“Clues to new particles and new forces of the Universe”

Announced by a team of scientists – including Italian, US and British researchers important indications of the existence of new particles and new forces of the Universe that could pave the way for new physics. The study was presented today at the Fermi National Accelerator Laboratory (FermiLab) in Batavia, near Chicago, by the team of researchers which is composed of a total of 200 scientists from 35 institutions from 7 different countries. It is about “a new and precise measure” of the magnetic properties of the muon – elementary particle belonging to the lepton family, very similar to the electron but with a mass about 200 times greater – which provides “new evidence” in favor of the existence of physical phenomena not described by the Standard Model, the reference theory for the explanation of subatomic processes. The expected result, obtained at the end of the first analysis campaign of the data acquired by the Muon g-2 experiment, was announced today at Fermilab
– the US center for research in particle physics that hosts the experiment – during the presentation of the results of the experiment. The international collaboration responsible for Muon g-2, of which the Italian National Institute of Nuclear Physics has been one of the main members since its inception, was able to obtain a measurement of the so-called anomalous magnetic moment of the muon with unprecedented precision, confirming the discrepancies with the predictions of the Standard Model already highlighted in a previous experiment conducted at Brookhaven National Laboratory, near New York, and concluded in 2001.

“The experiment consists in the search for a new physics inside the quantum vacuum. Qthese studies are leading us towards the possible identification of new particles or new interactions that can explain the presence of dark matter in the Universe “, physicist Marco Incagli explains to time.news, head of the Italian group of the Muon g-2 experiment and researcher at the Infn section of Pisa. “We know that the universe is composed only in a small part of ordinary matter, the one that also composes us human beings, but there is another part of matter that we know to be there but that has yet to be discovered, dark matter. this new study of ours indicates a possible presence of further particles and additional forces in addition to the known ones “adds Incagli who recalls how” basic research constitutes the foundations for creating new technologies useful to humanity “.

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Scientists point out that the present measure of Muon g-2 achieves a statistical significance of 3.3 sigma, or standard deviations, and its combination with the result of the predecessor experiment brings the significance of the discrepancy to 4.2 sigma, just under the 5 sigma considered the threshold to be able to announce a discovery. “This fundamental result – they observe – represents an important and exciting clue of the possible presence of still unknown forces or particles, a question that has fueled discussions among researchers for decades “. “The extremely high-precision measurement we obtained with our experiment was long overdue by the entire international particle physics community. Waiting for the results of the analyzes on the various data sets recently acquired by the experiment and on those that will be collected in the near future, it already offers us a possible opening towards a new physics “says Graziano Venanzoni co-spokesperson of the Muon g-2 experiment. and researcher of the Infn Section of Pisa. “Infn can be proud of this undertaking, 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 “, underlines Venanzoni.

Muons, which are naturally generated in the interaction of cosmic rays with the Earth’s atmosphere, can be produced in large numbers by the Fermilab accelerator and injected into the Muon g-2 magnetic storage ring, 15 meters in diameter, where they are circulated thousands of times with a speed close to that of light. Like electrons, muons are also endowed with spin and possess a magnetic moment, that is, they produce a magnetic field similar to that of a compass needle. Inside the Muon g-2 ring, the muon’s magnetic moment acquires a precession motion around the direction of the magnetic field, similar to that of a spinning top. The experiment measures the frequency of this muon precession with very high precision. The Standard Model predicts that for each particle the value of the magnetic moment is proportional to a certain number, called ‘gyromagnetic factor g’, and that its value is slightly different from 2, hence the name ‘g-2’ or ‘gyromagnetic anomaly’ given to this type of measurement. The result of Muon g-2 shows a difference between the measured value of ‘g-2’ for muons and that predicted by the Standard Model, whose prediction is based on the calculation of the interactions of muons with ‘virtual’ particles that form and they annihilate themselves continuously in the void that surrounds them. The discrepancy between the experimental result and the theoretical calculation it could therefore be due to unknown particles and interactions that the Standard Model does not take into account. With the result presented today, obtained thanks to the first set of data collected by Muon g-2 (Run 1), the experiment has therefore taken an important step towards confirming the existence of new physics phenomena.

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“The precision measurement requires a sophisticated, continuous calibration of the calorimeters, that is the injection of short laser pulses that guarantee the stability of the response, up to 1 part in 10,000”, explains Michele Iacovacci, researcher of the Muon g-2 collaboration and of the Infn Section of Naples. Made in Italy, in collaboration with the National Institute of Optics of the CNR, and financed by the Infn, the innovative laser calibration system it represented a significant step forward compared to those previously in use and was one of the fundamental ingredients to obtain the result published today in the Physical Review Letter. In addition to the development and implementation of this system, the Infn, one of the founders of the collaboration, has played and continues to play a central role in the Muon g-2 experiment which together with hundreds of scientists from 35 institutions from 7 different countries. “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, and has seen active the structures of the Infn of Naples, Pisa, Rome Tor Vergata, Trieste, Udine, and of the National Laboratories of Frascati, and in the subsequent analysis, with original contributions by very valid young researchers “finally underlines the physicist Marco Incagli.


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