By studying an exotic atom called muonium, researchers hope that the misbehavior of muons can cause grains to spill onto the Standard Model of particle physics. To make muonium, they use the most intense continuous beam of low-energy muons in the world at the Paul Scherrer PSI Institute. Research published in Nature Communications.
The muon is often described as the heavy electron’s cousin. A more apt description might be her rogue relationship. Since its discovery uttered the phrase “whoever commanded” (Isidor Isaac Rabi, Nobel laureate), the muon has been fooling scientists with its law-breaking behaviour.
The most famous misdemeanor of a muon is wobbling slightly in a magnetic field: An anomalous magnetic moment grabbed headlines in Fermilab’s 2021 muon g-2 experiment. The muon also posed a problem when it was used to measure the radius of a proton – resulting in a very different value from previous measurements and what became known as the proton radius puzzle.
However, rather than being chastised, the muon is cherished for its surprising behavior, making it a potential candidate for revealing new physics beyond the Standard Model.
With the aim of understanding the strange behavior of a muon, researchers from PSI and ETH Zurich turned to an exotic atom known as muonium. A muon consists of a positive muon with an electron orbiting around it, which is similar to hydrogen but much simpler. While a hydrogen proton is composed of quarks, the positive muon has no basic structure. This means that it provides a very clean model system by which to sort these problems: for example, by obtaining very precise values for fundamental constants such as the mass of the muon.
“With muonium, since we can measure its properties very precisely, we can try to detect any deviation from the Standard Model. And if we see it, we can then conclude which of the theories beyond the Standard Model are applicable or not,” explains Paolo Crivelli of ETH Zurich, who leads The study was supported by a Consolidator grant from the European Research Council under the Mu-MASS project.
This is only possible in one place in the world
The main challenge for making these measurements very precisely is to have an intense beam of muonium particles so that statistical errors can be minimized. Making a lot of meonium, which occasionally lasts only two microseconds, is not easy. There is only one place in the world where there are enough low-energy positive muons to create this: PSI’s Swiss Muon source.
“To make muons efficiently, we need to use slow muons. When they are first produced, they operate at a quarter of the speed of light. Then we need to slow them down by a factor of a thousand without losing them. At PSI, we have perfected this art. We have the most intense continuous source of low-energy muons So we’re uniquely positioned to make these measurements,” says Thomas Prokscha, who heads the Low Energy Muons group at PSI.
In the low-energy muon beamline, slow muons pass through a thin target where they capture electrons to form a muon. As they emerge, Crivelli’s team is waiting to examine their properties using microwave and laser spectroscopy.
A slight change in energy levels could be the key
A property of muonium that researchers can study in such detail is its energy levels. In the latest publication, the teams were able for the first time to measure the transition between very specific sublevels of energy in a muonium. In isolation from other so-called hyperfine levels, the transition can be modeled very cleanly. The ability to measure it will now facilitate other precision measurements: in particular, to obtain an improved value for an important quantity known as load displacement.
A convection shift is a slight change in certain energy levels in hydrogen relative to where they “should” be as predicted by classical theory. This shift has been explained with the advent of quantum electrodynamics (the quantum theory of how light and matter interact). However, as discussed, in hydrogen the protons – which have an infrastructure – complicate things. Ultra-fine load displacement measured in muoniums can lead to a test of quantum electrodynamics theory.
There are more. A muon is nine times lighter than a proton. This means that effects related to nuclear mass, such as how a particle bounces after absorbing a photon of light, are enhanced. Undetectable in hydrogen, the path to these values at high resolution in muonium could enable scientists to test specific theories that would explain muon G2 anomalies: for example, the existence of new particles such as scalar bosons or vector gauge bosons.
Put the muon on the scale
However intriguing the potential of this may be, the team has an even larger target in its sights: the weight of the muon. To do this, they will measure a different transition in the muonium to a thousand times greater accuracy than ever before.
An ultra-accurate value for a muon’s mass — the target is one part in a billion — will support ongoing efforts to reduce uncertainty even further for muon G2. Crivelli explains: “The muon mass is a fundamental parameter that we cannot predict with theory, and as experimental precision improves, we desperately need it.” to an improved value of the muon mass as an input to the calculations.
The measurement could also lead to a new value for the Rydberg constant—an important fundamental constant in atomic physics—that is independent of hydrogen spectroscopy. This may explain the discrepancies between measurements that have given rise to the mystery of the proton radius, and perhaps even solve it once and for all.
The muonium spectrometer is ready to fly with Project Impact
Given that the main limitation of such experiments is the production of enough ammonium to minimize statistical errors, the outlook for this research on PSI is bright.
“With the higher-intensity muon beams planned for the IMPACT project, we can go to a resolution factor of a hundred years higher, and that would be very interesting for the Standard Model,” says Prokscha.
Gianluca Janka et al, Measurement of Transition Frequency from 2S1/2, F = 0 to 2P1/2, F = 1 Cases in Muonium, Nature Communications(2022). DOI: 10.1038/s41467-022-34672-0
Provided by the Paul Scherrer Institute
the quote: muonium study to reveal new physics beyond the Standard Model (2022, November 29) Retrieved November 30, 2022 from https://phys.org/news/2022-11-muonium-reveal-physics-standard.html
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