Periodic Reporting for period 4 - Mu-MASS (Muonium Laser Spectroscopy)
Période du rapport: 2023-08-01 au 2024-01-31
Despite its incredible success, the Standard Model of particle physics does not provide a complete description of nature. For example, it does not explain the origin of dark matter, dark energy, the baryon asymmetry in the universe, or include gravity.
Muonium, an exotic atom composed of an anti-muon and an electron, is a unique system for addressing these outstanding questions. Unlike normal atoms, which have a nucleus made of neutrons and protons with a complicated substructure that cannot be calculated from first principles, the energy spectra of muonium, composed of two elementary point-like particles, can be predicted to a very high accuracy using quantum electrodynamics.
Therefore, if one can measure its properties very precisely by means of spectroscopy using microwaves or lasers, muonium becomes an ideal object to search for new physics beyond the Standard Model. In fact, if a difference between the theoretical predictions and experimental results is found, it could hint at new processes not considered in the calculations.
A notable example that is being scrutinized is the long-standing discrepancy between the Standard Model prediction and the Brookhaven National Laboratory measurement of the anomalous muon magnetic moment (g-2), which has recently been confirmed at Fermilab.
High-precision spectroscopy of muonium is the main objective of the Mu-MASS experiment, funded through this ERC grant. From the measurement of its spectroscopic properties, one can also extract improved values for fundamental constants such as the muon mass and an independent determination of the Rydberg constant and the fine structure constant, free of nuclear finite-size effects.
Muonium, an exotic atom composed of an anti-muon and an electron, is a unique system for addressing these outstanding questions. Unlike normal atoms, which have a nucleus made of neutrons and protons with a complicated substructure that cannot be calculated from first principles, the energy spectra of muonium, composed of two elementary point-like particles, can be predicted to a very high accuracy using quantum electrodynamics.
Therefore, if one can measure its properties very precisely by means of spectroscopy using microwaves or lasers, muonium becomes an ideal object to search for new physics beyond the Standard Model. In fact, if a difference between the theoretical predictions and experimental results is found, it could hint at new processes not considered in the calculations.
A notable example that is being scrutinized is the long-standing discrepancy between the Standard Model prediction and the Brookhaven National Laboratory measurement of the anomalous muon magnetic moment (g-2), which has recently been confirmed at Fermilab.
High-precision spectroscopy of muonium is the main objective of the Mu-MASS experiment, funded through this ERC grant. From the measurement of its spectroscopic properties, one can also extract improved values for fundamental constants such as the muon mass and an independent determination of the Rydberg constant and the fine structure constant, free of nuclear finite-size effects.
The Mu-MASS experiment was proposed at the Paul Scherrer Institute (PSI) in 2019. This laboratory was chosen for our experiment because PSI hosts the most intense muon beamlines in the world. The ERC funding was pivotal for the Mu-MASS approval by the PSI scientific committee and for attracting collaborators with the essential expertise required for this very challenging endeavor. Since then, Mu-MASS has been running at the Low Energy Muon (LEM) facility. After successfully demonstrating the production and detection of a 2S metastable muonium beam, we measured the muonium Lamb shift at an unprecedented level of precision. Because our measured value matches the theoretical calculations within one standard deviation, we could set stringent limits on Lorentz/CPT violation in the muonic sector, which could hold the key to explaining the baryon asymmetry in the universe.
We could also place constraints on the possible mass and coupling of new bosons, which could explain the muon g-2 anomaly by acting as mediators between normal (Standard Model) and dark matter. Additionally, we demonstrated for the first time the production of muonium in the 3S state and the measurements of the 2S hyperfine and fine muonium structures, which also test new physics scenarios in a complementary way. The techniques and experimental setups we developed during this project set the base for a new generation of measurements of muonium spectroscopic properties at a level where bound state QED recoil corrections, not accessible in hydrogen, could be tested and new physics scenarios could be further probed.
Furthermore, the ERC funding allowed us to make tremendous progress toward a 1000-fold improvement in the determination of the 1S-2S transition frequency of muonium. This will provide the best determination of the muon mass at 1 ppb. This fundamental constant cannot be predicted by theory but is an essential input for calculations; therefore, an improved value of the muon mass is necessary for more accurate studies involving muons. The most important milestones were the demonstration of the required stable generation of 25 W deep-UV continuous wave light at 244 nm (a world record high power at this wavelength). Moreover, we demonstrated that the background of the Mu-MASS detection scheme could be kept below 1 event/day as required by the experiment. The experiment is now ready for data taking, and the results are expected in the near future.
We could also place constraints on the possible mass and coupling of new bosons, which could explain the muon g-2 anomaly by acting as mediators between normal (Standard Model) and dark matter. Additionally, we demonstrated for the first time the production of muonium in the 3S state and the measurements of the 2S hyperfine and fine muonium structures, which also test new physics scenarios in a complementary way. The techniques and experimental setups we developed during this project set the base for a new generation of measurements of muonium spectroscopic properties at a level where bound state QED recoil corrections, not accessible in hydrogen, could be tested and new physics scenarios could be further probed.
Furthermore, the ERC funding allowed us to make tremendous progress toward a 1000-fold improvement in the determination of the 1S-2S transition frequency of muonium. This will provide the best determination of the muon mass at 1 ppb. This fundamental constant cannot be predicted by theory but is an essential input for calculations; therefore, an improved value of the muon mass is necessary for more accurate studies involving muons. The most important milestones were the demonstration of the required stable generation of 25 W deep-UV continuous wave light at 244 nm (a world record high power at this wavelength). Moreover, we demonstrated that the background of the Mu-MASS detection scheme could be kept below 1 event/day as required by the experiment. The experiment is now ready for data taking, and the results are expected in the near future.
Technology
• We have developed novel cryogenic muonium converters and new excitation and detection schemes for muonium, which will significantly enhance precision studies in this field in the coming years.
• We achieved a world-record high power in the deep ultraviolet by constructing a new laser system and new optics at this wavelength. Innovation in deep-UV laser technology benefits precision scientific measurements, materials processing, biomedicine, and various industrial and medical applications.
Conceptual
• We introduced a new approach for high-precision spectroscopy of atoms in challenging UV environments.
• We developed a novel scheme for two-photon optical Ramsey spectroscopy for muonium, with potential applications to other exotic systems such as positronium.
Knowledge
• We have updated values of muonium spectroscopic properties to enable improved comparison between theory and future experimental data.
• We established new constraints on CPT and Lorentz symmetries.
• We derived new constraints on new bosons relevant to the muon g-2 discrepancy and dark matter.
• We have developed novel cryogenic muonium converters and new excitation and detection schemes for muonium, which will significantly enhance precision studies in this field in the coming years.
• We achieved a world-record high power in the deep ultraviolet by constructing a new laser system and new optics at this wavelength. Innovation in deep-UV laser technology benefits precision scientific measurements, materials processing, biomedicine, and various industrial and medical applications.
Conceptual
• We introduced a new approach for high-precision spectroscopy of atoms in challenging UV environments.
• We developed a novel scheme for two-photon optical Ramsey spectroscopy for muonium, with potential applications to other exotic systems such as positronium.
Knowledge
• We have updated values of muonium spectroscopic properties to enable improved comparison between theory and future experimental data.
• We established new constraints on CPT and Lorentz symmetries.
• We derived new constraints on new bosons relevant to the muon g-2 discrepancy and dark matter.