Bs-Bs mixing in grand unified models

We study Bs-Bs mixing in supersymmetry grand unified SO(10), SU(5) models where the mixings among the second and third generation squarks arise due to the existence of flavor violating sources in the Dirac and Majorana couplings which are responsible for neutrino mixings. We find that when the branching ratio of tau-->mugamma decay is enhanced to be around the current experimental bound, Bs-Bs mixing may also contain large contribution from supersymmetry in the SO(10) boundary condition. Consequently, the phase of Bs-Bs mixing is large (especially for small tanbeta and large scalar mass m0) and can be tested by measuring CP asymmetries of Bs decay modes.     

Economic models for Dirac neutrinos in grand unified theories

It is shown that dimension five non-renormalizable interactions can produce light Dirac neutrinos in an extension of the minimal SU(5) GUT containing additional SU(5) singlets and global U(1) symmetries.     

Construction of grand unified models under maximal subalgebras

A construction of grand unified models of the strong, weak and electromagnetic interactions is described based on the transformation properties of the group generators under a maximal subgroup decomposition without recourse to large representation matrices or to the specific algebraic structures of some classical Lie-groups, such as the Clifford algebra associated with the orthogonal groups or the octonionic structure of the exceptional groups. To illustrate the procedure an explicit construction is given of the SU(5) model useful in the discussion of higher rank groups, of SO(10) under the maximal subalgebras SU(2)_L × SU(2)_R × SU(4)_c and SU(5) × U(1)_r and of the exceptional group E₆ under SU(3)_L × SU(3)_R × SU(3)_c and SO(10) × U(1)_t. The construction procedure can be used as well with any classical Lie-group.     

“Finite” grand unified models with a stable ultraviolet fixed point

Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 22–26, July, 1990.     

Breakdown of scaling in grand unified theories

We investigate the collapse of asymptotic freedom inS U (5) grand unified theory and itsS U (3) _c colour subgroup. It is shown that the change from a decreasing to an increasing gauge coupling constant, in the deep Euclidean, is dramatically sudden.     

Transition from weak to strong coupling in QCD and in grand unified models at high temperature

We investigate the weak-coupling expansion at high temperature for relativistic quantum field theories. Boson self-energies generally acquire a non-analytic dependence on the coupling due to the infrared plasmon effect. We show how to sum loops within loops for such thermodynamic quantities as pressure and energy density. Recent high-precision Monte Carlo simulations of quarkless SU(3) gauge theory are compared to the weak-coupling expansion and an excellent agreement is obtained for the high-temperature phase. Weak-coupling calculations are also carried out for supersymmetric grand unified models. We find indications that such models are often strongly coupled already at the unification temperature. This may have a significant effect on models of the early universe.     

Baryon number generation in grand unified theories

Grand unified models of elementary particle interactions suggest that there was an early epoch during the Big Bang while the temperature was slightly less than the Planck temperature, during which no known interactions were in thermal equilibrium. This epoch was probably followed by a brief period during which baryon number violating forces were in equilibrium and could have annihilated any previously existing net baryon number. As these forces dropped out of equilibrium, a CP violating component could have generated the observed baryon to entropy ratio of O(10⁻⁹).     

Hierarchical quark and lepton masses in grand unified theories

A general scheme by which fermions in grand unified theories may acquire radiative hierarchical masses is suggested. The observed hierarchy of fermion masses (≈10^?3–10² GeV) appears as a direct reflection of the physics taking place in the 10¹⁴–10¹⁹ GeV energy range. ASU (10) example is shown in which quark and leptons acquire their masses by means of radiative graphs.     

Cosmological black hole production in grand unified theories

Cosmological black hole production is reviewed with due consideration to interactions assumed relevant in the very early Universe. In the framework of the standard hot big bang and grand unified theories it is shown that primordial black holes would form too prolifically to be consistent with observations, if they are present in an extended mass range. This poses some constraints on the form of fluctuations present in the very early Universe, be they primordial or induced spontaneously by phase transitions in the cooling Universe.     

Low-energy neutral current phenomenology and grand unified theories

We derive necessary and sufficient conditions to be satisfied by any expanded electroweak gauge model in order to reproduce the standard model low-energy neutral current predictions. These conditions imply several constraints on the neutral gauge boson masses and quantum number assignments for the ordinary fermions. Using these conditions, we prove that the popular grand unified theories based on the gauge groups SO(10) and E₆ can only accomodate trivial extensions of the standard model. As a consequences, if any of these grand unified models works at some energy scale, present low-energy neutral current phenomenology implies that the Z-boson must be produced with the expected mass and couplings to the ordinary fermions. Any additional neutral gauge boson (with the possible exception of very heavy ones) could only be produced in hadronic collisions and it would not decay in e⁺e^?.     

The hierarchy problem in supersymmetric grand unified theories

Supersymmetric models are discussed with a view to solving the hierarchy problem in grand unified theories. Several types of light scalars are obtained and classified. A definition of naturalness is given for supersymmetric grand unified models, and a semi-realistic O(10) model is presented.     

Supersymmetric grand unified theories and the early universe

Consequences of supersymmetric, grand unified theories (with supersymmetry unnbroken at the tree level) for the production of baryons and magnetic monopoles in the early universe are discussed.