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CHAPTER 1
Introduction
The weak interaction is responsible for a large number of physical processes: nuclear β-decay, numerous decays of elementary particles, reactions induced by neutrinos from accelerators and nuclear reactors, and also some subtle effects involving parity violation in γ-decays of nuclei and in atomic optical spectra. All known leptons and hadrons are subject to the weak interaction. It plays an important role in such astrophysical phenomena as the sun’s burning and supernova explosions. Some of the weak processes were already put to use (for example, the angular asymmetry in the muon decay is a promising new tool in chemistry). Mainly, however, our interest in the weak interaction is rooted not in its possible applications but in the hope that its study will ultimately yield a unified theory of elementary particles and of the interactions between them. And although it would be very difficult today to predict any practical consequences of such a unified theory, there can be no doubt of their utmost importance.
In contrast to “stronger” interactions, namely the strong and electromagnetic, the weak interaction violates a number of conservation laws. Among the quantum numbers that are not conserved are space parity P, charge conjugation parity C, combined inversion parity CP, strangeness, charm, and some others.
The standard theory of weak interactions is based on the analogy with the electromagnetic interaction which is produced by the electromagnetic current coupled to the photon (see
fig. 1.1). Likewise, the weak interaction is postulated to result from weak currents being coupled to the so-called intermediate bosons W
+, W
−, Z. Intermediate bosons have not yet been found experimentally; however, this does not point to a defect in the theory since the expected masses are of the order of 100 GeV, the energies of the existing accelerators being well below their production thresholds. The W
+ and W
− bosons are coupled to charged currents which change the charges of particles involved. Such are the currents
and their hermitian conjugate currents
The last two currents, for instance, interact by exchanging a virtual W-boson and yield the muon decay
(
fig. 1.2). The Z
0 bosons are created by neutral currents of the types
and so on, involving identical ingoing and outgoing particles. Neutral currents are responsible, for example, for the scattering
vμe →
vμe (
fig. 1.3). Both the charged and the neutral currents include a leptonic and a hadronic part. At present we know six leptons which are naturally grouped into three pairs:
Fig. 1.1.
so that each lepton has its neutrino counterpart. Each lepton enters the charged current j with the appropriate neutrino:
This current emits W
+ bosons and absorbs W
− bosons. The hermitian conjugate current
emits W
− bosons and absorbs W
+ bosons. The neutral leptonic current
contains six terms:
The two leptonic currents given above are responsible for the processes involving both leptons
and antileptons
This follows from the properties of the relevant operators. For example, the operator
ē creates an electron and annihilates a positron, while the operator
e creates a positron and annihilates an electron. Operators of other particles act in a similar manner.
Fig. 1.2.
Fig. 1.3.
1.1. Quark currents
Hadrons are represented in weak currents by quarks. According to quark theory, all known hadrons consist of quarks of five types (five flavors): u, d, s, c and b. Theoretical arguments, however, point to the existence of a sixth quark t, so that in analogy to the six leptons, the six quarks form three pairs:
We recall that the charges of quarks u, c, and t are
and those of quarks d, s, and b are
The quark structure is uud for the proton, udd for the neutron,
for the
π+ meson, and so on. Strange particles include s-quarks (for instance,
), and charmed particles include c-quarks (for example,
). Particles with hidden charm, such as the J/
ψ meson, are represented by
The structure of the T-meson is
No hadrons with single b-quarks have so far been found*, and only very scant indirect information is available on the weak interaction of the b- and t-quarks (se...
