is scanned at a frequency of 22.5 W in the range of 200–800 MHz. There are black lines for fitting, but gray lines do not contribute 3S. The error bar corresponds to the counting statistics error. The colored areas represent a 1/2 conversion from 2S− 2P, i.e. 583 MHz (blue), 1140 MHz (orange), 1326 MHz (green), and combined 3S− 3P1/2 (yellow). The data point of TL OFF is not shown in the figure, but is included in the fit; it is located at 20.4(4) × 10−4. Source: Natural Communication (2022). DOI: 10.1038/s41467-022-34672-0
By studying a singular atom called muonium, researchers hope that misbehaving muons will sprinkle beans on the standard model of particle physics . To make muonium, they used the world's most intense continuous low-energy muon beam at Paul Scherrer Institute PSI. The study was published on Nature Communications.
muons are usually described as heavy cousins of electrons. A more appropriate description might be its rogue relationship. Since its discovery sparked the word "Who ordered" ( Nobel Prize winner Isidor Isaac Rabbi), muons have been confusing scientists with their illegal antics.
The most famous misdemeanor of the muon is that it swings a little too much in the magnetic field: its abnormal magnetic moment made headlines in the 2021 MUMON g-2 experiment at Fermi Labs. When muons are used to measure the radius of protons, it also causes obvious troubles - producing values that are quite different from previous measurements and the so-called proton radius puzzles.
However, the muon is not punished, but is cherished for its surprising behavior, which makes it likely a candidate to reveal new physics outside the Standard Model.
To understand the strange behavior of muons, researchers at ETH Zurich turned to a singular atom called muons. The μonium is formed from a normal muon orbited by electrons, similar to hydrogen, but is much simpler. The protons of hydrogen are composed of quarks and , while the normal muons of μonium have no sub-structure. This means it provides a very clean model system to solve these problems: for example, by obtaining extremely precise fundamental constant values, such as the mass of muons.
"For μonium, because we can measure its properties so accurately, we can try to detect any deviation from the standard model. If we see this, we can infer which theories that go beyond the standard model are feasible," explains Paolo Crivelli of ETH Zurich, who is leading the study, which was funded by the integrators of the European Research Council within the framework of the Mu-MASS project.
By making precise measurements in a singular atom called μonium, Crivelli and Prokscha aim to use muons to understand puzzling results, which may in turn reveal gaps in the laws of physics as we know it. To make the measurements, they used the world's most intense and continuous low-energy muon source at the Paul Scherrer Institute PSI in , Switzerland. Source: Paul Scheller Institute/Mahir Zambegovitch
There is only one place in the world that is possible
One major challenge to perform these measurements very accurately is having a strong μonium particle beam in order to reduce statistical errors. Making a large amount of μonium, by the way, only lasts for two microseconds, is not simple. There is a place in the world where there are enough low-energy positive muons to create this: the Swiss muon source of PSI.
" To effectively make muons, we need to use slow muons. When they are first produced, they are one quarter of the speed of light. Then we need to slow them down a thousand times without losing them. At PSI, we have perfected the art. We have the strongest continuous source of low-energy muons in the world.Therefore, we have unique advantages in performing these measurements, "Thomas, Head of PSI Low Energy Muon Group Prokscha said.
on a low-energy muon beam line, slow muons pass through a thin foil target, where they pick up electrons to form muons. When they appear, Crivelli’s team is waiting to use microwave and laser spectroscopy to detect their properties. Small changes in energy levels in
may be key
researchers were able to study the properties of μonium in such detail as its energy levels. In recent publications, the teams were able to Measuring the transition between certain very specific energy sub-levels in μonium. Isolated from other so-called ultrafine levels, the transition can be modeled very cleanly. The ability to measure it now will help other precision measurements: in particular, obtain improved values of important quantities called lamb displacements.
lamb displacement is a slight change in the position of certain energy levels in hydrogen relative to the “should” predicted by classical theory. This transition becomes explained with the advent of quantum electrodynamics ( quantum theory of how light and matter interact). However, as discussed, in hydrogen, protons - Has substructure - Complicate things. The ultra-precise lamb displacement measured in μonium can test quantum electrodynamic theory.
and more. Muons are nine times lighter than protons. This means that effects related to nuclear mass, such as how particles backlash after absorbing photons, will be enhanced. Undetectable in hydrogen, a pathway to reaching these values with high precision in μonium can enable scientists to test certain theories that explain muon g-2 anomalies: for example, the presence of new particles, such as scalar or vector canonical bosons.
Put the muon on the scale
None How exciting this potential is, the team has a bigger goal: weighing muons. To do this, they will measure different transitions of μonium, with a thousand times more accurate than ever. The ultra-high accuracy value of
muon mass (the goal is one billionth of a billion) will support ongoing efforts to further reduce the uncertainty of muon g-2. "Muson mass is a fundamental parameter that we cannot predict with theory, so as experimental accuracy increases, we urgently need to increase the value of muon mass as input for calculations," Crivelli explained.
measurements may also lead to new values of the Ridber constant - An important fundamental constant in atom physics - Independent of hydrogen spectroscopy. This could explain the differences between the measurements that lead to the proton radius puzzle, and might even solve it once and for all.
Muonium spectroscopy prepared to fly with the IMPACT project
Given that the main limitation of such experiments is to produce enough μonium to reduce statistical errors, the prospects for this study of PSI look bright.
"Our accuracy may be increased by a hundred times by planning a high-intensity muon beam for the IMPACT project, which will become very interesting for the standard model," Prokscha said.
More: Gianluca Janka et al., measured the transition frequency from 2S1/2, F = 0 to 2P1/2, F = 1 states, natural communication (2022). DOI: 10.1038/s41467-022-34672-0
Journal information: Natural communication
