B Decays, the Unitarity Triangle, and the Universe

Abstract

A review is given of recent developments in the physics of flavor. Current constraints on the Cabibbo-Kobayashi-Maskawa matrix are discussed and related to the recent measurements of ǫ ′ /ǫ, sin 2β and K + → π + νν. A brief review is given of the connection between CP violation in B decays and elec- Plenary talk presented at theThe past year has seen a number of exciting experimental developments which have advanced our understanding of the physics of flavor. These include the confirmation by KTeV [1] and NA48 [2] of the NA31 result [3] for Re(ǫ ′ /ǫ) in K L → ππ, the first measurement by CDF [4] of sin 2β in B → J/ψK S , and the observation of a single event in K → π + νν by BNL-AGS-E787 [5]. In addition, 1999 has seen the startup of the next generation of e + e − B Factory experiments: BaBar at PEP-II (SLAC), BELLE at KEK-B (KEK), and CLEO-III at CESR (Cornell). The first physics results from these machines are expected in 2000. The fixed target experiment HERA-B, operating in the HERA accelerator at DESY, will soon be taking physics data as well. For high statistics studies of the K system, KLOE at the DAΦNE φ Factory (Frascati) has also begun to take data this year.Having been assigned such an ambitious title, my plan for this talk is to locate these various developments within a broader context. How do they all fit together and relate to each other? What do they signify for the next decade of particle physics? And most important, what is the role of such "low-energy" high energy physics in the anticipated era of new discoveries at the Tevatron and LHC? In short, what do we know now about the physics of flavor, and where do we hope to go in the future? The Standard Model at low energiesWe begin by recalling what the Standard Model looks like at "low" energies, by which I mean renormalization scales µ below about 10 GeV. I will refer to this theory as the Low Energy Standard Model, or LESM. At these energies we have a theory with five quarks (u, d, s, c, b) and six leptons (e, µ, τ, ν e , ν µ , ν τ ). There is unbroken SU(3) C × U(1) EM gauge symmetry, with eight massless gluons and a massless photon. There are Dirac masses for the quarks and charged leptons, and perhaps also Majorana masses for the neutrinos. The gauge interactions and mass terms are renormalizable operators in the effective lagrangian. With the exception of neutrino masses, these interactions also conserve individual flavor quantum numbers.