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Abstract:

If the expectation value of the right-handed (rhd) sneutrino comes to dominate the universe, its decay naturally leads to successful leptogenesis, as well as significant dilution of dangerous inflationary relics, such as the gravitino. The resulting baryon asymmetry is independent of other cosmological initial conditions. This attractive variant of leptogenesis requires at least one of the rhd neutrinos to have small Yukawa coupling and to have mass ~ 10^6 GeV, much smaller than the grand unified (GUT) scale. We show that these features naturally arise in the context of independently motivated and successful 5d orbifold GUTs with inverse-GUT-scale-sized extra dimensions. Rhd neutrinos are realized as bulk fields N_i with 5d bulk masses, while Yukawa couplings and lepton-number-violating masses for the N_i are localized at the SM boundary. The exponential suppression of the would-be N_i zero-modes leads to the desired small 4d Yukawa couplings and small masses for the rhd neutrino states. The see-saw prediction for the lhd neutrino mass scale is automatically maintained. We show that this realization of rhd neutrinos is nicely accommodated within an attractive orbifold-GUT flavour model, where all flavour hierarchies have a geometrical origin.

Abstract:

This biennial Review summarizes much of Particle Physics. Using data from previous editions, plus 2205 new measurements from 667 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. This edition features expanded coverage of CP violation in B mesons and of neutrino oscillations. For the first time we cover searches for evidence of extra dimensions (both in the particle listings and in a new review). Another new review is on Grand Unified Theories. A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: http://pdg.lbl.gov.

Abstract:

In supersymmetric grand unified theories (GUTs) based on S^1/(Z_2 x Z'_2) orbifold constructions in 5 dimensions, Standard Model (SM) matter and Higgs fields can be realized in terms of 5d hypermultiplets. These hypermultiplets can naturally have large bulk masses, leading to a localization of the zero modes at one of the two branes or to an exponential suppression of the mass of the lowest-lying non-zero mode. We demonstrate that these dynamical features allow for the construction of an elegant 3-generation SU(5) model in 5 dimensions that explains all the hierarchies between fermion masses and CKM matrix elements in geometrical terms. Moreover, if U(1)_\chi (where SU(5) x U(1)_\chi \subset SO(10)) is gauged in the bulk, but broken by the orbifold action at the SM brane, the right-handed neutrino mass scale is naturally suppressed relative to M_GUT. Together with our construction in the charged fermion sector this leads, via the usual see-saw mechanism, to a realistic light neutrino mass scale and large neutrino mixing angles.