电荷对称破缺

1930年 ,物理学家海森伯曾认为 ,中子与质子实质上是在质量上具有微小差别的同一种粒子 ,他将两者统称为“核子” .近代的核物理学家赞同这样的观点 ,即在大量的核反应中 ,如果用中子来代替质子时 ,核反应过程将以相同方式继续进行下去 ,反之亦然 .但这种相似性在某些情况下会发生破坏 ,因为它将导致电荷对称破缺 ,简称为CSB .2 0 0 3年 4月 ,在美国费城召开的美国物理学会的会议上 ,有两个实验室分别独立地宣布 ,他们完成了对电荷对称破缺的实验观测 .一个是美国印第安那大学回旋加速器实验室 (IUCF) .IUCF的E .Stephenson教授宣…     

Charge ordering due to magnetic symmetry breaking

It is argued that both transitions observed in 50% doped manganites, at the Néel temperature (T(N)) and the so-called charge ordering temperature (T(CO)), are magnetic. T(N) corresponds to the order-disorder transition, which takes place between ferromagnetic zigzag chains, while the coherent motion of spins within the chains is destroyed only around T(CO). The magnetic structure below T(CO) is highly anisotropic. It is dressed by the lattice distortion and leads to the huge anisotropy of the electronic structure, which explains stability of this state as well as the form of the charge-orbital pattern above T(N). The type of phase transition at T=T(N) is determined by lattice interactions.     

Spontaneous symmetry breaking vacuum energy in cosmology

The gravitational effect of spontaneous symmetry breaking vacuum energy density is investigated by subtracting the flat space-time contribution from the energy in the curved space-time. We find that the remaining effective energy-momentum tensor is too small to cause the acceleration of the universe, although it satisfies the characteristics of dark energy. However, it could provide a promising explanation to the puzzle of why the gravitational effect produced by the huge symmetry breaking vacuum energy in the electroweak theory has not been observed, as it has a sufficiently small value (smaller than the observed cosmic energy density by a factor of 1032).     

Charge symmetry breaking in np-->dpi(0)

The forward-backward asymmetry in np-->dpi(0), which must be zero in the center-of-mass system if charge symmetry is respected, has been measured to be [17.2+/-8.0(stat)+/-5.5(syst)]x10(-4), at an incident neutron energy of 279.5 MeV. This observable is compared to recent chiral effective field theory calculations, with implications regarding the du quark mass difference.     

Light quark mass differences and charge symmetry breaking in hyperon-nucleon interaction

Quantum chromodynamics based isovector isoscolar particle mixing arising out of isospin violating quark mass differences has been used to construct a new charge-symmetry breaking Λ-N interaction element. It appears that vector meson exchanges between N and a physical Λ rather than pseudoscalar exchanges provide the largest contribution to explain the binding energy difference between the mirror hyper-nuclei _ΛHe⁴ and _ΛH⁴. Prediction of the model to Λn and Λp scattering lengths have been compared with those obtained from the combined analysis of ΛN scattering and s-shell hyperfragment data. We find satisfactory agreement of our results.     

Search for up-down quark mass differences via isospin symmetry breaking

We performed a series of experiments employing the two pion production reactions p + d → ³He + π⁰ and p + d → ³H + π⁺. The two isopin related reactions differ by a Clebsch Gordan coefficient of 2. We searched for deviations in this ratio in complete angular distributions in the range of the Δ excitation as well as in the vicinity of the η production threshold.     

Quark strong interactions as a possible mechanism of symmetry breaking in weak interactions

A new mechanism for symmetry breaking is suggested which is connected with the negative contribution of quark loops to the vacuum energy and with the strong interactions of quarks with gluons diminishing this effect. The “gluon mechanism” of symmetry breaking makes it possible to estimate the mass of the heaviest quark (～ 60 GeV) and the mass of the Higgs boson (～ 7 GeV).     

Spontaneous symmetry breaking and energy gap generated by variables at infinity

Spontaneous symmetry breaking in the presence of long range instantaneous interactions is studied and the general mechanism underlying it is clarified. A characteristic feature is that the algebraic dynamics does not leave any essentially local algebra stable, i.e. variables at infinity get involved in the time evolution of local variables, so that in each irreducible representation the time evolution fails to be symmetric. For continuous symmetries, the Fourier transform of the vacuum expectation value of charge commutators is related to the energy spectrum at low momenta and a generalized Goldstone theorem is proved which explains the generation of energy gap. This energy gap is further shown to be governed by a classical dynamics at infinity, equivalently by the group generated by the effective Hamiltonian and the charge. Explicit examples are discussed.Work supported in part by INFN, Sezione di Pisa     

Solvent-induced symmetry breaking

Symmetry breaking can be induced in a number of ways including interactions with a solvent. An example is the triiodide ion which is centrosymmetric in the gas phase. Molecular dynamics simulations of the triiodide ion in solution have been used to investigate the extent of symmetry breaking in a variety of solvents. We find that the triiodide ion loses its symmetry in water, ethanol, and methanol which form hydrogen bonds with the ion. This results in a localization of charge at one end of the ion and breaking of the geometric symmetry. The extent of symmetry breaking increases as the temperature is lowered. Correlation times for interconversion are reported and the energetics of symmetry breaking are presented. Analogies are made with second-order phase transitions.     

Absence of symmetry breaking for systems of rotors with random interactions

We prove that Gibbs states for the Hamiltonian , with thes _x varying on theN-dimensional unit sphere, obtained with nonrandom boundary conditions (in a suitable sense), are almost surely rotationally invariant if withJ _xy i.i.d. bounded random variables with zero average, 1 in one dimension, and 2 in two dimensions.     

Gravitation and spontaneous symmetry breaking

It is pointed out that the Higgs field may be supplanted by an ordinary Klein-Gordon field conformally coupled to the space-time curvature, and with very small, real, rest mass. Provided there is a bare cosmological constant of order of its square mass, this field can induce spontaneous symmetry breaking with a mass scale that can be as large as the Planck-Wheeler mass, but may be smaller. It can thus play a natural role in grand unified theories. In the theory presented here the physical cosmological constant is small, being of order of the squared mass, and can meet observational constraints without having to be cancelled accurately. The physical gravitational constant differs somewhat from the coupling constant in Einstein's equation, and is temperature dependent in the broken symmetry regime. Symmetry restoration occurs at high temperature.Research supported by the Arnow Chair in Astrophysics.     

Discrete symmetry and GUT breaking

We study the supersymmetric GUT models in which the supersymmetry and GUT gauge symmetry can be broken by a discrete symmetry. First, with the ansatz that there exist discrete symmetries in the branes' neighborhoods, we discuss the general reflection symmetries and GUT breaking on and . In those models, the extra dimensions can be large and the KK states can be set arbitrarily heavy. Second, considering that the extra space manifold is the annulus or the disc , we can define any symmetry and break any 6-dimensional N=2 supersymmetric SU(M) models down to the 4-dimensional N=1 supersymmetric models for the zero modes. In particular, there might exist the interesting scenario on where just a few KK states are light, while the others are relatively heavy. Third, we discuss the complete global discrete symmetries on and study the GUT breaking. Received: 12 February 2002 / Published online: 14 June 2002     

Space-time geometry and symmetry breaking

We look for solutions of the Einstein-Yang-Mills equations in a 4 + D dimensional space-time. We find solutions where the first 4 dimensions are a flat Minkowskian space-time, while the D others are a compact, space-like manifold of small size. Such solutions can be obtained for an arbitrary compact gauge group K and are invariant under a sub-group G of K related to the space-time geometry. This shows that 4 + D dimensional gravity can give a mechanism for the super-strong symmetry breaking needed in grand unified field theories without introducing Higgs scalars.     

Dynamical electroweak symmetry breaking

In theories of dynamical electroweak symmetry breaking, the electroweak interactions are broken to electromagnetism by the vacuum expectation value of a fermion bilinear. These theories may thereby avoid the introduction of fundamental scalar particles, of which we have no examples in nature. In this note, we review the status of experimental searches for the particles predicted in technicolor, topcolor, and related models.