Minimal Supersymmetric Seesaw Model Research Articles

OBSERVABLE ELECTRON EDM AND LEPTOGENESIS

In the context of the minimal supersymmetric seesaw model, the CP-violating neutrino Yukawa couplings might induce an electron electric dipole moment (EDM). The same interactions may also be responsible for the generation of the observed baryon asymmetry of the universe via leptogenesis. In the framework of mSUGRA, we identify in a model-independent way those patterns within the seesaw models which predict, barring unnatural cancelations, an electron EDM at a level probed by planned laboratory experiments and show that negative searches on τ→eγ decay may provide the strongest upper bound on the electron EDM. We also conclude that a possible future detection of the electron EDM is incompatible with thermal leptogenesis, even when flavor effects are accounted for.

Constraints on SUSY seesaw parameters from leptogenesis and lepton flavor violation

We study the constraints on a minimal supersymmetric seesaw model imposed by neutrino data, charged lepton flavor violation, thermal leptogenesis, and perturbativity. We show that it is possible to constrain the three heavy Majorana neutrino masses as well as the complex Yukawa coupling matrix. Our results provide a first step towards a seesaw benchmark model for further phenomenological studies and model building.

Sneutrino brane inflation and leptogenesis

Modifications to the Friedmann equation in brane cosmology can have important implications for early universe phenomena such as inflation and the generation of the baryon asymmetry. We study a simple scenario of chaotic brane inflation where, in a minimal supersymmetric seesaw model, the scalar superpartner of a heavy singlet Majorana neutrino drives inflation and, simultaneously, generates the required lepton asymmetry through its direct out-of-equilibrium decays after the inflationary era. For a gravitino mass in the range 100 GeV - 1 TeV, we find that successful nucleosynthesis and leptogenesis in this framework require that the 5D Planck mass is in the range M5 ~ 10^10 - 10^13 GeV and the reheating temperature Trh ~ 10^6 - 10^8 GeV.

New parametrization of the seesaw mechanism and applications in supersymmetric models

We present a new parametrization of the minimal seesaw model, expressing the heavy-singlet neutrino Dirac Yukawa couplings ${(Y}_{\ensuremath{\nu}}{)}_{\mathrm{ij}}$ and Majorana masses ${M}_{{N}_{i}}$ in terms of effective light-neutrino observables and an auxiliary Hermitian matrix H. In the minimal supersymmetric version of the seesaw model, the latter can be related directly to other low-energy observables, including processes that violate charged lepton flavor and $\mathrm{CP}.$ This parametrization enables one to respect the stringent constraints on muon-number violation while studying the possible ranges for other observables by scanning over the allowed parameter space of the model. Conversely, if any of the lepton-flavor-violating process is observed, this measurement can be used directly to constrain ${(Y}_{\ensuremath{\nu}}{)}_{\mathrm{ij}}$ and ${M}_{{N}_{i}}.$ As applications, we study flavor-violating $\ensuremath{\tau}$ decays and the electric dipole moments of leptons in the minimal supersymmetric seesaw model.

Observable consequences of partially degenerate leptogenesis

In the context of the seesaw mechanism, it is natural that the large solar and atmospheric neutrino mixing angles originate separately from large 2×2 mixings in the neutrino and charged-lepton sectors, respectively, and large mixing in the neutrino couplings is in turn more plausible if two of the heavy singlet neutrinos are nearly degenerate. We study the phenomenology of this scenario, calculating leptogenesis by solving numerically the set of coupled Boltzmann equations for out-of-equilibrium heavy singlet neutrino decays in the minimal supersymmetric seesaw model. The near-degenerate neutrinos may weigh ≲108 GeV, avoiding the cosmological gravitino problem. This scenario predicts that Br(μ→eγ) should be strongly suppressed, because of the small singlet neutrino masses, whilst Br(τ→μγ) may be large enough to be observable in B-factory or LHC experiments. If the light neutrino masses are hierarchical, we predict that the neutrinoless double-β decay parameter mee≈Δmsol2sin2θ12.

Leptogenesis and the violation of lepton number and CP at low energies

In the context of the minimal supersymmetric seesaw model, we study the implications of the current neutrino data for thermal leptogenesis, ββ 0 ν decay, and leptonic flavour- and CP-violating low-energy observables. We express the heavy singlet-neutrino Dirac–Yukawa couplings ( Y ν ) ij and Majorana masses M N i in terms of the light-neutrino observables and an auxiliary Hermitian matrix H, which enables us to scan systematically over the allowed parameter space. If the lightest heavy neutrino N 1 decays induce the baryon asymmetry, there are correlations between the M N 1 , the lightest active neutrino mass and the primordial lepton asymmetry ϵ 1 on the one hand, and the ββ 0 ν decay parameter m ee on the other hand. However, leptogenesis is insensitive to the neutrino oscillation phase. We find lower bounds M N 1 ≳10 10 GeV for the normal light-neutrino mass hierarchy, and M N 1 ≳10 11 GeV for the inverted mass hierarchy, respectively, indicating a potentially serious conflict with the gravitino problem. Depending on M N 1 , we find upper (upper and lower bounds) on the lightest active neutrino mass for the normal (inverted) mass hierarchy, and a lower bound on m ee even for the normal mass ordering. The low-energy lepton-flavour- and CP-violating observables induced by renormalization are almost independent of leptogenesis. The electron–electric dipole moment may be close to the present bound, reaching d e∼10 −(27−28) e cm in our numerical examples, while d μ may reach d μ∼10 −25 e cm.

CP violation in the minimal supersymmetric seesaw model

We study CP violation in the lepton sector of the supersymmetric extension of the Standard Model with three generations of massive singlet neutrinos with Yukawa couplings Y ν to lepton doublets, in a minimal seesaw model for light neutrino masses and mixing. This model contains six physical CP-violating parameters, namely the phase δ observable in oscillations between light neutrino species, two Majorana phases φ 1,2 that affect ββ 0 ν decays, and three independent phases appearing in Y νY ν † , that control the rate of leptogenesis. Renormalization of the soft supersymmetry-breaking parameters induces observable CP violation at low energies, including T-odd asymmetries in polarized μ→ eee and τ→ℓℓℓ decays, as well as lepton electric dipole moments. In the leading-logarithmic approximation in which the massive singlet neutrinos are treated as degenerate, these low-energy observables are sensitive via Y ν †Y ν to just one combination of the leptogenesis and light-neutrino phases. We present numerical results for the T-odd asymmetry in polarized μ→ eee decay, which may be accessible to experiment, but the lepton electric dipole moments are very small in this approximation. To the extent that the massive singlet neutrinos are not degenerate, low-energy observables become sensitive also to two other combinations of leptogenesis and light-neutrino phases, in this minimal supersymmetric seesaw model.