There is interesting news from Switzerland as the LHCb experiment team has announced evidence of a violation of lepton universality, which would be the first experimental evidence of a difference in the interactions of different leptons.

Let me begin with a quick review of leptons and their properties. At the start of the twentieth century, it was discovered that electricity was carried by sub-atomic particles known as electrons. In the following decades, it was proven that these electrons are also responsible for atomic structure, chemical reactions, the colour of elements and molecules, and many other things. When scientists started studying cosmic rays from space, they discovered the muon, which was identical in every way to the electron except for being ~200 times as heavy. As particle accelerators explored even higher energies, they discovered a third particle, called either a tauon or a tau-lepton, which was also identical in every way except for a mass ~3000 times more massive than the electron.

However all three of these particles, known collectively as leptons, behaved in the same way aside from their masses. All high energy reactions that could produce one of them would produce the other two at the same rate (aside from issues related to needing more energy to produce heavier particles). At the subatomic level, their charges and interactions are identical.

This leads to the question of whether they are truly identical, or have subtle differences. The BELLE and BaBAr experiments tried to measure this by looking at the decay rates of heavy particles known as B-mesons, but did not see a statistically significant difference between decays to electrons and decays to muons.

Now the LHCb experiment is claiming that they have seen a difference. They have observed numerous decays of the B+ particle, focusing on decays to a lighter particle called a kaon, and a lepton - antilepton pair. If leptons have the same properties, then half of these decays will produce electrons and half will produce muons. (The tau is too heavy for this decay). However what they have actually observed is that muons are produced only about 75% as much as electrons.

The Standard Model of particle physics cannot account for this, but there are many extensions that could. We do not yet know why nature provides three copies of the lepton, but that mechanism could explain the difference. There are also theories that include additional Higgs bosons or weak-interaction bosons that could alter the relative rates. There are even suggestions that the presence of hidden higher dimensions in our Universe could cause this discrepancy. 

For now this is just a preliminary result, and the researchers themselves admit that there is about a 1 in a 100 chance that this is nothing more than a fluke result that has no meaning. However they will continue to observe these rare decays, and try to improve their results (or prove them to be false, as all scientists should endeavor to do with their own work). If their result does hold up and prove to be valid, it will be an interesting new probe into physics beyond the accepted models of particle physics.

Only time and data will tell if this is a real effect, or just a statistical oddity.