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.     

Neutrinoless double beta-decay

The neutrinoless double β-decay of nuclei is reviewed. We discuss neutrino mixing and 3 × 3 PMNS neutrino mixing matrix. Basic theory of neutrinoless double β-decay is presented in some details. Results of different calculations of nuclear matrix element are discussed. Experimental situation is considered. The Appendix is dedicated to E. Majorana (brief biography and his paper in which the theory of Majorana particles is given)     

Theory of neutrinoless double beta decay—A brief review

Neutrinoless double decay (0νββ-decay) is a unique probe for lepton number conservation and neutrino properties. This is a process with long and interesting history with important implications for particle physics and cosmology, but its observation is still elusive. The search for the 0νββ-decay represents the new frontiers of neutrino physics, allowing to determine the Majorana nature of neutrinos and to fix the neutrino mass scale and possible CP violation effects, which could explain the matter-antimatter asymmetry in the Universe. At present a complete theory is missing and, thus, to motivate and guide the experiments the mechanism mediated by light neutrinos is mostly considered. The subject of interest is an effective mass of Majorana neutrinos, which can be deduced from the measured half-life, once this process is definitely observed. The accuracy of the determination of this quantity is mainly determined by our knowledge of the nuclear matrix elements. There is a request to evaluate them with high precision, accuracy and reliability. Recently, there is an increased interest to the resonant neutrinoless double electron capture, which may also establish the Majorana nature of neutrinos. This possibility is considered as alternative and complementary to searches for the 0νββ-decay.     

Q value related mass determinations using a Penning trap

We report here about measurements of reaction and decay Q values by precise determination of pairs of atomic masses. These were performed with the Penning trap mass spectrometer SMILETRAP. Measurements with Penning traps give reliable and accurate masses, in particular Q values, due to the fact that certain systematic errors to a great deal cancel in the mass difference between the two atoms defining the Q value. Some Q values that are of fundamental interest will be discussed here, for example, a new Q value for the ⁶Li (n,γ) ⁷Li reaction, for the β-decay of tritium, related to properties of the electron neutrino mass, and for the neutrino-less double β-decay of ⁷⁶Ge, related to the question of whether the neutrino is a Majorana particle or not. In case of the latter two we report the most accurate Q values, namely 18,589.8(12) eV for the tritium decay and 2,038.997(46) keV for the neutrino-less double β-decay of ⁷⁶Ge.     

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 neutrino masses in the three-flavor Pauli model

A special Majorana model for three neutrino flavors is developed on the basis of the Pauli transformation group. In this model, the neutrinos possess a partially conserved generalized lepton (Pauli) charge that makes it possible to discriminate between neutrinos of different type. It is shown that, within the model in question, a transition from the basic “mass” representation, where the average value of this charge is zero, to the representation associated with physical neutrinos characterized by specific Pauli “flavor” charges establishes a relation between the neutrino mixing angles θ _{mix, 12}, θ _{mix, 23}, and θ _{mix, 13} and an additional relation between the Majorana neutrino masses. The Lagrangian mass part, which includes a term invariant under Pauli transformations and a representation-dependent term, concurrently assumes a “quasi-Dirac” form. With allowance for these relations, the existing set of experimental data on the features of neutrino oscillations makes it possible to obtain quantitative estimates for the absolute values of the neutrino masses and the 2β-decay mass parameter m _ββ and a number of additional constraints on the neutrino mixing angles.     

Neutrinoless double-beta decay and related topics

The fundamental importance of searching for neutrinoless double-beta decay (0νββ-decay) is widely recognized. Observation of the decay would tell us that the total lepton number is not conserved and that, consequently, neutrinos are massive Majorana fermions. A brief history of the double-beta decay is presented. The 0νββ-decay is discussed in context of neutrino oscillation data. The perspectives of the experimental 0νββ-decay searches are analyzed. The importance of reliable determination of the 0νββ-decay nuclear matrix elements is pointed out. The problem of distinguishing of the light-neutrino exchange, heavy-neutrino exchange and the trilinear R-parity breaking supersymmetric ( \not R_p \not R_p SUSY) mechanisms of the 0νββ-decay is addressed. Further, the process of resonant neutrinoless double-electron capture (0νɛɛ) is revisited. Arguments are presented that an experimental search for the 0νɛɛ might be feasible.     

Mössbauer antineutrinos: Recoilless resonant emission and absorption of electron antineutrinos

Basic questions concerning phononless resonant capture of monoenergetic electron antineutrinos (Mössbauer antineutrinos) emitted in bound-state β-decay in the ³H-³He system are discussed. It is shown that lattice expansion and contraction after the transformation of the nucleus will drastically reduce the probability of phononless transitions and that various solid-state effects will cause large line broadening. As a possible alternative, the rare-earth system ¹⁶³Ho-¹⁶³Dy is favoured. Mössbauer-antineutrino experiments could be used to gain new and deep insights into several basic problems in neutrino physics.     

Theoretical study of β-decay of a negative tritium ion

A calculation of the probability of the main β-decay channel of a negative tritium ion is described as one of the processes that can affect the β-spectrum of a tritium source near the end point. The appropriate energy parameters have been calculated. This process should be taken into account in interpreting measured β-spectra near the end point in connection with determining the neutrino rest mass. Zh. éksp. Teor. Fiz. 112, 1537–1542 (November 1997)     

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.     

On the limit of nuclear stability — The region of double magic100Sn and bound-state β-decay of187Re75+

Experiments which led to the discovery of the heaviest, self-conjugated double magic nucleus¹⁰⁰Sn and the bound-state β-decay of completely ionized¹⁸⁷Re are presented.¹⁰⁰Sn was produced by fragmentation of 1.1 A GeV¹²⁴Xe beams, separated and implanted in a 4π Si-stack detector. From its decay a half-lifeT _1/2=0.94(+0.54, −0.27) s and a β⁺ endpoint energy ofE _β=3.4(+0.7, −0.3) MeV were determined for the 0⁺−1⁺ Gamov-Teller transition. Completely stripped¹⁸⁷Re⁷⁵⁺ was produced and stored in a coolerring with an energy of 351 A MeV for several hours. The products of bound-state β-decay of¹⁸⁷Re⁷⁵⁺ were detected by two independent methods, which allowed to determine a half-life ofT _1/2=33±6 y for this decay, which is of importance for the calibration of the Re/Os nucleosynthesis chronometer.     

Fundamental weak interaction studies using polarised nuclei and ion traps

Two experiments to search for new physics beyond the standard model for electroweak interactions by measuring correlations between different spin and momentum vectors in nuclear β-decay are discussed. In the first experiment the correlation between the emission asymmetry and the longitudinal polarisation of positrons emitted by polarised nuclei is determined. This type of measurement is sensitive to the presence of right-handed currents but also to possible scalar and tensor-type currents in the weak interaction. The aim of the second experiment is to determine the βν-correlation in β-decay by measuring the energy spectrum of the recoil ions, using a Penning trap and a retardation spectrometer. In this case the focus is on the search for scalar currents in the weak interaction. The results of the experiments presented here are complementary to results from experiments in muon decay and at high-energy colliders. This revised version was published online in July 2006 with corrections to the Cover Date.     

Beta–neutrino correlation experiments on laser trapped 38mK, 37K

Laser trapping and cooling techniques are now being applied to the study of nuclear β-decay at several labs. A magneto-optical trap (MOT) provides a localized source of atoms suspended in space, so the low-energy recoiling nuclei can freely escape and be detected in coincidence with the β. This allows reconstruction of the neutrino momentum, and the deduction of the β-v correlation in a more direct fashion than previously possible. In addition, the nuclei can be polarized by atomic techniques, opening a new class of spin correlation measurements to test the degree to which parity is maximally violated in the weak interaction. Our present experiment has detected several hundred thousand recoil-β⁺ coincidences from the 0⁺ → 0⁺ pure Fermi decay of ^38mK, produced at the on-line isotope separators TISOL and ISAC at TRIUMF. Our goal is to set constraints on non-Standard Model scalar bosons competitive with high-energy colliders and more conventional β-v correlation experiments. This revised version was published online in August 2006 with corrections to the Cover Date.     

Probing the neutrino mass matrix in next-generation neutrino oscillation experiments

We review the current status of the neutrino mass and mixing parameters needed to reconstruct the neutrino mass matrix. A comparative study of the precision in the measurement of oscillation parameters expected from the next-generation solar, atmospheric, reactor-and accelerator-based neutrino experiments is presented. We discuss the potential of 0νββ experiments in determining the neutrino mass hierarchy and the importance of a better ϑ ₁₂ measurement for it. The text was submitted by the author in English.     

An experiment for the measurement of the bound- β-decay of the free neutron

The hyperfine-state population of hydrogen after the bound-β-decay of the neutron directly yields the neutrino left-handedness or a possible right-handed admixture and possible small scalar and tensor contributions to the weak force. Using the through-going beam tube of a high-flux reactor, a background free hydrogen rate of ca. 3s^-1 can be obtained. The detection of the neutral hydrogen atoms and the analysis of the hyperfine states is accomplished by Lamb shift source type quenching and subsequent ionization. The constraints on the neutrino helicity and the scalar and tensor coupling constants of the weak interaction can be improved by a factor of ten.     

On the non-linear dependence of photocarrier radiometry signals from Si wafers on the intensity of the laser beam

The subject of this research was the dependence of the infrared photocarrier radiometric (PCR) signal on the intensity of the exciting super-bandgap laser beam. It has been shown that the amplitude of the PCR signal is proportional to the intensity to a power β, such that 1≤β≤2. The power dependence of the amplitude is an important indicator of the photoexcited carrier recombination physics, specifically in semiconductors ranging between monopolar (β = 1) and bipolar (β = 2) limits. The study was made with laser beams of varying power and spotsize and wafers with different transport parameters. It has been found that the conventional approach using β = 1 is inadequate and inconsistent with experimental slopes of amplitude vs. power.     

Neutrinoless double-β decay: Status and future

A brief summary of the status of neutrino masses, mixing, and oscillations is presented. Neutrinoless double β decay is considered. Predictions for the effective Majorana mass are reviewed. A possible test of the calculations of nuclear matrix elements of the 0νββ decay is proposed. The text was submitted by the authors in English.     

Analytic Calculation of Neutrino Mass Eigenvalues

Implication of the neutrino oscillation search for the neutrino mass square difference and mixing are discussed. We have considered the effective Majorana mass m _ee , related for ββ _0ν decay. We find limits for neutrino mass eigen value m _i in the different neutrino mass spectrum,which explain the different neutrino data.