Double Beta Decay Rate Research Articles

Predicting neutrinoless double beta decay

We give predictions for the neutrinoless double beta decay rate in a simple variant of the ${A}_{4}$ family symmetry model. We show that there is a lower bound for the $\ensuremath{\beta}{\ensuremath{\beta}}_{0\ensuremath{\nu}}$ amplitude even in the case of normal hierarchical neutrino masses, corresponding to an effective mass parameter $|{m}_{ee}|\ensuremath{\ge}0.17\sqrt{\ensuremath{\Delta}{m}_{\mathrm{ATM}}^{2}}$. This result holds both for the $CP$ conserving and $CP$ violating cases. In the latter case we show explicitly that the lower bound on $|{m}_{ee}|$ is sensitive to the value of the Majorana phase. We conclude therefore that in our scheme, $\ensuremath{\beta}{\ensuremath{\beta}}_{0\ensuremath{\nu}}$ may be accessible to the next generation of high sensitivity experiments.

Seesaw right-handed neutrino as the sterile neutrino for LSND

We show that a double seesaw framework for neutrino masses with $\ensuremath{\mu}\ensuremath{-}\ensuremath{\tau}$ exchange symmetry can lead to one of the right-handed seesaw partners of the light neutrinos being massless. This can play the role of a light sterile neutrino, giving a $3+1$ model that explains the Liquid Scintillator Neutrino Detector (LSND) results. We get a very economical scheme, which makes it possible to predict the full $4\ifmmode\times\else\texttimes\fi{}4$ neutrino mass matrix if $CP$ is conserved. Once $CP$ violation is included, the effect of the LSND mass range sterile neutrino is to eliminate the lower bound on neutrinoless double beta decay rate which exists for the three neutrino case with inverted mass hierarchy. The same strategy can also be used to generate a natural $3+2$ model for LSND, which is also equally predictive for the $CP$ conserving case in the limit of exact $\ensuremath{\mu}\ensuremath{-}\ensuremath{\tau}$ symmetry.

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.

Next-generation germanium spectrometer background reduction techniques at 2 Me

The Majorana project, a next-generation 76Ge neutrinoless double-beta decay experiment being undertaken by a large international collaboration, has the goal of measuring the neutrinoless double-beta decay rate by observing monochromatic events at 2039 keV in 500 kg of isotopically enriched 76Ge gamma-ray spectrometers. In order to achieve the desired sensitivity limit, the background in the 2037-2041 keV region must be reduced to <1 event per year in the entire germanium array. The effects of various background reduction techniques, and the combination thereof, to produce a huge 76Ge spectrometer array with virtually zero background are discussed.

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.

Operator analysis of neutrinoless double beta decay

We study the effective operators of the standard model fields which would yield an observable rate of neutrinoless double beta decay. We particularly focus on the possibility that neutrinoless double beta decay is dominantly induced by lepton-number-violating higher dimensional operators other than the Majorana neutrino mass. Our analysis can be applied to models in which neutrinoless double beta decay is induced either by a strong dynamics or by quantum gravity effects at a fundamental scale near the TeV scale as well as the conventional models in which neutrinoless double beta decay is induced by perturbative renormalizable interactions.

Predictions on the neutrinoless double beta decay from the leptogenesis via the LHu flat direction

If the baryon asymmetry in the present universe is generated by decays of the LH u flat direction, the observed baryon asymmetry requires the mass of the lightest neutrino to be much smaller than the mass scale indicated from the atmospheric and solar neutrino oscillations. Such a small mass of the lightest neutrino leads to a high predictability on the rate of the neutrinoless double beta (0 νββ) decay. In this Letter we show general predictions on the 0 νββ decay in the leptogenesis via the LH u flat direction.

Neutrino masses from operator mixing

We show that in theories that reduce, at the Fermi scale, to an extension of the standard model with two doublets, there can be additional dimension five operators giving rise to neutrino masses. In particular there exists a singlet operator which can not generate neutrino masses at tree level but generates them through operator mixing. Under the assumption that only this operator appears at tree level we calculate the neutrino mass matrix. It has the Zee mass matrix structure and leads naturally to bimaximal mixing. However, the maximal mixing prediction for solar neutrinos is very sharp even when higher order corrections are considered. To allow for deviations from maximal mixing a fine tuning is needed in the neutrino mass matrix parameters. However, this fine tuning relates the departure from maximal mixing in solar neutrino oscillations with the neutrinoless double beta decay rate.

Searching for the CP-violation associated with Majorana neutrinos

The effective Majorana mass which determines the rate of the neutrinoless double beta decay, |<m>|, is considered in the case of three-neutrino mixing and massive Majorana neutrinos. Assuming a rather precise determination of the parameters characterizing the neutrino oscillation solutions of the solar and atmospheric neutrino problems has been made, we discuss the information a measurement of |<m>| > (0.005 - 0.010) eV can provide on the value of the lightest neutrino mass and on the CP-violation in the lepton sector. The implications of combining a measurement of |<m>| with future measurement of the neutrino mass in tritium beta-decay experiments for the possible determination of leptonic CP-violation are emphasized.

Absolute neutrino mass determination

We discuss four approaches to the determination of absolute neutrino mass. These are the measurement of the zero-neutrino double beta decay rate, of the tritium decay end-point spectrum, of the cosmic ray spectrum above the GZK cutoff (in the Z-burst model), and the cosmological measurement of the power spectrum governing the CMB and large scale structure. The first two approaches are sensitive to the mass eigenstates coupling to the electron neutrino, whereas the latter two are sensitive to the heavy component of the cosmic neutrino background. All mass eigenstates are related by the $\Delta m^2$'s inferred from neutrino oscillation data. Consequently, the potential for absolute mass determination of each of the four approaches is correlated with the other three, in ways that we point out.

Signal of neutrinoless double beta decay, neutrino spectrum and oscillation scenarios.

The lower and upper bounds on the neutrinoless double beta (0nu-2beta) decay rate are obtained, as functions of the parameters of neutrino oscillations and of the lightest neutrino mass. The constraints on these parameters from the search for the 0nu-2beta transition, as well as from the interpretation of solar and atmospheric neutrino data in terms of oscillations, can be conveniently represented in one unitarity triangle. This representation helps to clarify the cases when the 0nu-2beta rate is small; the crucial dependence on the scenarios assumed for solar neutrino oscillations and on the neutrino spectrum is emphasized. We consider hierarchical and non-hierarchical neutrino spectra, and discuss their interest in view of future searches of the 0nu-2beta decay.

Nuclear moments for the neutrinoless double beta decay

A derivation of the neutrinoless double beta decay rate, specially adapted for nuclear structure calculations, is presented. It is shown that the Fourier-Bessel expansion of the hadronic currents, jointly with angular momentum recoupling, leads to very simple final expressions for the nuclear form factors. This greatly facilitates the theoretical estimate of the half-life. Our approach does not require the closure approximation, which, however, can be implemented if desired. The method is exemplified for ββ decay 48Ca → 48Ti, both within the QRPA and a shell-model like model.

Nuclear structure calculations of two-neutrino double-beta decay transitions to excited final states

Abstract The half-lives for two-neutrino double-beta decay transitions to the excited 2 1 + and 0 1 + states for several nuclei are calculated in the framework of a second quasi-random-phase approximation (QRPA). In addition, other single-beta and B (E2) transitions are evaluated and compared with experimental data. We find satisfactory agreement with data when we tried to describe unitarily, with the same set of parameters, all the computed physical quantities. The sum rule, which is fulfilled accurately in the framework of the proton-neutron QRPA, is violated insignificantly by our method. The higher-order QRPA terms only influence the β + strengths which enter the double-beta decay rate formula, shifting them to higher QRPA energies.

Neutrinoless double beta decay within the quasiparticle random-phase approximation with proton-neutron pairing.

We have investigated the role of proton-neutron pairing in the context of the quasiparticle random phase approximation formalism. This way the neutrinoless double beta decay matrix elements of the experimentally interesting A = 48, 76, 82, 96, 100, 116, 128, 130, and 136 systems have been calculated. We have found that the inclusion of proton-neutron pairing influences the neutrinoless double beta decay rates significantly, in all cases allowing for larger values of the expectation value of light neutrino masses. Using the best presently available experimental limits on the half lifetime of neutrinoless double beta decay we have extracted the limits on lepton number violating parameters. \textcopyright{} 1996 The American Physical Society.

Double-beta decay of 48Ca revisited

The 48Ca → 48Ti two neutrino double-beta decay rate is calculated exactly in the full fp shell with the hamiltonian describing well the spectroscopy of the A = 48 system. The resulting half-life is very near the experimental lower limit. In order to find out whether the shell model can at all accommodate longer 2 ν half-lives we modify the hamiltonian in several ways, keeping, however, the acceptable agreement with the main spectroscopic indicators intact. We conclude that, within such an approach, the longest possible half-life is about 10 20 yr, only a factor of three longer than the present limit. We stress that any possible modifications of the hamiltonian must be restricted by the comparison with available data on energy levels and transitions rates.

Multi-majoron modes for neutrinoless double-beta decay

We construct two new classes of models for double beta decay, each of which leads to an electron spectrum which differs from the decays which are usually considered. One of the classes has a spectrum which has not been considered to date, and which is softer than the usual two-neutrino decay of the Standard Model. We construct illustrative models to show how other phenomenological bounds can be accommodated. We typically find that, although all other bounds can be satisfied, the predicted double-beta decay rate only in one class of these models is at best large enough to be just detectable in current experiments.

A new search for neutrinoless ββ decay with a thermal detector

A 334 g TeO 2 crystal has been operated at a temperature of around 10 mK for more than one year in a low intrinsic radioactivity dilution refrigerator installed in the Gran Sasso Underground Laboratory. From the spectrum collected in 9234 h of effective running time we improve our limit on neutrinoless double beta decay of 130Te by an order of magnitude with respect to our previous experiment. It is the most stringent on this nucleus and excludes a large contribution of the neutrinoless mode to the rate of double beta decay found in geochemical experiments. Upper limits on the effective neutrino mass and on the contributions of right banded currents are given.

SO(10) grand unification model for degenerate neutrino masses

We propose an SO(10) scheme where neutrino masses can simultaneously explain the solar and atmospheric neutrino deficits, together with a hot dark matter component. In our scheme the ν e , ν μ , and ν τ are approximately degenerate with a mass of about 2 eV, which can lead to an observable neutrinoless double beta decay rate. The model is based on a realization of the seesaw mechanism in which the main contribution to the light neutrino masses is universal, due to a suitable SU(2) horizontal symmetry, while the splittings between ν e and ν μ explain the solar neutrino deficit and that between ν μ and ν τ explain the atmospheric neutrino anomaly.

Quasiparticle random phase approximation analysis of the double beta decay ofMo100to the ground state and excited states ofRu100

The beta decay rate of the ${1}^{+}$ ground state of $^{100}\mathrm{Tc}$ to the ground and excited states of $^{100}\mathrm{Mo}$ and $^{100}\mathrm{Ru}$ has been calculated using a combination of the charge-conserving and charge-nonconserving modes of the quasiparticle random phase approximation theory. These results, as well as the calculated E2 decay properties of $^{100}\mathrm{Mo}$ and $^{100}\mathrm{Ru}$, are compared with data. In addition, the two-neutrino double beta decay rates of $^{100}\mathrm{Mo}$ to the ground state and excited states of $^{100}\mathrm{Ru}$ are evaluated and analyzed using available experimental data. For completeness, the neutrinoless double beta decay rate of $^{100}\mathrm{Mo}$ is calculated and used to extract the value of the effective neutrino mass and the parameters of a general weak-interaction Hamiltonian.

Theoretical bounds for majoron emission from nuclear double-beta decay processes

Theoretical upper limits for the neutrino-majoron coupling constant, g, are extracted from a systematic calculation of neutrinoless double-beta decay rates for transitions in 48Ca, 76Ge, 82Se, 128Te, 130Te and 136Xe. The nuclear wavefunctions which we have used to compute the theoretical bounds on g have been tested in calculations involving two-neutrino and neutrinoless double-beta decay rates. The resulting value is of the order of g approximately=10-4, in good agreement with recently reported data by the Heidelberg-Moscow-LNGS collaboration.