Modern Physics Letters A

The instability of protons is a crucial prediction of supersymmetric GUTs. We review the decay in minimal supersymmetric SU(5), which is dominated by dimension-five operators, and discuss the implications of the failure of Yukawa unification for the decay rate. In a consistent SU(5) model, where SU(5) relations among Yukawa couplings hold, the proton decay rate can be several orders of magnitude smaller than the present experimental bound. Finally, we discuss orbifold GUTs, where proton decay via dimension-five operators is absent. The branching ratios of dimension-six decay can significantly differ from those in four dimensions.

The ground-state properties of Zr and Sn isotopes are studied within the relativistic mean field theory. Zr and Sn isotopes have received tremendous attention due to various reasons, including the predicted giant halos in the neutron-rich Zr isotopes, the unique feature of being robustly spherical in the region of ¹⁰⁰Sn~¹³²Sn and the particular interest of Sn isotopes to nuclear astrophysics. Furthermore, four (semi-) magic neutron numbers, 40, 50, 82 and 126, make these two isotopic chains particularly important to test the pairing correlations and the deformations in a microscopic model. In the present work, we carry out a systematic study of Zr and Sn isotopes from the proton drip line to the neutron drip line with deformation effects, pairing correlations and blocking effects for nuclei with odd number of neutrons properly treated. A constrained calculation with quadrupole deformations is performed to find the absolute minimum for each nucleus on the deformation surface. All ground-state properties, including the separation energies, the odd–even staggerings, the nuclear radii, the deformations and the single-particle spectra are analyzed and discussed in detail.

A new symmetry-preserving loop regularization method proposed in Ref. 1 is further investigated. It is found that its prescription can be understood by introducing a regulating distribution function to the proper-time formalism of irreducible loop integrals. The method simulates in many interesting features to the momentum cutoff, Pauli–Villars and dimensional regularization. The loop regularization method is also simple and general for the practical calculations to higher loop graphs and can be applied to both underlying and effective quantum field theories including gauge, chiral, supersymmetric and gravitational ones as the new method does not modify either the Lagrangian formalism or the spacetime dimension of original theory. The appearance of characteristic energy scale M_c and sliding energy scale μ_s offers a systematic way for studying the renormalization-group evolution of gauge theories in the spirit of Wilson–Kadanoff and for exploring important effects of higher dimensional interaction terms in the infrared regime.

Noncommutative U(1) gauge theory in four dimensions is shown to be equivalent in some scaling limit to an ordinary nonlinear sigma model in two dimensions. The model in this regime is solvable and the corresponding exact beta function is found. We also show that classical U(n) gauge theory on can be approximated by a sequence of ordinary (d-2)-dimensional Georgi–Glashow models with gauge groups U(n(L+1)), where L+1 is the matrix size of the regularized noncommutative plane .

We study the radiative h⁰(A⁰)→γγ decays in the framework of the general two-Higgs doublet model, the so-called model III. We estimate the decay width of the processes at the order of magnitude of 10^-2GeV for the intermediate values of the Yukawa couplings. We observe that the analysis of the two-photon system provides a comprehensive information for the complex nature of the Yukawa couplings and therefore, the source of the CP violation.

Limits on the anomalous magnetic moment and the electric dipole moment of the τ lepton are calculated through the reaction e⁺e^-→τ⁺τ^-γ at the Z₁-pole and in the framework of a left–right symmetric model. The results are based on the recent data reported by the L3 collaboration at CERN LEP. Due to the stringent limit of the model mixing angle ϕ, the effect of this angle on the dipole moments is quite small.