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A new nontrivial U(1) gauge extension of the Minimal Supersymmetric Standard Model (MSSM) is proposed which automatically conserves baryon number and lepton number, and solves the mu problem. Naturally small Dirac neutrino masses are also…
A discrete R-symmetry often appears as an exact gauge symmetry in the low energy effective theory of superstring theories. We search for such discrete R-symmetries from a phenomenological point of view and find that Z_{9R} and Z_{18R} are…
We explicitly show the analogy between the symmetry breaking scheme for the GUT flipped $SU(5)$ with that of the Weinberg-Salam theory of electroweak interactions. This allows us to construct the embedded defect spectrum of the theory…
Even if SUSY is not present at the Electro-Weak scale, string theory suggests its presence at some scale M_{SS} below the string scale M_s to guarantee the absence of tachyons. We explore the possible value of M_{SS} consistent with gauge…
We present an extension of the Standard Model to the local gauge group $SU(3)_c\otimes SU(4)_L\otimes U(1)_X$ with a family non-universal treatment and anomalies canceled among the three families in a nontrivial fashion. The mass scales,…
We propose the existence of a hidden or dark sector besides the standard model (SM) of particle physics, whose members (both fermionic and bosonic) obey a local SU(2)$_{\rm H}$ gauge symmetry while behaving like a singlet under the SM gauge…
In the context of $\mathrm{E}_{6}$ Grand Unified Theories (GUTs), an intriguing possibility for symmetry breaking to the Standard Model (SM) group involves an intermediate stage characterized by either…
We show how gauge coupling unification near the Planck scale $M_P\sim 10^{19}$ GeV can be achieved in the framework of supersymmetry, facilitating a full unification of all forces with gravity. Below the conventional GUT scale $M_{GUT}\sim…
We study a new mechanism to dynamically break supersymmetry in the E8xE8 heterotic string. As discussed recently in the literature, a long-lived, meta-stable non-supersymmetric vacuum can be achieved in an N=1 SQCD whose spectrum contains a…
An $SU(2)_N$ extension ($N$ stands for neutral) of the Standard Model (SM) is proposed with an additional $U(1)=S^{'}$ global symmetry, which stabilizes the lightest of the vector boson ($X,\bar{X}$) as dark matter (DM) through unbroken…
We study models of supersymmetric grand unification based on the SO(10) gauge group. We investigate scenarios of non-universal gaugino masses including models containing a mixture of two representations of hidden sector chiral superfields.…
An additional $Z_6$ symmetry hidden in the fermion and Higgs sectors of the Standard Model has been found recently. It has a singular nature and is connected to the centers of the SU(3) and SU(2) subgroups of the gauge group. A lattice…
A five dimensional $SU(6)$ grand gauge-Higgs unification compactified on $S^1/Z_2$ is discussed. We propose new sets of the $SU(6)$ representations where the quarks and leptons in one generation are embedded and there is no extra massless…
We carry a systematic study of possible extensions of the standard model based on the gauge group SU(3)_c X SU(4)_L X U(1)_X. We consider both models with particles with exotic electric charges and models which do not contain exotic…
We show that chiral matters in the minimal supersymmetric standard model (MSSM) with semi-realistic flavor structures can be obtained form 7-brane configurations of magnetized super Yang-Mills (SYM) theory on a toroidal orbifold $R^{1,3}…
There is good support for the embedding of the Standard Model fermions in the chiral 16 representation of SO(10). Such an embedding is provided by the realistic free fermionic heterotic-string models. In this talk we demonstrate the…
We give a classification of 3-family SO(10) and E_6 grand unification in string theory within the framework of conformal field theory and asymmetric orbifolds. We argue that the construction of such models in the heterotic string theory…
We propose low energy 6-dimensional N=2 supersymmetric SU(6) models on $M^4\times T^2/(Z_2)^3$ and $M^4\times T^2/(Z_2)^4$, where the orbifold $SU(3)_C\times SU(3)$ model can be embedded on the boundary 4-brane. For the zero modes, the…
We provide a simple solution to the $\mu/B_\mu$ problem in the gauge-mediated Next-to-Minimal Supersymmetric Standard Model. In this model the messenger sector contains one pair of $3+\bar 3$ and one pair of $2+\bar 2$ messengers. These two…
We perform a model-independent analysis of the spontaneously broken phase of an $SU(2)\times U(1)$ supersymmetric gauge theory, by using a non-linear parametrization of the Goldstone sector of the theory. The non-linear variables correspond…