Ultra-rare events

Particle collisions like this were used to make this measurement. The two red lines on the top of the figure are the two muons from a Bs meson decay. The yellow lines are all the other particles created in the collision.

Over the years, we have heard of the extraordinary scientific possibilities provided by the LHC’s very high energy. However, of equal value is its large number of collisions. When one is looking for a very rare and particular kind of collision, the best way to find it is to make many, many collisions. That increases the chances that a rare one will occur.

One particle that is of special interest is called a Bs (pronounced “B sub s”) meson. This particle contains a bottom antiquark and a strange quark (or a bottom quark and a strange antiquark). While there are many ways in which it can decay, it is forbidden in the simplest calculations to decay into a pair of muons. In more complex calculations, however, it turns out that such a decay is possible, though exceedingly rare. Bs mesons are predicted by the Standard Model to decay into pairs of muons only once out of every 280 million decays. The first experimental evidence for this process was announced by the LHCb experiment in November 2012.

Ordinarily such a rare decay would be of only academic interest, but there are many candidate theories for physics beyond the Standard Model that predict this unusual decay will occur far more often. These theories include several models that incorporate supersymmetry and another that involves hypothetical particles called leptoquarks.

Because detecting two muons is very easy for the CMS experiment, it was natural for it to attempt such an analysis. While identifying muons is quite straightforward, the analysis was exceedingly challenging. Using all the data recorded in 2011 and 2012 (during which about 3 quadrillion collisions occurred in the detector, each containing tens, hundreds or even thousands of particles), only 30 or so decays of Bs mesons were identified as having the sought-after properties.

The result of these studies was that the fraction of Bs mesons that decayed in this particular way was completely consistent with predictions of the Standard Model. Comparable studies of another type of meson called Bd told a similar story. The agreement between the data and the Standard Model prediction sets severe constraints on many proposed theories for new physics.

Don Lincoln

These US CMS scientists contributed to this analysis.
Keith Ulmer is giving today’s Wine and Cheese presentation on the CMS measurement of Bs mesons decaying into pairs of muons (Bs→μμ).