Double Beta Decay Rate Research Articles

Neutrinoless double beta decay rates in the presence of light sterile neutrinos

We investigate neutrinoless double-beta decay (0νββ) in minimal extensions of the Standard Model of particle physics where gauge-singlet right-handed neutrinos give rise to Dirac and Majorana neutrino mass terms. We argue that the standard treatment of these scenarios, based on mass-dependent nuclear matrix elements, is missing important contributions to the 0νββ amplitude. First, new effects arise from the exchange of neutrinos with very small (ultrasoft) momenta, for which we compute the associated nuclear matrix elements for the decays of 76Ge and 136Xe. These contributions can dominate the 0νββ rate in cases with light sterile neutrinos. The ultrasoft terms are also relevant in the more standard scenario of just three light Majorana neutrinos where they lead to a 10% reduction of the total 0νββ amplitude. Secondly, we highlight the importance of short-range terms associated with medium-heavy sterile neutrinos and provide explicit formulae that can be used in phenomenological analyses. As examples we discuss impact of these new effects in several explicit scenarios, including a realistic 3 + 2 model with two right-handed gauge-singlet neutrinos.

Survey of neutrino flavor predictions and the neutrinoless double beta decay funnel

The neutrinoless double beta decay experimental effort continues to make tremendous progress with hopes of covering the inverted neutrino mass hierarchy in coming years and pushing from the quasidegenerate hierarchy into the normal hierarchy. As neutrino oscillation data are starting to suggest that the mass ordering may be normal, we may well be faced with staring down the funnel of death: a region of parameter space in the normal ordering where—for a particular cancellation among the absolute neutrino mass scale, the Majorana phases, and the oscillation parameters—the neutrinoless double beta decay rate may be vanishingly small. To answer the question of whether this region of parameter space is theoretically preferred, we survey five broad categories of flavor model structures which make various different predictions for parameters relevant for neutrinoless double beta decay to determine how likely it is that the rate may be in this funnel region. We find that a non-negligible fraction of predictions surveyed are at least partially in the funnel region. Our results can guide model builders and experimentalists alike in focusing their efforts on theoretically motivated regions of parameter space. Published by the American Physical Society 2024

νDoBe — A Python tool for neutrinoless double beta decay

We present νDoBe, a Python tool for the computation of neutrinoless double beta decay (0νββ) rates in terms of lepton-number-violating operators in the Standard Model Effective Field Theory (SMEFT). The tool can be used for automated calculations of 0νββ rates, electron spectra and angular correlations for all isotopes of experimental interest, for lepton-number-violating operators up to and including dimension 9. The tool takes care of renormalization-group running to lower energies and provides the matching to the low-energy effective field theory and, at lower scales, to a chiral effective field theory description of 0νββ rates. The user can specify different sets of nuclear matrix elements from various many-body methods and hadronic low-energy constants. The tool can be used to quickly generate analytical and numerical expressions for 0νββ rates and to generate a large variety of plots. In this work, we provide examples of possible use along with a detailed code documentation. The code can be accessed through:GitHub: https://github.com/OScholer/nudobeOnline User-Interface: https://oscholer-nudobe-streamlit-4foz22.streamlit.app/

A simplest seesaw model of the TM1 and μ-τ reflection symmetries for the neutrino masses and leptogenesis

In this paper, following the Occam’s razor principle, we have put forward a very simple form of the Dirac neutrino mass matrix in the minimal seesaw model with the right-handed neutrino mass matrix being diagonal ; it has one texture zero and only contains three real parameters, whose values can be determined from the neutrino oscillation experimental results. Such a model leads to a neutrino mass matrix that obeys the TM1 and μ-τ reflection symmetries simultaneously. In this way all the lepton flavor mixing parameters except for are predicted; the value of is predicted by the TM1 symmetry, while those of , δ, ρ and σ by the μ-τ reflection symmetry. And the neutrino masses are predicted to be of the NO case with , for which all three light neutrino masses will be pinned down with the help of the experimental results for the neutrino mass squared differences. For these results, the effective Majorana neutrino mass that controls the rate of the neutrinoless double beta decay is predicted to be 1.6 or 3.8 meV in the case of σ = 0 or π/2. We have also studied the implications of the model for leptogenesis. It turns out that only in the two-flavor leptogenesis regime (which holds in the temperature range 109–1012 GeV) can leptogenesis have a chance to be successful. And a successful leptogenesis can be achieved at GeV in the case of σ = π/2, but not in the case of σ = 0.

Study of neutrino mass matrices with vanishing trace and one vanishing minor

In this work we carry out a systematic texture study of the neutrino mass matrix with the ansatzes - (i) one vanishing minor and (ii) the zero sum of the mass eigenvalues with the CP phases (henceforth vanishing trace). There are six possible textures of a neutrino mass matrix with one vanishing minor. The viability of each texture is checked with $3\sigma$ values of current neutrino data by drawing scatter plots. In our analysis we are motivated to use the ratio of solar to atmospheric mass-squared differences $R_{\nu}$ for its precise measurement (and also the atmospheric mixing angle $\theta_{23}$) to constrain phenomenologically first the Dirac CP phase $\delta$ in the range of $0^{\degree}-360^{\degree}$ for a given texture with the solutions of the constraint equations. Subsequently we employ this constrained $\delta$ to determine the range of completely unknown Majorana CP Phases ($\alpha$ and $\beta$) for all the viable textures. We also check the neutrinoless double beta decay rate, $|m_{ee}|$ and the Jarlskog invariant, $J_{cp}$ for the textures. Finally the symmetry realization of all the viable textures under the flavor symmetry group $Z_5$ via seesaw mechanism is implemented along with the FN mechanism to determine mass hierarchy structure.

A predictive and testable unified theory of fermion masses, mixing and leptogenesis

We consider a minimal non-supersymmetric SO(10) Grand Unified Theory (GUT) model that can reproduce the observed fermionic masses and mixing parameters of the Standard Model. We calculate the scales of spontaneous symmetry breaking from the GUT to the Standard Model gauge group using two-loop renormalisation group equations. This procedure determines the proton decay rate and the scale of U(1)B−L breaking, which generates cosmic strings and the right-handed neutrino mass scales. Consequently, the regions of parameter space where thermal leptogenesis is viable are identified and correlated with the fermion masses and mixing, the neutrinoless double beta decay rate, the proton decay rate, and the gravitational wave signal resulting from the network of cosmic strings. We demonstrate that this framework, which can explain the Standard Model fermion masses and mixing and the observed baryon asymmetry, will be highly constrained by the next generation of gravitational wave detectors and neutrino oscillation experiments which will also constrain the proton lifetime.

Light sterile neutrinos, left-right symmetry, and 0νββ decay

We investigate neutrinoless double beta (0νββ) decay rates in minimal left-right symmetric models in presence of relatively light right-handed neutrinos. By use of an effective field theory approach, we systematically include all contributions in the model as well as the dependence of the decay amplitude on the masses of right-handed neutrinos. In type-I and type-II seesaw scenarios, we analyze the impact of right-handed neutrinos heavier than about 10 MeV, showing that this effect can lead to a detection of 0νββ decay in the next-generation experiments even for the normal hierarchy and a relatively large right-handed scale set by the mass of hypothetical right-handed gauge bosons. Finally, we comment on a possible connection between light right-handed neutrinos and the strong CP problem.

Single- and Double-Charge Exchange Reactions and Nuclear Matrix Element for Double-Beta Decay

Neutrino properties such as the Majorana nature and the masses, which go beyond the standard model, are derived from the experimental double-beta decay (DBD) rate by using the DBD nuclear matrix element (NME). Theoretical evaluations for the NME, however, are very difficult. Single-charge exchange reactions (SCERs) and double-charge exchange reactions (DCERs) are used to study nuclear isospin (τ) and spin (σ) correlations involved in the DBD NME and to theoretically calculate the DBD NME. Single and double τσ NMEs for quasi-particle states are studied by SCERs and DCER. They are found to be reduced with respect to the quasi-particle model NMEs due to the τσ correlations. The impact of the SCER- and DCER-NMEs on the DBD NME is discussed.

The role of the transfer of nucleons in driving double charge exchange reactions

Transfer is an excellent tool to get insights into the short-range correlations on nucleons in a nuclear state. Within the context of direct reactions, the double charge exchange reactions have recently gained attention once their matrix elements might be associated with the double-beta decay rates. This class of reaction can occur from two completely distinctive mechanisms. They can take place by nucleons exchange or driven by mesons exchange between the projectile and target nuclei. Once the double charge exchange driven by multi-nucleon or mesons exchanges can compete with each other, it is crucial to analyze the contribution of the multi-nucleon transfer in this type of reaction to verify its relevance on the measured cross sections.

Left-Right Symmetry and Leading Contributions to Neutrinoless Double Beta Decay.

We study the impact of the mixing (LR mixing) between the standard model W boson and its hypothetical, heavier right-handed parter W_{R} on the neutrinoless double beta decay (0νββ decay) rate. Our study is done in the minimal left-right symmetric model assuming a type-II dominance scenario with charge conjugation as the left-right symmetry. We then show that the 0νββ decay rate may be dominated by the contribution proportional to this LR mixing, which at the hadronic level induces the leading-order contribution to the interaction between two pions and two charged leptons. The resulting long-range pion exchange contribution can significantly enhance the decay rate compared to previously considered short-range contributions. Finally, we find that even if future cosmological experiments rule out the inverted hierarchy for neutrino masses, there are still good prospects for a positive signal in the next generation of 0νββ decay experiments.

Nuclear structure model for double-charge-exchange processes

A new model, based on the BCS approach, is specially designed to describe nuclear phenomena $(A,Z)\rightarrow (A,Z\pm 2)$ of double-charge exchange (DCE). After being proposed, and applied in the particle-hole limit, by one of the authors (F. Krmpoti\'c [1]), so far it was never been applied within the BCS mean-field framework, nor has its ability to describe DCE processes been thoroughly explored. It is a natural extension of the pn-QRPA model, developed by Halbleib and Sorensen [2] to describe the single $\beta$-decays $(A,Z)\rightarrow (A,Z\pm 1)$, to the DCE processes. As such, it exhibits several advantages over the pn-QRPA model when is used in the evaluation of the double beta decay (DBD) rates. For instance, i) the extreme sensitivity of the nuclear matrix elements (NMEs) on the model parametrization does not occur, ii) it allows to study NMEs, not only for the fundamental state in daughter nuclei, as the pn-QRPA model does, but also for all final $0^+$ and $2^+$ states, accounting at the same time their excitation energies and the corresponding DBD Q-values, iii) together with the DBD-NMEs it provides also the energy spectra of Fermi and Gamow-Teller DCE transition strengths, as well as the locations of the corresponding resonances and their sum rules, iv) the latter are relevant for both the DBD and the DCE reactions, since the involved nuclear structure is the same; this correlation does not exist within the pn-QRPA model. As an example, detailed numerical calculations are presented for the $(A,Z)\rightarrow (A,Z+ 2)$ process in $^{48}$Ca $\rightarrow ^{48}$Ti and the $(A,Z)\rightarrow (A,Z- 2)$ process in $^{96}$Ru $\rightarrow ^{96}$Mo, involving all final $0^+$ states and $2^+$ states.

Loop-enhanced rate of neutrinoless double beta decay

Neutrino masses can be generated radiatively. In such scenarios their masses are calculated by evaluating a self-energy diagram with vanishing external momentum, i.e. taking only the leading order term in a momentum expansion. The difference between the full self-energy and the mass is experimentally difficult to access, since one needs off-shell neutrinos to observe it. However, massive Majorana neutrinos that mediate neutrinoless double beta decay (0νββ) are off-shell, with the virtuality of order 100 MeV. If the energy scale of the self-energy loop is of the order of this virtuality, the amplitude of double beta decay can be modified by the unsuppressed loop effect. This can have a drastic impact on the interpretation of future observations or limits of the 0νββ decay.

Neutrinoless Double-β Decay with Nonstandard Majoron Emission.

We present a novel mode of neutrinoless double-β decay with emission of a light Majoron-like scalar particle ϕ. We assume it couples via an effective seven-dimensional operator with a (V+A) lepton current and (V±A) quark currents leading to a long-range contribution that is unsuppressed by the light neutrino mass. We calculate the total double-β decay rate and determine the fully differential shape for this mode. We find that future double-β decay searches are sensitive to scales of the order Λ_{NP}≈1 TeV for the effective operator and a light scalar m_{ϕ}<0.2 MeV, based on ordinary double-β decay Majoron searches. The angular and energy distributions can deviate considerably from that of two-neutrino double-β decay, which is the main background. We point out possible ultraviolet completions where such an effective operator can emerge.

Simplest scoto-seesaw mechanism

By combining the simplest (3,1) version of the seesaw mechanism containing a single heavy “right-handed” neutrino with the minimal scotogenic approach to dark matter, we propose a theory for neutrino oscillations. The “atmospheric” mass scale arises at tree level from the seesaw, while the “solar” oscillation scale emerges radiatively, through a loop involving the “dark sector” exchange. Such simple setup gives a clear interpretation of the neutrino oscillation lengths, has a viable WIMP dark matter candidate, and implies a lower bound on the neutrinoless double beta decay rate.

Neutrinoless double beta decay and the baryon asymmetry of the Universe

We discuss the impact of the observation of neutrinoless double beta decay on the washout of lepton number in the early universe. Neutrinoless double beta decay can be triggered by a large number of mechanisms that can be encoded in terms of Standard Model effective operators which violate lepton number by two units. We calculate the contribution of such operators to the rate of neutrinoless double beta decay and correlate it with the washout of lepton number induced by the same operators in the early universe. We find that the observation of a non-standard contribution to neutrinoless double beta decay, i.e. not induced by the standard mass mechanism of light neutrino exchange, would correspond to an efficient washout of lepton number above the electroweak scale for many operators up to mass dimension 11. Combined with Standard Model sphaleron transitions, this would render many baryogenesis mechanisms at higher scales ineffective.

Search for creation of electrons in lab

We examine the physics motivations to search for lepton number violations in the laboratory, in particular by studying the important nuclear transition called neutrinoless double beta decay, which consists in the creation of a pair of electrons. We address some major shortcomings of the Standard Model of electroweak interactions, arguing that most of them can be solved simply within extended models with Majorana neutrinos. At the same time, we discuss the difficulties of obtaining precise predictions for the neutrinoless double beta decay rate and strengthen the case of probing Majorana neutrino masses of about 10 meV.

Improved description of the 2νββ -decay and a possibility to determine the effective axial-vector coupling constant

An improved formalism of the two-neutrino double-beta decay ($2\nu\beta\beta$-decay) rate is presented, which takes into account the dependence of energy denominators on lepton energies via the Taylor expansion. Till now, only the leading term in this expansion has been considered. The revised $2\nu\beta\beta$-decay rate and differential characteristics depend on additional phase-space factors weighted by the ratios of $2\nu\beta\beta$-decay nuclear matrix elements with different powers of the energy denominator. For nuclei of experimental interest all phase-space factors are calculated by using exact Dirac wave functions with finite nuclear size and electron screening. For isotopes with measured $2\nu\beta\beta$-decay half-life the involved nuclear matrix elements are determined within the quasiparticle random phase approximation with partial isospin restoration. The importance of correction terms to the $2\nu\beta\beta$-decay rate due to Taylor expansion is established and the modification of shape of single and summed electron energy distributions is discussed. It is found that the improved calculation of the $2\nu\beta\beta$-decay predicts slightly suppressed $2\nu\beta\beta$-decay background to the neutrinoless double-beta decay signal. Further, a novel approach to determine the value of effective weak-coupling constant in nuclear medium $g^{\rm eff}_{\rm A}$ is proposed.

Fundamental Physics with Electroweak Probes of Nuclei

The past decade has witnessed tremendous progress in the theoretical and computational tools that produce our understanding of nuclei. A number of microscopic calculations of nuclear electroweak structure and reactions have successfully explained the available experimental data, yielding a complex picture of the way nuclei interact with electroweak probes. This achievement is of great interest from the pure nuclear-physics point of view. But it is of much broader interest too, because the level of accuracy and confidence reached by these calculations opens up the concrete possibility of using nuclei to address open questions in other sub-fields of physics, such as, understanding the fundamental properties of neutrinos, or the particle nature of dark matter.In this talk, I will review recent progress in microscopic calculations of electroweak properties of light nuclei, including electromagnetic moments, form factors and transitions in between lowlying nuclear states along with preliminary studies for single- and double-beta decay rates. I will illustrate the key dynamical features required to explain the available experimental data, and, if time permits, present a novel framework to calculate neutrino-nucleus cross sections for A > 12 nuclei.

The NEXT high pressure xenon gas TPC for neutrinoless double beta decay searches

A high pressure xenon gas time projection chamber with electroluminescent amplification (EL HPGXe TPC) offers for the search of the neutrinoless double beta decay (0νββ): an excellent energy resolution (0.5−0.7% FWHM at the Qββ) and tracking capabilities as demonstrated by the NEXT collaboration using two kg-scale prototypes. The NEXT collaboration is constructing an EL HPGXe TPC capable to hold 100 kg (NEXT-100) of xenon isotopically enriched in 136Xe. Since October 2016, a first phase of the NEXT experiment, called NEW detector, is running with depleted 136Xe at the Laboratorio Subterraneo de Canfranc. The NEW detector is a scale 1:2 in size of the NEXT-100 detector and uses same materials and photosensors. It will be used to perform a characterization of the 0νββ backgrounds and a measurement of the standard double beta decay rate. Recent results on the characterization of the NEW detector performance using data from calibration sources are presented.

Scalar-mediated double beta decay and LHC

The decay rate of neutrinoless double beta (0νββ) decay could be dominated by Lepton Number Violating (LNV) short-range diagrams involving only heavy scalar intermediate particles, known as “topology-II” diagrams. Examples are diagrams with diquarks, leptoquarks or charged scalars. Here, we compare the LNV discovery potentials of the LHC and 0νββ-decay experiments, resorting to three example models, which cover the range of the optimistic-pessimistic cases for 0νββ decay. We use the LHC constraints from dijet as well as leptoquark searches and find that already with 20/fb the LHC will test interesting parts of the parameter space of these models, not excluded by the current limits on 0νββ-decay.