Measuring the muon anomalous magnetic moment discrepancy with the muon g-2 and MUonE experiments
Lead Research Organisation:
University of Liverpool
Department Name: Physics
Abstract
Despite its many successes, the Standard Model (SM) of particle physics is known to be incomplete. The search for physics from beyond the SM (BSM) is increasingly urgent, with many SM extensions ruled out by collider experiments. Precision tests of the SM can complement direct searches for BSM phenomena. By comparing ultra-precise experimental measurements with theoretical predictions, we can look for gaps in the SM.
A quantity of particular interest is the anomalous magnetic moment of the muon, aµ, which parameterises the strength of the interaction between the muon's spin and a magnetic field. It can be measured and predicted with equal precision, and is sensitive to all SM interactions. Calculations involving every particle and interaction in the SM are used to predict aµ, so a difference between measurement and prediction could indicate BSM physics.
Recently, the Fermilab g-2 experiment published a measurement of aµ with a precision of 0.20 ppm. This is the most precise measurement ever made at a particle accelerator, and is in excellent agreement with the previous result in 2021 from the same experiment. The measurement has attracted huge interest worldwide, since it appears to deviate from the currently-accepted SM prediction, aµth, at the 5s level. However, recent advances in the methods used to perform this challenging calculation appear consistent with the experiment and could signify a problem with the theory. It is crucial to verify aµth in order to confirm the existence of a SM-breaking discrepancy.
This project has two aims that will address this challenging problem:
confirm the experimental value with the full dataset from the muon g-2 experiment, and
confirm the theoretical value by performing an independent measurement of the part of the SM prediction with the highest uncertainty, using the MUonE experiment.
The target precision for the full g-2 dataset is 0.14 ppm, with approximately equal statistical and systematic uncertainties. One part of this project will focus on performing the final measurement of the muon-weighted magnetic field and reducing the associated uncertainties to ensure this ambitious target is met. The final publication from g-2 is expected by 2026.
As well as reducing the experimental uncertainty, it is crucial to confirm the theoretical value and improve the precision on the SM prediction. The second aim of this fellowship is to perform a direct measurement of the leading-order hadronic contribution to aµth with the new MUonE experiment at CERN. The hadronic contribution dominates the uncertainty on aµth, and different treatments of this term lead to the current disagreement between SM predictions. MUonE will measure this term in a novel and entirely independent way, with competitive uncertainty, in order to resolve the theoretical tension. The experiment is highly challenging, requiring excellent control of systematic uncertainties as well as developments in simulation techniques from the theory community.
This project will shed light on one of the most famous puzzles in particle physics today. Confirming the discrepancy as a sign of BSM physics would be one of the most important discoveries in decades. Alternatively, the confirmation of a problem with the previously well-understood theoretical treatment would also be a major result, with consequences for other areas of the SM, and may expose conflicts with other experimental data. In either case, both the g-2 and MUonE measurements are among the most highly-anticipated results in the next 5 years.
A quantity of particular interest is the anomalous magnetic moment of the muon, aµ, which parameterises the strength of the interaction between the muon's spin and a magnetic field. It can be measured and predicted with equal precision, and is sensitive to all SM interactions. Calculations involving every particle and interaction in the SM are used to predict aµ, so a difference between measurement and prediction could indicate BSM physics.
Recently, the Fermilab g-2 experiment published a measurement of aµ with a precision of 0.20 ppm. This is the most precise measurement ever made at a particle accelerator, and is in excellent agreement with the previous result in 2021 from the same experiment. The measurement has attracted huge interest worldwide, since it appears to deviate from the currently-accepted SM prediction, aµth, at the 5s level. However, recent advances in the methods used to perform this challenging calculation appear consistent with the experiment and could signify a problem with the theory. It is crucial to verify aµth in order to confirm the existence of a SM-breaking discrepancy.
This project has two aims that will address this challenging problem:
confirm the experimental value with the full dataset from the muon g-2 experiment, and
confirm the theoretical value by performing an independent measurement of the part of the SM prediction with the highest uncertainty, using the MUonE experiment.
The target precision for the full g-2 dataset is 0.14 ppm, with approximately equal statistical and systematic uncertainties. One part of this project will focus on performing the final measurement of the muon-weighted magnetic field and reducing the associated uncertainties to ensure this ambitious target is met. The final publication from g-2 is expected by 2026.
As well as reducing the experimental uncertainty, it is crucial to confirm the theoretical value and improve the precision on the SM prediction. The second aim of this fellowship is to perform a direct measurement of the leading-order hadronic contribution to aµth with the new MUonE experiment at CERN. The hadronic contribution dominates the uncertainty on aµth, and different treatments of this term lead to the current disagreement between SM predictions. MUonE will measure this term in a novel and entirely independent way, with competitive uncertainty, in order to resolve the theoretical tension. The experiment is highly challenging, requiring excellent control of systematic uncertainties as well as developments in simulation techniques from the theory community.
This project will shed light on one of the most famous puzzles in particle physics today. Confirming the discrepancy as a sign of BSM physics would be one of the most important discoveries in decades. Alternatively, the confirmation of a problem with the previously well-understood theoretical treatment would also be a major result, with consequences for other areas of the SM, and may expose conflicts with other experimental data. In either case, both the g-2 and MUonE measurements are among the most highly-anticipated results in the next 5 years.
