As I scrolled through the physics papers posted on the arXiv this week, one in particular caught my eye – “Measuring Antimatter Gravity with Muonium”. Possibly, this was because I wrote a bit about antimatter and gravity last week¹. But also, there was an unfamilar word.



“Muonium?” I thought “What’s that then?”

Muons I knew about already of course, and so will you if you read these articles regularly. They are fundamental particles similar to electrons but about 200 times heavier, and they decay in a couple of microseconds, producing an electron, a neutrino and an antineutrino.



the first chance of measuring the effects of gravity on second-generation matter

Muons are what we call “second generation” matter. For reasons which are not clear, the fundamental particles of matter come in a repeated pattern of three generations, each heavier than the last. The muon is the second generation version of the electron. (The third generation version is called the tau.)

Frequently produced in particle collisions, either when particles from outer space hit the upper atmosphere, or in accelerators in laboratories on Earth, muons are rather versatile. They provide a number of unique ways to probe physical laws. It turns out muonium is another useful trick of theirs. It is an atom-style combination of an electron (negative electric charge) and an antimuon (positive electric charge). In the same way that an electron and a proton bound together make hydrogen, an electron and an antimuon make muonium². The stuff is actually made whenever antimuons are travelling slowly enough with electrons nearby. One place that this happens a lot is at the Paul Scherrer Institute (PSI) in Villigen, Switzerland, where they have made something of an industry out of doing physics with muons.



A particle-beam of slow muonium is apparently under development at PSI, and the proposal which caught my eye describes one way to make use of this to measure the effects of gravity on the muonium - basically by watching how the beam bends under gravity. Since the antimuon is 200 times heavier that the electron, its mass dominates, and therefore this a study of the effects of gravity on second generation antimatter.



Gravity is very, very weak compared to other forces, so detecting its action on tiny elementary particles is very difficult. It certainly requires an electrically neutral object, otherwise stray electric fields will overwhelm the minuscule gravational effects. Other studies are underway with positronium (an electron and its antiparticle bound together) and antihydrogen, but muonium has advantages. One the one hand, because the antimuon is heavier and dominates the electron, the (anti)gravity effects should be bigger. On the other hand, muonium is much easier to make than antihydrogen, because the antimuon is lighter than an antiproton. This means that it should be possible to reduce the statistical and other uncertainties in the measurement, just by having lots and lots ofmuonium atoms to work with. Last but not least, it offers the first chance of measuring the effects of gravity on second-generation particles.



In summary, muonium is an atomic bound-state of leptons, consisting of a mix of first-generation matter and second-generation antimatter. Therefore I think there’s a strong case that this article violates Betteridge’s Law of Headlines. Muonium is, indeed, the most exotic atom.

¹Coincidentally, on looking at the paper, I saw it was a write up of meeting held at UCL in the summer, which I attended some of but missed this talk. We do have an positronium beam of our own and a lot of activity in this area (link added 21/2/2016), though I don’t work on it myself.

² Electrons and antimuons don’t annihilate with each other immediately, because they are not each other’s antiparticles. In any case, they are orbiting round each other in a sense, so they don’t immediately ‘collide’.



Jon Butterworth’s book Smashing Physics, about his involvement in the discovery of the Higgs boson, is available as “Most Wanted Particle” in Canada & the US and was shortlisted for the Royal Society Winton Prize for Science Books.



