Intriguing results from precision tests with muons

Lars Eklund, member of the LHCb collaboration at CERN in Switzerland and Andrzej Kupsc and Stefan Leupold, members of the Muon g-2 Theory Initiative at Fermilab in USA participated in the experimental and theoretical work that showed the differences between the standard model predictions and the measured values.

“The standard model of particle physics has provided us with a detailed description of the elementary particles that build up matter and their interactions, which has been confirmed by thousands of measurements. Yet some observations, mostly of astronomical and cosmological nature, suggest that part of the story about the composition of the Universe is not fully revealed to us”, says Lars Eklund, professor at the Nuclear Physics Division at Uppsala University, and continues.

“An important way of searching for physics beyond the standard model is to study quantities that can be both calculated and measured precisely, and look for differences. Two recent results from studies of muons are examples of these kinds of tests.”

The result from the LHCb experiment

The LHCb experiment at the Large Hadron Collider at CERN specialises in precision measurements of decays of particles that contain charm and beauty quarks. On 23 March 2021, the LHCb collaboration presented a measurement showing a difference in rate of a specific decay with muons in the final state as compared to the same decay but with electrons in the final state. The standard model predicts that both types of decays should occur at almost exactly the same rate due to the universality between electrons and muons.

“In contrast, the experimental result indicates a significant deviation from that prediction. In technical terms, experiment and theory show a 3.1 standard deviation difference which means that the probability of this being just a statistical fluctuation is only about 0.1%. This suggests that, in contrast to the standard-model prediction, muons might not behave in the very same way as electrons”, says Lars Eklund.

The Muon g-2 experiment and Theory Initiative

The g-2 experiment at Fermilab measures the strength of the intrinsic magnetic dipole of the muon, which can be done to a great precision. A large body of theoretical work was coordinated through the Muon g-2 Theory Initiative, to reach an equally precise result on the standard model calculation of the dipole moment.

“This theory result is based on data from many experimental studies of hadronic processes over the years with significant involvement of all researchers from our Nuclear Physics Division”, says Andrzej Kupsc, researcher at the Nuclear Physics Division.

On 7 April 2021, the Muon g-2 experiment reported a measurement of the magnetic moment of the muon that deviates from the standard model prediction with a significance of 4.2 standard deviations.

“The achievement is twofold. First, the Fermilab experiment confirmed completely independently the previous results obtained at the Brookhaven National Laboratory (BNL), USA”, says Stefan Leupold, professor at the Nuclear Physics Division and continues.

“Second, the corresponding standard-model prediction has been improved in recent years. Together with the experimental results from BNL and Fermilab this points to an increased deviation between experiment and standard model. The probability that this deviation is just a statistical fluctuation is about 1 in 40000.”

Confirmations by future measurements

A research programme lays ahead to confirm or refute these deviations between theory and experiment. The g-2 experiment is continuing to collect data and will reduce the uncertainties on their measurement. At the same time, experimental and theoretical work is continuing to reduce the uncertainty on the standard model prediction.

“The LHCb experiment will measure other similar decays to compare those with electrons and those with muons to see if an analogous difference in behaviour is observed in them. An upgraded version of the LHCb experiment will start to collect data next year to further increase the precision of the measurement. If these deviations stand the test of time, they may be the first signs of physics beyond the standard model that we see now”, says Lars Eklund.

More information about the LHCb experiment

More information about the Muon g-2 experiment

Theresé Herrgård