Conversion In Nuclei Research Articles

Minimal flavor violation in the lepton sector

We extend the notion of minimal flavor violation to the lepton sector. We introduce a symmetry principle which allows us to express lepton flavor violation in the charged lepton sector in terms of neutrino masses and mixing angles. We explore the dependence of the rates for flavor changing radiative charged lepton decays ( ℓ i → ℓ j γ ) and μ-to- e conversion in nuclei on the scales for total lepton number violation, lepton flavor violation and the neutrino masses and mixing angles. Measurable rates are obtained when the scale for total lepton number violation is much larger than the scale for lepton flavor violation.

ResonantτLeptogenesis with Observable Lepton Number Violation

We consider a minimal extension of the standard model with one singlet neutrino per generation that can realize resonant leptogenesis at the electroweak scale. In particular, the baryon asymmetry in the Universe can be created by lepton-to-baryon conversion of an individual lepton number, for example, that of the tau lepton. The current neutrino data can be explained by a simple CP-violating Yukawa texture. The model has several testable phenomenological implications. It contains heavy Majorana neutrinos at the electroweak scale, which can be probed at e+ e- linear colliders, and predicts e- and mu-lepton-number-violating processes, such as 0nu betabeta decay, mu --> e gamma, and mu-e conversion in nuclei, with rates that are within reach of experimental sensitivity.

NEUTRINOLESS DOUBLE BETA DECAY AND LEPTON FLAVOR VIOLATION

I argue that muon lepton flavor violating (LFV) processes (such as μ → eγ and μ → e conversion in nuclei) provide a useful diagnostic tool to determine whether or not the effective neutrino mass can be safely extracted from the rate of neutrinoless double beta decay.

Lepton flavor violation in extra dimension models

Models involving large extra spatial dimension(s) have interesting predictions on lepton flavor violating processes. We consider some five-dimensional (5D) models which are related to neutrino mass generation or address the fermion masses hierarchy problem. We study the signatures in low energy experiments that can discriminate the different models. The focus is on muon-electron conversion in nuclei $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$ and $\ensuremath{\mu}\ensuremath{\rightarrow}3e$ processes and their $\ensuremath{\tau}$ counterparts. Their links with the active neutrino mass matrix are investigated. We show that in the models we discussed the branching ratio of $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$ like rare process is much smaller than the ones of $\ensuremath{\mu}\ensuremath{\rightarrow}3e$ like processes. This is in sharp contrast to most of the traditional wisdom based on four-dimensional (4D) gauge models. Moreover, some rare tau decays are more promising than the rare muon decays.

Nuclear(μ−,e+)conversion mediated by Majorana neutrinos

We study lepton number violating (LNV) process of muon-positron conversion in nuclei mediated by the exchange of light and heavy Majorana neutrinos. Nuclear structure calculations have been carried out for the case of experimentally interesting nucleus 48Ti in the framework of renormalized proton-neutron Quasiparticle Random Phase Approximation. We demonstrate that the imaginary part of the amplitude of light Majorana neutrino exchange mechanism gives an appreciable contribution to the muon-positron conversion rate. This specific feature is absent in the allied case of neutrinoless double beta decay. Using the present neutrino oscillations, tritium beta decay, accelerator and cosmological data we derived the limits on the effective masses of light <m>_{\mu e} and heavy <M_N^{-1}>_{\mu e} neutrinos. The expected rates of muon-positron nuclear conversion, corresponding to these limits, were found to be so small that even within a distant future the muon-positron conversion experiments will hardly be able to detect the neutrino signal. Therefore, searches for this LNV process can only rely on the presence of certain physics beyond the trivial extension of the Standard Model by inclusion of massive Majorana neutrinos.

Neutrinoless Double Beta Decay and Lepton Flavor Violation

We point out that extensions of the standard model with low scale (approximately TeV) lepton number violation (LNV) generally lead to a pattern of lepton flavor violation (LFV) experimentally distinguishable from the one implied by models with grand unified theory scale LNV. As a consequence, muon LFV processes provide a powerful diagnostic tool to determine whether or not the effective neutrino mass can be deduced from the rate of neutrinoless double beta decay. We discuss the role of mu-->egamma and mu-->e conversion in nuclei, which will be studied with high sensitivity in forthcoming experiments.

Lepton flavor violation without supersymmetry

We study the lepton flavor violating (LFV) processes mu -> e gamma, mu -> 3e, and mu -> e conversion in nuclei in the left-right symmetric model without supersymmetry and perform the first complete computation of the LFV branching ratios B(mu -> f) to leading non-trivial order in the ratio of left- and right-handed symmetry breaking scales. To this order, B(mu -> e gamma) and B(mu -> e) are governed by the same combination of LFV violating couplings, and their ratio is naturally of order unity. We also find B(mu -> 3 e)/B(mu -> e) \sim 100 under slightly stronger assumptions. Existing limits on the branching ratios already substantially constrain mass splittings and/or mixings in the heavy neutrino sector. When combined with future collider studies and precision electroweak measurements, improved limits on LFV processes will test the viability of low-scale, non-supersymmetric LFV scenarios.

Neutrino-induced lepton flavor violation in gauge-mediated supersymmetry breaking

Gauge-mediated supersymmetry breaking is known to greatly suppress flavor changing neutral current effects. However, we show that gauge mediation in the context of leptogenesis implies potentially large lepton flavor violating signals. If the heavy right-handed neutrinos that participate in leptogenesis are lighter than the messenger scale of gauge mediation, they will induce flavor off-diagonal masses to the sleptons which in turn can induce large effects in $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}e\ensuremath{\gamma},\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\tau}}\ensuremath{\mu}\ensuremath{\gamma},$ and $\ensuremath{\mu}\ensuremath{-}e$ conversion in nuclei. We demonstrate this result and compute numerically the lepton-flavor violating decay and conversion rates in scenarios of direct gauge mediation.

(μ−,μ+)conversion in nuclei as a probe of new physics

We present a detailed analysis of the muonic analogue of neutrinoless double beta decay $({\ensuremath{\mu}}^{\ensuremath{-}},{\ensuremath{\mu}}^{+})$ conversion. We study several lepton number violating (LNV) mechanisms potentially triggering this process: exchange by light and heavy Majorana neutrinos as well as exchange by supersymmetric particles participating in R-parity violating interactions. The nuclear structure is taken into account within the renormalized quasiparticle random phase approximation method. This is the first realistic treatment of nuclear structure aspects of the $({\ensuremath{\mu}}^{\ensuremath{-}},{\ensuremath{\mu}}^{+})$ conversion. We estimate the rate of this process utilizing the existing experimental constraints on the parameters of the underlying LNV interactions and conclude that the $({\ensuremath{\mu}}^{\ensuremath{-}},{\ensuremath{\mu}}^{+})$ conversion will hardly be detectable in the near future experiments.

Current nuclear physics issues in studying the neutrinoless (μ −, e −) conversion in nuclei

The open nuclear physics problems in investigating the exotic μ − e conversion process are reviewed and discussed. A formulation of the nuclear matrix elements, in a compact way for all basic μ − → e − operators originating from a general (nucleon-level) effective Lagrangian, is also presented. The consideration of the nuclear deformation, which had not been taken into account in previous calculations performed with normal QRPA, is examined. As an example the low-lying spectrum of 72 Ge isotope, which has been chosen for future calculation in the context of the deformed Hartree-Fock method, is preliminary discussed.

Non-Linear Excitation and Decay of Low-Energy Nuclear Isomers Produced under Femtosecond Laser-Plasma Interaction

The properties of internal electronic conversion of nucleus excited in hot dense laser plasma are considered. The important role of dynamics of plasma charge state in residual gas is shown. We are also considering the role of multi-quanta photoexcitation of low-energy nuclear levels in this plasma.

The exotic double-charge exchange μ−→e + conversion in nuclei

The formalism for the neutrinoless ( μ −,e +) conversion is investigated in detail and the relevant nuclear matrix elements for light intermediate neutrinos in the case of 27Al( μ −,e +) 27Na are calculated. The nucleus 27Al is going to be used as a stopping target in the MECO experiment at Brookhaven, one of the most sensitive probes expected to reach a sensitivity in the branching ratio of the order 10 −16 within the next few years. The relevant transition operators are constructed utilizing a variety of mechanisms present in current gauge theories, with emphasis on the intermediate neutrinos, both light and heavy, and heavy SUSY particles. The nuclear wave functions, both for the initial state and all excited final states are obtained in the framework of 1s–0d shell model employing the well-known and tested Wildenthal realistic interaction. In the case of the light intermediate neutrinos the transition rates to all excited final states up to 25 MeV in energy are calculated. We find that the imaginary part of the amplitude is dominant. The total rate is calculated by summing over all these partial transition strengths. We also find that the rate due to the real part of the amplitude is much smaller than the corresponding quantity found previously by the closure approximation.

Phenomenology of the neutrino-mass-giving Higgs triplet and the low-energy seesaw violation of lepton number

Small realistic Majorana neutrino masses can be generated via a Higgs triplet ( ξ ++, ξ +, ξ 0) without having energy scales larger than M ∗= O(1) TeV in the theory. The large effective mass scale Λ in the well-known seesaw neutrino-mass operator Λ −1( LLΦΦ) is naturally obtained with Λ∼M ∗ 2/μ, where μ is a small scale of lepton-number violation. In theories with large extra dimensions, the smallness of μ is naturally obtained by the mechanism of “shining” if the number of extra dimensions n⩾3. We study here the Higgs phenomenology of this model, where the spontaneous violation of lepton number is treated as an external source from extra dimensions. The observable decays ξ ++→ l i + l j + will determine directly the magnitudes of the { ij} elements of the neutrino mass matrix. The decays ξ +→ W + J 0 and ξ 0→ ZJ 0, where J 0 is the massless Goldstone boson (Majoron), are also possible, but of special importance is the decay ξ 0→ J 0 J 0 which provides stringent constraints on the allowed parameter space of this model. Based on the current neutrino data, we also predict observable rates of μ–e conversion in nuclei.

B-quark mediated neutrinoless μ−–e− conversion in presence of R-parity violation

We found that in supersymmetric models with R-parity non-conservation the b-quarks may appreciably contribute to exotic neutrinoless muon-electron conversion in nuclei via the triangle diagram with two external gluons. This allowed us to extract previously overlooked constraints on the third generation trilinear R-parity violating parameters significantly more stringent than those existing in the literature.

Lepton-flavor violation in supersymmetric models with trilinearR-parity violation

Supersymmetry with R-parity violation (RPV) provides an interesting framework for naturally accommodating small neutrino masses. Within this framework, we discuss the lepton-flavour violating (LFV) processes mu -> e gamma, mu -> eee, and mu -> e conversion in nuclei. We make a detailed study of the observables related to LFV in different RPV models, and compare them to the expectations of R-conserving supersymmetry with heavy right-handed neutrinos. We show that the predictions are vastly different and uniquely characterise each model, thus providing a powerful framework for experimentally distinguishing between different theories of LFV. Besides the obvious possibility of amplified tree-level generation of mu -> eee and mu -> e conversion in nuclei, we find that even in the case where these processes arise at the one-loop level, their rates are comparable to that of mu -> e gamma, in clear contrast to the predictions of R-conserving models. We conclude that in order to distinguish between the different models, such a combined study of all the LFV processes is necessary, and that measuring P-odd asymmetries in polarised mu -> eee can play a decisive role. We also comment on the intriguing possibility of RPV models yielding a large T-odd asymmetry in the decay of polarised mu -> eee.

Verifiable model of neutrino masses from large extra dimensions

We propose a new scenario of neutrino masses with a Higgs triplet (xi(++),xi(+),xi(0)) in a theory of large extra dimensions. Lepton number violation in a distant brane acts as the source of a very small trilinear coupling of xi to the standard Higgs doublet in our brane. Small realistic Majorana neutrino masses are naturally obtained with the fundamental scale M(*) approximately O(1) TeV, foretelling the possible discovery of xi (m(xi) less, similarM(*)) at future colliders. Decays of xi(++) into same-sign dileptons are fixed by the neutrino mass matrix. Observation of &mgr;-e conversion in nuclei is predicted.

Exotic μ−−e− conversion in nuclei and R-parity violating supersymmetry

The flavor violating $\mu^--e^-$ conversion in nuclei is studied within the minimal supersymmetric standard model. We focus on the R-parity violating contributions at tree level including the trilinear and the bilinear terms in the superpotential as well as in the soft supersymmetry breaking sector. The nucleon and nuclear structure have consistently been taken into account in the expression of the $\mu^--e^-$ conversion branching ratio constructed in this framework. We have found that the contribution of the strange quark sea of the nucleon is comparable with that of the valence quarks. From the available experimental data on $\mu^--e^-$ conversion in $^{48}$Ti and $^{208}$Pb and the expected sensitivity of the MECO experiment for $^{27}$Al we have extracted new stringent limits on the R-parity violating parameters.

$\mu-e$ conversion in nuclei in the left-right supersymmetric model

We investigate the contributions to $\mu - e $ conversion in nuclei in a supersymmetric model with left-right symmetry, motivated by the new data on neutrino oscillations. We study the dependence of the conversion rate on the various parameters of the model, and show that light-mass or large $\tan \beta$ scenarios are severely restricted. We analyse the effect of several popular mecahnisms of neutrino mixing on the conversion rate as well as the influence of the right-handed scale on the conversion rate. We compare the conversion rate to the branching ratio for $\mu \rightarrow e \gamma$ and discuss their relative accessibility at future experiments, their sensitivity to various parameters of the model, as well as their relative importance in providing signals for new Physics.

Model-independent constraints on leptoquarks from rareμandτlepton processes

We perform a model-independent analysis so as to constrain the leptoquark (LQ) models from negative searches for $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}e\ensuremath{\gamma},$ $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}3e$ decays (and analogous processes in the $\ensuremath{\tau}$ sector), and coherent $\ensuremath{\mu}\ensuremath{-}e$ conversion in nuclei. We considerably improve some constraints obtained by analyses known in the literature, analyses which we show have by far underestimated the LQ contributions to the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}3e.$ In particular we find that the coherent $\ensuremath{\mu}\ensuremath{-}e$ conversion in nuclei mediated by the photon-conversion mechanism and the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}3e$ decay are golden plates where the flavor-changing leptoquark couplings, involving the second and third quark generations, can be strongly constrained. This is due to the fact that these processes get the enhancements by large $\mathrm{log}{(m}_{q}^{2}{/m}_{\mathrm{LQ}}^{2})$ terms which are induced by the so-called ``photon-penguin'' diagrams. These enhancements, which produce a mild Glashow-Iliopoulos-Maiani (GIM) suppression in the amplitudes, have not been taken into account in the previous analyses. We show that the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}e\ensuremath{\gamma}$ decay can set weaker constraints on the LQ models and this is because its amplitude is strongly GIM suppressed by the terms of order ${O(m}_{q}^{2}{/m}_{\mathrm{LQ}}^{2}).$ We also present the results for the corresponding constraints in the \ensuremath{\tau} sector. Finally, the prospects of the future muon experiments for the improvement of the present bounds are analyzed and discussed.

Lepton flavor violation and bilinear r-parity violation

We examine some flavor changing processes such as rare leptonic decays of the long-lived neutral kaon, muon-electron conversion in nuclei and radiative muon decay which are induced by the combined effects of bilinear and trilinear R-parity violations. These processes are used to put strong constraints on certain products of the bilinear and trilinear couplings. We also discuss the constraints on R-parity violation from neutrino masses and compare them with the constraints from flavor changing decay processes. Large range of parameter space satisfying the constraints from neutrino masses can be excluded by the flavor changing processes considered in this paper, and also vice versa.