The Standard Model (SM) of particles and interactions provides a successful description of most of the matter we know of. However, physicists have known for many years that it is not complete: the SM predicted massless neutrinos, and has no place for dark matter. A new result from the BaBar experiment at the Stanford Linear Accelerator Center (SLAC) could possibly provide another problem for the SM—and would place severe constraints on a popular alternative theory, supersymmetry (SUSY).

As described in a new paper in Physical Review Letters, the BaBar collaboration measured a decay process of the bottom (b) quark, the second-heaviest such particle. This decay process produces leptons, the class of particles including electrons, neutrinos, muons (a common product of cosmic rays), and taus. The latest BaBar results indicate more taus were produced than the SM predicted. However, the results were also inconsistent with the predictions of the simplest form of SUSY. While the uncertainties on these results are still large, they are similar to earlier data from the Belle Collaboration in Japan.

Although SLAC's main accelerator is no longer used for collisions, it can inject electrons and positrons into two storage rings that cross paths at the BaBar detector. The energies of these collisions are tuned to produce B mesons, which contain a bottom quark.

The BaBar experiment was designed for b quark physics. (The name even refers to b quarks and their antiquarks, written as b and pronounced "B-bar.") The current results focused on a particular decay of a b quark into a D or D* meson, a charged lepton, and a neutrino.

Muons and taus (also known as the tauon or tau lepton) are leptons with the same electric charge as electrons, but are much more massive: the muon is about 200 times the mass of an electron, while the tauon has about 3500 times the electron's mass. (Neutrinos, on the other hand, are neutral leptons with very tiny masses.) While muons and electrons are common decay products in particle experiments, taus are produced only rarely, and they quickly decay into their lighter lepton cousins.

Even though taus are rare, when they are produced in the decay of particles containing b quarks, they provide sensitive tests of their decay, and thus can be used to test theories on the frontiers of the SM and its alternatives.



According to current theories, ordinary matter (as opposed to dark matter) fall into two categories: leptons, which are fundamental, and hadrons, which are made of quarks. Hadrons made of two quarks (actually a quark and an antiquark) are the mesons, while those with three quarks are the baryons. Protons and neutrons are baryons, and their quarks are the stable "up" (u) and "down" (d) varieties. According to current theories, ordinary matter (as opposed to dark matter) fall into two categories: leptons, which are fundamental, and hadrons, which are made of quarks. Hadrons made of two quarks (actually a quark and an antiquark) are the mesons, while those with three quarks are the baryons. Protons and neutrons are baryons, and their quarks are the stable "up" (u) and "down" (d) varieties. There are three generations of both leptons and quarks. The lightest generation includes the electron along with the u and d quarks; the heaviest generation contains the tau, the b quark, and the top (t) quark. In between lies the generation with the muon, the strange quark, and the charm quark. Particle physics is trippy.



The SM predicts the frequency at which charged leptons produced in this decay process will be taus: roughly 20 percent for the decays that produce a D meson, and 23 percent for the D* mode. In contrast, the researchers at BaBar found 31 percent taus for the D and 25 percent for the D*—a significant excess. One possible way to explain this difference is to introduce an additional Higgs boson beyond the one predicted by the SM (which we've only just found). A common version of SUSY introduces four Higgs particles, one of which could produce the extra tauons seen in an experiment like this. However, these particular BaBar data did not agree with the predictions of the SUSY model, either.

The BaBar results were in the range of three standard deviations, or "3 σ," which in particle physics means they are significant, but not yet definite. Thus, it's too soon to sign the death certificate for the popular version of SUSY. (The SM, of course, is already known to be incomplete, so its overthrow by these data isn't anything radical.) We'll have to watch future experiments carefully—not least since superstring theory, which currently lies well beyond the realm of experimental tests, depends upon SUSY.

Physical Review Letters, 2012. DOI: 10.1103/PhysRevLett.109.101802 (About DOIs).

Thanks to Richard Ruiz of the University of Pittsburgh for his help in understanding the results of this paper. Any errors of course are my own.