Flavour physics in the age of the LHC

Abstract

This is a written version of a series of lectures aimed at graduate students and postdoctoral fellows in particle physics theory and experimental physics familiar with the basics of the Standard Model. We explain the many reasons for the interest in flavour physics. We describe flavour physics and the related CP violation within the Standard Model, and explain how the B-factories proved that the Kobayashi–Maskawa mechanism dominates the CP violation that is observed in meson decays. We explain the implications of flavour physics for new physics. We emphasize the ‘new physics flavour puzzle’. As an explicit example, we explain how the recent measurements of D0–D mixing constrain the supersymmetric flavour structure. We explain how the ATLAS and CMS experiments can solve the new physics flavour puzzle and perhaps shed light on the Standard Model flavour puzzle. Finally, we describe various interpretations of the neutrino flavour data and their impact on flavour models. 1 What is flavour? The term ‘flavours’ is used, in the jargon of particle physics, to describe several copies of the same gauge representation, namely several fields that are assigned the same quantum charges. Within the Standard Model, when thinking of its unbroken SU(3)C × U(1)EM gauge group, there are four different types of particles, each coming in three flavours: – Up-type quarks in the (3)+2/3 representation: u, c, t. – Down-type quarks in the (3)−1/3 representation: d, s, b. – Charged leptons in the (1)−1 representation: e, μ, τ . – Neutrinos in the (1)0 representation: ν1, ν2, ν3. The term ‘flavour physics’ refers to interactions that distinguish between flavours. By definition, gauge interactions, namely interactions that are related to unbroken symmetries and mediated therefore by massless gauge bosons, do not distinguish among the flavours and do not constitute part of flavour physics. Within the Standard Model, flavour-physics refers to the weak and Yukawa interactions. The term ‘flavour parameters’ refers to parameters that carry flavour indices. Within the Standard Model, these are the nine masses of the charged fermions and the four ‘mixing parameters’ (three angles and one phase) that describe the interactions of the charged weak-force carriers (W±) with quark– antiquark pairs. If one augments the Standard Model with Majorana mass terms for the neutrinos, one should add to the list three neutrino masses and six mixing parameters (three angles and three phases) for the W± interactions for lepton–antilepton pairs. The term ‘flavour universal’ refers to interactions with couplings (or to flavour parameters) that are proportional to the unit matrix in flavour space. Thus, the strong and electromagnetic interactions are flavour universal1. An alternative term for ‘flavour universal’ is ‘flavour blind’. The term ‘flavour diagonal’ refers to interactions with couplings (or to flavour parameters) that are diagonal, but not necessarily universal, in the flavour space. Within the Standard Model, the Yukawa interactions of the Higgs particle are flavour diagonal in the mass basis. In the interaction basis, the weak interactions are also flavour universal, and one can identify the source of all flavour physics in the Yukawa interactions among the gauge-interaction eigenstates.