A friend of mine forwarded a popular physics article to me this morning titled "A Crack In The Standard Model". My first thought was that it was either a crackpot article that wasn't worth reading, or that it was the biggest breakthrough of the last thirty years. As it turned out, it was neither.

The Standard Model of Particle Physics was developed through the 20th century, as new particles and new forces were discovered. However the last new particle discovered in an experiment was the top quark in 1995, which had been predicted to exist since the 1960s, and the last new force carriers to be discovered were back in the 1970s and 1980s, but they had also been predicted to exist in the 1960s. The Higgs boson was the final piece in the puzzle, but as with the others, most people have known it would eventually be discovered since its prediction in 1964. While there are numerous proposals for modifications to the Standard Model, there hasn't been a strong candidate for a new particle or new force in the last forty years or more. (Dark matter does not count, since there are thousands of candidates and no strong evidence to differentiate between them).

However the experimental data that lead to this article may genuinely be a sign of a new force beyond the four we know of. Unfortunately the evidence just isn't strong enough yet for confirmation.

One of the interesting probes of new physics is the decay of the B-mesons. B-mesons are a group of heavy particles which can be produced in large quantities, and which due to their large mass can decay to a wide variety of other particles. They have been used to study everything from symmetry violations, to evidence of higher dimensions, to possible models of light mass dark matter (which I worked on extensively as a graduate student :) )

Now they have yielded a fascinating mystery. Both the SLAC and LHC particle accelerators have been producing large numbers of these B-mesons and counting up the number and types of particles produced in their decays. In particular in this study, they have been counting how many muons are produced and how many tau leptons are produced.

According to all known models of particle physics, these two particle species are identical aside from their masses. They both interact in exactly the same manner with the weak nuclear force. And that means that, aside from the effects of their mass difference, the muons and the taus should be produced in equal amounts. (Being heavier, the production of taus is suppressed relative to the much lighter muons. But when this difference is removed, they should be equivalent)

Except that is not what has been seen. The preliminary results seem to show more taus than would be expected, suggesting there is some production mechanism that affects the two particles differently. At this point the results are between 2 and 3 sigma, which means that there is still a chance that it is nothing more than a statistical fluctuation that does not reflect reality, but it has also been seen in two separate experiments analyzed by different people. That would seem to improve the possibility that it is a genuine effect that has been observed.

There are four known forces in the Standard Model. The electromagnetic force and weak nuclear force affect both particles the same way, so that isn't the source. And these two particles are completed immune to the effects of strong nuclear forces, while gravity affects them differently but is far too weak to be observable in these experiments. So it would appear to be a fifth force!

At this point the data is not good enough to test theories, but there are a few possibilities. 

One possibility is the presence of a charged Higgs boson. By now most people are aware of the neutral Higgs boson that was discovered a few years ago, but some models - especially supersymmetric models - predict that there will be another Higgs boson which carries an electric charge. This charged Higgs can be produced in a B-meson decay, and can then decay into either a tau or a muon. Except that the effect is stronger for heavier particles, such as the tau lepton. This would generate a tau lepton excess as seen.

Another possibility is the heavy-W models. Some models of extra dimensional physics include a copy of the weak nuclear force that is much weaker than the usual one. This would appear as a fifth force, but unfortunately most models affect both the tau and the muon equally. It can be made to explain this excess, but it requires a lot of tweaking of the model and is not a prediction of the theory.

A third possibility is the higher dimensional graviton. In models that contain additional hidden dimensions of space, there are predictions of particles called either radions or Kaluza-Klein gravitons which interact with particles in our Universe. These particles interact more with heavier particles, just like the Higgs bosons do, and so could explain the excess. The effect is usually considered to be too weak to explain this excess, but there are models in which the interactions are stronger. Unfortunately as with the charged Higgs models, there are too many variations to make a clear prediction that can be compared with the experimental data.

So we really cannot say yet what has been observed. Maybe it is a statistical fluctuation that means nothing. Maybe it is a sign of higher dimensions. Maybe it is an indication that the tau and muon have some fundamental difference that no one has discovered yet.

All we can say is that it is an interesting result, and one that will be worth watching in future.