Future Neutrinoless Double Beta Decay Research Articles

Solar neutrino background in high-pressure gaseous 82SeF6 TPC neutrinoless double beta decay experiments* *Supported in part by the National Natural Science Foundation of China (12105110) and National Key Research and Development Program of China (2022YFA1604703)

In this study, the possibility of observing a solar neutrino background in a future neutrinoless double beta decay experiment using a high-pressure gaseous 82SeF6 TPC is investigated. Various contributions are simulated, and possible features that could be used for event classification are discussed; two types of backgrounds are identified. The rate of multi-site background events is approximately 0.63 events/(ton·yr) in a 30 keV ROI window. This background could be effectively reduced to less than 0.0001 events/(ton·yr) (95% C.L.) while maintaining a high signal efficiency of 93% by applying a selection based on the number of clusters and energy of the leading cluster. The rate of the single-electron background events is approximately 0.01 events/(ton·yr) in the ROI. Assuming a reduction factor of 10 for the single-electron background events obtained via the algorithms developed for radioactive background rejection, the total background induced by the solar neutrino would be 0.001 events/(ton·yr), which is sufficiently small for conducting ton-level experiments.

Impact of nuclear matrix element calculations for current and future neutrinoless double beta decay searches

Nuclear matrix elements (NME) are a crucial input for the interpretation of neutrinoless double beta decay data. We consider a representative set of recent NME calculations from different methods and investigate the impact on the present bound on the effective Majorana mass mββ by performing a combined analysis of the available data as well as on the sensitivity reach of future projects. A crucial role is played by the recently discovered short-range contribution to the NME, induced by light Majorana neutrino masses. Depending on the NME model and the relative sign of the long- and short-range contributions, the current 3σ bound can change between mββ< 40 meV and 600 meV. The sign-uncertainty may either boost the sensitivity of next-generation experiments beyond the region for mββ predicted for inverted mass ordering or prevent even advanced setups to reach this region. Furthermore, we study the possibility to distinguish between different NME calculations by assuming a positive signal and by combining measurements from different isotopes. Such a discrimination will be impossible if the relative sign of the long- and short-range contribution remains unknown, but can become feasible if mββ ≳ 40 meV and if the relative sign is known to be positive. Sensitivities will be dominated by the advanced 76Ge and 136Xe setups assumed here, but NME model-discrimination improves if data from a third isotope is added, e.g., from 130Te or 100Mo.

Uncovering a chirally suppressed mechanism of 0νββ decay with LHC searches

∆L = 2 lepton number violation (LNV) at the TeV scale could provide an alternative interpretation of positive signal(s) in future neutrinoless double beta (0νββ) decay experiments. An interesting class of models from this point of view are those that at low energies give rise to dimension-9 vector operators and a dimension-7 operator, both of whose 0νββ-decay rates are “chirally suppressed”. We study and compare the sensitivities of 0νββ-decay experiments and LHC searches to a simplified model in this class of TeV-scale LNV that is also SU(2)L × U(1)Y gauge invariant. The searches for 0νββ decay, which are here diluted by a chiral suppression of the vector operators, are found to be less constraining than LHC searches whose reach is increased by the assumed kinematic accessibility of the mediator particles. For the chirally suppressed dimension-7 operator generated by TeV-scale mediators, in contrast, 0νββ-decay searches place strong constraints on the size of the new Yukawa coupling. Signals of this model at the LHC and 0νββ-decay experiments are entirely uncorrelated with the observed neutrinos masses, as these new sources of LNV give negligible contributions to the latter. We find the prospects for the high-luminosity LHC and ton-scale 0νββ-decay experiments to uncover the chirally-suppressed mechanism with TeV-scale LNV to be promising. We also comment on the sensitivity of the 0νββ-decay lifetime to certain unknown low-energy constants that in the case of dimension-9 scalar operators are expected to be large due to non-perturbative renormalization.

Neutrino mass and mass ordering: no conclusive evidence for normal ordering

The extraction of the neutrino mass ordering is one of the major challenges in particle physics and cosmology, not only for its implications for a fundamental theory of mass generation in nature, but also for its decisive role in the scale of future neutrinoless double beta decay experimental searches.It has been recently claimed that current oscillation, beta decay and cosmological limits on the different observables describing the neutrino mass parameter space provide robust decisive Bayesian evidence in favor of the normal ordering of the neutrino mass spectrum [1].We further investigate these strong claims using a rich and wide phenomenology, with different sampling techniques of the neutrino parameter space.Contrary to the findings of Jimenez et al. [1], no decisive evidence for the normal mass ordering is found.Neutrino mass ordering analyses must rely on priors and parameterizations that are ordering-agnostic: robust results should be regarded as those in which the preference for the normal neutrino mass ordering is driven exclusively by the data, while we find a difference of up to a factor of 33 in the Bayes factors among the different priors and parameterizations exploited here.An ordering-agnostic prior would be represented by the case ofparameterizations sampling over the two mass splittings and a mass scale,or thosesampling over the individual neutrino masses via normal prior distributions only.In this regard, we show that the current significance in favor of the normal mass ordering should be taken as 2.7σ (i.e. moderate evidence), mostly driven byneutrino oscillation data.Let us stress that, while current data favor NO only mildly, we do not exclude the possibility that this may change in the future.Eventually, upcoming oscillation and cosmological data may (or may not) lead to a more significant exclusion of IO.

Unraveling the left-right mixing using 0νββ decay and collider probes

In the context of the minimal left-right symmetric model, we study the interplay between current and future neutrinoless double beta ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$) decay experiments, long-lived particle searches at the LHC main detectors ATLAS/CMS, and the proposed far detector MATHUSLA. The heavy Majorana neutrino can be produced in association with an electron from the decay of $W$ boson for a nonzero left-right mixing and subsequently decays into another electron with the same charge and jets. Owing to the suppression of large right-handed charged gauge boson ${W}_{R}$ mass, the heavy neutrinos could be long-lived. We show that long-lived particle (LLP) searches for heavy Majorana neutrinos in the same-sign dilepton channel at the LHC can be used to extend ${W}_{R}$ boson mass reach relative to the reach of the Keung-Senjanovic (KS) process. Finally, we show that sensitivities of LLP searches at the high-luminosity LHC with main detectors ATLAS/CMS are competitive with those of future $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ decay searches.

Leptogenesis, fermion masses and mixings in a SUSY SU(5) GUT with D4 flavor symmetry

We propose a model of fermion masses and mixings based on SU(5) grand unified theory (GUT) and a D4 flavor symmetry. This is a highly predictive 4D SU(5) GUT with a flavor symmetry that does not contain a triplet irreducible representation. The Yukawa matrices of quarks and charged leptons are obtained after integrating out heavy messenger fields from renormalizable superpotentials while neutrino masses are originated from the type I seesaw mechanism. The group theoretical factors from 24- and 45-dimensional Higgs fields lead to ratios between the Yukawa couplings in agreement with data, while the dangerous proton decay operators are highly suppressed. By performing a numerical fit, we find that the model captures accurately the mixing angles, the Yukawa couplings and the CP phase of the quark sector at the GUT scale. The neutrino masses are generated at the leading order with the prediction of trimaximal mixing while an additional effective operator is required to account for the baryon asymmetry of the universe (BAU). The model is remarkably predictive because only the normal neutrino mass ordering and the lower octant of the atmospheric angle are allowed while the CP conserving values of the Dirac neutrino phase δCP are excluded. Moreover, the predicted values of the effective Majorana mass mββ can be tested at future neutrinoless double beta decay experiments. An analytical and a numerical study of the BAU via the leptogenesis mechanism is performed. We focused on the regions of parameter space where leptogenesis from the lightest right-handed neutrino is successfully realized. Strong correlations between the parameters of the neutrino sector and the observed BAU are obtained.

Measurement of the scintillation and ionization response of liquid xenon at MeV energies in the EXO-200 experiment

Liquid xenon (LXe) is employed in a number of current and future detectors for rare event searches. We use the EXO-200 experimental data to measure the absolute scintillation and ionization yields generated by $\gamma$ interactions from $^{228}$Th (2615~keV), $^{226}$Ra (1764~keV) and $^{60}$Co (1332~keV and 1173~keV) calibration sources, over a range of electric fields. The $W$-value that defines the recombination-independent energy scale is measured to be $11.5~\pm~0.5$~(syst.)~$\pm~0.1$~(stat.) eV. These data are also used to measure the recombination fluctuations in the number of electrons and photons produced by the calibration sources at the MeV-scale, which deviate from extrapolations of lower-energy data. Additionally, a semi-empirical model for the energy resolution of the detector is developed, which is used to constrain the recombination efficiency, i.e., the fraction of recombined electrons that result in the emission of a detectable photon. Detailed measurements of the absolute charge and light yields for MeV-scale electron recoils are important for predicting the performance of future neutrinoless double beta decay detectors.

Growth and Optical Properties of a Cs2Mo2O7 Single Crystal

A Cs2Mo2O7 single crystal, a molybdenum-based crystal as research and development scintillation material for future neutrinoless double beta decay experiments under milli-Kelvin temperatures, was grown by the Czochralski method. The grown crystal was prepared as a sample for measurements of its optical properties. The sample was cooled down from room temperature to 10 K, and temperature dependence of emission spectrum and decay time were measured. We used a light emitting diode as an excitation source for the emission spectra measurements and a pulsed-type laser for the decay time due to its high intensity. The intensity of emission spectrum and the decay time increased as the temperature decreased. We estimated the optical energy bandgap via a transmittance measurement at room temperature. As far as we know, it is the first time to grow a large Cs2Mo2O7 crystal and to study its luminescence properties. Moreover, we grew a well-studied Li2MoO4 crystal as a reference to compare the light output of the Cs2Mo2O7 at 10 K. Through these results, the Cs2Mo2O7 crystal found to be a candidate scintillator for the future neutrinoless double beta decay experiments.

Extracting Majorana properties from strong bounds on neutrinoless double beta decay

Assuming that neutrinos are Majorana particles, we explore what information can be inferred from future strong limits (i.e. non-observation) for neutrinoless double beta decay. Specifically we consider the case where the mass hierarchy is normal and the different contributions to the effective mass $\langle m \rangle_{ee}$ partly cancel. We discuss how this fixes the two Majorana CP phases simultaneously from the Majorana Triangle and how it limits the lightest neutrino mass $m_1$ within a narrow window. The two Majorana CP phases are in this case even better determined than in the usual case for larger $\langle m \rangle_{ee}$. We show that the uncertainty in these predictions can be significantly reduced by the complementary measurement of reactor neutrino experiments, especially the medium baseline version JUNO/RENO-50. We also estimate the necessary precision on $\langle m \rangle_{ee}$ to infer non-trivial Majorana CP phases and the upper limit $\langle m \rangle_{ee} \lesssim 1\,\mbox{meV}$ sets a target for the design of future neutrinoless double beta decay experiments.

Single molecule fluorescence imaging as a technique for barium tagging in neutrinoless double beta decay

Background rejection is key to success for future neutrinoless double beta decay experiments. To achieve sensitivity to effective Majorana lifetimes of ∼ 1028 years, backgrounds must be controlled to better than 0.1 count per ton per year, beyond the reach of any present technology. In this paper we propose a new method to identify the birth of the barium daughter ion in the neutrinoless double beta decay of 136Xe. The method adapts Single Molecule Fluorescent Imaging, a technique from biochemistry research with demonstrated single ion sensitivity. We explore possible SMFI dyes suitable for the problem of barium ion detection in high pressure xenon gas, and develop a fiber-coupled sensing system with which we can detect the presence of bulk Ba++ ions remotely. We show that our sensor produces signal-to-background ratios as high as 85 in response to Ba++ ions when operated in aqueous solution. We then describe the next stage of this R&D program, which will be to demonstrate chelation and fluorescence in xenon gas. If a successful barium ion tag can be developed using SMFI adapted for high pressure xenon gas detectors, the first essentially zero background, ton-scale neutrinoless double beta decay technology could be realized.

Left-right symmetry and lepton number violation at the Large Hadron electron Collider

We show that the proposed Large Hadron electron Collider (LHeC) will provide an opportunity to search for left-right symmetry and establish lepton number violation, complementing current and planned searches based on LHC data and neutrinoless double beta decay. We consider several plausible configurations for the LHeC -- including different electron energies and polarizations, as well as distinct values for the charge misidentification rate. Within left-right symmetric theories we determine the values of right-handed neutrino and gauge boson masses that could be tested at the LHeC after one, five and ten years of operation. Our results indicate that this collider might probe, via the $\Delta L =2$ signal $e^-p\to e^+jjj$, Majorana neutrino masses up to 1 TeV and $W_R$ masses up to 6.5 TeV. Interestingly, part of this parameter space is beyond the expected reach of the LHC and of future neutrinoless double beta decay experiments.

A flexible scintillation light apparatus for rare events searches

FLARES (a Flexible scintillation Light Apparatus for Rare Event Searches) is a project for an innovative detector technology to be applied to rare event searches, and in particular to neutrinoless double beta decay experiments. Its novelty is the enhancement and optimization of the collection of the scintillation light emitted by ultra-pure crystals through the use of arrays of high performance silicon photodetectors cooled to 120 K. This would provide scintillation detectors with ~1% level energy resolution, with the advantages of a technology offering relatively simple low cost mass scalability and powerful background reduction handles, as requested by future neutrinoless double beta decay experimental programs.

Hybrid textures of neutrino mass matrix under the lamppost of latest neutrino and cosmology data

In this paper, we study all possible neutrino mass matrices with one zero element and two equal nonzero elements, known as hybrid texture neutrino mass matrices. In the diagonal charged lepton basis, we consider 39 such possible cases which are consistent with the latest neutrino data. Using the two constraints on neutrino mass matrix elements imposed by hybrid textures, we numerically evaluate the neutrino parameters like the lightest neutrino mass [Formula: see text], one Dirac CP phase [Formula: see text] and two Majorana CP phases [Formula: see text], [Formula: see text] by using the global fit [Formula: see text] values of three mixing angles and two mass squared differences. We then constrain this parameter space by using the cosmological upper bound on the sum of absolute neutrino masses given by Planck experiment. We also calculate the effective neutrino mass [Formula: see text] for this region of parameter space which can have relevance in future neutrinoless double beta decay experiments. We finally discriminate between these hybrid texture mass matrices from the requirement of producing correct baryon asymmetry through type I seesaw leptogenesis. We also constrain the light neutrino parameter space as well as the lightest right-handed neutrino mass from the constraint on baryon asymmetry of the Universe from Planck experiment.

Neutrino masses from modified bimaximal mixing

The bimaximal (BM) neutrino mixing matrix was formulated in order to accommodate the data of the experimental results which indicate that both solar and atmospheric neutrino oscillation in vacuum are near maximal. But, after the T2K and Daya Bay Collaborations reported that the mixing angle [Formula: see text] is nonzero and relatively large, many authors have modified the neutrino mixing matrix in order to accommodate experimental data. We modified the BM mixing matrix by introducing a simple perturbation matrix into BM mixing matrix. The modified BM mixing matrix can proceed the mixing angles which are compatible with the global fit analysis data and by imposing the [Formula: see text]–[Formula: see text] symmetry into mass matrix from modified BM, we have the neutrino mass in normal hierarchy (NH): [Formula: see text]. Using the neutrino masses that obtained from neutrino mass matrix in the scheme of modified BM and imposing the constraint exact [Formula: see text] symmetry into neutrino mass matrix, we cannot have compatible squared-mass differences for both [Formula: see text] and [Formula: see text] as dictated by experimental results. In order to proceed the neutrino masses that can predict correctly the squared-mass difference, we introduce a small parameter [Formula: see text] into neutrino mass matrix. The obtained neutrino masses are in agreement with the squared-mass difference as dictated by experimental results. The predicted neutrino effective mass: [Formula: see text] in this paper can be tested in the future neutrinoless double beta decay.

FLARES: A flexible scintillation light apparatus for rare event searches

FLARES is a project for an innovative detector technology to be applied to rare event searches, and in particular to neutrinoless double beta decay experiments. Its novelty is the enhancement and optimization of the collection of the scintillation light emitted by ultra-pure crystals through the use of arrays of high performance silicon photodetectors cooled to 120K. This would provide scintillation detectors with 1% level energy resolution, with the advantages of a technology offering relatively simple low cost mass scalability and powerful background reduction handles, as requested by future neutrinoless double beta decay experimental programs. The performances of a first production of matrices of Silicon Drift Detectors are presented and discussed in this paper.

A golden [formula omitted] model of leptons with a minimal NLO correction

We propose a new A5 model of leptons which corrects the LO predictions of Golden Ratio mixing via a minimal NLO Majorana mass correction which completely breaks the original Klein symmetry of the neutrino mass matrix. The minimal nature of the NLO correction leads to a restricted and correlated range of the mixing angles allowing agreement within the one sigma range of recent global fits following the reactor angle measurement by Daya Bay and RENO. The minimal NLO correction also preserves the LO inverse neutrino mass sum rule leading to a neutrino mass spectrum that extends into the quasi-degenerate region allowing the model to be accessible to the current and future neutrinoless double beta decay experiments.

Effect of SiO2 coating in bolometric Ge light detectors for rare event searches

In germanium-based light detectors for scintillating bolometers, a SiO2 anti-reflective coating is often applied on the side of the germanium wafer exposed to light with the aim to improve its light collection efficiency. In this paper, we report about a measurement, performed in the temperature range 25–35mK, of the light-collection increase obtained thanks to this method, which resulted to be of the order of 20%. The procedure followed has been carefully selected in order to minimize systematic effects. The employed light sources have the same spectral features (peaking at ∼630nm wavelength) that will characterize future neutrinoless double beta decay experiments on the isotope 82Se and based on ZnSe crystals, such as LUCIFER. The coupling between source and light detector reproduces the configuration used in scintillating bolometers. The present measurement clarifies the role of SiO2 coating and describes a method and a set-up that can be extended to the study of other types of coatings and luminescent materials.

Development and Characterization of CdZnTe Detectors for Neutrino Physics Research

CdZnTe crystals contain 9 double beta decay isotopes and can serve simultaneously as source and detector in a search for neutrino-less double beta decay. In particular, 116Cd and 130Te are suitable isotopes in such a search due to their high Q-values. The endpoint of the beta spectra resulting from double-beta decay of these isotopes is well above natural gamma lines which constitute backgrounds to a potential signal. Detectors for neutrino-less double beta decay searches require good energy resolution and effective background rejection. Both properties can be realized with position sensitive pixilated detectors that have particle tracking capabilities. CdZnTe detectors are promising detectors to satisfy these criteria and have the additional advantage of room temperature operation. We are developing and characterizing the performance of co-planar and pixilated CdZnTe detectors, study their charged particle tracking capabilities and evaluate their use in future neutrino-less double beta decay search experiments. Results from our laboratory measurements will be presented.

MiniBooNE results and neutrino schemes with 2 sterile neutrinos: possible mass orderings and observables related to neutrino masses

The MiniBooNE and LSND experiments are compatible with each other when two sterile neutrinos are added to the three active ones. In this case there are eight possible mass orderings. In two of them both sterile neutrinos are heavier than the three active ones. In the next two scenarios both sterile neutrinos are lighter than the three active ones. The remaining four scenarios have one sterile neutrino heavier and another lighter than the three active ones. We analyze all scenarios with respect to their predictions for mass-related observables. These are the sum of neutrino masses as constrained by cosmological observations, the kinematic mass parameter as measurable in the KATRIN experiment, and the effective mass governing neutrinoless double beta decay. It is investigated how these non-oscillation probes can distinguish between the eight scenarios. Six of the eight possible mass orderings predict positive signals in the KATRIN and future neutrinoless double beta decay experiments. We also remark on scenarios with three sterile neutrinos. In addition we make some comments on the possibility of using decays of high energy astrophysical neutrinos to discriminate between the mass orderings in presence of two sterile neutrinos.

A NEUTRINO MASS MODEL WITH REFLECTION SYMMETRY

We demonstrate that an SU (2)L× U (1)Y model with the same particle content as Standard Model (SM) and discrete reflection symmetry between second and third generations of leptons gives rise to charged lepton and neutrino mass matrices which can accommodate the present solar, atmospheric, WMAP neutrino experimental results. The model predicts the value of |U13| which could be tested in neutrino factories and the effective Majorana neutrino mass &lt;mee&gt; which is at the lower end of the present experimental value. Neutrino masses are generated through dim=5 operators and the scale of which are constrained by the value of &lt;mee&gt;. If, in future neutrinoless double beta decay experiments namely, MOON, EXO, GENIUS shift the lower bound on &lt;mee&gt; by one order, the present model will fail to accommodate the solar neutrino mixing angle due to LMA solution.