Flavor Physics in the Standard Model and Beyond

In the “Standard Model” (SM) we have a very successful mathematical model of the known particles of matter and of the strong, weak and electromagnetic forces that act on them. Take a look at the Review of Particle Physics. You will find page upon page of tabulations of properties of elementary particles and their reactions, many measured with remarkable precision. And all of it is explained by the Standard Model!

The SM has 18 adjustable parameters. This is quite excellent for a model that predicts a myriad reactions, often with spectacular precision, including dependence on all sorts of kinematic variables, for example, angular and energy dependence of the outcome of collisions between elementary particles. So you could say, wow, all those predictions with only 18 adjustable parameters. But we particle theorists are quite ambitious, so we'd like to reduce the number. Most of the parameters of the SM have to do with the masses of the quarks and leptons and with the quark mixing angles (these measure how they share in the weak interactions). The branch of particle physics that deals with phenomena associated with transitions between types of quarks is called “Flavor physics” --- because we call “flavors” the different types of quarks. So if you know the proton is made of two “up” (or “u”) quarks and one “down “ (or “d”) quark, you should know that “u” and “d” are two different “flavors.” The complete list of flavors in ascending order according to their mass is up, down, strange, charm, bottom (or beauty, in Europe) and top (or truth in Europe, although this has fallen out of use of late).

Much of the 90s I spent developing tools for producing precise predictions of the SM that involve flavor physics and could be tested experimentally. Moreover, the techniques allowed for precise determination of some of the 18 fundamental parameters. I was motivated by strong experimental programs dedicated to this type of physics, BaBar in SLAC and Belle in KEK (Japan). See my old page.

But now that we have established that the SM is pretty good at explaining all of these flavor transitions, and have extracted the parameters with excellent precision (well, most of them), I have turned my attention to other questions. sm flavor-photoThe figure shows the result of a study with then CERN postdocs Redi and Villadoro of a model we invented, in the same publication, that attempts to explain the flavor parameters of the SM through dynamics. We are not the first to do this, but our model is very different from others, so it is worth exploring. The figure shows, as a function of two parameters of the model, the region that is definitely not ruled out experimentally (in green), the region that may or may not be ruled out, depending of additional details (in yellow) and contours of fixed mass of a new particle, the t' (in red).