Quark-lepton universality

It is apparent that the numbers and masses of the flavor and quark-lepton transforming gauge bosons are larger than those of the SU(5) minimal model. This means that the value of a is lower, and assuming that the duration of the inflationary period is fixed, the scale for the expansion of the universe is reduced. This means that there is the enhanced prospect for deviations from flatness. So one may presume that the universe started as a small 3-sphere with a large curvature, where the inflationary period flattened out the universe, but maybe not completely. This leaves open the prospect that if before inflation that if the universe were open or closed, k = 1, that the universe today still contains this structure on a sufficiently large scale. The closer to flatness the universe is, the tighter are the constraints on the masses of particles in the early universe. 

It led to a prediction that the number of different sorts of neutrino (equivalent in standard particle physics to the number of families of quarks and leptons) is less than 4 and probably no more than 3. This prediction was subsequently confirmed (subject to slight reservations about differences between effective numbers of neutrino species in the laboratory and in the early Universe) by measurements of the width or lifetime of the Z° boson at CERN in 1990. 

Thus there are 12 matter particles, the six quarks and the six leptons (antiparticles such as the positron are not counted separately). Although this is a vast improvement over the situation in 1960, when physicists had hundreds of particles to contend with, scientists do not yet believe that they have found the key to the universe. Simply too much remains to be explained. Physicists don t know why there should be 12 matter particles, and not more or fewer. They don t know why the particles have the masses they do. And they don t know why there should be four forces in nature, rather than three or six, or why they have the strengths that they do. 

Early stages of the universe are listed in Table 15.2. Primeval matter was merging into elementary particles, huge amounts of energy were released and the big bang immediately caused a rapid expansion of the universe. Within about 1 s the temperature decreased markedly, matter and antimatter annihilated each other, quarks combined into mesons and baryons and enoimous amounts of energy were liberated causing further expansion. Formation of the first protons and leptons is assumed after about 1 s, when the temperature of the early universe was about lO K. 

We argue that to know the universe, students should know its fundamental composition. In this unit, we break the universe into its fundamental building blocks. First, we introduce the concept of the smallest piece of matter. But to do so, we do not simply make a list fundamental particles. Rather we explore the subplot of discovery. How did humans come to know of the fundamental parts of matter Starting with Democritus s atom and going through a sequence of scientists (Dalton, Thomson, Rutherford, Bohr, Pauli, Fermi) we end with our current state of knowledge six quarks and six leptons. 

Around "time zero" the Universe consisted of an immensely dense, hot sphere of photons, quarks and leptons, and their antiparticles, in thermal equilibrium, particles being created... 

Thus, the modem cosmological (still mainly hypothetical) picture of the worid gives us unknown or poorly studied forms of matter self-preservation which is the cause and source of the existence of stars in a galaxy, congestions of galaxies, super congestions, and other forms of baryonic matter (quarks, bosons, leptons) whieh constitutes the observable Universe. 

We here enlarge the standard model to include the weak and electromagnetic interactions of hadrons. We encounter serious techniceil problems if we try to restrict ourselves to the original three quarks, u,d,s. In particular we find unwanted neutral strangeness-changing currents in the theory. These difficulties are eliminated by the introduction of a new charm quark c. There is then a very attractive universality between the two lepton doublets (e-) ( - ) quark doublets formed from... 

The discovery of a third generation lepton, the t, recreates technical problems which are resolved by the discovery of a third generation quark, the b (for bottom ). Assuming that the hitherto unobserved partners Ur and the top quark t really exist, we have an enlarged lepton-quark universality between three generations of leptons and three generations of quarks. 

In accord with the assumption of exact universality between leptons and bare quarks, the interaction of the bare quarks with the W boson will be given by terms analogous to (4.2.26), which are, of course, diagonal in flavour. But these terms, when re-expressed in terms of the fields in (9.7.15) become non-diagonal in flavour. 

To the extent that lepton-quark universality holds, the mere counting of decay modes (the lepton modes, Fig. 14.6(o), contributing twice and the hadronic Cabibbo favoured mode. Fig. 14.6(6), contributing three times because of colour) yields the very naive prediction... 

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