Minimal SU Research Articles

Operator analysis for proton decay in supersymmetric SO(10) GUT models

Nonrenormalizable operators both account for the failure of down quark and charged lepton Yukawa couplings to unify and reduce the proton decay rate via dimension-five operators in minimal SUSY SU(5) GUT. We extend the analysis to SUSY SO(10) GUT models.

PROTON DECAY IN SUPERSYMMETRIC GUT MODELS

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.

Implications of constraints on mass parameters in the Higgs sector of the nonlinear supersymmetric SU(5) model

The Higgs sector of the minimal nonlinear supersymmetric SU(5) model contains three mass parameters. Although these mass parameters are essentially free at the electroweak scale, they might have particular values if they evolve from a particular constraints at the GUT scale through the RG equations. By assuming a number of simple constraints on these mass parameters at the GUT scale, we obtain their values at the electroweak scale through the RG equations in order to investigate the phenomenological implications. Some of them are found to be consistent with the present experimental data.

Gravity-assisted exact unification in minimal supersymmetric SU(5) and its gaugino mass spectrum

Minimal supersymmetric SU(5) with exact unification is naively inconsistent with proton decay constraints. However, it can be made viable by a gravity-induced non-renormalizable operator connecting the adjoint Higgs boson and adjoint vector boson representations. We compute the allowed coupling space for this theory and find natural compatibility with proton decay constraints even for relatively light superpartner masses. The modifications away from the naive SU(5) theory have an impact on the gaugino mass spectrum, which we calculate. A combination of precision linear collider and large hadron collider measurements of superpartner masses would enable interesting tests of the high-scale form of minimal supersymmetric SU(5).

Higgs bosons in the RG-improved minimal nonlinear supersymmetric SU(5) model: their mass bounds and productions at e+e− colliders

The renormalization group equations of the Higgs sector of the minimal nonlinear supersymmetric SU(5) model are derived and numerically solved in the scheme. Evolving the parameters of the model from the GUT scale down to the electroweak scale, the allowed regions of the parameters are determined, in particular the quartic coupling constant λ. The mass bounds, corrections to tree-level mass sum rules and productions of the Higgs bosons at e+e− colliders are investigated for the c.m. energy of LEP1, LEP2 and future linear colliders up to 2000 GeV.

Mass predictions based on a supersymmetric SU(5) fixed point

I examine the possibility that the third generation fermion masses are determined by an exact fixed point of the minimal supersymmetric SU(5) model. When one-loop supersymmetric thresholds are included, this unified fixed point successfully predicts the top quark mass, $175\ifmmode\pm\else\textpm\fi{}2\mathrm{GeV},$ as well as the weak mixing angle. The bottom quark mass prediction is sensitive to the supersymmetric thresholds; it approaches the measured value for $\ensuremath{\mu}<0$ and very large unified gaugino mass. The experimental measurement of the tau lepton mass determines $\mathrm{tan}\ensuremath{\beta},$ and the strong gauge coupling and fine structure constant fix the unification scale and the unified gauge coupling.

Nonanomalous discreteRsymmetry and a light gravitino

We discuss nonanomalous R-symmetry in the supersymmetric grand unified theories. In particular, we explore anomaly-free solutions predicting the gravitino mass in the range of $10^{-3} eV \lsim m_{3/2} \lsim 1 TeV$ when the $\mu$-parameter is fixed to be $\mu \simeq 1 TeV$. In the minimal SU(5) GUT, we have shown that $\mu \simeq 1 TeV$ is obtained only if the gravitino is ultralight with mass $m_{3/2} \sim 10^{-3} eV$. If extra fields ${\bf 5}\oplus{\bf 5^*}$ or ${\bf 10}\oplus{\bf 10^*}$ are introduced, many solutions predicting $m_{3/2} \gsim 10^{-3} eV$ are found. The R-parity is violated due to the vacuum expectation value of the superpotential, but it is controlled by the discrete R-symmetry. We find that the R-parity violating couplings are naturally suppressed much below the experimental bounds for some charge assignments. These charge assignments predict light gravitino with masses of order ${\cal O}(10^{-3} eV)$--${\cal O}(1 MeV)$. These discrete R-symmetries can be considered as solutions to the $\mu$-problem in low energy supersymmetry breaking models such as the gauge mediation.

Proton decay in a consistent supersymmetric [formula omitted] GUT model

It is widely believed that minimal supersymmetric SU(5) GUTs have been excluded by the SuperKamiokande bound for the proton decay rate. However, in the minimal model, the theoretical prediction assumes unification of Yukawa couplings, Y d = Y e , which is known to be badly violated. We analyze the implications of this fact for the proton decay rate. In a consistent SU(5) model with higher dimensional operators, where SU(5) relations among Yukawa couplings hold, the proton decay rate can be several orders of magnitude smaller than the present experimental bound.

Gauge coupling unification in grand unified theories with anomalous U1 symmetry.

We show that in the framework of grand unified theory (GUT) with anomalous U(1)(A) gauge symmetry, the success of the gauge coupling unification in the minimal SU(5) GUT is naturally explained, even if the mass spectrum of superheavy fields does not respect SU(5) symmetry. Because the unification scale for most realizations of the theory becomes smaller than the usual GUT scale, it suggests that the present level of experiments is close to that sufficient to observe proton decay via dimension 6 operators, p-->e+pi.

LMA MSW solution in the minimal SU(3) L×U(1) N gauge model

The minimal SU(3) L×U(1) N gauge model for (ν,ℓ,ℓ C) equipped with the (approximate) L e−L μ−L τ symmetry and a discrete Z 4 symmetry is found to provide radiative neutrino masses compatible with the LMA MSW solution.

Unification of Higgs and gauge fields in five dimensions

We construct realistic theories in which the Higgs fields arise from extra-dimensional components of higher-dimensional gauge fields. In particular, we present a minimal 5D SU(3) C × SU(3) W model and a unified 5D SU(6) model. In both cases the theory is reduced to the minimal supersymmetric standard model below the compactification scale, with the two Higgs doublets arising from the 5D gauge multiplet. Quarks and leptons are introduced in the bulk, giving Yukawa couplings without conflicting with higher-dimensional gauge invariance. Despite the fact that they arise from higher-dimensional gauge interactions, the sizes of these Yukawa couplings can be different from the 4D gauge couplings due to wave-function profiles of the matter zero modes determined by bulk mass parameters. All unwanted fields are made heavy by introducing appropriate matter and superpotentials on branes, which are also the source of intergenerational mixings in the low-energy Yukawa matrices. The theory can accommodate a realistic structure for the Yukawa couplings as well as small neutrino masses. Scenarios for supersymmetry breaking and the μ-term generation are also discussed.

Atmospheric neutrino mixing and b–τ unification

Extrapolating the τ and b masses in the MSSM tends to give for their ratio at the GUT scale a number around 1.2, for most viable values tanβ, rather than the minimal SU(5) prediction of 1. We suggest that this may be due to large off-diagonal elements in the charged lepton mass matrix ML that can also explain the large atmospheric neutrino mixing angle. Several simple models with definite predictions for mτ/mb(MGUT) are presented.

Flipping SU(5) out of trouble

Minimal supersymmetric SU(5) GUTs are being squeezed by the recent values of αs, sin2θW, the lower limit on the lifetime for p→ν̄K decay, and other experimental data. We show how the minimal flipped SU(5) GUT survives these perils, accommodating the experimental values of αs and sin2θW and other constraints, while yielding a p→e/μ+π0 lifetime beyond the present experimental limit but potentially accessible to a further round of experiments. We exemplify our analysis using a set of benchmark supersymmetric scenarios proposed recently in a constrained MSSM framework.

Proton decay in minimal supersymmetric SU(5)

We systematically study proton decay in the minimal supersymmetric SU(5) grand unified theory. We find that although the available parameter space of soft masses and mixings is quite constrained, the theory is still in accord with experiment.

U(1) flavor symmetry and proton decay in supersymmetric standard model

Issues of proton decay are examined in supersymmetric standard model with U(1) flavor symmetry. Dimension five proton-decay operators which arise generically are controlled by the flavor symmetry. We show that unlike the minimal supersymmetric SU(5) case the proton decay modes containing charged lepton can have large branching ratios if the dimension five operators of left-handed type dominate. Measuring the branching ratio of the electron mode to the muon mode may reveal the mechanism of the neutrino mass generation. The case with vanishing charges for Higgs doublets marginally survives the present experimental bound on proton life time.

Not even decoupling can save the minimal supersymmetric SU(5) model

We make explicit the statement that Minimal Supersymmetric SU(5) has been excluded by the Super-Kamiokande search for the process $p \to K^{+} \overline{\nu}$. This exclusion is made by first placing limits on the colored Higgs triplet mass, by forcing the gauge couplings to unify. We also show that taking the superpartners of the first two generations to be very heavy in order to avoid flavor changing neutral currents, the so-called ``decoupling'' idea, is insufficient to resurrect the Minimal SUSY SU(5). We comment on various mechanisms to further suppress proton decay in SUSY SU(5). Finally, we address the contributions to proton decay from gauge boson exchange in the Minimal SUSY SU(5) and flipped SU(5) models.

On the stability of the classicalvacua in a minimal SU(5) five-dimensional supergravity model

We consider a five-dimensional supergravity model with SU(5) gaugesymmetry and the minimal field content. Studying the arising scalarpotential we find that the gauging of the U(1)R symmetry of thefive-dimensional supergravity causes instabilities. Lifting theinstabilities, the vacua are of anti-de-Sitter type and SU(5) isbroken along with supersymmetry. Keeping the U(1)R ungauged thepotential has flat directions along which supersymmetry is unbroken.

Realistic supersymmetric SU(6)

We present an example of SUSY SU(6) GUT, which predicts an excellent value for $\al_s(M_Z)(\simeq 0.119)$, in comparison with the value $\al_s^0(M_Z)\simeq 0.126$ of minimal SUSY SU(5). A crucial role is played by the vectorlike multiplets from the matter sector, whose masses lie below the GUT scale. For a realistic pattern of fermion masses, the adjoint scalar of SU(6) has VEV along the $SU(4)\times SU(2)\times U(1)$ direction. This also offers a natural resolution of the doublet-triplet (DT) splitting through the pseudo Goldstone Boson mechanism.

Semi-simple unification on / orientifold in the type IIB supergravity

The semi-simple unification model based on SU(5)_GUT \times U(3)_H gauge group is an interesting extension of the minimal SU(5)_GUT grand unification theory (GUT), since it solves the two serious problems in the standard GUT: the triplet-doublet splitting problem and the presence of dangerous dimension five operators for proton decay. Here, the extra U(3)_H gauge interaction plays a crucial role on the GUT breaking. In this paper, we show that the full multiplet structure of the U(3)_H sector required for the desired GUT breaking is reproduced naturally on T^6/Z_12 orientifold in the type IIB supergravity with a D3-D7 system. The SU(5)_GUT vector multiplet lives on D7-branes and the U(3)_H sector resides on D3-branes. We also show that various interesting features in the original SU(5)_GUT \times U(3)_H model are explained in the present brane-world scenario. A possible extension to the type IIB string theory is also discussed.

The interplay between neutrinos and charged leptons in the minimal SU(3) ×U(1) gauge model

In the minimal SU(3)_LxU(1)_N gauge model with a global L_e-L_mu-L_tau (=L') symmetry and a discrete Z_4 symmetry, it is found that the interplay between neutrinos and charged leptons contained in triplets of \psi^i=(\nu^i_L, \ell^i_L, \ell^{ci}_L) (i=1,2,3) naturally leads to the large mixing angle (LMA) MSW solution. The model includes two (anti)sextet Higgs scalars, S^(0) with L'=0 and S^(+) with L'=2, which, respectively, couple to \psi^1\psi^{2,3} for the electron mass and masses of atmospheric neutrinos and to \psi^{2,3}\psi^{2,3} for the \mu- and \tau-masses and one-loop radiative neutrino masses relevant to solar neutrinos. This mechanism is realized by utilizing an additional residual discrete symmetry supplied by explicitly broken L', which guarantees the absence of tree-level neutrino mass terms of the \psi^{2,3}\psi^{2,3}-type. Pure rotation effects due to the diagonalization of neutrino and charged-lepton mass matrices are estimated to yield \Delta m^2_\odot/\Delta m^2_{atm} \leq (m_e/m_\mu)^{3/2}=O(10^{-4}) but the radiative effects supersede the rotation effects to yield \Delta m^2_\odot/\Delta m^2_{atm}=O(10^{-2}) as the LMA solution.