Neutrino flavours

Unlike the charged leptons, which can be detected from the continuous track defined by the ionisation of atoms as they traverse matter, neutrinos are never directly observed; they are only detected through their weak interactions. Different neutrino flavours can only be distinguished by the flavours of charged lepton produced in charged-current weak interactions. Consequently, the electron neutrino νe, is defined as the neutrino state produced in a charged-current weak interaction along with an electron. Similarly, by definition, the weak charged-current interactions of a νe will produce an electron. For many years it was assumed that the νe, νμ and ντ were massless fundamental particles. This assumption was based, at least in part, on experimental evidence. For example, it was observed that the interactions of the neutrino/antineutrino produced along with a positron/electron in a nuclear β-decay, would produce an electron/positron as indicated in Figure 13.1.

The need for the Higgs boson

The apparent violation of unitarity in the e+e− → W+W− cross section was resolved by the introduction of the Z boson. A similar issue arises in the W+W− → W+W− scattering process, where the cross section calculated from the Feynman diagrams shown in Figure 17.1 violates unitarity at a centre-of-mass energy of about 1 TeV. The unitarity violating amplitudes originate from WL WL → WL WL scattering, where the W bosons are longitudinally polarised. Consequently, unitary violation in WW scattering can be associated with the W bosons being massive, since longitudinal polarisation states do not exist for massless particles. The unitarity violation of the WL WL → WL WL cross section can be cancelled by the diagrams involving the exchange of a scalar particle, shown in Figure 17.2. In the Standard Model this scalar is the Higgs boson.

The weak charged-current interaction

At the fundamental level, QED and QCD share a number of common features. Both interactions are mediated by massless neutral spin-1 bosons and the spinor part of the QED and QCD interaction vertices have the same ū(p′)γμ u(p) form. The charged-current weak interaction differs in almost all respects. It is mediated by massive charged W± bosons and consequently couples together fermions differing by one unit of electric charge. It is also the only place in the Standard Model where parity is not conserved. The parity violating nature of the interaction can be directly related to the form of the interaction vertex, which differs from that of QED and QCD.

Parity

The parity operation is equivalent to spatial inversion through the origin, x → −x.

The Standard Model of particle physics represents one of the triumphs of modern physics. With the discovery of the Higgs boson at the LHC, all of the particles in the Standard Model have now been observed. The main aim of this book is to provide a broad overview of our current understanding of particle physics. It is intended to be suitable for final-year undergraduate physics students and also can serve as an introductory graduate-level text. The emphasis is very much on the modern view of particle physics with the aim of providing a solid grounding in a wide range of topics.

Our current understanding of the sub-atomic Universe is based on a number of profound theoretical ideas that are embodied in the Standard Model of particle physics. However, the development of the Standard Model would not have been possible without a close interplay between theory and experiment, and the structure of this book tries to reflects this. In most chapters, theoretical concepts are developed and then are related to the current experimental results. Because particle physics is mostly concerned with fundamental objects, it is (in some sense) a relatively straightforward subject. Consequently, even at the undergraduate level, it is quite possible to perform calculations that can be related directly to the recent experiments at the forefront of the subject.

Pedagogical approach

In writing this textbook I have tried to develop the subject matter in a clear and accessible manner and thought long and hard about what material to include.