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Z. Chacko

Publications and source records attributed to Z. Chacko.

48 records · Page 3Linked to original sources
arXiv1999

Gaugino Mediated Supersymmetry Breaking

We consider supersymmetric theories where the standard-model quark and lepton fields are localized on a "3-brane" in extra dimensions, while the gauge and Higgs fields propagate in the bulk. If supersymmetry is broken on another 3-brane, supersymmetry breaking is communicated to gauge and Higgs fields by direct higher-dimension interactions, and to quark and lepton fields via standard-model loops. We show that this gives rise to a realistic and predictive model for supersymmetry breaking. The size of the extra dimensions is required to be of order 10-100 times larger than fundamental scale (e.g. the string scale). The spectrum is similar to (but distinguishable from) the predictions of "no-scale" models. Flavor-changing neutral currents are naturally suppressed. The μterm can be generated by the Giudice-Masiero mechanism. The supersymmetric CP problem is naturally solved if CP violation occurs only on the observable sector 3-brane. These are the simplest models in the literature that solve all supersymmetric naturalness problems.

hep-ph↗
arXiv1999

Massive Higher-dimensional Gauge Fields as Messengers of Supersymmetry Breaking

We consider theories with one or more compact dimensions with size r > 1/M, where M is the fundamental Planck scale, with the visible and hidden sectors localized on spatially separated "3-branes." We show that a bulk U(1) gauge field spontaneously broken on the hidden-sector 3-brane is an attractive candidate for the messenger of supersymmetry breaking. In this scenario scalar mass-squared terms are proportional to U(1) charges, and therefore naturally conserve flavor. Arbitrary flavor violation at the Planck scale gives rise to exponentially suppressed flavor violation at low energies. Gaugino masses can be generated if the standard gauge fields propagate in the bulk; μand Bμterms can be generated by the Giudice-Masiero or by the VEV of a singlet in the visible sector. The latter case naturally solves the SUSY CP problem. Realistic phenomenology can be obtained either if all microscopic parameters are order one in units of M, or if the theory is strongly coupled at the scale M. (For the latter case, we estimate parameters by extending "naive dimensional analysis" to higher-dimension theories with branes.) In either case, the only unexplained hierarchy is the "large" size of the extra dimensions in fundamental units, which need only be an order of magnitude. All soft masses are naturally within an order of magnitude of m_{3/2}, and trilinear scalar couplings are negligible. Squark and slepton masses can naturally unify even in the absence of grand unification.

hep-ph↗
arXiv1999

Sterile Neutrinos in E_6 and a Natural Understanding of Vacuum Oscillation Solution to the Solar Neutrino Puzzle

If Nature has chosen the vacuum oscillation solution to the Solar neutrino puzzle, a key theoretical challenge is to understand the extreme smallness of the $Δm^2_{ν_e-ν_X}$ ($\sim 10^{-10} eV^2$) required for the purpose. We find that in a class of models such as [SU(3)]^3 or its parent group E_6, which contain one sterile neutrino, $ν_{is}$ for each family, the $Δm^2_{ν_i-ν_{is}}$ is proportional to the cube of the lepton Yukawa coupling. Therefore fitting the atmospheric neutrino data then predicts the $ν_e-ν_{es}$ mass difference square to be $\sim (\frac{m_e}{m_μ})^3 Δm^2_{atmos}$, where the atmospheric neutrino data is assumed to be solved via the $ν_μ-ν_{μs}$ oscillation. This provides a natural explanation of the vacuum oscillation solution to the solar neutrino problem.

hep-ph↗
arXiv1999

Realistic Anomaly-mediated Supersymmetry Breaking

We consider supersymmetry breaking communicated entirely by the superconformal anomaly in supergravity. This scenario is naturally realized if supersymmetry is broken in a hidden sector whose couplings to the observable sector are suppressed by more than powers of the Planck scale, as occurs if supersymmetry is broken in a parallel universe living in extra dimensions. This scenario is extremely predictive: soft supersymmetry breaking couplings are completely determined by anomalous dimensions in the effective theory at the weak scale. Gaugino and scalar masses are naturally of the same order, and flavor-changing neutral currents are automatically suppressed. The most glaring problem with this scenario is that slepton masses are negative in the minimal supersymmetric standard model. We point out that this problem can be simply solved by coupling extra Higgs doublets to the leptons. Lepton flavor-changing neutral currents can be naturally avoided by approximate symmetries. We also describe more speculative solutions involving compositeness near the weak scale. We then turn to electroweak symmetry breaking. Adding an explicit μterm gives a value for Bμthat is too large by a factor of order 100. We construct a realistic model in which the μterm arises from the vacuum expectation value of a singlet field, so all weak-scale masses are directly related to m_{3/2}. We show that fully realistic electroweak symmetry breaking can occur in this model with moderate fine-tuning.

hep-ph↗
arXiv1998

A New Doublet-Triplet Splitting Mechanism for Supersymmetric SO(10) and Implications for Fermion Masses

We present a new mechanism for doublet-triplet splitting in supersymmetric SO(10) models using a missing vev pattern which is different from the one used in the currently popular Dimopoulos-Wilczek method. In our method, the doublets in a {$\bf 16,\bar{16}$} pair are the ones split from the rest of the multiplet and are then mixed with the doublets from one or two {\bf 10}'s giving rise to the doublets {$\bf H_u$} and {$\bf H_d$} of the standard model. This approach provides a natural way to understand why top quark is so much heavier than the bottom quark. It also enables us to generate both hierarchical and nonhierarchical pattern for neutrino masses, the latter being of interest if neutrino is the hot component of the dark matter of the universe. We construct a simple, realistic model based on this idea. The model uses only simple representations, has no unwanted flat directions and maintains coupling constant unification as a prediction.

hep-ph↗
arXiv1998

Calculable Dynamical Supersymmetry Breaking on Deformed Moduli Spaces

We consider models of dynamical supersymmetry breaking in which the extremization of a tree-level superpotential conflicts with a quantum constraint. We show that in such models the low-energy effective theory near the origin of moduli space is an O'Raifeartaigh model, and the sign of the mass-squared for the pseudo-flat direction at the origin is calculable. We analyze vector-like models with gauge groups SU(N) and Sp(2N) with and without global symmetries. In all cases there is a stable minimum at the origin with an unbroken U(1)_R symmetry.

hep-th↗
arXiv1998

Doublet-Triplet Splitting in Supersymmetric SU(6) by Missing VEV Mechanism

We present a realistic supersymmetric SU(6) model which implements doublet-triplet splitting by the missing vev mechanism. The model makes use of only the simplest representations, requires no fine tuning of parameters and maintains coupling constant unification as a prediction. Fermion masses also emerge in a very straightforward manner. This is the first time that the missing vev mechanism has been realized in the context of SU(6).

hep-ph↗
arXiv1998

Economical Doublet-Triplet Splitting and Strong Suppression of Proton Decay in SO(10)

We present a new approach to realizing the Dimopoulos-Wiczek mechanism for doublet-triplet splitting in supersymmetric SO(10). The method can be used to achieve strong suppression of proton decay in a straightforward manner and is relatively economical; in particular the particle spectrum required for its implementation is consistent with the constraints from string compactification at Kac-Moody level two. We construct two examples of realistic models based on this idea. These models have no unwanted flat directions and do not make use of any nonrenormalizable operators to stabilize the vacuum thereby maintaining coupling constant unification as a prediction. The first model is characterised by a simple discrete symmetry that guarantees naturalness at the renormalizable level while also eliminating all potentially dangerous R-parity violating effects. The second model predicts the strong suppression of Higgsino mediated proton decay. In both models the light MSSM doublets emerge from two different {\bf 10}'s so that there is considerable flexibility in constructing realistic fermion masses and CKM angles.

hep-ph↗
arXiv1998

Dynamical determination of the unification scale by gauge-mediated supersymmetry breaking

We propose a mechanism for generating the GUT scale dynamically from the Planck scale. The idea is that the GUT scale is fixed by the vacuum expectation value of a "GUT modulus" field whose potential is exactly flat in the supersymmetric limit. If supersymmetry is broken by gauge mediation, a potential for the GUT modulus is generated at 2 loops, and slopes away from the origin for a wide range of parameters. This potential is stabilized by Planck-suppressed operators in the Kahler potential, and the GUT scale is fixed to be of order M_* / (4π^2) (where M_* ~ 10^{18} GeV is the reduced Planck scale) independently of the supersymmetry breaking scale. The cosmology of this scenario is acceptable if there is an epoch of inflation with reheat temperature small compared to the supersymmetry-breaking scale. We construct a realistic GUT that realizes these ideas. The model is based on the gauge group SU(6), and solves the doublet-triplet splitting problem by a sliding singlet mechanism. The GUT sector contains no dimensionful couplings or tuned parameters, and all mass scales other than the Planck scale are generated dynamically. This model can be viewed as a realistic implementation of the inverted hierarchy mechanism.

hep-ph↗
arXiv1998

Supersymmetric $SU(2)_L\times SU(2)_R \times SU(4)_c$ and observable neutron-antineutron oscillation

We show that in a large class of supersymmetric $SU(2)_L\times SU(2)_R\times SU(4)_c$ models with the see-saw mechanism for neutrino masses and an R-parity conserving vacuum, there are diquark Higgs bosons with masses ($M_{qq}$) near the weak scale even though the scale of $SU(2)_R\times SU(4)_c$ symmetry breaking is around $10^{10}$ GeV. This happens because these masses ($M_{qq}$) arise out of higher dimensional operators needed to stabilize the charge conserving vacuum in the model. This feature has the interesting implication that the $ΔB=2$ processes such as neutron-anti-neutron oscillation can have observable rates while at the same time yielding neutrino masses in the range of current interest.

hep-ph↗
arXiv1997

Sparticle Spectroscopy and Phenomenology in a New Class of Gauge Mediated Supersymmetry Breaking Models

Recently, a proposal (by R.N.M. and S.N.) was made for a new class of gauge mediated supersymmetry breaking (GMSB) models where the standard model gauge group is embedded into the gauge group $SU(2)_L\times U(1)_{I_{3R}}\times U(1)_{B-L}$ (or $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$) at the supersymmetry breaking scale $Λ$. Supersymmetry breaking is transmitted to the visible sector via the same fields that are responsible for gauge symmetry breaking rather than by vector-like quarks and leptons as in the conventional GMSB models. These models have a number of attractive properties such as exact R-parity conservation, non-vanishing neutrino masses and a solution to the SUSYCP (and strong CP) problem. In this paper, we present the detailed sparticle spectroscopy and phenomenological implications of the various models of this class that embody the general spirit of our previous work but use a larger variety of messenger fields. A distinct characteristic of this class of models is that unlike the conventional GMSB ones, the lightest neutralino is always the NLSP leading to photonic events in the colliders.

hep-ph↗