Investigation of double-beta decay at the Institute of Theoretical and Experimental Physics (ITEP, Moscow)

Investigation of neutrinoless double-beta (2β0ν) decay is presently being considered as one of the most important problems in particle physics and cosmology Interest in the problem was quickened by the observation of neutrino oscillations. The results of oscillation experiments determine the mass differences between different neutrino flavors, and the observation of neutrinoless decay may fix the absolute scale and the hierarchy of the neutrino masses. Investigation of 2β0ν decay is the most efficient method for solving the problem of whether the neutrino is a Dirae or a Majorana particle, Physicists from the Institute of Theoretical and Experimental Physics (ITEP, Moscow) have been participating actively in solving this problem. They initiated and pioneered the application of semiconductor detectors manufactured from enriched germanium to searches for the double-beta decay of ⁷⁶Ge. Investigations with ⁷⁶Ge provided the most important results. At present, ITEP physicists are taking active part in four very large projects, GERDA. Majorana, EXO, and NEMO, which are capable of recording 2β0ν decay at a Majorana neutrino mass of 〈m _ν〉 ≈ 10^?2 eV.     

Double-beta decay and neutrino mass

Recent achievements in the study of double-beta (ββ) decay are presented. We discuss the potential of this process to search, beyond Standard Model physics, for the QRPA-based methods used for the calculation of the relevant nuclear matrix elements and the derivation of the neutrino mass from both ββ-decay calculations and neutrino oscillation and cosmological data. The key position of the ββ-decay experiments in resolving the neutrino absolute mass is highlighted.     

Majorana neutrinos, <Emphasis Type="Italic">CP</Emphasis> violation,neutrinoless double beta and tritium beta decays

If the present or upcoming searches for neutrinoless double beta ((ββ)_0ν) decay give a positive result, the Majorana nature of massive neutrinos will be established. From the determination of the value of the (ββ)_0ν-decay effective Majorana mass parameter (|〈m〉|), it would be possible to obtain information on the type of neutrino mass spectrum. Assuming 3-ν mixing and massive Majorana neutrinos, we discuss the information that a measurement of, or an upper bound on, |〈m〉| can provide on the value of the lightest neutrino mass m₁. With additional data on the neutrino masses obtained in ³H β-decay experiments, it might be possible to establish whether the CP symmetry is violated in the lepton sector. This would require very high precision measurements. If CP invariance holds, the allowed patterns of the relative CP parities of the massive Majorana neutrinos would be determined.     

Predictions of 2ν and 0ν double beta decay rates for nuclei with A ⩾ 70

Microscopic calculations of double beta (ββ) decay matrix elements are presented for the first time for all potential ββ decaying nuclei with A ? 70. This replaces previous phase-space estimates for most of these nuclei. It is found that nuclear structure effects considerably influence the ββ decay rates. The best candidates for 2ν as well as 0νββ decay experiments are selected.     

Double-β decay of116Cd

The NEMO-2 tracking detector located in the Fréjus Underground Laboratory was designed as a prototype of the detector NEMO-3 to study 0ν and 2ν double-beta decay (ββ) physics. After ten months of nearly continuous running with an enriched cadmium source (0.92 mol·y of¹¹⁶Cd) aββ2ν half-life ofT _1/2=(3.75±0.35(stat)±0.21(syst))·10¹⁹ y was measured. Limits with 90% CL on the¹¹⁶Cd half-lives of 5.0·10²¹ y forββ0ν decay and of 1.2·10²¹ y forββ0νχ ⁰ decay with a Majoron (χ ⁰) were obtained. Theoretical predictions for 0ν and 2ν decays of¹¹⁶Cd are also presented.     

Searching the absolute neutrino mass in tritium β-decay—interplay between nuclear, atomic and molecular physics

The paper treats recent experiments measuring the endpoint region of tritium β-decay with high resolution, sensitivity and background rejection, using an electrostatic filter with adiabatic magnetic collimation. The spectra are analysed with respect to the neutrino mass. These results form the primary source for the present upper limit of the neutrino mass m _ν ?<?2 eV quoted by the particle data group. Particular attention is paid to the decisive influence which atomic and molecular physics effects take on the results. A brief outlook on future experiments is given.     

Towards the meV limit of the effective neutrino mass in neutrinoless double-beta decays

We emphasize that it is extremely important for future neutrinoless double-beta(0νββ)decay experiments to reach the sensitivity to the effective neutrino mass|mββ|≈1 meV.With such a sensitivity,it is highly possible to discover the signals of 0νββ decays.If no signal is observed at this sensitivity level,then either neutrinos are Dirac particles or stringent constraints can be placed on their Majorana masses.In this paper,assuming the sensitivity of|mββ|≈1 meV for future 0νββ decay experiments and the precisions on neutrion oscillation parameters after the JUNO experiment,we fully explore the constrained regions of the lightest neutrino mass m1 and two Majorana-type CP-violating phases{ρ,σ}.Several important conclusions in the case of normal neutrino mass ordering can be made.First,the lightest neutrino mass is severely constrained to a narrow range m1∈[0.7,8]meV,which together with the precision measurements of neutrino mass-squared differences from oscillation experiments completely determines the neutrino mass spectrum m2∈[8.6,11.7]meV ing phases is limited to ρ∈[130°,230°],which cannot be obtained from any other realistic experiments.Third,the sum of three neutrino masses is found to beΣ≡m1+m2+m3∈[59.2,72.6]meV,while the effective neutrino mass for beta decays turns out to be mβ≡(|Ue1|2m1^2+|Ue2|2m2^2+|Ue3|2m3^2)1/2∈[8.9,12.6]meV.These observations clearly set up the roadmap for future non-oscillation neutrino experiments aiming to solve the fundamental problems in neutrino physics.     

Double beta decays and neutrino nuclear responses

Neutrino-less double beta decays (0νββ), which violate the lepton number conservation law by ΔL=2, are unique and realistic probes for studying the Majorana nature of neutrinos, the ν mass spectrum and the absolute mass scale, the lepton sector CP and others beyond the standard electro-weak theory. Neutrino nuclear responses (nuclear matrix elements M^0ν) are crucial to extract neutrino properties of the particle physics interests from experimental 0νββ rates. Subjects discussed include 0νββ processes and Majorana neutrinos, the present and future 0νββ experiments, and neutrino nuclear responses (M^0ν) and charge exchange reactions.     

揭开中微子和反中微子的马约拉纳神秘面纱——无中微子双贝塔衰变低温晶体量热器实验

中微子可能的马约拉纳粒子属性超出了目前标准模型的范畴,是粒子物理与核物理研究领域最重要的科学问题之一。无中微子双贝塔衰变(0vββ)实验是能够确定中微子马约拉纳属性的唯一途径。0vββ的发现可以揭示中微子绝对质量、轻子数破缺、物质—反物质不对称等一系列自然奥秘,是当今粒子物理与核物理研究的前沿课题。在探索无中微子双贝塔衰变的可选择实验方案中,低温晶体量热器具有高能量分辨率、高运行稳定性和低辐射本底的技术优势,成为新一代0vββ实验最具竞争力的探测器技术之一。文章首先介绍无中微子双贝塔衰变的研究历史,之后介绍低温晶体量热器及其最先进的代表——CUORE实验,最后展望关于我国锦屏地下实验室开展低温晶体量热器0vββ实验研究的前景。     

Limits of Majorana neutrino mass from combined analysis of data from <Superscript>76</Superscript>Ge and <Superscript>136</Superscript>Xe neutrinoless double beta decay experiments

We present effective Majorana neutrino mass limits <m _ββ> obtained from the joint analysis of the recently published results of ⁷⁶Ge and ¹³⁶Xe neutrinoless double beta decay (0νββ) experiments, which was carried out by using the Bayesian calculations. Nuclear matrix elements (NMEs) used for the analysis are taken from the works, in which NMEs of ⁷⁶Ge and ¹³⁶Xe were simultaneously calculated. This reduced systematic errors connected with NME calculation techniques. The new effective Majorana neutrino mass limits <m _ββ> less than [85.4–197.0] meV are much closer to the inverse neutrino mass hierarchy region.     

Double beta decay of 96Zr

After 10357 h of running the NEMO-2 tracking detector with an isotopically enriched zirconium source (0.084 mol yr of ⁹⁶Zr), a ββ2ν decay half-life of T_1/2=(2.1^+0.8(stat)_−0.4(stat)±0.2(syst))·10¹⁹ y was measured. Limits with a 90% C.L. on the ⁹⁶Zr half-lives of 1.0·10²¹ y for ββ0ν decay to the ground state, 3.9·10²⁰ y to the 2⁺ excited state and 3.5·10²⁰ y for ββ0νχ⁰ decay with a Majoron (χ⁰) were obtained. The data also provide direct limits at the 90% C.L. for the ⁹⁴Zr half-lives. These limits are 1.1·10¹⁷ y for ββ2ν decay to the ground state, 1.9·10¹⁹ y for ββ0ν decay to the ground state and 2.3·10¹⁸ y for ββ0νχ⁰ decay to ground state.     

Neutrinos in the time of Higgs

In this paper, the recent progress in the determination of neutrino oscillation parameters and future prospects have been discussed. The tiny neutrino masses as inferred from oscillation data and cosmology cannot be explained naturally by the Higgs mechanism and warrant some new physics. The latter can be connected to the Majorana nature of the neutrinos which can be probed by neutrinoless double beta decay (0 νββ). The paper also summarizes the latest experimental results in 0 νββ and discusses some implications for the left–right symmetric model which could be a plausible new physics scenario for the generation of neutrino masses.     

Neutrino mass from laboratory: contribution of double beta decay to the neutrino mass matrix

Double beta decay is indispensable to solve the question of the neutrino mass matrix together with ν oscillation experiments. The most sensitive experiment - since eight years the HEIDELBERG-MOSCOW experiment in Gran-Sasso - already now, with the experimental limit of m_ν < 0.26 eV practically excludes degenerate ν mass scenarios allowing neutrinos as hot dark matter in the universe for the smallangle MSW solution of the solar neutrino problem. It probes cosmological models including hot dark matter already now on the level of future satellite experiments MAP and PLANCK. It further probes many topics of beyond SM physics at the TeV scale. Future experiments should give access to the multi-TeV range and complement on many ways the search for new physics at future colliders like LHC and NLC. For neutrino physics some of them (GENIUS) will allow to test almost all neutrino mass scenarios allowed by the present neutrino oscillation experiments.     

Search for neutrino radiative decay with a prototype Borexino detector

Results of background measurements with a prototype of the Borexino detector were used to obtain bounds on the lifetime of radiative neutrino decay ν_H→ν_L+γ. The new lower limit for the lifetime of pp and ⁷Be neutrinos is τ_c.m.(ν_H→ν_L+γ) /mν≥4.2×10³ s eV^?1(α)= 0). It is more than an order of magnitude stronger than the value obtained in previous experiments using nuclear reactors and accelerators.     

Nuclear matrix elements for rare decays

Neutrinoless double electron capture (0νECEC) is being vigorously investigated because of the possibility of it telling us something about the absolute mass scale of the neutrino. The resonant 0νECEC is particularly interesting due to the potentially huge enhancement of its decay rate by a resonance condition. Recently the mass differences of two atom pairs were measured in order to study the enhancement of the 0νECEC rates of ⁷⁴Se and ¹¹²Sn. The associated nuclear matrix elements were also evaluated. The neutrino mass can also be detected by using beta decays with low Q values. Related to this we have investigated the second-forbidden decay branch of ¹¹⁵In with its ultra-low Q value. Open questions about nuclear and atomic contributions to the associated decay rate emerge.     

Direct neutrino mass measurements

The determination of the neutrino rest mass plays an important role at the intersections of cosmology, particle physics and astroparticle physics. This topic is currently being addressed by two complementary approaches in laboratory experiments. Neutrinoless double beta decay experiments probe whether neutrinos are Majorana particles and determine an effective neutrino mass value. Single beta decay experiments such as KATRIN and MARE investigate the spectral shape of β-decay electrons close to their kinematic endpoint in order to determine the neutrino rest mass with a model-independent method. Owing to neutrino flavour mixing, the neutrino mass parameter appears as an average of all neutrino mass eigenstates contributing to the electron neutrino. The KArlsruhe TRItium Neutrino experiment (KATRIN) is currently the experiment in the most advanced status of commissioning. Applying an ultra-luminous molecular windowless gaseous tritium source and an integrating high-resolution spectrometer of MAC-E filter type, it allows β-spectroscopy close to the T ₂ end-point with unprecedented precision and will reach a sensitivity of 200 meV/c ² (90% C.L.) on the neutrino rest mass.     

A large scale double beta and dark matter experiment: On the physics potential of GENIUS

The physics potential of GENIUS, a recently proposed double beta decay and dark matter experiment is discussed. The experiment will allow to probe neutrino masses down to 10^?(2–3) eV. GENIUS will test the structure of the neutrino mass matrix, and therefore implicitly neutrino oscillation parameters comparable or superior in sensitivity to the best proposed dedicated terrestrial neutrino oscillation experiments. If the 10^-3 eV level is reached, GENIUS will even allow to test the large angle MSW solution of the solar neutrino problem. Even in its first stage GENIUS will confirm or rule out degenerate or inverted neutrino mass scenarios, which have been widely discussed in the literature as a possible solution to current hints on finite neutrino masses and also test the ν_e ? ν_μ hypothesis of the atmospheric neutrino problem. GENIUS would contribute to the search for R-parity violating SUSY and right-handed W-bosons on a scale similar or superior to LHC. In addition, GENIUS would largely improve the current 0νββ decay searches for R-parity conserving SUSY and leptoquarks. Concerning cold dark matter (CDM) search, the low background anticipated for GENIUS would, for the first time ever, allow to cover the complete MSSM neutralino parameter space, making GENIUS competitive to LHC in SUSY discovery. If GENIUS could find SUSY CDM as a by-product it would confirm that R-parity must be conserved exactly. GENIUS will thus be a major tool for future non-accelerator particle physics.     

Grand unification in the minimal left-right symmetric extension of the standard model

The simplest minimal left-right symmetric extension of the Standard Model is studied in the high energy limit. Some consequences of the Grand Unification hypothesis are explored assuming that the parity-breaking scale is the only relevant energy between the electroweak scale and the unification point. While the model is shown to be compatible with the observed neutrino phenomenology, the parity-breaking scale and the heavy boson masses are predicted to be above 107 TeV, which is beyond the reach of present-day experiments. Below that scale, only an almost sterile right-handed neutrino with a mass M(ν _R ) of ≈100 TeV is allowed.     

Ikeda sum rule,self-consistency and double-beta decay in the renormalized quasiparticle random phase approximation

The effect of the inclusion of ground-state correlations into the QRPA equation of motion for the two-neutrino double-beta (ββ_2ν) decay is carefully analyzed. The resulting model, called renormalized QRPA (RQRPA), does not collapse near the physical value of the nuclear force strength in the particle-particle channel, as happens with the ordinary QRPA. Still, the ββ_2ν transition amplitude is only slightly less sensitive on this parameter in the RQRPA than that in the plain QRPA. It is argued that this fact reveals once more than the characteristic behavior of the ββ_2ν transition amplitude within the QRPA is not an artifact of the model, but a consequence of the partial restoration of the spin-isospin SU(4) symmetry. It is shown that the price paid for bypassing the collapse in the RQRPA is the violation of the Ikeda sum rule.