Strong isospin breaking of the nucleon and delta masses on the lattice

Strong isospin breaking in the spectrum of the nucleons and deltas can be studied in lattice QCD with the help of chiral perturbation theory. At leading order in the chiral expansion, the mass splittings between the proton and neutron and between the deltas are linear in the quark mass difference. The next-to-leading order contributions to these splittings vanish even away from the strong-isospin limit. Therefore, any non-linear quark mass dependence of these mass splittings is a signal of the next-to-next-to-leading order mass contributions, thus providing access to low energy constants at this order. We determine the mass splittings of the nucleons and deltas in two-flavor, heavy baryon chiral perturbation theory to next-to-next-to-leading order. We also derive expressions for the nucleon and delta masses in partially quenched chiral perturbation theory to the same order. The resulting mass expressions will be useful both for the extrapolation of lattice data on baryon masses, and for the study of strong isospin breaking.     

A radiative model with a naturally mild neutrino mass hierarchy

Many neutrino mass models postulate the existence of at least two extra fermions in order to account for the measured solar and atmospheric mass splittings. In these models, however, the predicted hierarchy between the two mass splittings is generically much larger than the observed one, unless extra flavor symmetries are introduced. We present in this Letter a radiative neutrino mass model consisting of the Standard Model extended by one heavy fermionic singlet and two scalars which predicts, under very general conditions, a neutrino mass hierarchy in qualitative agreement with the experimental value.     

Grand unification with large supersymmetry breaking

We give general criteria for the magnitudes of mass splittings between supersymmetric partners compatible with the desired hierarchy of mass scales in GUTs. These splittings arise naturally when masses are generated radiatively from a Higgs sector breaking supersymmetry through the O'Raifeartaigh mechanism. We construct explicit GUTs in which the supersymmetry-breaking scale may easily be as large as the Planck mass.     

A QCD relation between electromagnetic mass differences of charmed mesons and strange baryons

Relations between hadron mass splittings are obtained from the assumption that the ratio of the electromagnetic and strong contributions to hardon spin splittings depends only upon the ratio of the electromagnetic to the strong couplings of the quark-quark or quark-antiquark pairs involved. This ratio is the same in charmed mesons and strange baryons when the values of color and electric charges of the standard colored quark model are used. Predictions for D¹⁺?D¹⁰ and ∑^1?-∑¹⁺ mass splittings obtained are in good agreement with present experimental data and can be tested much better when better data on D and D¹ masses become available.     

Quarkonia with a variational method

A variational method, based in independent minimization of energy levels, is applied to quantum mechanical quarkonium systems. A discussion is presented of the method, with emphasis on quark mass dependence of energy levels (Feynman-Hellman theorem), on the behaviour of wave functions at the origin (Martin's theorem) and on the ordering of energy levels. The potential used in the applications is the Coulomb (4-vector) + Linear (4-scalar) potentials, the fine and hyperfine splittings being included by means of the Fermi-Breit hamiltonian. An attempt is made for an overall treatment of the known splittings (radials-wave, hyperfine and fine splittings) for all quarkonium systems, using an approximately flavour independent potential. Thep-wave splittings, as well as the hyperfine splittings, are shown to be particularly sensitive to the nature and mass dependence of the potential. In particular, asymptotic freedom can be more easily tested there. On the other hand, radial excitations provide the place where the set up of short range potential effects should be first detected. Some results concerning the ordinary mesons are presented, and it is specifically pointed out that strong restrictions exist for the masses of the ρ′ and theA ₁. Predictions for the toponium family are also presented.     

Breit夸克势的不同次正规化与ηc-J/ψ等的质量劈裂

用夸克势模型研究结构相同而自旋和轨道量子数不同的介子之间质量劈裂是检验势模型有效性的重要手段之一. 在以往的用各种夸克势模型计算质量劈裂工作中, 当轻介子和重介子一起计算时, π-ρ很容易劈裂, 而η_c-J/ψ等的劈裂都很 难达到实验值. 这里首先用正规化形状因子μ²/(q²+μ²), 对完整的动量空间中的Breit夸克势的第三项实施二次正规化, 除了第一项 库仑势和第七项常数项势, 对其余的项实施一次正规化, 然后用来计算 质量劈裂. 研究计算发现, 只有当屏蔽质量μ取为关于 折合质量μ_r=m_r m_j/(m_r+m_j) 的三阶多项式时, 轻介子π-ρ和重介子η_c-J/ψ, η_b-Υ(1s), 还有χ_c0-χ_c1-χ_c2 等的劈裂 精确达到实验值, 同时其他介子质量也都比以往得到较大的改善. 因此, 本文给出了一个有效的夸克势模型.     

Breit势的正规化与ηc-J/ψ的劈裂

研究自旋和轨道量子数不同的介子之间的质量劈裂对检验夸克势模型非常重要。 在以往的夸克势模型计算中,η_c-J/ψ的质量劈裂都很难达到实验的值。 用正规化形状因子μ2/(q2+μ2)， 在对完整的动量空间中的Breit夸克势实施一次正规化下研究了η_c-J/ψ和新介子ηb与Υ(1s)之间的质量劈裂。 结果表明， 正规化形状因子中的屏蔽质量μ应与介子折合质量有关， 当将其展开为介子折合质量的三阶多项式时， 上述介子质量劈裂可以精确重现实验的结果。另外， 由于完整的Breit势包含有自旋-轨道耦合相互作用的项，因而还可以研究χc0,χc1和χc2之间的质量劈裂。 The study of the splittings between the masses of mesons with different spin and orbit quantum numbers is important for checking the quark potential model. In the previous calculations with quark potential models, the splitting between ηc and J/ψ is however too small to the experimental result. In this paper the mass splitting of ηc J/ψ and the splitting between the new meson ηb and Υ(1s) are investigated with the complete Breit quark potential regularized by applying the form factor μ2/(q2+μ2) one time in momentum space. In addition, because the complete Breit potential includes the spin orbit interaction, it can be used to investigate the splittings among χc0, χc1 and χc2. The investigated results indicate that the screen mass μ in the form factor of regularization is related to the reduced mass of meson. The splittings of η_c-J/ψ, ηb Υ, and χc0 χc1 χc2 can reproduce the experimental results with high accuracy when the screen mass is expanded to the third order polynomial of meson reduced mass.     

Mass splitting in supersymmetric theories from an effective lagrangian point of view

Gauge and global internal symmetries, even when spontaneously broken, restrict the possible mass splittings within supermultiplets. A systematic method of analysis is presented which is applicable both for 〈F〉- and 〈D〉-type supersymmetry breaking, with emphasis on avoiding unacceptable splittings within the quark and lepton supermultiplets. Expressions for gluino and photino masses are also obtained.     

Hyperfine interactions as clues to the structure of elementary particles

The history of hyperfine interactions in hadron physics is reviewed. The recent treatment of hyperfine splittings in meson spectroscopy by Frank and O'Donnell is generalized and applied to baryons as well as mesons by the use of techniques developed for treating hyperfine interactions in atomic physics. New relations between meson and baryon mass splittings are obtained following from the assumption that mesons and baryons are made of the same quarks and have the same color hyperfine interactions at the quark level, with corrections due to color factors and differences between baryon and meson wave functions. Work supported by the U.S. Department of Energy, Division of High Energy Physics, Contract W-31-109-ENG-38 and by the Minerva Foundation, Munich, Germany. 1985–86 Argonne Fellow on leave from the Wiezmann Institute of Science, Rehovot, Israel.     

"Intrinsic" acceptor ground state splitting in silicon: an isotopic effect

One of the oldest open questions in semiconductor physics is the origin of the small splittings of the neutral acceptor ground state in silicon which lead to a distribution of doublet splittings rather than the fourfold-degenerate state of Gamma(8) symmetry expected in the absence of perturbations. Here we show that these acceptor ground state splittings are absent in the photoluminescence spectra of acceptor bound excitons in isotopically purified 28Si, demonstrating conclusively the surprising result that the splittings previously observed in natural Si result from the randomness of the Si isotopic composition.     

Calculation of hyperfine splitting in mesons using configuration interaction approach

The spin–spin mass splitting of light, heavy and mixed mesons are described within a good accuracy in the potential model with screened potential. We conclude that the long-range part of the potential cannot be pure scalar and that a vector–scalar mixture is favored. Excellent spin–spin splittings of heavy quarkonia are obtained with the same parameters as the ones which give the correct average mass spectrum. The results are obtained by going beyond the usually used perturbation method, namely using a configuration interaction approach. Received: 19 September 2000 / Revised version: 19 April 2001 / Published online: 3 August 2001     

A mechanism for fermion mass generation

We present a mechanism for fermion mass generation, and apply it to the lepton sector of the Weinberg-Salam model. Extra fields, necessary to produce the large mass splittings of the observed fermions, form bound states more massive than the electroweak scale.     

重夸克偶素质量谱的相对论夸克模型研究（已撤稿）

使用量子场论中准势途径的相对论夸克模型,计算了重夸克偶素的质量谱.所采用的同味正反夸克间的有效相互作用包括通常的直接单胶子交换、单胶子湮没及具有相对论修正的标量和矢量线性禁闭的混合.得到了与实验上肯定的数据符合较好的拟合计算结果，并讨论了湮没项和相对论修正各项对能谱超精细分裂的作用. 关键词： 重夸克偶素 质量谱 相对论修正 湮没势     

Neutrino masses in the left–right supersymmetric model

We show that in a left–right supersymmetric model with a Higgs structure that supports the see-saw mechanism, the neutrinos get additional contributions to their masses at one loop level. The mechanism responsible is analogous to the Grossman–Haber see-saw mechanism, but the additional mass terms are proportional to the mass difference of the right-handed sneutrinos. We show that the data on both the solar and the atmospheric neutrinos can be accommodated by either two almost degenerate right-handed sneutrinos, or two heavy sneutrino with different, but still relatively small, mass splittings. We discuss the implications of this result for the masses and mixings of the heavy sneutrinos, and the soft-breaking parameters of the left–right supersymmetric model.     

A phenomenological bag model with variable bag pressure

We examine the consequences of a variable (density-dependent) bag pressure term and a fixed hadronic size in the phenomenological MIT bag model for hadron spectroscopy. Mass spectrum of the low-lying baryons and mesons, baryon magnetic moments and the hadron mass splittings are estimated. These are found to be in closer agreement with experiment than the MIT results.     

Nuclear relativistic Hartree-Fock approximation with weakened pion tensor force

Properties linked to the single-particle energies, as nuclear spectra, spin-orbit splittings and shell gaps are investigated in the framework of the relativistic Hartree-Fock approximation with pseudovector coupling for the πN vertex. The role of an effective mass of pions moving in the nuclear medium and its relationship with the strength of pion tensor force is discussed. A simple method to reduce the contribution of this tensor force that considerably improves the single-particle spectrum of nuclei is proposed.     

Electromagnetic mass differences between baryons in potential models

We calculate explicitly the electromagnetic mass splittings induced within the ground state baryons by the mass difference between theu andd quarks. This is performed in the framework of the non-relativistic potential model with an accurate treatment of the three-body problem. We also discuss the charge radius of the neutron.     

Are spin-dependent forces in charmonium generated through scalar-vector admixture?

I show that if the spin-dependent confining forces are generated through scalar-vector admixture then the ³p_J splittings restrict the vector to scalar ratio to be less than half. Further it is shown that an accurate measurement of the ¹p₁ mass can clearly distinguish this model from vector and scalar exchange models.