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The muon is often described as the electron's heavy cousin. A more appropriate description might be its rogue relation. Since its discovery triggered the words "who ordered that" (Isidor Isaac Rabi, Nobel laureate), the muon has been bamboozling scientists with its law-breaking antics. The muon's most famous misdemeanour is to wobble slightly too much in a magnetic field: its anomalous magnetic moment hit the headlines with the 2021 muon g-2 experiment at Fermilab. The muon also notably caused trouble when it was used to measure the radius of the proton – giving rise to a wildly different value to previous measurements and what became known as the proton radius puzzle. Yet rather than being chastised, the muon is cherished for its surprising behaviour, which makes it a likely candidate to reveal new physics beyond the Standard Model.
Aiming to make sense of the muon's strange behaviour, researchers from PSI and ETH Zurich turned to an exotic atom known as muonium. Formed from a positive muon orbited by an electron, muonium is similar to hydrogen but much simpler. Whereas hydrogen's proton is made up of quarks, muonium's positive muon has no substructure. And this means it provides a very clean model system from which to sort these problems out: for example, by obtaining extremely precise values of fundamental constants such as the mass of the muon.
[...] The property of muonium that the researchers are able to study in such detail is its energy levels. In the recent publication, the teams were able to measure for the first time a transition between certain very specific energy sublevels in muonium. Isolated from other so-called hyperfine levels, the transition can be modelled extremely cleanly. The ability to now measure it will facilitate other precision measurements: in particular, to obtain an improved value of an important quantity known as the Lamb shift.
[...] However exciting the potential of this may be, the team have a greater goal in their sights: weighing the muon. To do this, they will measure a different transition in muonium to a precision one thousand times greater than ever before.
An ultra-high precision value of the muon mass - the goal is 1 part per billion - will support ongoing efforts to reduce uncertainty even further for muon g-2. "The muon mass is a fundamental parameter that we cannot predict with theory, and so as experimental precision improves, we desperately need an improved value of the muon mass as an input for the calculations," explains Crivelli.
The measurement could also lead to a new value of the Rydberg constant - an important fundamental constant in atomic physics - that is independent of hydrogen spectroscopy. This could explain discrepancies between measurements that gave rise to the proton radius puzzle, and maybe even solve it once and for all.
(Score: 5, Informative) by hubie on Saturday December 31, @03:43AM (4 children)
It is considered elementary in that it is a lepton and is thus not made up of anything inside (like quarks). It does decay, like a neutron, if there are no other forces holding it together, so if one pops free out of a collision of a proton with a molecule in the atmosphere, it will stick around only about a couple of microseconds before decaying into an electron and neutrinos. I think whether you want to call it a decay or a transformation is more semantics, but "decay" is the term physicists have used for many decades.
(Score: 4, Touché) by bzipitidoo on Saturday December 31, @11:42AM (3 children)
Decays like a neutron? But a neutron is not an elementary particle.
Don't be dismissive of this as only semantics. It's important to use accurate terminology, to head off misunderstandings. Part of why quantum mechanics is so mind-bending is the terminology. We have too much liking of the basic notion of particles, the idea that the fundamental units of matter are like tiny bricks, though we have learned these things can also be viewed as waves. Maybe a muon "decay" should be thought of as "evolving", similar to the use of that term in "stellar evolution" and not "biological evolution".
Bad terminology is everywhere. Just in physics are the notions of absolute zero temperature and the speed of light. These terms naturally lead to thoughts of, why can't the temperature be negative, colder than absolute zero, and why can't there be speeds faster than light? In the case of temperature, thinking of temperature as motion, the idea that there can be motion that is somehow slower than absolute stillness is nonsense. We do intuitively understand that speeds cannot be less than zero. Yet we persist in wishful thinking at the opposite end, of FTL. Our science fiction is full of FTL, from Star Trek's warp drive notion, to Star Wars hyperdrive sort of stuff. Lot of the time travel thinking may have arisen from the extrapolation of General Relativity to FTL speeds, in which for that to work, something is traveling backwards in time.
(Score: 4, Interesting) by hubie on Saturday December 31, @02:34PM
I do agree that using accurate terminology is important, but I still see this as semantics. Particle decay is when a particle, elementary or not, spontaneously turns into other particles. I don't think the science definition is any more complicated than that and that has how the term has been used for many many decades. I think "transforming" as being consistent with that, but I'm not convinced it is better. I think you are applying a very specific definition of decay, one where something falls apart into smaller pieces that were made up inside it, but there are other kinds of decay that don't follow that scheme, such as biological decay or the decay of a society or relationship.
I don't agree with your physics examples of bad terminology, though. For absolute zero, I think the "absolute" tells the non-specialist that's the bottom. If one has been exposed to a little bit of physics they might think of temperature as having to do with motion, and the idea of slower than zero motion does sound silly, but you have to remember that using "speed" in this context is itself bad terminology because there is no negative speed. Speed is a scalar, the magnitude of the velocity, and the positive/negative sign comes from the direction. In fact, temperature isn't just the average kinetic energy of something, but it is also tied into the entropy of a system and negative temperatures are actually a thing and are a consequence of how temperature is defined thermodynamically.
As for the speed of light, it is just that, the speed of light. It is a limit for us due to the singularity in the equation for the Lorentz transformation, but unless something has changed in the last few decades that I don't know about, the idea of faster-than-light physics is still a field of study (tachyons, negative mass, etc.). It is perhaps very very unlikely to be true, but who knows? I think FTL in science fiction arises from a much more practical cause, that you need FTL to have a range of interesting stories. You can't have great space cowboy epics if you can't reasonably travel from star system to star system, and a show like Star Trek Voyager would have been a VERY boring story if they couldn't travel FTL because they never would have made it to the next star system. Unless the author actually works the consequences of FTL into a story line, FTL in science fiction is simply the MacGuffin [wikipedia.org] to keep the plot moving.
(Score: 2) by PiMuNu on Saturday December 31, @03:07PM
(Score: 2) by PiMuNu on Saturday December 31, @03:08PM
> Decays like a neutron? But a neutron is not an elementary particle
(repost with white space this time)
You are correct, but there are no unstable, elementary particles that are well-known outside particle physics, so it is impossible to make a comparison that holds and is sensible when discussing decay. Here are the Feynman diagrams:
Neutron decay:
https://upload.wikimedia.org/wikipedia/commons/thumb/2/2e/Neutron_decay.png/240px-Neutron_decay.png [wikimedia.org]
Muon decays:
https://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Feynman_diagram_of_muon_to_electron_decay.svg/320px-Feynman_diagram_of_muon_to_electron_decay.svg.png [wikimedia.org]
Topologically they are the same process, except a neutron has "d" going to "u", whereas muon decay has "muon" going to "muon neutrino".