Time for a blog post that has been far too long coming! Remember the Quest for B Meson Decay? I wrote about this several months ago: the LHCb experiment had seen one of the rarest interactions in particle physics, the decay of the \(\mathrm{B}^0_s\) meson into a muon and antimuon, for the first time after 25 years of searching.

Lots of physicists were interested in this particular decay because it’s unusually good at distinguishing between different theories. The standard model (which incorporates only known particles) predicts that a muon and antimuon should be produced in about 3.56 out of every billion \(\mathrm{B}^0_s\) decays — a number known as the branching ratio. But many other theories that involve additional, currently unknown particles, predict drastically different values. A precise measurement of the branching ratio thus has the ability to either rule out lots of theoretical predictions, or provide the first confirmation on Earth of the existence of unknown particles!

Naturally, most physicists were hoping for the latter possibility — having an unknown particle to look for makes things exciting. But so far, the outlook doesn’t look good. Last November, LHCb announced their measurement of the branching ratio …

As if last week’s announcements of new Higgs results, B-dimuon decay, the rediscovered Y(4140), and all sorts of other goodies at HCP 2012 weren’t enough, there’s more big news from the world of experimental particle physics this week. A paper published just a few days ago in Physical Review Letters (here’s the PDF, and the arXiv page) describes the first observation ever of actual time reversal asymmetry: a difference between the behavior of a particular physical process and the time-reversed version of the same process.

Lest you get too excited, though: this has nothing to do with actual reversing of time, so it doesn’t mean time travel is possible or anything like that. And in fact, nobody in physics is the least bit surprised that it worked out the way it did. There is a theorem in physics called the CPT theorem (or sometimes PCT, or TCP, but not that TCP) which basically guaranteed that time reversal asymmetry had to show up somewhere. The theorem is suddenly getting a lot of attention in the news coverage of the discovery, but it’s technical enough that most people aren’t bothering to explain it. I …

Now that I’ve finished my quest to identify the misbehaving sinc function, I can bring you the latest news from the Hadron Collider Physics conference in Kyoto, Japan. This is a major conference at which several new results from the LHC experiments are being announced — an exciting time for the physics world indeed!

B meson-muon decay

The most interesting result to come out of the conference so far is an observation by the LHCb experiment of a \(\mathrm{B}^0_s\) meson, made of bottom and antistrange quarks, decaying to a muon-antimuon pair. This is a reaction that physicists have been searching for since 1987. This year, the LHCb collaboration has actually seen it happen — or at least, they’ve collected enough statistical evidence to be fairly confident (\(3.5\sigma\)) that it does happen — for the first time.

It’s not quite as dramatic a discovery as that makes it sound, though; the reason \(\mathrm{B}^0_s\to\mu^+\mu^-\) has never been seen before is that it’s incredibly rare. B mesons are not exactly easy to produce, and then once you’ve got one, the standard model predicts that only one out of every 300 million will …