Beware of penguins!

This is the signal from the decay of a Λb particle into a Λ particle and a pair of muons, observed with a significance of 5.8 sigma.

We know to be mindful of the geese around the Fermilab site, but penguins? Yes, they’re here, and they can be aggressive, especially against the Standard Model.

Penguin decays, which take their name from their diagramed shape, are a class of particle decays important in indirect searches for new particles or interactions at the Tevatron. If new particles or interactions exist, it’s very likely that they participate in these processes. For some types of new particles, searching the penguin decays might be the best chance to detect them. However, the penguin processes are very weak. Their effects can only be seen in rare decays, so scientists must search through a lot of data.

Sifting through 6.8 inverse femtobarns of data (roughly 10 trillion Tevatron collisions), a team of CDF scientists looked for a particularly noble species of penguins: the decays of particles containing a bottom quark (B hadrons) into lighter particles containing a strange quark and a pair of muons. These decays are special because they offer a full suite of measurement opportunities, each sensitive to a different facet of new physics. However, only a few per million B hadrons decay this way, resulting in only hundreds of signal events observed in the CDF data. Still, these are among the world’s largest samples available so far.

The data analysis, which uses 12 distinct decay modes, is challenging. But the results, totaling more than 20 different measurements, are very rewarding. The CDF scientists obtained the world’s best measurements of decay rates and their asymmetries. The asymmetries are the difference between the decay of the B particle and the B anti-particle, and they are especially sensitive to the presence of new particles. The CDF scientists also reported asymmetries that had not been previously measured.

As a very welcome bonus, the CDF team observed the first process involving a B baryon, which is a heavier cousin of the proton. They reconstructed 24±5 candidates for the decay of a Λb baryon into the Λ baryon and two muons , which makes this the rarest decay ever of a B baryon. Overall, the results are consistent with the predictions of the Standard Model. But this is far from being the end of the story, since the precision of measurements is still limited by the small event samples.

The results are described in two letters submitted to Physical Review Letters and will be shown at the upcoming Wine and Cheese seminar this week.

Learn more here and here.

The primary authors of this analysis, from left: Fumihiko Ukegawa, Hideki Miyake and Shinhong Kim, all from the University of Tsukuba in Japan.