Conversion In Nuclei Research Articles

Coherent muon-electron conversion in the dualized standard model

Muon-electron conversion in nuclei is considered in the framework of the Dualized Standard Model. The ratio $B_{\mu-e}$ of the conversion rate to the total muon capture rate is derived, and computed for several nuclei in a parameter-free calculation using parameters previously determined in different physical contexts. The values obtained all lie within the present experimental bounds, but some are so close as to seem readily accessible to experiments already being planned. Similar considerations are applied also to muon-electron conversion in muonium but give rates many orders of magnitude below the present experiment limit.

The inclusive μ− e conversion process with renormalized quasiparticle random-phase approximation

The inclusive process of the neutrinoless muon-to-electron conversion in nuclei is investigated by using the renormalized quasiparticle random-phase approximation (RQRPA). This approach is an improvement of the ordinary QRPA which restores the Pauli principle. It is a more reliable method for extremely small transition matrix elements as for the ( μ −, e −) conversion. Our calculations refer to a set of nuclei throughout the periodic table but we specifically investigate those isotopes for which experiments are done or planned. We study in particular 48 Ti which is at present used as a target at PSI in the new ongoing μ− e conversion experiment with the SINDRUM II spectrometer.

Neutrino physics at the turn of the millennium

Recent solar & atmospheric nu-data strongly indicate need for physics beyond the Standard Model. I review the ways of reconciling them in terms of 3-nu oscillations. Though not implied by data, bi-maximal nu-mixing models emerge as a possibility. SUSY with broken R-parity provides an attractive way to incorporate it, opening the possibility of testing nu-anomalies at high- energy colliders such as the LHC or at the upcoming long-baseline or nu- factory experiments. Reconciling, in addition, the LSND hint requires a fourth, light sterile neutrino, nus. The simplest are the most symmetric scenarios, in which 2 of the 4 neutrinos are maximally-mixed and lie at the LSND scale, while the others are at the solar scale. The lightness of nus, the nearly maximal atmospheric mixing, and the solar/atmospheric splittings all follow naturally from the assumed lepton-number symmetry and its breaking. These basic schemes can be distinguished at neutral-current-sensitive solar & atmospheric neutrino experiments such as SNO. However underground experiments have not yet proven neutrino masses, as there are many alternatives. For example flavour changing interactions can play an important role in the explanation of solar and contained atmospheric data and could be tested e.g through \mu \to e + \gamma, \mu-e conversion in nuclei, unaccompanied by neutrino-less double beta decay. Conversely, a short-lived numu might play a role in the explanation of the atmospheric data. Finally, in the presence of a nus, a long-lived heavy nutau could delay the time at which the matter and radiation contributions to the energy density of the Universe become equal, reducing density fluctuations on smaller scales, thus saving the standard CDM scenario, while the light nue, numu and nus would explain the solar & atmospheric data.

New constraints on R-parity violation from μ– e conversion in nuclei

We derive new constraints on the products of explicitly R-parity violating couplings λ and λ′ in MSSM from searches for μ– e conversion in nuclei. We concentrate on the loop induced photonic coherent conversion mode. For the combinations | λλ| which in μ– e conversion can be probed only at loop level our constraints are in many cases more stringent than the previous ones due to the enhancement of the process by large ln(m 2 f/m 2 f ̃ ). For the combinations of | λ′ λ′| the tree-level μ– e conversion constraints are usually more restrictive than the loop ones except for two cases which involve the third generation. With the expected improvements in the experimental sensitivity, the μ– e conversion will become the most stringent test for all the involved combinations of couplings.

μ– e conversion in nuclei versus [formula omitted]: an effective field theory point of view

Using an effective lagrangian description we analyze possible new physics contributions to the most relevant muon number violating processes: μ→e γ and μ– e conversion in nuclei. We identify a general class of models in which those processes are generated at one loop level and in which μ– e conversion is enhanced with respect to μ→e γ by a large ln( m 2 μ / Λ 2), where Λ is the scale responsible for the new physics. For this wide class of models bounds on μ– e conversion constrain the scale of new physics more stringently than μ→e γ already at present and, with the expected improvements in μ– e conversion experiments, will push it upwards by about one order of magnitude more. To illustrate this general result we give an explicit model containing a doubly charged scalar and derive new bounds on its couplings to the leptons.

State-by-state calculations for all channels of the exotic (μ-,e-) conversion process

The coherent and incoherent channels of the neutrinoless muon to electron conversion in nuclei, ${\ensuremath{\mu}}^{\ensuremath{-}}(A,Z)\ensuremath{\rightarrow}{e}^{\ensuremath{-}}{(A,Z)}^{*}$, are studied throughout the periodic table. The relevant nuclear matrix elements are computed by explicitly constructing all possible final nuclear states in the context of the quasiparticle random phase approximation. The obtained results are discussed in view of the existing at PSI and TRIUMF experimental data for ${}^{48}\mathrm{Ti}$ and ${}^{208}\mathrm{Pb}$ and compared with results obtained by (i) shell model sum-rule techniques, (ii) nuclear matter mapped into nuclei via a local density approximation, and (iii) earlier similar calculations.

Non-decoupling and lepton number violation in left-right models

We argue that large non-decoupling effects of heavy neutrinos can appear naturally in manifestly left-right symmetric models due to the minimization conditions of the scalar potential and the structure of vev's imposed by phenomenology. We derive constraints on off-diagonal light-heavy and heavy-heavy neutrino mixings from the searches for lepton violating decays μ → eγ, μ → ee − e + and μ − e conversion in nuclei. The most stringent limits come from the latter process because its amplitude shows a quadratic non-decoupling dependence on the heavy neutrino mass. Due to the suppression of right-handed currents by the large W R mass the present experiment are not sensitive to the intergenerational mixings between heavy neutrinos if M W R > 200 TeV.

New vector-scalar contributions to neutrinoless double beta decay and constraints on R-parity violation.

We show that in minimal supersymmetric standard model (MSSM) with R-parity breaking as well as in the left-right symmetric model, there are new observable contributions to neutrinoless double beta decay arising from hitherto overlooked diagrams involving the exchange of one W boson and one scalar boson. In particular, in the case of MSSM, the present experimental bounds on neutrinoless double beta decay lifetime improves the limits on certain R-parity violating couplings by about two orders of magnitude. It is shown that similar diagrams also lead to enhanced rates for $\mu^-\rightarrow e^+$ conversion in nuclei, which are in the range accessible to ongoing experiments.

Non-decoupling of heavy neutrinos and lepton flavour violation

We consider a class of models predicting new heavy neutral fermionic states, whose mixing with the light neutrinos can be naturally significant and produce observable effects below the threshold for their production. We update the indirect limits on the flavour non-diagonal mixing parameters that can be derived from unitarity, and show that significant rates are in general expected for one-loop-induced rare processes due to the exchange of virtual heavy neutrinos, involving the violation of the muon and electron lepton numbers. In particular, the amplitudes for μ- e conversion in nuclei and for μ → ee + e − show a non-decoupling quadratic dependence on the heavy neutrino mass M, while μ → eγ is almost independent of the heavy scale above the electroweak scale. These three processes are then used to set stringent constraints on the flavour-violating mixing angles. In all the cases considered, we point out explicitly that the non-decoupling behaviour is strictly related to the spontaneous breaking of the SU(2) symmetry.

Muon decays with lepton-number violation via vector leptoquark

The decays μ → eγ, μ → eγγ, and μ → ee e are analysed in the framework of the Pati-Salam type quark-lepton symmetry SU (4) v ⊗ SU (2) L ⊗ G R where the effects of mixing in the quark-lepton currents are taken into account. It is shown that the μ → eγγ and μ → ee e decays via the vector leptoquark have not a GIM-type suppression, while the μ → eγ decay has So, the specific hierarchy of the decay probabilities could take place Г (μ → ee e) ⪢ Г(μ → eγγ) ⪢ Г(μ → eγ) . The existing bounds on the vector leptoquark mass on the mixing matrix elements, based on the data for the μe conversion in nuclei and for the ratio of the K e2 and K μ2 decays allow to set the upper limits on the branching ratios at a level of 10 −18 for the μ → eγγ decay at a level of 10 −15 for the μ → ee e decay.

Muon number violating processes in single particle extensions of the standard model

We study the one-loop induced muon number processes when the standard model is minimally extended to include a SU(2) singlet of a charged scalar h + and a neutral fermion N. We find that μ → ey is more sensitive for the former model whereas μ − e conversion in nuclei for the latter. Effects of a scalar leptoquark y 1 3 and a heavy vector fermion E −, which induce tree level rare muon decays, are also discussed.

Neutrino mass physics

Present limits on neutrino masses are briefly reviewed, along with cosmological and astrophysical hints from dark matter, solar and atmospheric neutrino observations that suggest neutrino masses. These would imply many possible new phenomena such as neutrinoless ββ decay, lepton flavour violating processes such as neutrino oscillations, μ → eψ, μ → 3 e, μ → e conversion in nuclei, as well as two-body decays with the emission of a superweakly interacting spin zero particle, called majoron, e.g. μ → e + J. All of these processes may occur at levels consistent with present or planned experimental sensitivities. The underlying physics may also be probed at the high energies accessible at LEP, through related Z decay processes. Another possible, albeit quite indirect, manifestation of massive neutrinos is in Higgs physics. As an example I discuss the possibility of an invisibly decaying Higgs, a quite generic feature of majoron models where the lepton number is spontaneously violated close to the electroweak scale.

Formation of a -Hypernucleus and Transitions to Double- States

A scenario is given for the formation of ξ − states and the transitions to states with double-Λ in anticipation of observations, especially in the KEK-E224 experiment. First, the production cross sections of ξ − hypernuclear states by (K − , K + ) reactions are calculated within the framework of the distorted-wave impulse approximation. Next, the transition rates from ξ − hypernuclear states to possible double-Λ states are obtained, which are closely related to single- and double-Λ emissions after the ξ − p → ΛΛ conversion in nuclei

A discussion of the (μ −,e −) conversion

Coherent, incoherent and total rate calculations of the neutrinoless muon to electron conversion in nuclei are presented and discussed. For the most interesting nucleus from an experimental point of view, 48 Ti, the ( μ −, ρ −) matrix elements have been obtained by using quasi-particle RPA (i) by explicitly constructing the intermediate states (icoherent rate) or (ii) by RPA sum-rules (total rate). Also, for nuclei covering a wide range of the periodic table, the μ - ρ conversion matrix elements have been evaluated by using local density approximation taken into account Pauli blocking effects (incoherent rate) or by using electron scattering data for parametrizing the Fermi distribution (coherent rate). The results are compared to those found previously in the context of the shell model.

Can μ- e conversion in nuclei be a good probe for lepton-number violating Higgs couplings?

Motivated by the improving sensitivity, R, of experiments on μ Ti → e Ti and the enhanced Higgs nucleon interaction, we study this lepton number violating process induced by Higgs exchange. Taking the possible sensitivity, R ≅ 10 −16, we obtain the constraint on the Higgs-muon-electron vertex, κ μe , to be less than 2.4 × 10 −7 if the masses of the Higgs scalar and W gauge boson are the same. κ μe is also calculated for some models.

Update on the physics of neutrino mass

The present limits on neutrino masses are briefly reviewed, along with the cosmological and astrophysical hints from dark matter, solar and atmospheric neutrino observations that suggest neutrino masses. These would imply many possible new phenomena such as neutrinoless ββ decay, lepton flavour violating processes such as neutrino oscillations, μ → ργ, μ → 3 ρ, μ → ρ conversion in nuclei, as well as two-body decays with the emission of a superweakly interacting spin sero particle, called majoron, e.g. μ → ρ + J. All of these processes may occur at levels consistent with present or planned experimental sensitivities. The underlying physics may also be probed at the high energies accessible at LEP, through related Z decay processes. Another possible, albeit quite indirect, manifestation of massive neutrinos is in Higgs physics. As an example I discuss the possibility of an invisibly decaying Higgs, a quite generic feature of majoron models where the lepton number is spontaneously violated close to the electroweak scale.

μ-e conversion in nuclei and Z′ physics

Together with the existence of new neutral gauge bosons, models based on extended gauge groups (rank > 4) often predict also new charged fermions. A mixing of the known fermions with new states with exotic weak-isospin assignments (left-handed singlets and right-handed doublets) will induce tree level flavour changing neutral interactions mediated by Z exchange, while if the mixing is only with new states with ordinary weak-isospin assignments, the flavour changing neutral currents are mainly due to the exchange of the lightest new neutral gauge boson Z′. We show that the present experimental limits on μ-e conversion in nuclei give a nuclear-model-independent bound on the Z-e-μ vertex which is twice as strong as that obtained from μ → eee. In the case of E 6 models these limits provide quite stringent constraints on the Z′ mass and on the ZZ′ mixing angle. We point out that the proposed experiments to search for μ-e conversion in nuclei have good chances to find evidence of lepton flavour violation, either in the case that new exotic fermions are present at the electroweak scale, or if a new neutral gauge boson Z′ of E 6 origin lighter than a few TeV exists.

Coherent and incoherent (μ −, e −) conversion in nuclei

Coherent and incoherent (μ −, e −) conversion in nuclei is studied within the framework of several theories which violate flavour lepton number. A useful approach is followed which allows a factorization of the conversion widths into nuclear factors and other factors which depend only on the elementary process. The nuclear factors are evaluated in a wide range of nuclei allowing simple calculations of the conversion rates throughout the periodic table for a given theory with a minimum of work in the elementary sector. The coherent conversion is found to dominate the process. The results obtained modify appreciable previous results in the literature, particularly in the incoherent process.

Study of the flavour violating (μ −,e −) conversion in nuclei

The muon number violating (μ −, e −) conversion in the presence of nuclei is investigated. The effective operators resulting from various mechanisms, both photonic and non-photonic, are expressed in terms of nuclear coordinates: (i) for intermediate neutrino models, and (ii) for supersymmetric theories. Using these operators the ( A, Z) dependence of the corresponding widths is discussed throughout the periodic table. For the coherent process the dependence of the conversion rate on the nuclear parameters is given in terms of the elastic nuclear form factors, which are obtained in the context of shell model, taking into account finite nucleon-size effects. For the inclusive μ−e conversion process the total rates are calculated for closed-shell nuclei. The method of calculation utilizes microscopic non-energy-weighted sum rules (closure). The isospindependent part of the total (μ −, e −) rate is also related to that of the total (μ −, v μ capture rate, the latter being calculated with Primakoff's phenomenological method. The agreement between the two provides a good test of the reliability of the shell-model closure calculations. For non-closed-shell nuclei the relevant rates can be obtained by interpolation techniques from the functions obtained in the present work for closed-shell nuclei. Finally the experimentally interesting ratio of the coherent to the total μ−e conversion rate is discussed.

Lepton physics versus neutrino mass

The relationship between the strength of lepton flavour violating processes and the magnitude of the neutrino mass is rather model dependent. I review this question within different neutrino mixing models including superstring inspired models. Processes such as μ → e + γ, μ → 3 e, μ- e conversion in nuclei, etc. as well as lepton flavour violating Z° decays can occur even if the physical neutrinos are strictly massless. As a result, the corresponding rates are unconstrained by bounds on the neutrino mass that follow from laboratory, astrophysics and cosmology and can therefore be large. Leptonic CP violation may also occur even when the physical neutrinos are strictly massless.