Geometry of the effective Majorana neutrino mass in the 0νββ decay

The neutrinoless double-beta (0νββ) decay is a unique process used to identify the Majorana nature of massive neutrinos, and its rate depends on the size of the effective Majorana neutrino mass <m>_ee. We put forward a novel ‘coupling-rod’ diagram to describe <m>_ee in the complex plane, by which the effects of the neutrino mass ordering and CP-violating phases on <m>_ee are intuitively understood. We show that this geometric language allows us to easily obtain the maximum and minimum of |<m>_ee|. It remains usable even if there is a kind of new physics contributing to <m>_ee, and it can also be extended to describe the effective Majorana masses <m>_eμ, <m>_eτ, <m>_μμ, <m>_μτ and <m>_ττ which may appear in some other lepton-number violating processes.     

Renormalized boson expansion for the 2νββ decay

Two neutrino double beta (2) Gamow-Teller transitions, and , are treated with a many-body Hamiltonian involving a Bonn-type realistic two-body interaction. The states involved in these transitions are described by a pnRQRPA approach. Transition operators are expanded in first order in terms of renormalized bosons. For illustration the formalism is applied to the case Kr.     

Description of the 2νββ decay within a fully renormalised RPA approach

The RPA treatment of a many body Hamiltonian describing the states of even-even nuclei involved in a 2νββ decay is revisited. One shows that renormalizing the dipole two quasiparticle operators by accounting for new correlations in the ground state requires a simillar renormalization for the dipole density operators which results in activating new boson degress of freedom. Possible consequences on Ikeda sum rule and Gamow-Teller transition amplitude are suggested. A numerical application for a two levels model is presented     

The nuclear deformation versus the spin-flip like excitations and the suppression of the 2νββ decay amplitude

The suppression mechanism of the Gamow-Teller double beta decay amplitude M_GT is studied using a many body Hamiltonian which describes a composite system of protons and neutrons moving in a projected spherical single particle basis. Alike nucleons interact through pairing while protons and neutrons by a separable dipole-dipole force both in the particle-hole (ph) and particle-particle (pp) channels. The spin-flip and non-spin-flip components of the QRPA phonons have a differents contribution to the M_GT values. The relative magnitudes and phases depend both on the strength of the particle-particle interaction (g_pp) and on the nuclear deformation. The deformation yields a fragmentation of the M_GT value on one hand and washes out the separation of states of pure spin-flip and non-spin-flip structures. Due to this effect M_GT has only one fragmented resonance structure in the low part of the spectrum.     

The Gamow-Teller transitions in the96Pd→96Rh decay

Using the⁴⁰Ca(4.2 MeV/u) +⁶⁰Ni reaction and on-line mass separation, the decay properties of ₄₆ ⁹⁶ Pd₅₀ were reinvestigated. From a comparison of experimental and theoreticalβ ⁺/(EC +β ⁺) probability ratios, theQ _EC value was deduced to be 3,450 ± 150 keV. The strength was determined for four 0⁺→1⁺ Gamow-Teller beta transitions and found to be quenched as compared to predictions of the single-particle shell model. For⁹⁶Pd and⁹⁴Ru the sources of this quenching are discussed.     

Intensities of two-photon transitions in the 2ν2 band of NH3

We determine theoretically all the coincidences between the ¹⁴NH₃ν₂= 0−2 band and the spectrum of two CO₂ lasers; they can be induced either by a small frequency tuning of lasers or by a Stark tuning of molecular levels. For each two-photon transition we report the line intensity, the preferred laser lines and the necessary Stark field or laser tuning. The theoretical line intensities are compared to the results of an optoacoustic Doppler-free experiment.     

A study on the decay of ^72As

The excited states of the ⁷²Ge nucleus were investigated in radioactive decay of ⁷²As. Three new transitions with 1996.58, 2125.59 and 2255.49keV have been found for the first time. One γ-ray with 912.09keV has been placed in the decay scheme for the first time, and the placement of 1938.88, 2116.79, 2785.59, 2833.03, 2950.69 and 3338.00keV γ-rays are confirmed again in the present work. One new level at 2027.72keV excitation energy is proposed. The level scheme was established and for a number of levels spin-parity assignments are suggested on the basis of logft values and γ-branching ratios.     

A study of ultraviolet renormalon ambiguities in the determination of α s from τ decay

The divergent large-order behaviour of the perturbative series relevant for the determination of _s from decay is controlled by the leading ultraviolet (UV) renormalon. Even in the absence of the first infrared (IR) renormalon, an ambiguity of order ²/m ² is introduced. We make a quantitative study of the practical implications of this ambiguity. We discuss the magnitude of UV renormalon corrections obtained in the large-N _f limit, which, although unrealistic, is nevertheless interesting to some extent. We then study a number of improved approximants for the perturbative series, based on a change of variable in the Borel representation, such as to displace the leading UV renormalon singularity at a larger distance from the origin than the first IR renormalon. The spread of the resulting values of _s (m ²) obtained by different approximants, at different renormalization scales, is exhibited as a measure of the underlying ambiguities. Finally, on the basis of mathematical models, we discuss the prospects of an actual improvement, given the signs and magnitudes of the computed coefficients, the size of _s (m ²) and what is known of the asymptotic properties of the series. Our conclusion is that a realistic estimate of the theoretical error cannot go below _s (m ²)±0.060, or _s (m _Z ²)±0.006.On leave of absence from INFN, Sezione di Milano, Milan, Italy     

A phenomenological study of bottom-quark fragmentation in top-quark decay

Top-quark physics is one of the main fields of investigation at the Tevatron accelerator and, ultimately, at the LHC. We perform a phenomenological analysis of $t\bar{t}$ events at hadron colliders, with a focus on observables relying on bottom-quark fragmentation in top-quark decay. In particular, we investigate the B-lepton invariant-mass distribution in the dilepton channel and give an estimate of the contribution of bottom fragmentation to the Monte Carlo uncertainty on the top-quark mass reconstruction.     

Half-Life and the Nuclear Matrix Element of the 2νββ Decay in 76Ge

We apply the shell-model wave functions of ⁷⁶Ge to calculate the nuclear matrix elements and the half-life of the 2νββ decay of ⁷⁶Ge. Our result is comparable to the recent observed 2νββ decay half-life of ⁷⁶Ge. Furthermore it provides the upper limit of the effective neutrino mass is about 0.4eV with the recent experimental data of the 0νββ decay half-life.     

Thermal expansion anisotropy of β-TlInS2 crystals in the course of phase transitions

Basing on the thermal expansion studies performed for β-TlInS₂ crystals in the course of their phase transitions, we have found that the crystals aged for one year are composed from at least two polytypes, which undergo phase transitions at different temperatures. The detailed analysis of anisotropy of the thermal expansion demonstrates that all of the diagonal components of the thermal expansion tensor associated with the eigensystem have almost the same modules but differ by their signs. This leads to appearance of an elliptical conical surface of zero thermal expansion. The orientation of this surface does not depend on the temperature in the temperature interval under study. We have also found that the thermal expansion coefficients along the crystallographic a and b axes are close to zero.     

Fine structure in the α decay of202Rn

The α decay of mass-separated²⁰²Rn is studied at the ISOLDE separator. Time sequential α-singles spectra together with α-X-t and α-e-t coincidence events are collected. Fine structure in the α decay is observed and feeding of a low-lying 0 ₂ ⁺ state at 816 keV in¹⁹⁸Po is evidenced. This 0⁺ state can be interpreted as the bandhead of an intruder-state based deformed band, coexisting with the spherical groundstate band. Mixing between normal and intruder states is discussed. A preliminary α-decay study of²⁰⁰Rn did not yet reveal any fine structure.     

New transitions in the β-decay of 36Ca

The β-decay of the T _z = - 2 nucleus ³⁶Ca was studied at the LISE3 magnetic spectrometer at GANIL. Two new proton-emitting states have been detected and the other nine known βp and βγ transitions have been remeasured with improved resolution. A simulation with the GEANT code has been applied to this experimental setup. A comparison with shell model calculations is given. Received: 19 September 2000 / Accepted: 28 February 2001     

Observation of the decay Ks → 2γ and measurement of the decay rates KL → 2γ and KS → 2γ

The decay rates of K_L → 2γ and K_S → 2γ have been measured at the CERN SPS. The results are Γ(K_L→2γ)/χ(K_L→2π₀)=0.632±0.004±0.008 and Γ(K_S→2γ)/Γ(K_L→2γ)=2.3 ±1.0±0.4. This is the first observation of K_S→2γ decays.     

Influence of ocular dominance in the variability of the eye’s anterior segment measurements

The visual system tends to favour one eye over the other in perceptual or motor tasks. This effect, called ocular dominance, makes those small movements in one eye be smaller and more precise than in the other eye. These dynamic effects are usually small and static devices are not capable of detecting differences between both eyes. In the last years ophthalmic devices are becoming more and more precise, thus they can be sensible to such variability. The hypothesis posed here is that variability of measures acquired this way is affected by ocular dominance. With a Pentacam system we have measured several parameters of the anterior segment of the eye. Our findings show that variables measured for the dominant eye are less dispersive than for the non-dominant eye although the limited accuracy of the device can mask this effect. The trend is confirmed by a contrast experiment and by a previous work, so we accept the validity of our hypothesis. Our main conclusion is that systematic election of the right eye in analysis of reliability or reproducibility can bias the variability of results and consequently we suggest considering dominance effects.     

Considering collective degrees of freedom in the β decay of odd nuclei

The β decay of around 30 odd nuclei was calculated within the dynamic collective model using expressions for the reduced matrix elements of the weak-interaction Hamiltonian for U-type and V-type β decay obtained earlier by the authors. All of the many-phonon states of the yrast band of an even-even core contributing to the formation of the excited states of nuclei were taken into account. For β transitions with an intensity of >1%, the calculated logft values differ from the experimental values by no more than 0.2.     

Theoretical study of the radiative decay processes in H~+(D~+,T~+)–Be collisions

The potential energy curves of X~1Σ~+, A~1Σ~+, C~1Σ~+, and B~1Π are calculated with high-level MRDCI method, and the calculated spectroscopic constants of those states are in good agreement with most recent experimental data. On the basis of high precision PECs, the radiative processes of H~++ Be collisions are studied by using the fully quantum, optical potential and semiclassical methods in the energy ranges of 10~(-8) eV/u–0.1 eV/u, and the radiative decay, the radiative charge transfer,and the radiative association cross-sections are computed. It is found that the radiative association process is dominant in the energy region of 10~(-8) eV/u–0.02 eV/u, while radiative charge transfer becomes important at higher energies. Rich resonance structures are present in the radiative association and charge transfer cross-sections in the whole energy region considered, which result from the interaction between the quasi-bound rovibrational(J, v) states in the entrance channel with the final continuum state. Significant isotope effects have been found in the radiative decay processes of H~++ Be collisions.     

Charge distribution of the daughter atom in α decay of 210Po

The evaporation model developed by Russek and collaborators for multiple ionization of outer-shell electrons in ion-atom collisions has been applied to the charge distribution of the daughter atom produced by α decay of ²¹⁰Po. The calculated result indicates that most of the daughter atoms are positively charged ions with a mean charge of + 1.41. The other ionization mechanisms, the recoil effect and the shake-off effect, are also considered. In comparison with the recent experimental results, it is shown that the evaporation model predicts more adequately than the recoil effect and the shake-off effect.