Generalized interacting boson model and the collective behaviour in nuclei

The effect of including the high spin bosons on the manifestation of collective behaviour in nuclei is examined by plotting theB(E2; 2⁺→0⁺) rates as a function of neutron number for various values ofη, whereη is the highest angular momentum of the bosons included in the calculation.B(E2; 2⁺→0⁺) values of a large number of nuclei in various regions of the nuclear periodic table are calculated with a single value for the effective charge in the generalized scheme. Irreducible representations of SU(3) contained in the symmetric partition [N] of U(15) are worked out for integersN uptoN=15, to enable the explicit inclusion of theg boson into calculations. The experimentally observed odd-K bands in²³⁴U and¹⁸⁴W are described as a direct consequence of theg boson.     

Shape transitions in even Mo and Sm isotopes: Study in a new microscopic interacting boson model scheme

A simple dynamic procedure, based on the deformed Hartree-Fock solution of a nucleus, is presented to construct the IBM operators in microscopic basis. The parameters of these operators are evaluated by establishing a Marumori mapping from the truncated shell model space onto the boson space. The transitions from spherical to axial-rotor shape observed in the low-lying levels ofeven ^96–108Mo and^146–154Sm isotopes are reproduced qualitatively by applying this procedure with a fixed set of fermion input parameters to each chain. Variation of a few parameters in fermion space leads to quantitative agreement.     

Recent results from large electron positron collider

We summarize here the recent results from the four experiments at the large electron positron collider (LEP). These experiments provide precise measurements of theW and Z boson properties and their couplings to leptons and quarks. These measurements, together with measurements of the top quark andW boson masses in the Tevatron collider provide a stringent test of the standard electroweak theory. Searches for Higgs boson and supersymmetric particles have yielded null results so far giving rise to lower bounds in the parameter space.     

Testing the minimal supersymmetric standard model with the mass of the <Emphasis Type="Italic">W</Emphasis> boson

We review the currently most accurate evaluation of the W boson mass, M _W , in the minimal supersymmetric standard model (MSSM). It consists of a full one-loop calculation, including the complex phase dependence, all available MSSM two-loop corrections as well as the full standard model result. We analyse the impact of the phases in the scalar quark sector on M _W and compare the prediction for M _W based on all known higher-order contributions with the experimental results.      

Shell model description of interacting bosons

The interacting boson model, describing collective states of even-even nuclei, is introduced as a drastic truncation of large scale shell model calculations. The shell model hamiltonian can be diagonalized by using a correspondence, or mapping, of the nucleon states in the truncated space into states obtained by coupling proton and neutron s- and d-bosons. The equivalent boson hamiltonian in a simple case is obtained and diagonalized. Eigenstates with definite proton-neutron symmetry (good F-spin) emerge for certain values of proton and neutron numbers. In general the situation is more complex but the results obtained follow closely the experimental data.     

A correspondence between IBA-1 and IBA-2 models and electromagnetic transitions in the decay of some erbium isotopes

The interacting boson approximation IBA-1 model space, in which neutron and proton degrees of freedom are not distinguished, can be considered as a subspace of the IBA-2 model space. Using the microscopic background of the IBA-2 model, a correspondence can be established between IBA-1 and IBA-2 model space. Since the space of the IBA-1 model can be regarded as a subspace of the IBA-2 model there is a unique way to ‘Project’ the operators of the IBA-2 model onto those of IBA-1. This projection can be carried out using theF-spin formalism. In the IBA-2 model, the lowest states are indeed fully symmetric, and using the calculations with the help of this projection, we explore the energy levels and the electric quadrupole transition probabilitiesB(E2; {ie393-01}) and γ-ray E2/M1 mixing ratios for selected transitions of^{162, 164, 166, 168, 170}Er. Owing to admixtures of non-fully-symmetric states in IBA-2, we renormalized the parameters (ε) and (κ). This is the first time we show that this projection can be applied to some heavier isotopes and the results obtained for^{162, 164, 166, 168, 170}Er isotopes are reasonably in good agreement with the previous experimental values.     

Flavour-changing neutral currents in models with extra<Emphasis Type="Italic">Z′</Emphasis> boson

New neutral gauge bosonsZ′ are the features of many models addressing the physics beyond the standard model. Together with the existence of new neutral gauge bosons, models based on extended gauge groups (rank > 4) often predict new charged fermions also. A mixing of the known fermions with new states, with exotic weak-isospin assignments (left-handed singlets and right-handed doublets) will induce tree-level flavour-changing neutral interactions mediated byZ exchange, while if the mixing is only with new states with ordinary weak-isospin assignments, the flavour-changing neutral currents are mainly due to the exchange of the new neutral gauge bosonZ′. We review flavour-changing neutral currents in models with extraZ′ boson. Then we discuss some flavour-changing processes forbidden in the standard model and new contributions to standard model processes.     

T = 0 versusT = 1 pairing in 0(36) limit of IBM-4 for heavyN = Z nuclei

In theO(36) limit of the interacting boson model including spin-isospin degrees of freedom (IBM-4), starting with a group chain that preservess andd boson spins and isospins together with a simple mixing hamiltonian, it is shown that the model generates, for heavyN =Z nuclei, even-even to odd-odd staggering in the number ofT = 0 pairs in the ground states for moderate difference in the basicT = 0 andT = 1s-boson pair energies; the staggering disappears when the energy difference is large.     

Shell model half-lives for r-process N = 82 nuclei

We have performed shell model calculations of the half-lives and neutron-branching probabilities of the r-process waiting-point nuclei at the magic neutron number N = 82 . These new calculations use a larger model space than previous shell model studies and an improved residual interaction which is adjusted to recent spectroscopic data around A = 130 . Our shell model results give a good account of all experimentally known half-lives and Q _β -values for the N = 82 r-process waiting-point nuclei. Our half-life predictions for the N = 82 nuclei with Z = 42-46 agree well with recent estimates based in the energy-density functional method.     

Distinguishing a SM-like MSSM Higgs boson from SM Higgs boson at muon collider

We explore the possibility of distinguishing the SM-like MSSM Higgs boson from the SM Higgs boson via Higgs boson pair production at future muon collider. We study the behavior of the production cross-section in SM and MSSM with Higgs boson mass for various MSSM parameters tan β and m _A . We observe that at fixed CM energy, in the SM, the total cross-section increases with the increase in Higgs boson mass whereas this trend is reversed for the MSSM. The changes that occur for the MSSM in comparison to the SM predictions are quantified in terms of the relative percentage deviation in cross-section. The observed deviations in cross-section for different choices of Higgs boson masses suggest that the measurements of the cross-section could possibly distinguish the SM-like MSSM Higgs boson from the SM Higgs boson.      

基于微观IBM对电子-核散射及核电磁性质的研究(Ⅱ) ME理论方案

从一种微观作用玻色子模型（IBM）玻色子集体态子空间映射出费米子集体态子空间,通过假定玻色子算符形式以及使物理算符在两集体态子空间中对应归一化基矢间矩阵元相等,给出了从费米子单体算符导出玻色子单体算符的一种微观理论方法（ME方法）,文中以获取玻色子结构函数亦即确定玻色子形式核跃迁电荷／电流密度算符为例对此作出了详细介绍,利用微观IBM提供的波函数可在玻色子态空间中求出核跃迁电荷／电流密度,结合电子－核散射以及核电磁跃迁的形式理论,可建立研究电子－核散射各种形状因子与微分散射截面以及核约化跃迁几率,电磁多极矩,核态g因子等物理量的理论方案,在微观sdgIBM-1下利用该方案初步计算了146Nd核21^ 态到o 1^ 态的跃迁电荷密度以及约化跃迁几率,理论结果与实验值符合较好。     

q-Anharmonic oscillator with quartic interaction

The first order perturbative correction to the energy levels of a boson realization of aq-oscillator due to a quartic term in the potential energy is evaluated. We also discuss the statistical mechanics ofq-anharmonic oscillators in the case where the parameterq deviates slightly from unity.     

Center-of-mass problem in truncated configuration interaction and coupled-cluster calculations

The problem of center-of-mass (CM) contaminations in ab initio nuclear structure calculations using configuration interaction (CI) and coupled-cluster (CC) approaches is analyzed. A rigorous and quantitative scheme for diagnosing the CM contamination of intrinsic observables is proposed and applied to ground-state calculations for ⁴He and ¹⁶O. The CI and CC calculations for ¹⁶O based on model spaces defined via a truncation of the single-particle basis lead to sizable CM contaminations, while the importance-truncated no-core shell model based on the N_maxΩ space is virtually free of CM contaminations.     

The collective bands of positive parity states in odd-A (fp) shell nuclei

The low-lying collective bands of positive parity states in (fp) shell nuclei are described in the deformed Hartree-Fock method by projecting states of definite angular momenta from ‘the lowest energy intrinsic states in (sd)⁻¹ (fp) ⁿ⁺¹ configurations. The modified Kuo-Brown effective interaction for (fp) shell and modified surface delta interaction (MSDI) for a hole in (sd) shell with a particle in (fp) shell have been used. The collective bands of states are in general well reproduced by the effective interactions. The excitation energies of the band head states are however off by about one MeV. The calculated magnetic moments of the band headj=3/2⁺ states are in reasonable agreement with experiment. Using effective chargese _p=1.33e ande _n=0.64e we get fairly good agreement forE(2) transitions. The hinderedM(1) transition strengths are reproduced to the correct order however they are slightly higher compared to experiment.     

Microscopic study of low-lying collective bands in77Kr

The structure of the collective bands in⁷⁷Kr is investigated within our deformed shell model (DSM) based on Hartree-Fock states. The different levels are classified into collective bands on the basis of their B(E2) values. The calculatedK = 5/2⁺ ground band agrees reasonably well with the experiment. An attempt has been made to study the structure of the 3-quasiparticle band based on large J state in this nucleus. The calculated collective bands, the B(E2), and B(M1) values are compared with available experimental data. The nature of alignments in the low-lying bands is also analyzed.     

A simple model of superheavy nuclei

I present a simple algebraic model of superheavy and superdeformed nuclei, produced through heavy-ion collisions, based on a microscopic evaluation of the effective boson numbers in the actinide and superheavy regions. The relevant calculations have been performed within the framework of a deformed shell model, including the pairing interaction between like-particles. As far as the actinide isotopes are concerned, the theoretical boson numbers are compared with the corresponding empirical estimates, obtaining a good agreement. The calculated boson numbers are used to predict collective spectra and electromagnetic transition intensities for actinide — including fission isomers — and superheavy nuclei, by using the interacting boson model (IBM).     

Microscopic structure of <Superscript>126</Superscript>Ba in IBM terms

The yrast band of the nonaxially deformed ¹²⁶Ba nucleus is described by the Hamiltonian of the interaction boson model. Its parameters are calculated on the basis of a microscopic theory within a spherical mean field, and residual interactions that include pairing and multipole factorized forces. Each state of the yrast band is considered independently of others, allowing us to study variations in the superfluid properties of the nucleus and the quasiparticle structure of collective D phonons with spin. The calculations are performed in an expanded configuration space that includes the collective D phonon states, and noncollective states in which an additional phonon of positive parity whose spin assumes values of 0 to 6 is present along with the D phonons. It is shown that the collective Hamiltonian parameters cannot be reproduced without considering the effect of the noncollective states.     

Microscopic interacting boson model calculations for even-even <Superscript>128–138</Superscript>Ce nuclei

In this study, we determined the most appropriate Hamiltonian that is needed for the present calculations of energy levels and B(E2) values of ^128–138Ce nuclei which have a mass around A≅130 using the interacting boson model (IBM). Using the best-fitted values of parameters in the Hamiltonian of the IBM-2, we have calculated energy levels and B(E2) values for a number of transitions in ^{128,130,132,134,136,138}Ce. The results were compared with the previous experimental and theoretical (PTSM model) data and it was observed that they are in good agreement. Also some predictions of this model have better accuracy than those of PTSM model. It has turned out that the interacting boson approximation (IBA) is fairly reliable for calculating spectra in the entire set of ^{128,130,132,134,136,138}Ce isotopes and the quality of the fits presented in this paper is acceptable.