Lepton Flavor Violation and the Origin of the Seesaw Mechanism

The right--handed neutrino mass matrix that is central to the understanding of small neutrino masses via the seesaw mechanism can arise either (i) from renormalizable operators or (ii) from nonrenormalizable or super-renormalizable operators, depending on the symmetries and the Higgs content of the theory beyond the Standard Model. In this paper, we study lepton flavor violating (LFV) effects in the first class of seesaw models wherein the \nu_R Majorana masses arise from renormalizable Yukawa couplings involving a B-L = 2 Higgs field. We present detailed predictions for \tau -> \mu + \gamma and \mu -> e + \gamma branching ratios in these models taking the current neutrino oscillation data into account. Focusing on minimal supergravity models, we find that for a large range of MSSM parameters suggested by the relic abundance of neutralino dark matter and that is consistent with Higgs boson mass and other constraints, these radiative decays are in the range accessible to planned experiments. We compare these predictions with lepton flavor violation in the second class of models arising entirely from the Dirac Yukawa couplings. We study the dependence of the ratio r \equiv B(\mu -> e+\gamma)/B(\tau ->\mu +\gamma) on the MSSM parameters and show that measurement of r can provide crucial insight into the origin of the seesaw mechanism.Comment: 20 pages, Revtex, 7 figure

A 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 $(Y_\nu)_{ij}$ 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 flavour and 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-flavour-violating process is observed, this measurement can be used directly to constrain $(Y_\nu)_{ij}$ and $M_{N_i}.$ As applications, we study flavour-violating $\tau$ decays and the electric dipole moments of leptons in the minimal supersymmetric seesaw model.Comment: Important references adde

Ginzburg-Landau functional for nearly antiferromagnetic perfect and disordered Kondo lattices

Interplay between Kondo effect and trends to antiferromagnetic and spin glass ordering in perfect and disordered bipartite Kondo lattices is considered. Ginzburg-Landau equation is derived from the microscopic effective action written in three mode representation (Kondo screening, antiferromagnetic correlations and spin liquid correlations). The problem of local constraint is resolved by means of Popov-Fedotov representation for localized spin operators. It is shown that the Kondo screening enhances the trend to a spin liquid crossover and suppresses antiferromagnetic ordering in perfect Kondo lattices and spin glass ordering in doped Kondo lattices. The modified Doniach's diagram is constructed, and possibilities of going beyond the mean field approximation are discussed.Comment: 18 pages, RevTeX, 7 EPS figures include

Flavour Violation in SUSY SU(5) GUT at Large tan beta

We study flavour violation in the minimal SUSY SU(5) GUT assuming all the third generation Yukawa couplings to be due to the renormalizable physics above GUT scale. At large $\tan\beta,$ as suggested by Yukawa unification in SU(5), sizable flavour violation in the left (right) slepton (down squark) sector is induced due to renormalization effects of down type Yukawa couplings between GUT and Planck scales in addition to the flavour violation in the right slepton sector. The new flavour physics contribution to $K-\bar K,$ $B-\bar B$ mixing is small but might be of phenomenological interest in the case of $b\to s\gamma.$ The sign of the latter contribution is the same as the sign of the dominant chargino contribution, thus making the constraints on SUSY scale coming from $b\to s\gamma$ somewhat more restrictive. The most important feature of the considered scenario is the large rate of lepton flavour violation. Given the present experimental constraints, the $\mu\to e\gamma$ and $\mu-e$ conversion branching ratios are above the sensitivity of the planned experiments unless the SUSY scale is pushed above one TeV.Comment: 22 pages, 7 figure

Charged lepton Flavor Violation in Supersymmetry with Bilinear R-Parity Violation

The simplest unified extension of the Minimal Supersymmetric Standard Model with bi-linear R-parity violation naturally predicts a hierarchical neutrino mass spectrum, suitable to explain atmospheric and solar neutrino fluxes. We study whether the individual violation of the lepton numbers L_{e,mu,tau} in the charged sector can lead to measurable rates for BR(mu->e gamma)and $BR(tau-> mu gamma). We find that some of the R-parity violating terms that are compatible with the observed atmospheric neutrino oscillations could lead to rates for mu->e gamma measurable in projected experiments. However, the Delta m^2_{12} obtained for those parameters is too high to be compatible with the solar neutrino data, excluding therefore the possibility of having measurable rates for mu->e gamma in the model.Comment: 29 pages, 8 figures. Constraint from solar neutrino data included, conclusions changed respect v

Softening the Supersymmetric Flavor Problem in Orbifold GUTs

The infra-red attractive force of the bulk gauge interactions is applied to soften the supersymmetric flavor problem in the orbifold SU(5) GUT of Kawamura. Then this force aligns in the infra-red regime the soft supersymmetry breaking terms out of their anarchical disorder at a fundamental scale, in such a way that flavor-changing neutral currents as well as dangerous CP-violating phases are suppressed at low energies. It is found that this dynamical alignment is sufficiently good compared with the current experimental bounds, as long as the diagonalization matrices of the Yukawa couplings are CKM-like.Comment: 15 pages,4 figure

Gauge and Scheme Dependence of Mixing Matrix Renormalization

We revisit the issue of mixing matrix renormalization in theories that include Dirac or Majorana fermions. We show how a gauge-variant on-shell renormalized mixing matrix can be related to a manifestly gauge-independent one within a generalized ${\bar {\rm MS}}$ scheme of renormalization. This scheme-dependent relation is a consequence of the fact that in any scheme of renormalization, the gauge-dependent part of the mixing-matrix counterterm is ultra-violet safe and has a pure dispersive form. Employing the unitarity properties of the theory, we can successfully utilize the afore-mentioned scheme-dependent relation to preserve basic global or local symmetries of the bare Lagrangian through the entire process of renormalization. As an immediate application of our study, we derive the gauge-independent renormalization-group equations of mixing matrices in a minimal extension of the Standard Model with isosinglet neutrinos.Comment: 31 pages, LaTeX, uses axodraw.st

Search for W' bosons decaying to an electron and a neutrino with the D0 detector

This Letter describes the search for a new heavy charged gauge boson W' decaying into an electron and a neutrino. The data were collected with the D0 detector at the Fermilab Tevatron proton-antiproton Collider at a center-of-mass energy of 1.96 TeV, and correspond to an integrated luminosity of about 1 inverse femtobarn. Lacking any significant excess in the data in comparison with known processes, an upper limit is set on the production cross section times branching fraction, and a W' boson with mass below 1.00 TeV can be excluded at the 95% C.L., assuming standard-model-like couplings to fermions. This result significantly improves upon previous limits, and is the most stringent to date.Comment: submitted to Phys. Rev. Let

Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV

Results are presented from a search for a W' boson using a dataset corresponding to 5.0 inverse femtobarns of integrated luminosity collected during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV. The W' boson is modeled as a heavy W boson, but different scenarios for the couplings to fermions are considered, involving both left-handed and right-handed chiral projections of the fermions, as well as an arbitrary mixture of the two. The search is performed in the decay channel W' to t b, leading to a final state signature with a single lepton (e, mu), missing transverse energy, and jets, at least one of which is tagged as a b-jet. A W' boson that couples to fermions with the same coupling constant as the W, but to the right-handed rather than left-handed chiral projections, is excluded for masses below 1.85 TeV at the 95% confidence level. For the first time using LHC data, constraints on the W' gauge coupling for a set of left- and right-handed coupling combinations have been placed. These results represent a significant improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe

Search for the associated production of a b quark and a neutral supersymmetric Higgs boson which decays to tau pairs

We report results from a search for production of a neutral Higgs boson in association with a $b$ quark. We search for Higgs decays to $\tau$ pairs with one $\tau$ subsequently decaying to a muon and the other to hadrons. The data correspond to 2.7fb$^{-1}$ of \ppbar collisions recorded by the D0 detector at $\sqrt{s} = 1.96$TeV. The data are found to be consistent with background predictions. The result allows us to exclude a significant region of parameter space of the minimal supersymmetric model.Comment: Submitted to Phys. Rev. Letter