We investigate the impact of TeV-scale matter belonging to complete vectorlike multiplets of unified groups on the lightest Higgs boson in the MSSM. We find that consistent with perturbative unification and electroweak precision data the mass m h can be as large as 160 GeV. These extended MSSM models can also render the little hierarchy problem less severe, but only for lower values of m h ( 125) GeV. We present estimates for the sparticle mass spectrum in these models.

We reconsider the constraints on Universal Extra Dimensions (UED) models arising from precision electroweak data. We take into account the subleading contributions from new physics (expressed in terms of the X, Y . . . variables), as well as two loop corrections to the Standard Model ρ parameter. For the case of one extra dimension, we obtain a lower bound on the inverse compactification scale M = R −1 of 600 GeV (at 90% confidence level), with a Higgs mass of 115 GeV. However, in contradiction to recent claims, we find that this constraint is significantly relaxed with increasing Higgs mass, allowing for compactification scales as low as 300 GeV. LEP II data does not affect significantly these results. * On a leave of absence from:

We explore the sparticle and Higgs spectroscopy of an SU(5) inspired extension of the constrained minimal supersymmetric standard model (CMSSM). The universal soft parameter m 0 is replaced by m5 and m 10 , where m5 and m 10 denote universal soft scalar masses associated with fields in the five and ten dimensional representations of SU(5). The special case m5 ≪ m 10 yields a rather characteristic sparticle spectroscopy which can be tested at the LHC. We highlight a few benchmark points in which the lightest neutralino saturates the WMAP bound on cold dark matter abundance.

A new SO(10) unified model is proposed based on a one step breaking of SO(10) to the Standard Model gauge group SU(3) C × SU(2) L × U(1) Y using a single 144 of Higgs. The symmetry breaking occurs when the SU(5) 24-plet component of 144 develops a vacuum expectation value. Further, it is possible to obtain from the same 144 a light Higgs doublet necessary for electro-weak symmetry breaking using recent ideas of string vacua landscapes and fine tuning. Thus the breaking of SO(10) down to SU(3) C ×U(1) em can be accomplished with a single Higgs. We analyze this symmetry breaking pattern in the nonsupersymmetric as well as in the supersymmetric SO(10) model. In this scenario masses of the quarks and leptons arise via quartic couplings. We show that the resulting mass pattern is consistent with experimental data, including neutrino oscillations. The model represents an alternative to the currently popular grand unified scenarios.

We show that the Standard Model Lagrangian, including small neutrino masses, has an anomaly-free discrete Z 6 symmetry. This symmetry can emerge naturally from (I 3 R + L i + L j − 2L k ) gauge symmetry (L i is the ith lepton number) and ensure the stability of the nucleon even when the threshold of new physics Λ is low. All ∆B = 1 and ∆B = 2 (B is the baryon number) effective operators are forbidden by the Z 6 symmetry. ∆B = 3 operators are allowed, but they arise only at dimension 15. We estimate the lifetime for "triple nucleon decay" resulting from these operators and find that Λ can be as low as 10 2 GeV. We suggest a simple mechanism for realizing reasonable neutrino masses and mixings even with such a low scale for Λ.

We consider the unification of gauge, Higgs as well as the matter fields in a 6D N = 2 supersymmetric SU(8) gauge theory. The gauge symmetry SU(8) is broken down to SU(4) × SU(2) L × SU(2) R × U(1) 2 in 4D through T 2 /Z 6 orbifold compactification, and the theory is reduced to 4D N = 1 supersymmetric Pati-Salam model. The electroweak Higgs fields as well as the third family of fermions are unified in the 6D N = 2 gauge multiplet. The 6D bulk gauge interaction provides both gauge and Yukawa interactions for the third family predicting α 1 = α 2 = α 3 = α t = α b = α τ at the unification scale, in good agreement with experiment. Incorporation of the first and second family as well as other orbifolds are also briefly discussed.

In the minimal supersymmetric Standard Model with seesaw neutrino masses we show how R-parity can emerge naturally as a discrete gauge symmetry. The same discrete symmetry explains the smallness of the µ-term (the Higgsino mass parameter) via the Giudice-Masiero mechanism. The discrete gauge anomalies are cancelled by a discrete version of the Green-Schwarz mechanism. The simplest symmetry group is found to be Z 4 with a charge assignment that is compatible with grand unification. Several other Z N gauge symmetries are found for N = 10, 12, 18, 36 etc, with some models employing discrete anomaly cancellation at higher Kac-Moody levels. Allowing for a flavor structure in Z N , we show that the same gauge symmetry can also explain the observed hierarchy in the fermion masses and mixings.

The axion solution to the strong CP problem makes use of a global Peccei-Quinn U (1) symmetry which is susceptible to violations from quantum gravitational effects. We show how discrete gauge symmetries can protect the axion from such violations. PQ symmetry emerges as an approximate global symmetry from discrete gauge symmetries. Simple models based on Z N symmetries with N = 11, 12, etc are presented realizing the DFSZ axion and the KSVZ axion. The discrete gauge anomalies are cancelled by a discrete version of the Green-Schwarz mechanism. In the supersymmetric extension our models provide a natural link between the SUSY breaking scale, the axion scale, and the SUSY-preserving µ term.