Strong contribution to octet baryon mass splittings

We calculate the _md−_mu$m_d-m_u$ contribution to the mass splittings in baryonic isospin multiplets using SU(3) chiral perturbation theory and lattice QCD. Fitting isospin-averaged perturbation theory functions to PACS-CS and QCDSF-UKQCD Collaboration lattice simulations of octet baryon masses, and using the physical light-quark mass ratio _mu/_md$m_u/m_d$ as input, allows _Mn−_Mp$M_n-M_p$, _MΣ−−_MΣ+$M_{{\Sigma }^{-}}-M_{{\Sigma }^{+}}$ and _MΞ−−_MΞ0$M_{{\Xi }^{-}}-M_{{\Xi }^0}$ to be evaluated from the full SU(3) theory. The resulting values for each mass splitting are consistent with the experimental values after allowing for electromagnetic corrections. In the case of the nucleon, we find _Mn−_Mp=2.9±0.4 MeV$M_n-M_p=2.9\pm 0.4\text{MeV}$, with the dominant uncertainty arising from the error in _mu/_md$m_u/m_d$.     

One-gluon-exchange effects on semileptonic decay of baryon octet

We study one-gluon-exchange (OGE) effects ong _A /g _V of weak semileptonic decay processes of theSU(3) baryon octet. We especially investigate the systematic behavior of OGE effects on all the possible decay processes and search for better overall agreements between theory and experiment. We adopt the MIT bag model to describe quarks confined in a baryon. We also use a formulation of the perturbative QCD in which gluon field is also confined in the bag. Our calculations respect the boundary conditions which achieve the confinement of each field within the bag. The magnitudes of our corrections tog _A /g _V have a variation of 1 ～30% depending on the decay, processes. They have process-dependence quite different from that of the uncorrected values ofg _A /g _V primarily governed by theSU(6) symmetry. These OGE effects ong _A /g _V reflectSU(6) asymmetric spin-spin correlation between quarks due to the color magnetic interactions. Especially, the OGE corrections tog _A /g _V are very important in the processes, \(\Sigma ^ - \to n + l^ - + \bar v_l ,\Sigma ^ - \to p + l^ - + \bar v_l \) and \(\Xi ^ - \to \Xi ^0 + e^ - + \bar v_e \) . Consequently, we clarify that the OGE effects can sizably improve theoretical values ofg _A /g _V in favor of overall agreements with the present experimental data of semileptonic decays of the baryon octet, at least, in the MIT bag model.     

Mass spectrum of a baryon octet is linear

It is argued that the mass spectrum of a baryon octet is linear, consistent with a Gell-Mann-Okubo type relation for the squared masses of the baryons obtained previously be Balázs and Nicolsecu. The mass spectrum of a baryon decuplet is briefly discussed. On sabbatical leave from School of Physics and Astronomy. Tel Aviv University, Ramat Aviv, Israel. Also at Department of Physics, Bar-Ilan University, Ramat-Gan, Israel.     

Heavy baryon chiral perturbation theory and the weak nonleptonic p-wave decays of the baryon octet

Considerable confusion in the literature about heavy baryon chiral perturbation theory, its convergence properties, and appropriate calculational techniques, make it timely to discuss the nonleptonic p-wave decays of the octet of baryons. We present model independent results.     

Electromagnetic form factors of the baryon octet in the perturbative chiral quark model

We apply the perturbative chiral quark model at one loop to analyze the electromagnetic form factors of the baryon octet. The analytic expressions for baryon form factors, which are given in terms of fundamental parameters of low-energy pion-nucleon physics (weak pion decay constant, axial nucleon coupling, strong pion-nucleon form factor), and the numerical results for baryon magnetic moments, charge and magnetic radii are presented. Our results are in good agreement with experimental data.Received: 7 January 2003, Revised: 4 November 2003, Published online: 15 April 2004PACS: 12.39.Ki Relativistic quark model - 13.40.Gp Electromagnetic form factors - 14.20.Dh Protons and neutrons - 14.20.Jn Hyperons     

Weak form factors for semileptonic octet baryon decays in the chiral quark model

We study the weak vector and axial-vector form factors of first- and second-class currents for the semileptonic octet baryon decays in the spirit of the chiral quark model. Our results for the weak magnetism form factors are consistent with the conserved vector current (CVC) results. The induced pseudotensor form factors, which are highly model dependent, are small. The overall performance of the chiral quark model is quite good and in general agreement with existing experimental data. Received: 9 March 1998 / Published online: 12 August 1998     

Pseudoscalar meson and baryon octet interaction with strangeness zero in the unitary coupled-channel approximation

The interaction of the pseudoscalar meson and the baryon octet is investigated by solving the Bethe-Salpeter equation in the unitary coupled-channel approximation. In addition to the Weinberg-Tomozawa term, the contribution of the s-and u-channel potentials in the-wave approximation are taken into account. In the sector of isospin I=1/2 and strangeness S =0, a pole is detected in a reasonable region of the complex energy plane of ■ in the center-of-mass frame by analyzing the behavior of the scattering amplitude, which is higher than the ηN threshold and lies on the third Riemann sheet. Thus, it can be regarded as a resonance state and might correspond to the N(1535) particle of the Particle Data Group(PDG) review. The coupling constants of this resonance state to the πN,ηN,KΛ and KΣ channels are calculated, and it is found that this resonance state couples strongly to the hidden strange channels. Apparently, the hidden strange channels play an important role in the generation of resonance states with strangeness zero. The interaction of the pseudoscalar meson and the baryon octet is repulsive in the sector of isospin I = 3/2 and strangeness S = 0, so that no resonance state can be generated dynamically.     

Complex mixing of baryon resonances

We employ the Wigner-Weisskopf method for a simultaneous treatment of decays, mass splittings and mixings of baryon resonances. Complex mixing matrices naturally arise in this context. Most of the amplitudes at resonance become also complex now and we make predictions for their phases. In spite of the doubling of the number of parameters in the mixing matrices the well0known discrepancies between decay and spectrum analyses still persist.     

Violation of isotopic invariance in strong interactions and mass spectrum of the 1/2+ baryon octet

The ideas developed by Gell-Mann and Okubo in studying violation of unitary symmetry are used to describe violation of isototic invariance in strong interactions. The present consideration is performed for the example of the mass spectrum of the octet formed by baryons of spin-parity 1/2⁺: only for this family are the widths of its particles much less than the scale of the effects being investigated, their masses being known from experiments to a fairly high precision. The Gell-Mann-Okubo formula is generalized in such a way that relations both for the splitting between the isomultiplets of the octet and for the mass splitting within these isomultiplets follow from the new formula. Moreover, a relation between masses that describes their electromagnetic splitting and which coincides in form with the Coleman-Glashow relation also follows from this formula. The relations obtained for the masses of the baryons belonging to the octet in question are satisfied to a precision not poorer than 3%.     

Configuration mixing in Mo isotopes

An extension of the neutron-proton interacting boson model which takes into account the breaking of shell or subshell closures through two-particie-two-hole excitations is applied to the even-even Mo isotopes (Z = 42). In this calculation, ⁹⁰₄₀Zr₅₀ is taken as doubly magic and two-proton excitations across the subshell at Z = 40 are considered. Level spectra and electromagnetic properties are investigated. The results support the hypothesis of an interplay of two configurations in these isotopes.     

Flavour mixing of neutrinos and baryon asymmetry of the universe

We investigate baryogenesis in the ν  MSM, which is the Minimal Standard Model (MSM) extended by three right-handed neutrinos with Majorana masses smaller than the weak scale. In this model the baryon asymmetry of the universe (BAU) is generated via flavour oscillation between right-handed neutrinos. We consider the case when BAU is solely originated from the CP violation in the mixing matrix of active neutrinos. We perform analytical and numerical estimations of the yield of BAU, and show how BAU depends on mixing angles and CP violating phases. It is found that the asymmetry in the inverted hierarchy for neutrino masses receives a suppression factor of about 4% comparing with the normal hierarchy case. It is, however, pointed out that, when θ₁₃=0${\theta }_{13}=0$ and θ₂₃=π/4${\theta }_{23}=\pi /4$, baryogenesis in the normal hierarchy becomes ineffective, and hence the inverted hierarchy case becomes significant to account for the present BAU.     

The pseudoscalar meson and baryon octet interaction with strangeness S= −2 in the unitary coupled-channel approximation

The interaction of the pseudoscalar meson and the baryon octet with strangeness $S=-2$ and isospin $I=1/2$ is investigated by solving the Bethe–Salpeter equation in the infinite and finite volume respectively. It is found that there is a resonance state generated dynamically, which owns a mass of about 1550 MeV and a large decay width of 120–200 MeV. This resonance state couples strongly to the π${\rm{\Xi }}$ channel. Therefore, it might not correspond to the ${\rm{\Xi }}(1620)$ particle announced by Belle collaboration. At the same time, this problem is studied in the finite volume, and an energy level at 1570 MeV is obtained, which is between the ${\pi }{\rm{\Xi }}$ and $\bar{K}{\rm{\Lambda }}$ thresholds and independent of the cubic box size.     

Radiative decays with light scalar mesons and singlet–octet mixing in ChPT

We study different types of radiative decays involving f₀(980) and a₀(980) mesons within a unified ChPT-based approach at one-loop level. Light scalar resonances, which are seen in the ππ, πη and KK̄ channels of the φ(1020) radiative decays and in the J/ψ decays are responsible for key questions of low-energy dynamics in the strong interaction sector, and the decays φ(1020)→γa₀(980), φ(1020)→γf₀(980), a₀(980)→γγ and f₀(980)→γγ are of interest for current experimental programs in Jülich, Frascati and Novosibirsk. From the theoretical point of view it is important to verify whether light scalar mesons are members of some flavor octet or nonet. We find a value for the mixing angle dictated by consistency with experiment and the coupling structures of the ChPT Lagrangian. The decay widths f₀(980)/a₀(980)→γρ(770)/ω(782), which have not been experimentally studied yet, are predicted. We also obtain several relations between the widths, which hold independently of the coupling constants and which represent a fingerprint of the model. PACS 11.30.Hv; 12.39.Fe; 13.30.Eg; 14.40.-n     

Configuration mixing in low-lying spectra of carbon hypernuclei

<正>The spectroscopy of hypernuclear low-lying states is very important to understand the structure of hypernuclei and the hyperon impurity effect in atomic nuclei.Several novel phenomena about Λ hyperon have already been discovered by studying energy spectra and electromagnetic transitions of low-lying states in p-shell hypernuclei.One of them is the