as the results now have to be cross-checked by other research groups. But what is the bottom line?
It is important to understand that for the discovery of New Physics,beyond the Standard Model, there is scientific consensus that the discrepancy between theory and measurement should be as large as five sigma - a statistical measure that equates to a probability of about 1 in 3.5 million.
This is interesting: Scientists from CERN are on the verge of discovering "new physics"
In the case of a muon, measurements of its magnetic fielddeviated from existing theoretical predictions by about 3.7 sigma. It's certainly intriguing, but not enough to announce the collapse of the Standard Model. So in the future, researchers intend to improve both measurements and theory, in the hope of either reconciling theory and measurement, or increasing sigma to a level that would allow the announcement of the discovery of New Physics.