T.S. Kosmas

Enhanced μ−–e− conversion in nuclei in the inverse seesaw model

Abstract We investigate nuclear μ − – e − conversion in the framework of an effective Lagrangian arising from the inverse seesaw model of neutrino masses. We consider lepton flavour violation interactions that arise from short range (non-photonic) as well as long range (photonic) contributions. Upper bounds for the L f -parameters characterizing μ − – e − conversion are derived in the inverse seesaw model Lagrangian using the available limits on the μ − – e − conversion branching ratio, as well as the expected sensitivities of upcoming experiments. We comment on the relative importance of these two types of contributions and their relationship with the measured solar neutrino mixing angle θ 12 and the dependence on θ 13 . Finally we show how the L f μ − – e − conversion and the μ − → e − γ rates are strongly correlated in this model.

Nuclear aspects of neutral current non-standard ν-nucleus reactions and the role of the exotic μ−→e− transitions experimental limits

Abstract The nuclear aspects of flavour changing neutral current (FCNC) processes, predicted by various new-physics models to occur in the presence of nuclei, are examined by computing the relevant nuclear matrix elements within the context of the quasi-particle RPA using realistic strong two-body forces. One of our aims is to explore the role of the non-standard interactions (NSI) in the leptonic sector and specifically: (i) in lepton flavour violating (LFV) processes involving the neutral particles ν l and ν ˜ l , l = e , μ , τ and (ii) in charged lepton flavour violating (cLFV) processes involving the charged leptons l − or l + . As concrete nuclear systems we have chosen the stopping targets of μ − → e − conversion experiments, i.e. the 48 Ti nucleus of the PRIME/PRISM experiment at J-PARC and the 27 Al of the COMET at J-PARC as well as of the Mu2e at Fermilab. These experiments have been designed to reduce the single event sensitivity down to 10 −16 –10 −18 in searching for charged lepton mixing events. Our goal is, by taking advantage of our detailed nuclear structure calculations and using the present limits or the sensitivity of the aforementioned exotic μ − → e − experiments, to put stringent constraints on the parameters of NSI Lagrangians.

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

Abstract We found that in supersymmetric models with R-parity non-conservation ( R / p SUSY) the b-quarks may appreciably contribute to exotic neutrinoless μ−–e−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 / p parameters significantly more stringent than those existing in the literature.

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

Abstract The flavor violating μ − − 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 μ − − 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 μ − − 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.

Study of the muon number violating (μ−, e− conversion in a nucleus by using quasi-particle RPA

Abstract The exotic (μ−, e−) conversion reaction for the 48Ti nucleus is studied in the framework of the quasi-particle random phase approximation (QRPA). For the non-coherent processes the relevant total rate is calculated by summing over partial rates for all the possible intermediate states constructed in the above approximation. For the coherent process the contribution is obtained by using an uncorrelated BCS vacuum. In order to check the validity of closure approximation, which is almost unavoidable in shell-model calculations, we also evaluate the total (μ−, e−) conversion rates by QRPA sum-rules by first explicitly calculating a suitable mean excitation energy of the nucleus. The influence of the ground state correlations to the (μ−, e−) conversion matrix elements is estimated by using a correlated RPA vacuum. The fraction of the transition rate of the coherent process for each of these methods is calculated and the results are compared to those found previously by using shell-model closure approximation.

Probing neutrino magnetic moments at the Spallation Neutron Source facility

Majorana neutrino electromagnetic properties are studied through neutral current coherent neutrino-nucleus scattering. We focus on the potential of the recently planned COHERENT experiment at the Spallation Neutron Source to probe muon-neutrino magnetic moments. The resulting sensitivities are determined on the basis of a chi^2 analysis employing realistic nuclear structure calculations in the context of the quasi-particle random phase approximation. We find that they can improve existing limits by half an order of magnitude. In addition, we show that these facilities allow for Standard Model precision tests in the low energy regime, with a competitive determination of the weak mixing angle. Finally, they also offer the capability to probe other electromagnetic neutrino properties, such as the neutrino charge-radius. We illustrate our results for various choices of experimental setup and target material.

The exotic μ−→e− conversion rates by explicit renormalized quasiparticle RPA calculations.

Abstract We investigate the μ−e conversion matrix elements by explicit calculations within the renormalized quasiparticle random phase approximation (RQRPA). In this approach, by promoting the quasi-Boson approximation mostly used in ordinary QRPA, the Pauli-principle is restored to a large extent and the nucleon-nucleon ground state correlations are reliably taken into account. We have pointed out that the usual QRPA includes quite strong ground state correlations and it, therefore, overestimates their effect on the μ−e conversion rates and especially on the coherent mode, which is the most important μ−e conversion channel. The rate of this channel is currently measured in the SINDRUM II experiment at PSI and is expected to be extracted in the designed MECO experiment at Brookhaven. By combining our nuclear calculations with the existing experimental limits on the branching ratio Rμe− for 208 Pb and 48 Ti we put constraints on the flavour violating parameters entering the μ−e conversion effective currents in modern gauge theories.

Neutrino transition magnetic moments within the non-standard neutrino–nucleus interactions

Abstract Tensorial non-standard neutrino interactions are studied through a combined analysis of nuclear structure calculations and a sensitivity χ 2 -type of neutrino events expected to be measured at the COHERENT experiment, recently planned to operate at the Spallation Neutron Source (Oak Ridge). Potential sizeable predictions on transition neutrino magnetic moments and other electromagnetic parameters, such as neutrino milli-charges, are also addressed. The non-standard neutrino–nucleus processes, explored from nuclear physics perspectives within the context of quasi-particle random phase approximation, are exploited in order to estimate the expected number of events originating from vector and tensor exotic interactions for the case of reactor neutrinos, studied with TEXONO and GEMMA neutrino detectors.

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

Abstract 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.

State-by-state calculations for all channels of the exotic ({mu}{sup {minus}},e{sup {minus}}) conversion process

The coherent and incoherent channels of the neutrinoless muon to electron conversion in nuclei, {mu}{sup {minus}}(A,Z){r_arrow}e{sup {minus}}(A,Z){sup {asterisk}}, 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 {sup 48}Ti and {sup 208}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. {copyright} {ital 1997} {ital The American Physical Society}