无中微子双贝塔衰变：寻找马约拉纳中微子之路

无中微子双贝塔衰变是目前粒子物理与核物理学家积极寻找的一种极其稀有的原子核衰变模式。它的发现将验证中微子是否是其本身的反粒子,也就是通常指的马约拉纳费米子。同时这一物理过程破坏轻子数守恒,也可以为宇宙初期的正反物质不对称性提供重要的条件。鉴于其极重要的物理意义,国际上多个实验组利用不同的探测器技术,在多种不同的目标同位素中寻找这一突破粒子物理标准模型的稀有衰变。目前主流实验还未发现确定的无中微子双贝塔衰变信号,但对其半衰期的限制已经达到了1026年量级。国内近期也开展了一系列预研实验,期望在未来几年内可以确定一到两个切实可行的实验方案,开展吨级实验。     

轻子与非轻子弱相互作用的唯象描述

弱相互作用的重要特性之一是奇异粒子的非轻子衰变与π介子的轻子衰变有着统一的强度。另外,大家知道,轻子耦合还有两个特点与普适V-A费米作用理论(以下简称UFI)有偏离。第一,β衰变的矢量耦合常数G_V,没有重正化效应,因而与μ衰变的耦合常数G_μ相等;而轴矢耦合常数G_A则有一小的重正化效应,G_A=-1.25G_V.第二,奇异粒子的轻子衰变几率比UFI所预言的小一个数量级,因而异数改变流的耦合强度约为异数守恒流强度的1/4.在通常的流—流耦合模型或者流—中间玻色子耦合模型中,上述三个特性不能相互协调。例如,在李政道的中间玻色子理论中,由轻子衰变确定各种流的耦合强度以后,奇异粒子非轻子衰变几率将比实验小一个数量级。为了克服这一矛盾,一般需要引入多一倍的中间玻色子。     

原子核多体问题的玻色子-费米子描述

本文建立了一种在处理原子核多体问题时可以灵活地采用玻色变数与费米变数的理论形式,它原则上允许应用于与原来的费米型态空间等价的任何一个物理态空间.文中借助于玻色子和费米子产生湮没算符之间通常的对易反对易关系式,构成一个广义态空间,再从中建立与原来的费米型态空间等价的物理子空间系列,给出了各个不同的等价物理子空间之间的变换关系,并求出各类算符在这种子空间中的等效算符.     

关于J/ψ→Gb+γ和J/ψ→e+ē过程

基于J/ψ辐射产生胶球态Gb和湮没为e+ē对两过程的中间态和末态都不涉及任何夸克强子而只与强作用的真空性质密切相关这一特性，从另一种角度讨论了这两个过程.利用高能强作用软过程中修改后的pomeron场论模型和相关的最大非微扰强作用反应假定，并考虑到轻的和重的组分夸克在结构上的差异而在模型参数中作出相应的变更后，提出了这两个过程的另一种物理机制，计算了它们的衰变宽度，定出了胶球态与非微扰胶子间的耦合数值，并就此进行了简单的讨论. 关键词： J/ψ衰变 pomeron 非微扰     

不同边界条件下的封闭矩形声腔的结构-声耦合分析

分析了由2块四边弹性支承的弹性板及4块刚性板构成的封闭矩形声腔的结构-声耦合特性。通过在弹性板边界施加假想的连续分布弹簧系统模拟板的不同边界和连接条件,利用汉密尔顿函数和瑞利-李兹方法,充分考虑了两弹性板之间以及弹性板与腔体内声场之间的耦合。腔体内的声压响应和弹性板的振动速度与Kim的实验结果能较好的吻合,验证了本文理论推导的正确性。最后,通过对耦合声场的分析计算,表明两弹性板之间为弱耦合,耦合声场主要由受到外激励的结构所决定;支承板的线弹簧的刚度变化对耦合声场的影响较旋转弹簧大。     

半空间球面波函数叠加法重构声源直接辐射声场

提出了基于半空间球面波函数叠加的声场重构方法,以重构含有限声阻抗边界半空间中声源直接辐射的声场。在半空间中多极子声源声压场的解析解的基础上,构造出以边界声阻抗为参量的半空间球面波函数的正交基;通过求逆获得半空间总声压解的基函数系数,同时也获得声源直接辐射声场即自由空间中的基函数系数,进而重构出声源直接辐射的声场。在边界声阻抗已知和边界声阻抗未知两种条件下,对该方法进行了仿真验证和参数分析,并在全消声室内进行了实验验证。结果表明,所提方法能重构出半空间中典型声源即球形声源和平面声源的直接辐射声场;该方法在边界声阻抗已知时的重构精度与稳定性高于在边界声阻抗未知时的情形。      

关于J/ψ→Gb+γ和J/ψ→e+(-e)过程

基于J/ψ辐射产生胶球态Gb和湮没为e-e对两过程的中间态和末态都不涉及任何夸克强子而只与强作用的真空性质密切相关这一特性,从另一种角度讨论了这两个过程.利用高能强作用软过程中修改后的pomeron场论模型和相关的最大非微扰强作用反应假定,并考虑到轻的和重的组分夸克在结构上的差异而在模型参数中作出相应的变更后,提出了这两个过程的另一种物理机制,计算了它们的衰变宽度,定出了胶球态与非微扰胶子间的耦合数值,并就此进行了简单的讨论.     

四代层子弱耦合矩阵的问题

本文讨论了四代层子的弱耦合矩阵的问题。在几种可能性中,选择了四代左手流方案。建议了一种带电弱流的耦合矩阵,它是K-M矩阵的直接推广。为检验这个弱耦合矩阵做了两个例子。例一,用它算了K介子衰变的CP破坏。例二,用水平规范算出了以层子质量表示的四代弱耦合矩阵。一点意外的收获是b层子的主要衰变道是b→c+W^-。而在三代情况下,完全相同的方案得出的b层子的主要衰变道是:b→u+W^-,因此,如果尔后的实验选择了这种水平规范的方案,那么由三代层子扩充到四代层子则找到了依据。     

基于联合波叠加法的浅海信道下圆柱壳声辐射研究

针对浅海信道下弹性结构声辐射预报尚无高效可靠的研究方法,提出了一种浅海信道下弹性结构声辐射快速预报的联合波叠加法.该方法结合了浅海信道传输函数、多物理场耦合数值计算法和波叠加法理论,运用该方法可对浅海信道下弹性结构辐射声场进行快速预报.经数值法和解析解法验证后,从信道下辐射源、环境影响和辐射声场测量的角度研究分析了浅海信道下弹性圆柱壳的声辐射特性,阐释了进行浅海信道下结构声辐射研究的必要性.研究结果表明,仅在低频浅海信道下弹性结构可近似等效为点源,信道上下边界对声场产生显著的耦合影响,高频段的空间声场指向性分布尤为明显,垂直线列阵进行信道下结构辐射声功率测量时,测量结果受到信道环境边界和潜深的影响较大.     

SU(4)⊕SU(2)⊕U(1)统一模型

本文提出一种SU(4)SU(2)U(1)统一模型。在此模型中,层子和轻子放于同一个多重态中,所以它们有耦合,但这种耦合不会导致质子衰变,只对高能散射有效应。此模型预言了有两种中性中间玻色子,还算得了sin~2θ_w=0.25,并可使中微子有合理的质量。     

Different sound decay patterns and energy feedback in coupled volumes

Different non-exponential decays such as the concave and the convex double sloped decays in the coupled rooms provide distinct sound qualities. These are commonly considered to occur in the less reverberant sub-room and the more reverberant sub-room, respectively. However, numerical simulations and experiments in this paper show that the demarcation line is not located along the physical boundaries (e.g., the partition and the coupling aperture), but in the more reverberant sub-room. The sound field with the concave double sloped decay penetrates into the auxiliary sub-room to an extent which is influenced by the difference between the two natural reverberations of the sub-rooms. Furthermore the sound energy flows in different regions are investigated, demonstrating how energy feedback leads to the concave double sloped decay.     

Modeling the sound transmission between rooms coupled through partition walls by using a diffusion model

In this paper, a modification of the diffusion model for room acoustics is proposed to account for sound transmission between two rooms, a source room and an adjacent room, which are coupled through a partition wall. A system of two diffusion equations, one for each room, together with a set of two boundary conditions, one for the partition wall and one for the other walls of a room, is obtained and numerically solved. The modified diffusion model is validated by numerical comparisons with the statistical theory for several coupled-room configurations by varying the coupling area surface, the absorption coefficient of each room, and the volume of the adjacent room. An experimental comparison is also carried out for two coupled classrooms. The modified diffusion model results agree very well with both the statistical theory and the experimental data. The diffusion model can then be used as an alternative to the statistical theory, especially when the statistical theory is not applicable, that is, when the reverberant sound field is not diffuse. Moreover, the diffusion model allows the prediction of the spatial distribution of sound energy within each coupled room, while the statistical theory gives only one sound level for each room.     

Statistical-acoustics models of energy decay in systems of coupled rooms and their relation to geometrical acoustics

An improved statistical-acoustics model of high-frequency sound fields in coupled rooms is developed by incorporating into prior models geometrical-acoustics corrections for both energy decay within subrooms and energy transfer between subrooms. The conditions under which statistical-acoustics models of coupled rooms are valid approximations to geometrical acoustics are examined by comparison of computational geometrical-acoustics predictions of decay curves in two- and three-room systems with those of both improved and prior statistical-acoustics models. The accuracy of the decay model used within subrooms is found to have a primary influence on the accuracy of predictions in coupled systems. Likewise, nondiffuse transfer of energy is shown to significantly affect decay of energy in systems of coupled rooms. The decrease in energy density of the reverberant field with distance from the source, which is predicted by geometrical acoustics, is found to result in spatial dependence of decay-curve shape for certain coupling geometries. Geometrical effects are shown to contribute to the failure of statistical-acoustics models in the case of strong coupling between subrooms; thus, previously proposed statistical-acoustics criteria cannot predict the point at which the models break down with consistent accuracy.     

Relaxation of sound fields in rooms of diffusely reflecting boundaries and its application in acoustical radiosity simulation

The acoustical radiosity method is a computationally expensive acoustical simulation algorithm that assumes an enclosure with ideal diffuse reflecting boundaries. Miles observed that for such an enclosure, the sound energy decay of every point on the boundaries will gradually converge to exponential manner with a uniform decay rate. Therefore, the ratio of radiosity between every pair of points on the boundaries will converge to a constant, and the radiosity across the boundaries will approach a fixed distribution during the sound decay process, where radiosity is defined as the acoustic power per unit area leaving (or being received by) a point on a boundary. We call this phenomenon the "relaxation" of the sound field. In this paper, we study the relaxation in rooms of different shapes with different boundary absorptions. Criteria based on the relaxation of the sound field are proposed to terminate the costly and unnecessary radiosity computation in the later phase, which can then be replaced by a fast regression step to speed up the acoustical radiosity simulation.     

A hybrid finite element – statistical energy analysis approach to robust sound transmission modeling

When considering the sound transmission through a wall in between two rooms, in an important part of the audio frequency range, the local response of the rooms is highly sensitive to uncertainty in spatial variations in geometry, material properties and boundary conditions, which have a wave scattering effect, while the local response of the wall is rather insensitive to such uncertainty. For this mid-frequency range, a computationally efficient modeling strategy is adopted that accounts for this uncertainty. The partitioning wall is modeled deterministically, e.g. with finite elements. The rooms are modeled in a very efficient, nonparametric stochastic way, as in statistical energy analysis. All components are coupled by means of a rigorous power balance. This hybrid strategy is extended so that the mean and variance of the sound transmission loss can be computed as well as the transition frequency that loosely marks the boundary between low- and high-frequency behavior of a vibro-acoustic component. The method is first validated in a simulation study, and then applied for predicting the airborne sound insulation of a series of partition walls of increasing complexity: a thin plastic plate, a wall consisting of gypsum blocks, a thicker masonry wall and a double glazing. It is found that the uncertainty caused by random scattering is important except at very high frequencies, where the modal overlap of the rooms is very high. The results are compared with laboratory measurements, and both are found to agree within the prediction uncertainty in the considered frequency range.     

Decay rates and wall absorption at low frequencies

This paper is concerned with evaluating the error of conventional estimates of the boundary absorption of rectangular enclosures, with particular reference to reverberation room sound power measurements. The reverberation process is examined theoretically; the relative contributions to the decay rate from different modes in a rectangular room are calculated from an ensemble average over rooms with nearly the same dimensions. It is shown that the traditional method of determining the absorption of the walls of reverberation rooms systematically underestimates the absorption at low frequencies; the error is computed from the ensemble average. Finally, an unbiased estimate of the sound power radiated by a source in a reverberation room is derived. This estimate involves measurement of the initial decay rates of the room and is, unlike the usual reverberation room sound power estimate, neither based on statistical diffuse field considerations nor on the normal mode theory.     

一种水平变化波导中声传播问题的耦合模态法

针对介质参数及海底边界水平变化波导中的声传播问题,本文基于多模态导纳法提出一种能量守恒且便于数值稳定求解的耦合模态方法.将声压表示为一组正交完备的本地本征函数之和,对声压满足的Helmholtz方程在本地本征函数上作投影,推导出关于声压模态系数的二阶耦合模态方程组.耦合矩阵直观描述水平变化因素对模态耦合的贡献.为避免直接求解二阶耦合模态方程组可能遇到的数值发散问题,将其重构为两个耦合的一阶演化方程组,引入导纳矩阵并使用Magnus数值积分方法获得稳定的声场解.利用该耦合模态方法数值计算水平变化波导中的声场,并与COMSOL参考解比较,结果表明该耦合模态理论能够精确求解水平变化波导中的点源及分布源传播问题.     

一种能量守恒的双向耦合简正波水声传播模型

针对非水平边界条件近似处理所引起的双向声场能量不守恒问题,提出了一种满足能量守恒的双向耦合简正波模型。在模型的建立中,严格考虑了非水平界面的边界条件,并依据介质运动方程和连续性方程推导了便于数值计算的耦合微分方程。同时,结合能量守恒方程和不同海底参数下的数值计算,对模型的准确性进行了研究。由理论分析和仿真结果可以看出,边界条件的严格考虑将导致边界修正项的产生,而边界修正项的引入确保了耦合系数满足对称性和模型所计算的声场满足能量守恒,使得本模型可充分考虑边界水平变化对声场的影响,实现非水平分层波导中双向场的准确计算。      

Applicability of locally reacting boundary conditions to porous material layer backed by rigid wall: Wave-based numerical study in non-diffuse sound field with unevenly distributed sound absorbing surfaces

To clarify the applicability of locally reacting boundary conditions in wave-based numerical analyses of sound fields in rooms, we numerically analyzed a non-diffuse sound field in a room with unevenly distributed sound absorbing surfaces and investigated the differences between the extended and local reactions. Each absorbing surface was a porous material layer backed by a rigid wall. Simulations were performed by the fast multipole boundary element method, a highly efficient boundary element method using the fast multipole method. At low frequencies, the extended and local reactions yielded similar reverberation decay curves because of the influence of the room. However, when the random incidence absorption coefficients were small at low frequencies or frequencies were high, the difference was greater than expected from the corresponding Eyring decay lines. We conclude at high frequencies, the locally reacting boundary conditions lead to a longer reverberation time than that expected from the absorption coefficient differences between the extended and local reactions. These differences were similar in sound-pressure-level and sound-intensity-level distributions, and in the oblique incidence absorption coefficient of the absorbing surfaces, but were increased at low frequencies.     

Accounting for delay of energy transfer between coupled rooms in statistical-acoustics models of reverberant-energy decay

A statistical-acoustics model for energy decay in systems of two or more coupled rooms is introduced, which accounts for the distribution of delay in the transfer of energy between subrooms that results from the finite speed of sound. The method extends previous models based on systems of coupled ordinary differential equations by using functional differential equations to explicitly model dependence on prior values of energy in adjacent subrooms. Predictions of the model are illustrated for a two-room coupled system and compared with the predictions of a benchmark computational geometrical-acoustics model.