The world of physics has been in turmoil for several weeks with rumors of the first results of an experiment that would open new horizons. The expectations were not disappointed. During a presentation held at the Fermi National Accelerator Laboratory (“FermiLab”) in Batavia, near Chicago, the first data of the Muon g-2 experiment were collected (read g minus two, ed) on precise measurements of the magnetic properties of the muon that provide new evidence in support of a “new physics”, in practice of the presence of phenomena hitherto not described by the so-called “Standard Model”, the most detailed theory available on the quantum world and on infinity small. The Muon g-2 experiment sees among the most important holdings the Italian one of the National Institute of Nuclear Physics (Infn).

The muons

Muons, generated in nature in the interaction of cosmic rays with the earth’s atmosphere, are particles similar to electrons, but with a mass about 200 times larger. The experiment measured the anomalous magnetic moment of the muon with extreme precision, confirming the differences already highlighted in a previous experiment in 2001. Combined the two measurements, the probabilities that the anomalies are not a statistical randomness become very high, making the hypothesis of the existence of hitherto unknown forces or particles that make it possible to overcome the impasse that has lasted for decades on the most hidden mysteries of matter and of the forces that govern our Universe.

The experiment

In practice, what was measured at Fermilab is the discrepancy between the experimental result and the calculation predicted by the «Standard Model» of the precession frequency of the muon’s magnetic moment around the direction of the magnetic field. The difference could therefore be due to unknown particles and interactions that the Standard Model does not take into account. Short laser pulses of the order of nanoseconds are used to obtain these ultra-precise measurements. The laser calibration system was created in Italy in collaboration with the National Optical Institute of the CNR and financed by the Infn.

Accuracy

“The very high precision measurement – 150 parts per billion – that we obtained with our experiment was long awaited by the entire international community of particle physics”, commented Graziano Venanzoni, co-spokesperson for the Muon g-2 experiment. and researcher at the Infn of Pisa. «The INFN can be proud of this feat, having played a decisive role in the whole experiment. A success largely thanks to the young researchers who, with their talent, ideas and enthusiasm, made it possible to obtain this first important result ». “We can be proud of the contribution that the Infn has been able to offer to this important discovery, both in the design and construction phase of the apparatus,” said Marco Incagli, of the Infn section of Pisa, national manager of Muon g-2.

Ma Nature slow down

But at the same time the data was announced, Nature published a theoretical calculation led by Zoltan Fodor of the University of Pennsylvania, according to which the magnetic field of muons behaves as predicted by the Standard Model. However, this theoretical result requires further verification.

April 7, 2021 (change April 7, 2021 | 22:47)

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