Grand Unified Theories

In General > s.a. symmetry breaking; types of Yang-Mills theories; unification.
* History: 1974, Idea proposed by Georgi and Glashow.
* Idea: Theories that unify the electroweak and strong interactions, usually described by a (Yang-Mills) gauge theory, where the interaction is mediated by a connection/potential belonging to (the Lie algebra of) a single group; Gravity is still not included in the unification.
* Structure: Like other gauge theories, they are based on a principal fiber bundle over spacetime (suitably compactified from boundary conditions), with a certain structure group G, and the quarks and leptons are described by a cross section of appropriate associated vector bundles; The interactions are mediated by connections, which are determined by the critical points of action integrals constructed as spacetime integrals of the curvature and its Hodge dual.
* Gauge group: Several groups have been proposed as strong candidates for grand unification, in particular SU(5), now known to be inadequate (1994); SO(10); Exceptional groups like E₆, which could come from the E₈ of string theory, E₇, and E₈.
* Symmetry breaking: All of the above are assumed to break to SU(3) SU(2) U(1) SU(3) U(1).
@ Reviews: Georgi SA(81)apr; Langacker PRP(81).
@ Textbooks: Cline & Mills 78; Zee 82; Ross 84; Kounnas et al 85.
@ Models: Georgi & Glashow PRL(74) [SU(5)]; Maraner MPLA(04)ht/03 [spacetime extensions of SO(10)]; Dorsner & Fileviez Pérez NPB(05)hp [non-susy SU(5)].
> Online resources: Wikipedia page.

Phenomenology > s.a. inflation scenarios; neutrino; monopoles.
* Motivation: (i) Observed family-structure; (ii) Meeting of the gauge couplings; (iii) Neutrino oscillations; (iv) The intricate pattern of masses and mixings of all fermions, including neutrinos; and (v) Need for B-L as a generator, to implement baryogenesis.
* Indications: 2000, Evidence favors grand unification along a particular route, based on the ideas of supersymmetry, SU(4)-color and left-right symmetry; thus it points to the relevance of an effective string-unified G(224) or SO(10)-symmetry.
* Successes: Prediction of sin²_W to within 5%; Elegant classification of particles.
* Problems: Quark/lepton mass ratios; Proton decay.
* Leptoquark: A hypothetical particle that turns quarks into leptons and vice versa, which arises naturally in GUT's; Depending on the model, they may form a singlet, a doublet, or a triplet (one particle may have charge +2/3, another –1/3); Their masses are estimated to be at least in the hundreds of GeV; If they exist, they may offer an explanation for the NuTeV anomaly in neutrino physics, and the LHC will search for them..
@ Coupling constants: Bennet & Nielsen IJMPA(94).
@ Cosmology: Singh FdP(83); Dorsner et al NPB(06).

Proton Decay > s.a. particle types.
* Decay modes: The dominant one may be -bar K⁺, with ⁺K⁰ being another possibility.
* Lifetime: In a typical (non-susy) version of GUT, the lifetime is predicted around 5 10²⁹ yr, while experimentally one gets > 10³⁰ yr and 5 10³¹–5 10³² yr for neutrino and neutrinoless decays, respectively; A conservative estimate is about 1 10³⁴ yr.
* Status: 1998, 1.6 10³³ yr, from Superkamiokande [@ Shiozawa et al PRL(98)].
@ References: Goldhaber et al Sci(80)nov*; Weinberg SA(81)jun; Sulak AS(82); Pati hp/00; Dorsner & Fileviez Pérez PLB(05)hp/04 [upper bound on lifetime]; Frampton MPLA(07) [in teravolt unification].

Beyond Regular GUTS > s.a. action for general relativity; cosmic strings; kaluza-klein; particle physics.
* Supersymmetric GUTs: Adding susy extende proton lifetime, among other benefits; > s.a. supersymmetric theories.
@ Supersymmetric GUTs: Sakellariadou & Rocher hp/04-in, Rocher & Sakellariadou JCAP(05) [and cosmic strings]; Mondragón & Zoupanos SIGMA(08)-a0802 [reduction of couplings].
@ And brane world: Duff IJMPA(01)ht/00-in; Berenstein ht/06.
@ And strings: Pati IJMPD(06).
@ Non-commutative: Aschieri et al NPB(03)ht/02; Calmet EPJC(07) [non-commutative spacetime].
@ Generalized: Froggatt et al NPB(94) [anti-grand unification, and fermion masses]; Spaans gq/97 [topological]; Chaves & Morales MPLA(00)ht/99 [with generalized Yang-Mills].

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