Neutrino Mass and Oscillation

The question of neutrino mass is one of the major riddles in particle physics. Recently, strong evidence that neutrinos have nonzero masses has been found. While tiny, these masses could be large enough to contribute significantly to the mass density of the universe. The evidence for nonvanishing neutrino masses is based on the apparent observation of neutrino oscillation -- the transformation of a neutrino of one type or "flavor" into one of another. We explain the physics of neutrino oscillation, and review and weigh the evidence that it actually occurs in nature. We also discuss the constraints on neutrino mass from cosmology and from experiments with negative results. After presenting illustrative neutrino mass spectra suggested by the present data, we consider how near- and far-future experiments can further illuminate the nature of neutrinos and their masses.Comment: 43 pages, 8 figures, to appear in the Annual Review of Nuclear and Particle Science, Vol. 49 (1999

Big Bang nucleosynthesis and physics beyond the Standard Model

The Hubble expansion of galaxies, the 2.73\dK blackbody radiation background and the cosmic abundances of the light elements argue for a hot, dense origin of the universe --- the standard Big Bang cosmology --- and enable its evolution to be traced back fairly reliably to the nucleosynthesis era when the temperature was of \Or(1) MeV corresponding to an expansion age of \Or(1) sec. All particles, known and hypothetical, would have been created at higher temperatures in the early universe and analyses of their possible effects on the abundances of the synthesized elements enable many interesting constraints to be obtained on particle properties. These arguments have usefully complemented laboratory experiments in guiding attempts to extend physics beyond the Standard SU(3)_{\c}{\otimes}SU(2)_{\L}{\otimes}U(1)_{Y} Model, incorporating ideas such as supersymmetry, compositeness and unification. We first present a pedagogical account of relativistic cosmology and primordial nucleosynthesis, discussing both theoretical and observational aspects, and then proceed to examine such constraints in detail, in particular those pertaining to new massless particles and massive unstable particles. Finally, in a section aimed at particle physicists, we illustrate applications of such constraints to models of new physics.Comment: 156 pages LaTeX, including 18 PostScript figures; uses ioplppt.sty, epsf, and personal style file (incl.); Revised and updated to include, e.g. implications of new deuterium observations in primordial clouds; 2-up PostScript version (78 pages) available at ftp://ftp.physics.ox.ac.uk/pub/local/users/sarkar/BBNreview.ps.gz ; to appear in Reports on Progress in Physic