Solar neutrino background in high-pressure gaseous ^(82)SeF_(6) TPC neutrinoless double beta decay experiments

In this study,the possibility of observing a solar neutrino background in a future neutrinoless double beta decay experiment using a high-pressure gaseous ^(82)SeF_(6) 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.

First results on ^(76)Ge neutrinoless double beta decay from CDEX-1 experiment

We report the first results on ^(76)Ge neutrinoless double beta decay from stage one of the China dark-matter experiment(CDEX).A p-type point-contact high-purity germanium detector with a mass of 994 g has been installed to detect neutrinoless double beta decay events, as well as to directly detect dark matter particles. An exposure of 304 kg d has been analyzed over a wide spectral band from 500 keV to 3 MeV. The average event rate obtained was about 0.012 counts per keV per kg per day over the 2.039 MeV energy range. The half-life of ^(76)Ge neutrinoless double beta decay derived based on this result is T^(0ν)1/2>6.4×10^(22) yr(90% C.L.). An upper limit on the effective Majorana-neutrino mass of 5.0 eV has been achieved.

Study of CdMoO4 crystal for a neutrinoless double beta decay experiment with ^116Cd and ^100Mo nuclides

The scintillation properties of a CdMoO4 crystal have been investigated experimentally.The fluorescence yields and decay times measured from 22 K to 300 K demonstrate that CdMoO4 crystal is a good candidate for an absorber for a bolometer readout,for both heat and scintillation signals.The results from Monte Carlo studies,taking the backgrounds from 2ν2β of^100 42Mo ^116 48Cd） and internal trace nuclides ^214Bi and ^208Tl into account,show that the expected sensitivity of a CdMoO4 bolometer for neutrinoless double beta decay experiments with an exposure of 100kg·years is one order of magnitude higher than those of the current sets of the limT1/2^0νββ of ^100 42Mo and ^116 48Cd.

Hiding neutrinoless double beta decay in the minimal seesaw model with the TM1 orμ-τreflection symmetry

In Asaka et al(2021 Phys.Rev.D 103,015014),Asaka,Ishida and Tanaka put forward an interesting possibility that the neutrinoless double beta decay can be hidden in the minimal seesaw model with the two right-handed neutrinos having a hierarchical mass structure:the lighter one is lighter enough than the typical Fermi-momentum scale of nuclei while the heavier one is sufficiently heavy to decouple from the neutrinoless double beta decay.Then,in the basis where the mass matrices of the charged leptons and right-handed neutrinos are diagonal,for some particular texture of the Dirac neutrino mass matrix M_(D),the neutrinoless double beta decay can be hidden.In this paper,on top of this specified model,we study the interesting scenario that M_(D)further obeys the TM1 symmetry orμ-τreflection symmetry which are well motivated by the experimental results for the neutrino mixing parameters.

NνDEx-100 conceptual design report

Observing nuclear neutrinoless double beta (0vββ) decay would be a revolutionary result in particle physics.Observing such a decay would prove that the neutrinos are their own antiparticles,help to study the absolute mass of neutrinos,explore the origin of their mass,and may explain the matter-antimatter asymmetry in our universe by lepton number violation.We propose developing a time projection chamber (TPC) using high-pressure ^(82)SeF_(6) gas and Topmetal silicon sensors for readout in the China Jinping Underground Laboratory (CJPL) to search for neutrinoless double beta decay of82Se,called the NvDEx experiment.Besides being located at CJPL with the world’s thickest rock shielding,NvDEx combines the advantages of the high Qββ(2.996 MeV) of82Se and the TPC’s ability to distinguish signal and background events using their different topological characteristics.This makes NvDEx unique,with great potential for low-background and high-sensitivity 0 vββsearches.NvDEx-100,a NvDEx experiment phase with 100 kg of SeF_(6)gas,is being built,with plans to complete installation at CJPL by 2025.This report introduces 0 vββ physics,the NvDEx concept and its advantages,and the schematic design of NvDEx-100,its subsystems,and background and sensitivity estimation.

Search for two-neutrino double-beta decay of^(136)Xe to the 0^(+)_(1)excited state of 136Ba with the complete EXO-200 dataset

A new search for two-neutrino double-beta(2νββ)decay of^(136)Xe to the 0+1 excited state of 136Ba is performed with the full EXO-200 dataset.A deep learning-based convolutional neural network is used to discriminate signal from background events.Signal detection efficiency is increased relative to previous searches by EXO-200 by more than a factor of two.With the addition of the Phase II dataset taken with an upgraded detector,the median 90%confidence level half-life sensitivity of 2νββdecay to the 0+1 state of 136Ba is 2.9×10^(24)yr using a total^(136)Xe exposure of 234.1 kg yr.No statistically significant evidence for 2νββdecay to the 0^(+)_(1)state is observed,leading to a lower limit of T2ν1/2(0^(+)→0^(+)_(1))>1.4×10^(24)yr at 90%confidence level,improved by 70%relative to the current world's best constraint.

Nuclear 0ν2βdecays in B-L symmetric SUSY model and in TeV scale left–right symmetric model

In this paper,we take the B-L supersymmetric standard model(B-LSSM)and TeV scale left–right symmetric model(LRSM)as two representations of the two kinds of new physics models to study the nuclear neutrinoless double beta decays(0ν2β)so as to see the senses onto these two kinds of models when the decays are taken into account additionally.Within the parameter spaces allowed by all the existing experimental data,the decay half-life of the nucleus^(76)Ge and^(136)Xe,T_(12)^(0ν)(^(76)Ge,^(136)Xe),is precisely calculated and the results are presented properly.Based on the numerical results,we conclude that there is greater room for LRSM type models than for B-LSSM type models in foreseeable future experimental observations on the decays.

Phenomenological advantages of the normal neutrino mass ordering

The preference of the normal neutrino mass ordering from the recent cosmological constraint and the global fit of neutrino oscillation experiments does not seem like a wise choice at first glance since it obscures the neutrinoless double beta decay and hence the Majorana nature of neutrinos.Contrary to this naive expectation,we point out that the actual situation is the opposite.The normal neutrino mass ordering opens the possibility of excluding the higher solar octant and simultaneously measuring the two Majorana CP phases in future 0ν2βexperiments.Especially,the funnel region will completely disappear if the solar mixing angle takes the higher octant.The combined precision measurement by the JUNO and Daya Bay experiments can significantly reduce the uncertainty in excluding the higher octant.With a typical O(meV)sensitivity on the effective mass|mee|,the neutrinoless double beta decay experiment can tell if the funnel region really exists and hence exclude the higher solar octant.With the sensitivity further improved to sub-meV,the two Majorana CP phases can be simultaneously determined.Thus,the normal neutrino mass ordering clearly shows phenomenological advantages over the inverted one.