arXiv subjects
Publications and source records attributed to Anna Rossi.
We discuss supersymmetric scenarios in which neutrino masses arise from effective d=6 operators in the Kahler potential (including SUSY-breaking insertions). Simple explicit realizations of those Kahler operators are presented in the context of the type II seesaw. An appealing scenario emerges upon identifying the seesaw mediators with SUSY-breaking messengers.
We investigate supersymmetric scenarios in which neutrino masses are generated by effective d=6 operators in the Kahler potential, rather than by the standard d=5 superpotential operator. First, we discuss some general features of such effective operators, also including SUSY-breaking insertions, and compute the relevant renormalization group equations. Contributions to neutrino masses arise at low energy both at the tree level and through finite threshold corrections. In the second part we present simple explicit realizations in which those Kahler operators arise by integrating out heavy SU(2)_W triplets, as in the type II seesaw. Distinct scenarios emerge, depending on the mechanism and the scale of SUSY-breaking mediation. In particular, we propose an appealing and economical picture in which the heavy seesaw mediators are also messengers of SUSY breaking. In this case, strong correlations exist among neutrino parameters, sparticle and Higgs masses, as well as lepton flavour violating processes. Hence, this scenario can be tested at high-energy colliders, such as the LHC, and at lower energy experiments that measure neutrino parameters or search for rare lepton decays.
We summarize the predictions for the radiative decays l_j->l_i γwithin the context of the supersymmetric type II seesaw mechanism considering universal boundary conditions for the soft SUSY breaking terms. The dependence on the low-energy neutrino parameters is discussed and the deviations from the analytical results for large $\tanβ$ analyzed.
We thoroughly discuss a new supersymmetric grand unified scenario of the triplet seesaw mechanism where the exchange of heavy SU(2)_W triplet states generates both neutrino masses and soft supersymmetry breaking terms. This framework, recently proposed by us in a previous work, is highly predictive since it contains only three free parameters connecting low-energy neutrino parameters, lepton and quark flavour violation, sparticle and Higgs boson spectra and electroweak symmetry breakdown. These three parameters are the triplet mass M_T, the effective supersymmetry breaking scale B_T and a coupling constant lambda. We perform a complete analysis of the parameter space taking into account the present experimental constraints and considering different types of neutrino spectrum. A special emphasis is given to the particular features of the sparticle and Higgs spectra and to the model independent predictions obtained for the processes mu-> e X, mu-> e conversion in nuclei, tau-> e Y and tau-> mu Y (X = gamma, e e, Y= gamma, e e, mu mu). In the appendices, we present some technical aspects relevant for our analysis.
We discuss a GUT realization of the supersymmetric triplet seesaw mechanism(recently proposed by us in hep-ph/0604083 and further analyzed in hep-ph/0607298) where the exchange of the heavy triplet states generates both neutrino masses and soft SUSY breaking terms.
We propose a novel supersymmetric unified scenario of the triplet seesaw mechanism where the exchange of the heavy triplets generates both neutrino masses and soft supersymmetry breaking terms. Our framework is very predictive since it relates neutrino mass parameters, lepton flavour violation in the slepton sector, sparticle and Higgs spectra and electroweak symmetry breakdown. The phenomenological viability and experimental signatures in lepton flavor violating processes are discussed.
The generation of electric dipole moments (EDMs) is addressed in the supersymmetric seesaw scenario realized through the exchange of SU(2)_W triplet states. In particular, we show that the triplet soft-breaking bilinear term can induce finite contributions to lepton and quark EDMs. Moreover, the peculiar flavour structure of the model allows us to predict the EDM ratios d_e/d_mu and d_mu/d_tau only in terms of the neutrino parameters.
We perform a detailed analysis of several mu-tau lepton flavour violating (LFV) processes, namely tau --> mu X (X = gamma, e+ e-, mu+ mu-, rho, pi, eta, eta'), Z --> mu tau and Higgs boson decays into mu tau. First we present a model independent operator analysis relevant to such decays, then we explicitly compute the LFV operator coefficients [and (g_μ-2)] in a general unconstrained MSSM framework, allowing slepton mass matrices to have large mu-tau entries. We systematically study the role and the interplay of dipole and non-dipole operators, showing how the rates and the mutual correlations of those LFV decays change in different regions of the MSSM parameter space.Values of the LFV branching ratios in the experimentally interesting range 10^{-9}-10^{-7} can be achieved. For at least two MSSM Higgs bosons, the branching ratio of the LFV decay into mu tau can reach values of order 10^{-4}.
We present two recent developments on lepton flavour violation in the MSSM. 1) The supersymmetric seesaw mechanism can be realized through the exchange of heavy SU(2)_W-triplet states, rather than `right-handed' neutrinos. In this scenario the ratio of the tau --> mu gamma (or tau --> e gamma) and mu --> e gamma rates are unambiguously predicted in terms of the low-energy neutrino parameters. 2) Lepton flavour violating decays of the Higgs bosons, (h, H, A) --> mu tau, are induced at one-loop level. The branching ratios can reach 10^{-4} for large tan(beta) and sizeable smuon-stau mixing. Interesting implications for LHC and correlations with tau --> mu gamma and tau --> 3 mu are found.
We compute the lepton flavour violating couplings of Higgs bosons in the Minimal Supersymmetric Standard Model, and show that they can induce the decays (h,H,A) --> mu tau at non-negligible rates, for large tan(beta) and sizeable smuon-stau mixing. We also discuss the prospects for detecting such decays at LHC and other colliders, as well as the correlation with other flavour violating processes, such as tau --> mu gamma and tau --> 3 mu.
We consider the supersymmetric seesaw mechanism induced by the exchange of heavy SU(2)_W triplet states, rather than `right-handed' neutrino singlets, to generate neutrino masses. We show that in this scenario the neutrino flavour structure tested at low-energy in the atmospheric and solar neutrino experiments is directly inherited from the neutrino Yukawa couplings to the triplets. This allows us to predict the ratio of the tau --> mu gamma (or tau --> e gamma) and mu --> e gamma decay rates in terms of the low-energy neutrino parameters. Moreover, once the model is embedded in a grand unified model, quark-flavour violation can be linked to lepton-flavour violation.
We provide an effective Lagrangian analysis of contact non-standard interactions of neutrinos with electrons, which can be effectively mediated by extra particles, and examine the associated experimental limits. At present, such interactions are strongly constrained only for $ν_μ$: the bounds are loose for $ν_e$ and absent for $ν_τ$. We emphasize the unique role played by the reaction $e^+e^-\to ν\barνγ$ in providing direct constraints on such non-standard interactions.
The present experimental status does not exclude weak-strength non-standard interactions of neutrinos with electrons. These interactions can be revealed in solar neutrino experiments. Our discussion covers several aspects related to this issue. First, we perform an analysis of the Super Kamiokande and SNO data to investigate their sensitivity to such interactions. In particular, we show that the ν_e oscillation into sterile neutrinos can be still allowed if ν_e has extra interactions of the proper strength. Second, we suggest that the Borexino detector can provide good signatures for these non-standard interactions. Indeed, in Borexino the shape of the recoil electron spectrum from the νe \to νe scattering essentially does not depend on the solar neutrino conversion details, since most of the signal comes from the mono-energetic ^7Be neutrinos. Hence, the partial conversion of solar ν_e into a a nearly equal mixture of ν_μand ν_τ, as is indicated by the atmospheric neutrino data, offers the chance to test extra interactions of ν_τ, or of ν_e itself.
We discuss the role played by the horizontal flavour symmetry in supersymmetric theories. In particular, we consider the horizontal symmetry $SU(3)_H$ between the three fermion families and show how this concept can help in explaining the fermion mass spectrum and mixing pattern in the context of SUSY GUTs.
We propose new textures for the fermion Yukawa matrices which are generalizations of the so-called Stech ansatz. We discuss how these textures can be realized in supersymmetric grand unified models with horizontal symmetry SU(3)_H among the fermion generations. In this framework the mass and mixing hierarchy of fermions (including neutrinos) can emerge in a natural way. We emphasize the central role played by the SU(3)_H adjoint Higgs field which reduces SU(3)_H to U(2)_H at the GUT scale. A complete SO(10)\times SU(3)_H model is presented in which the desired Yukawa textures can be obtained by symmetry reasons. The phenomenological implications of these textures are thoroughly investigated. Among various realistic possibilities for the Clebsch factors between the quark and lepton entries, we find three different solutions which provide excellent fits of the quark masses and CKM mixing angles. Interestingly, all these solutions predict the correct amount of CP violation via the CKM mechanism, and, in addition, lead to an appealing pattern of the neutrino masses and mixing angles. In particular, they all predict nearly maximal 23 mixing and small 12 mixing in the lepton sector, respectively in the range needed for the explanation of the atmospheric and solar neutrino anomaly.
We point out that the interactions of goldstinos with matter supermultiplets are a potential source of flavour violation, if fermion and sfermion mass matrices are not aligned and supersymmetry is spontaneously broken at a low scale. We study the impact of those couplings on low-energy processes such as μ\to e γ, μ\to eee, K \to μ^+ μ^-, K-\ov{K} transitions and analogous ones. Moreover, we address the issue of flavour violation in low-energy processes involving two goldstinos and two matter fermions, generalizing earlier results obtained in the flavour-conserving case.
We show that neutrino oscillations in matter are always accompanied by collective forces on the medium. This effect may produce interesting consequences for the background and the neutrino oscillations themselves. The force is maximal in the case of resonant neutrino conversion in the adiabatic regime. We study here the forces driven by $ν_e-ν_{μ,τ}$ and $ν_e-ν_s$ MSW conversion and shortly discuss their possible relevance for the dynamics of a type II supernova.
The comparison of the CKM mixing angles with the leptonic mixings implied by the recent atmospheric and solar neutrino data exhibits an interesting complementarity. This pattern can be understood in the context of the SU(5) grand unification, assuming that the fermion mass matrices have Fritzsch-like structures but are not necessarily symmetric. (The present contribution is based on the paper in ref. \cite{az}.)