Possible clues to new physics in early Muon g-2 results

A new and precise measurement of the magnetic properties of the muon - an elementary particle belonging to the lepton family, very similar to the electron, but with a mass about 200 times greater - provides new evidence in favor of the existence of physical phenomena not described by the Standard Model, the reference theory for explaining subatomic processes.

The long-awaited result, based on analysis of the first data take (Run1, 2018) of the Muon g-2 experiment, was just announced at a presentation held at the Fermi National Accelerator Laboratory (FermiLab) in Batavia, near Chicago, the U.S. center for particle physics research. The international Muon g-2 collaboration, of which INFN has been a leading member since its inception, has succeeded in obtaining a measurement of the so-called anomalous magnetic moment of the muon with unprecedented precision, confirming the discrepancies with Standard Model predictions already highlighted in the previous experiment conducted at Brookhaven National Laboratory (BNL), near New York, and concluded in 2001.

The present measurement of Muon g-2 achieves a statistical significance of 3.3 sigma, or standard deviations, and its combination with the result of the BNL experiment brings the significance of the discrepancy to 4.2 sigma, just under the 5 sigma needed to declare a discovery.

"The very high-precision measurement we obtained with our experiment was long awaited by the entire international particle physics community. As we await the results of the analysis on the various datasets recently acquired by the experiment and those that will be collected in the near future, it already offers us a possible glimmer toward new physics." says Graziano Venanzoni co-principal investigator of the Muon g-2 experiment and a researcher at the INFN Section in Pisa.

Muons, which are naturally generated in the interaction of cosmic rays with the Earth's atmosphere, can be produced in large numbers by Fermilab's system-accelerator and injected into Muon g-2's 15-meter-diameter storage ring, where they are circulated thousands of times with speeds close to that of light. The muon has a mass that is about 200 times that of the electron. Like the electron, it belongs to the lepton family, and although considered point-like it has a property (spin) that makes it similar to a small magnet.

Moving charge and magnet rotate when immersed in magnetic field. If the gyromagnetic factor g of the muon were exactly 2 every rotation of the velocity vector would correspond to an equal rotation (precession) of the spin, instead the spin overrides and in particular, in the extraordinarily uniform magnetic field of 1.45 Tesla of the Muon g-2 experiment, rotates about 12° more per revolution. After about 30 revolutions the velocity and spin realign, so everything repeats. The measurement of this additional spin rotation, proportional to the so-called muon magnetic anomaly _aμ =(g-2)/2, is the figure of merit of the Muon g-2 experiment.

As they circulate in the storage ring, in which a vacuum is practiced, muons interact with virtual particles that continuously appear and disappear in the quantum vacuum. Interactions with these very short-lived particles affect the value of the g factor, causing the spin precession to accelerate or slow down slightly. The Standard Model allows the magnetic moment of the muon to be calculated extremely accurately. However, if the quantum vacuum contains new forces or particles not contemplated in the Standard Model, this results in uncalculable corrections to the muon's g-factor and thus discrepancy between theory and measurement.

The muon is not stable; in addition to neutrino and antineutrino, its decay produces an electron that is preferentially emitted along the muon's spin direction. The Muon g-2 experiment therefore measures the decay electrons and extracts the spin spin profile from them. The experiment's 24 calorimeters, placed regularly along the storage ring, measure energy and arrival time of the electrons. Because the frequency of electron arrival varies from many millions down to a few hundred per second, continuous calibration of the calorimeters is necessary.

The Italian contribution to the experiment, funded by the National Institute of Nuclear Physics, was to build the laser calibration system. "The system," explains Michele Iacovacci, professor at the University Federico II in Naples, a researcher at INFN in Naples, and coordinator of the Neapolitan group collaborating on Muon g-2, "uses short light pulses, lasting a few nanoseconds, produced by lasers and appropriately injected into the calorimeters during data taking. Control of the stability of the calibration pulses is delegated to electronics capable of appreciating very small variations, up to 1 part in 10000, of the signals. "The design of the control electronics was developed thanks to the Electronics and Detectors Service of the Naples INFN section, specifically with the important contribution of Paolo di Meo of the Naples INFN. The realization of the laser management electronics and the calibration data acquisition (DAQ) system was coordinated by Stefano Mastroianni, a researcher at the INFN section of Naples and currently co-responsible for the DAQ of the Muon g-2 experiment.

Developed by the Italian INFN group of Muon g-2, in collaboration with the CNR National Institute of Optics, the innovative laser calibration system represented a major step forward from those previously used and was one of the key ingredients in achieving the result published today in Physical Review Letters.

"We can be proud of the contribution that INFN has been able to offer to this important discovery, both in the conception and construction phase of the apparatus, which saw the active participation of the Naples, Pisa, Rome Tor Vergata, Trieste, Udine, and Frascati National Laboratories, and in the subsequent analysis phase, with original contributions from very valid young researchers," says Marco Incagli, of the Pisa INFN section, national g-2 manager.

The Muon g-2 collaboration consists of 200 scientists from 35 institutions in 7 different countries. The topic of Muon g-2 is in the heartstrings of the Federico II University, which has supported and funded over the years, together with the Department of Physics and INFN Section of Naples, the organization of the FCCP Workshop "Flavor changing and conserving processes," in the three editions 2015, 2017 and 2019, at its Villa Orlandi conference center, Anacapri - Capri. The workshop has been a reference point for the international scientific community, both experimental and theoretical, involved in the Muon g-2 problem.

 

C. Polly, Fermilab April 7, 17:00 CET: https: //theory.fnal.gov/events/event/first-results-from-the-muon-g-2-experiment-at-fermilab/

Press Conference, Fermilab April 7, 7:00 p.m. CET: https: //fnal.zoom.us/j/95785531415?pwd=V1N4bjlNMEJTRlVvYzZMU2NJRzVXdz09

G. Venanzoni, CERN April 8, 16:00 CET : https://indico.cern.ch/event/1019685/

Images of Muon g-2 available here.

Information and details are at the Muon g-2 website.

Muon g-2 b-roll footage - https://www.youtube.com/watch?v=YhZa8-_sRdQ

FCCP2015: http: //fccp2015.na.infn.it/

FCCP2017: http: //fccp2017.na.infn.it/

FCCP2019: http: //fccp2019.na.infn.it/