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。     

Present and future strategies for neutrinoless double beta decay searches

The renewed interest shown in these days towards neutrinoless double beta decay, a lepton number violating process which can take place only if neutrinos are Majorana particles (ν = \(\bar \nu \)) with a nonvanishing mass, is justified by the fact that the Majorana nature of neutrinos is expected in many theories beyond the Standard Model. We also now know, thanks to the neutrino oscillation experiments, that neutrinos are in fact massive, as expected in these theories and not requested in the Standard Model. Moreover, since neutrino oscillation experiments measure only the absolute value of the difference of the square of the neutrino masses, the discovery of neutrinoless double beta decay would help to disentangle questions that still remain unsolved: what is the absolute mass scale of the neutrinos and which mass hierarchy (normal, inverted or quasi-degenerate) is the correct one?The scope of this paper is not only to review the present results reached in the field by the different groups and technologies worldwide, but also to illustrate and comment on the (near and long-term) future strategies that experimentalists are trying to pursue to reach the needed sensitivity required to explore the inverted hierarchy neutrino mass scale.     

Neutrinoless double beta decay in the LHC era

Once neutrinoless double beta decay is discovered, the question which mechanism triggers the decay becomes crucial for drawing any conclusion about the concrete physics underlying the process, like the neutrino Majorana mass. For example, in the minimal supersymmetric extension with R-parity violation both neutrino Majorana masses and superpartners can trigger the decay. We show that in this case, if the decay is triggered by superpartners, there exist good prospects to observe single slepton production at the LHC. Resonant single slepton production at the LHC can therefore discriminate between the neutrinoless double beta decay mechanism and others.     

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.     

Neutrinoless double beta decay and a new limit on the lepton number violation

We have calculated the neutrinoless double beta decay rate of ⁷⁶Ge. We take into account for the first time a relativistic correction to the nuclear current including weak magnetism. Its effect is to cancel a considerable part of the decay amplitude and we obtain less stringent upper limits on the neutrino Majorana mass and the right-handed weak leptonic current compared with previous calculations.     

Random coincidence of 2<Emphasis Type="Italic">ν</Emphasis>2<Emphasis Type="Italic">β</Emphasis> decay events as a background source in bolometric 0<Emphasis Type="Italic">ν</Emphasis>2<Emphasis Type="Italic">β</Emphasis> decay experiments

Two-neutrino double β decay can create an irremovable background even in high energy resolution detectors searching for neutrinoless double β decay due to random coincidence of 2ν2β events in the case of poor time resolution. Some possibilities for suppressing this background in cryogenic scintillating bolometers are discussed. It is shown that the present bolometric detector technologies enable one to control this form of background at the level required to explore the inverted hierarchy of the neutrino mass pattern, including the case of bolometers searching for the neutrinoless double β decay of ¹⁰⁰Mo, which is characterized by a relatively short two-neutrino double β decay half-life.     

New limits on double beta decay of 76Ge

New results are presented from an experiment on double beta decay of ⁷⁶Ge carried out with two Ge Li detectors in the Mont Blanc tunnel. No evidence has been found for neutrinoless double beta decay, and the obtained lower limit of the half lifetime (1.2 × 10²³ yr) is discussed in terms of the mass of Majorana neutrinos and of the possible presence of right-handed currents.     

Double-beta-decay experiments: Present status and prospects for the future

The present status of experiments seeking double-beta decay is surveyed. The results of the most sensitive experiments are discussed. Particular attention is given to describing the NEMO-3 detector, which is intended for seeking the neutrinoless double-beta decay of various isotopes (¹⁰⁰Mo, ⁸²Se, etc.) with a sensitivity as high as T _1/2 ～ 10²⁵ yr, which corresponds to a sensitivity to the Majorana neutrino mass at a level of 0.1 to 0.3 eV. The first results obtained with the NEMO-3 detector are presented. A review of the existing projects of double-beta-decay experiments where it is planned to reach a sensitivity to the Majorana neutrino mass at a level of 0.01 to 0.1 eV is given.     

The neutrinoless double-beta decay: A test for new physics

The neutrinoless double-beta decay is not allowed in the Standard Model (SM) but it is allowed in most Grand Unified Theories (GUTs). The neutrino must be a Majorana particle (identical with its antiparticle) and must have a mass to allow the neutrinoless double-beta decay. Apart of one claim that the neutrinoless double-beta decay in ⁷⁶Ge is measured, one has only upper limits for this transition probability. But even the upper limits allow to give upper limits for the electron Majorana neutrino mass and upper limits for parameters of GUTs and the minimal R-parity violating supersymmetric model. One further can give lower limits for the vector boson mediating mainly the right-handed weak interaction and the heavy mainly right-handed Majorana neutrino in left-right symmetric GUTs. For that, one has to assume that the specific mechanism is the leading one for the neutrinoless double-beta decay and one has to be able to calculate reliably the corresponding nuclear matrix elements. In the present contribution, one discusses the accuracy of the present status of calculating the nuclear matrix elements and the corresponding limits of GUTs and supersymmetric parameters.     

First results of the search for neutrinoless double-beta decay with the NEMO 3 detector

The NEMO 3 detector, which has been operating in the Fréjus underground laboratory since February 2003, is devoted to the search for neutrinoless double-beta decay (beta beta 0v). The half-lives of the two neutrino double-beta decay (beta beta 2v) have been measured for 100Mo and 82Se. After 389 effective days of data collection from February 2003 until September 2004 (phase I), no evidence for neutrinoless double-beta decay was found from approximately 7 kg of 100Mo and approximately 1 kg of 82Se. The corresponding limits are T1/2(beta beta0v) > 4.6 x 10(23) yr for 100Mo and T1/2(beta beta 0v) > 1.0 x 10(23) yr for 82Se (90% C.L.). Depending on the nuclear matrix element calculation, the limits for the effective Majorana neutrino mass are < 0.7-2.8 e/v for 100Mo and < 1.7-4.9 eV for 82Se.     

Majorana neutrino masses and the neutrinoless double-beta decay

Neutrinoless double-beta decay is forbidden in the Standard Model of electroweak and strong interaction but allowed in most Grand Unified Theories (GUTs). Only if the neutrino is a Majorana particle (identical with its antiparticle) and if it has a mass is neutrinoless double-beta decay allowed. Apart from one claim that the neutrinoless double-beta decay in ⁷⁶Ge is measured, one has only upper limits for this transition probability. But even the upper limits allow one to give upper limits for the electron Majorana neutrino mass and upper limits for parameters of GUTs and the minimal R-parity-violating supersymmetric model. One further can give lower limits for the vector boson mediating mainly the right-handed weak interaction and the heavy mainly right-handed Majorana neutrino in left-right symmetric GUTs. For that, one has to assume that the specific mechanism is the leading one for neutrinoless double-beta decay and one has to be able to calculate reliably the corresponding nuclear matrix elements. In the present work, one discusses the accuracy of the present status of calculating of the nuclear matrix elements and the corresponding limits of GUTs and supersymmetric parameters. The text was submitted by the author in English.     

Probing the mechanism of neutrinoless double beta decay with SuperNEMO

The SuperNEMO experiment is being designed to search for neutrinoless double beta decay. Its experimental technique of tracking and calorimetry provides the means to discriminate different underlying mechanisms for neutrinoless double beta decay by measuring the angular and energy distributions of electrons. The results of a study by the SuperNEMO Collaboration and F. Deppisch (in preparation) [7] for identifying light Majorana neutrino exchange and right-handed currents are presented.     

Extension of an empirical charged lepton mass relation to the neutrino sector

It is known that the charged lepton masses obey to high precision an interesting empirical relation (Koide relation). In turn, the light neutrino masses cannot obey such a relation. We note that if neutrinos acquire their mass via the seesaw mechanism, the empirical mass relation could hold for the masses in the Dirac and/or heavy Majorana mass matrix. Examples for the phenomenological consequences are provided. We furthermore modify the mass relation for light neutrino masses including their Majorana phases, and show that it can be fulfilled in this case as well, with interesting predictions for neutrinoless double beta decay.     

The Two-Nucleon Mechanism for the Neutrinoless Double Beta Decay and the BCS Nuclear Wave Function

A method of using the Bardeen-Cooper-Schrieffer (BCS) nuclear wave function to treat the two-nucleon mechanism for neutiinoless double beta decay process 0⁺ → 0⁺ is proposed.The neutrinoless decay mode and the neutrinoless decay accompanied by a Majoron emission mode of ⁸²Se are studikd. Our cdculated results show that to reproduce the experimental value of γ^(ov) > 1.8 × 10²² yr for neutrinoless double beta decai of ⁸²Se the Majorana neutrino mass m_v < 6.2 eV and the mixing parameter of right-handed current η < 7.0 × 10^-6 In the emission with a Majoron mode the effective Majoron coupling to neutrino is deduced from the experimental value of γ^(ov,H) > 4.4 × 10²⁰ yr for ⁸²Se with the result H0> < 6.2 × 10^-4.     

Neutrinoless double beta decay with small and hierarchical neutrino mass

We construct a model where neutrino Majorana masses are small and hierarchical but where neutrinoless double beta decay occurs at an observable rate potentially detectable by present day experiments.     

A multi-isotope $$0\nu 2\beta $$ bolometric experiment

There are valuable arguments to perform neutrinoless double beta (\(0\nu 2\beta \)) decay experiments with several nuclei: the uncertainty of nuclear-matrix-element calculations; the possibility to test these calculations by using the ratio of the measured lifetimes; the unpredictability of possible breakthroughs in the detection technique; the difficulty to foresee background in \(0\nu 2\beta \) decay searches; the limited amount of isotopically enriched materials. We propose therefore approaches to estimate the Majorana neutrino mass by combining experimental data collected with different \(0\nu 2\beta \) decay candidates. In particular, we apply our methods to a next-generation experiment based on scintillating and Cherenkov-radiation bolometers. Current results indicate that this technology can effectively study up to four different isotopes simultaneously (\(^{82}\)Se, \(^{100}\)Mo, \(^{116}\)Cd and \(^{130}\)Te), embedded in detectors which share the same concepts and environment. We show that the combined information on the Majorana neutrino mass extracted from a multi-candidate bolometric experiment is competitive with that achievable with a single isotope, once that the cryogenic experimental volume is fixed. The remarkable conceptual and technical advantages of a multi-isotope investigation are discussed. This approach can be naturally applied to the proposed CUPID project, follow-up of the CUORE experiment that is currently taking data in the Gran Sasso underground laboratory.     

Constraining the lightest neutrino mass and <Emphasis Type="BoldItalic">m</Emphasis> <Subscript> <Emphasis Type="BoldItalic">ee</Emphasis> </Subscript> from general lepton mass matrices

Despite spectacular advances in fixing the neutrino mass and mixing parameters through various neutrino oscillation experiments, we still have little knowledge about the magnitudes of some vital parameters in the neutrino sector such as the absolute neutrino mass scale, effective Majorana mass m_ee measured in neutrinoless double beta decay. In this context, the present work aims to make an attempt to obtain some bounds for m_ee and the lightest neutrino mass using fairly general lepton mass matrices in the Standard Model.     

No-go for detecting CP violation via neutrinoless double beta decay

We present a necessary condition on the solar oscillation amplitude for CP violation to be detectable through neutrinoless double beta (0νββ) decay. It depends only on the fractional uncertainty in the ν_e–ν_e element of the neutrino mass matrix. We demonstrate that even under very optimistic assumptions about the sensitivity of future experiments to the absolute neutrino mass scale, and on the precision with which nuclear matrix elements that contribute to 0νββ decay are calculable, it will be impossible to detect neutrino CP violation arising from Majorana phases.     

Majorana neutrinos and same-sign dilepton production at LHC and in rare meson decays

We discuss same-sign dilepton production mediated by Majorana neutrinos in high-energy proton–proton collisions for at the LHC energy TeV, and in the rare decays of the , and B mesons of the type . For the pp reaction, assuming one heavy Majorana neutrino of mass , we present discovery limits in the plane where are the mixing parameters. Taking into account the present limits from low-energy experiments, we show that at LHC one has sensitivity to heavy Majorana neutrinos up to a mass TeV in the dilepton channels , and , but the dilepton states will not be detectable due to the already existing constraints from neutrinoless double beta decay. We work out a large number of rare meson decays, both for the light and heavy Majorana neutrino scenarios, and argue that the present experimental bounds on the branching ratios are too weak to set reasonable limits on the effective Majorana masses. Received: 24 April 2001 / Published online: 29 June 2001     

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.