University of Edinburgh
Flavour Physics
E. Gardi
In parallel to the high-energy frontier, testing the Standard Model and searching for new physics proceed via a wide range of low-energy precision measurements. The history of particle physics provides numerous examples where new particles first revealed themselves through quantum correction, as virtual modes, and only later on were actually produced in scattering experiments.
Flavour physics studies the transition between generations. These occur in the Standard Model only via the charged weak current, involving the non-diagonal Cabbibo-Kobayashi-Maskawa (CKM) matrix elements: these transitions are rare and highly constrained and therefore provide a window to discover short-distance physics beyond the Standard Model, which potentially induces extra flavour changing transitions.
Over the past decade this has been a field of major experimental progress, led by the B factories BaBar (SLAC) and Belle (KEK, Japan), and the Fermilab Tevatron. So far, the Standard Model paradigm prevailed.
We are now in the beginning of a new era, with LHCb starting its operation. Super B factories in Japan and Italy will be the next phase, in a few years time. The Edinburgh experimental group has been an important player in this field for many years; It had an important role in BaBar, and is currently heavily involved in LHCb.
The success of the experimental program - the experiments recently completed, as well as the future ones - largely depends on our theoretical understanding of heavy and light hadrons, and especially the transition amplitudes governing B meson mixing and decay. These are controlled by QCD.
While much of the relevant physics is non-perturbative, and therefore requires numerical lattice simulations, certain questions require, or can be addressed by analytic methods. In particular, so are the fundamental questions of factorization and the computation of inclusive B-meson decay widths.
Inclusive B-meson decays
Inclusive B decay measurements of semileptonic b -> c and b -> u transitions have been instrumental in establishing the CKM picture. In addition inclusive rare decay measurements (flavour changing neutral currents) b -> s gamma, and b-> s l + l - , provide a stringent constraints on physics beyond the Standard Model. These measurements have therefore been central to the B-factory physics program, and in parallel, received much attention by the theory community over many years.
On the theory side inclusive decays are robust, and do not require detailed knowledge of the hadron wave functions: the well-established tools of the local Operator Product Expansion (OPE) and perturbation theory provide accurate predictions for total inclusive decay widths.
The main difficultly has been the fact that experimental measurements are limited to certain kinematic regions. Exploiting the potential of these measurements therefore relies on detailed calculations of decay spectra, not just the total widths. Our expertise is in using resummed QCD perturbation theory - Dressed Gluon Exponentiation (DGE) - to compute these spectra.
Inclusive B-Decay Spectra by DGE links:
Charmless semileptonic decay spectra
Heavy Flavor Averaging Group (semileptonic)
Factorization in exclusive B decays
Factorization is a general terms referring to the separation between long- and short-distance physics. The fundamental principle is that long-distance physics, which is non-perturbative, is quantum-mechanically incoherent with physics at short distance scales, which is well-described by perturbation theory. The implementation of this separation becomes complicated in processes that involve several hadrons.
Our main interest concerns the situation where a heavy quark (part of a heavy hadron) decays into light ones, ending up in highly-energetic light mesons. Such light-cone physics is captured by the ``lightcone distribution amplitudes'' of the hadrons involved, which is very hard to access on the lattice.