Diquarks from aU (3) L ×U (3) R invariant quark Lagrangian

An effective Lagrangian involving composite pseudoscalar and scalar mesons as well as scalar and pseudoscalardiquarks is derived from a chiralU(3) _L ×U(3) _R invariant four-quark theory using path integral methods. We obtain mass relations for the diquarks where all free parameters can be fixed in the meson sector. We also calculate weak diquark decay constants, and discuss baryon masses in terms of a quark-diquark model.     

Some rare processes in a model of composite quarks and leptons

The branching ratios of some rare processes, i.e. meson →μe, meson →meson μe, baryon → baryon μe, are calculated in a composite quark and lepton model. Use is made of an effective Hamiltonian which originates from hypercolor singlet 0^? and 1^? bound state exchange mechanism, acting at a mass scaleM _P andM _V respectively. Under the reasonable assumptionM _P =M _V onlyK _L →μe puts a significant limit on the composite scale.     

Baryon–Baryon and Meson–Baryon Potentials with QCD-Based Short-Range Cores

A unified potential model of baryon–baryon and meson–baryon interactions at low energies is proposed. In this model, the short-range cores which simulate recent lattice QCD calculations are introduced. In baryon–baryon sector, our potentials give a very good agreement with NN and YN scattering data and have very similar behavior to those from the lattice QCD calculations. The π N and KN interactions are also well described in our model.     

A comment on the phenomenology of the SU(3) Skymre model

We find that the realistic baryon mass spectrum in the SU(3) Skyrme model with massive 0^? mesons can be obtained only for F_π < 70 MeV. We fit m_Λ and and obtain baryon masses within the limits of ± 16% of the experimental val ues, but at the expense of very low ifF_π=46.32 MeV.     

Hadron masses and the sigma commutator in light of chiral perturbation theory

We analyze carefully the impact of non-analytic chiral corrections to the mass spectrum of the pseudoscalar meson octet J^P = 0^? and the baryon octet $\text{J}{}^{\mathrm{<mtext>P</mtext>}}\text{=}\text{1}\text{2}{}^{\mathrm{+}}$. We find that the quark mass ratios must lie in the range $\text{21 ≤}\text{m}{}_{\mathrm{<mtext>s</mtext>}}\text{m}\text{?}\text{≤ 32}$ and $\text{1.6 ≤}\text{m}{}_{\mathrm{<mtext>d</mtext>}}\text{m}{}_{\mathrm{<mtext>u</mtext>}}\text{≤ 2.2}$. We also calculate the analogous corrections to the pion-nucleon sigma commutator σ_πN. It turns out that the value σ_πN = 60 MeV is not compatible with the structure of the meson and baryon spectrum, unless the nucleon mass is smaller than 600 MeV in the chiral limit m_u = m_d = m_s = 0.     

QUARK MASS RENORMALIZATION IN LATTICE GAUGE THEORY (Ⅱ) BARYON MASS CALCULATION

Using the renormalized quark mass m(g) obtained by fixing the ρ meson mass, we calculate the masses of the baryon states as a function of 1/g² in lattice gauge theory with fermions by variational method. The results are in agreement with experimental values in the range of 1/g² between 0.1 to 0.9.     

Solitons bound to heavy mesons

We improve the bound state approach of the Skyrme model applied to the heavy baryons by adopting a static heavy meson picture where the soliton moves around the fixed heavy meson. This allows to take into account the center of mass corrections in a more consistent way. The bound state masses so obtained are comparable to the experimentally observed Λ _c and Λ _c ^* masses. A loosely bound state of a soliton with an antiflavored heavy meson is found, which leaves a possibility of the nonstrange pentaquark baryon(s).     

An su (6)w fit to the negative parity baryon decay rates

We present a least squares fit to the experimental data on decays of negative parity baryon resonances into a pseudoscalar meson and either a J^P = 1/2⁺ stable baryon or a J^P = 3/2⁺ decuplet member. We find that the s-waves and d-waves are separately in good agreement with the predictions of SU (6)_w ⊗ O(2)_Lz. Predictions are given regarding several as yet unobserved decay processes, and for those which concern hitherto undetected resonances, their possible detection is discussed.     

Octet-decuplet baryon mass splittings from self-consistent one-loop perturbation theory

The bag model of confined relativistic quarks in chiral-invariant interaction with scalar, pseudoscalar, vector, and pseudovector mesons, as well as gluons, is used to calculate the masses and wave functions of the spin-1/2 baryon octet and spin-3/2 decuplet, using selfconsistent Brillouin-Wigner bound state perturbation theory. Chiral symmetry breaking is invoked with the sigma model. SU (6) and SU (3) symmetries are broken by the experimental meson spectrum, and a strange quark mass. Mass corrections are calculated to one loop order, limited to the baryons of the octet and decuplet and the lowest lying mesons. Encouraging results are obtained, especially for theΔ — N and theΣ — Λ splittings. Convergence and stability have not been demonstrated, but are evidently improved by the self-consistency requirement. An initial parameter tuning gives a fit to all the octet and decuplet masses within ≦0.02 GeV, at the price of choosing the bag radius, the non-strange baryon input bag mass, and the strange quark mass. Even these small discrepancies can be dramatically reduced by fine-tuning the vector meson coupling and including an instanton contribution peculiar to theΛ.     

New results of experiments at the Tevatron

The most important results obtained by the D0 and CDF collaborations in 2011–2012 on the search for the Higgs boson, studying the properties of the t-quark, measuring the W-boson mass, studying meson and baryon states with b-quarks, and a search for a New Physics beyond the Standard Model, are discussed. A comparison with the experimental results from the LHC is given.     

SU (2N f) ⊗ O(3) light diquark symmetry and current-induced heavy baryon transition form factors

We study the current-induced bottom baryon to charm baryon transitions in the Heavy Quark Symmetry limit as m_q → ∞. Our discussion involves s-wave to s-wave as well as s-wave to p-wave transitions. Using a constituent quark model picture for the light diquark system with an underlying SU (2N_f) ? O(3) symmetry and the heavy quark symmetry we arrive at a number of new predictions for the reduced form factors that describe these transitions.     

Hadronization of quark systems

Hadronization in simple models of quark systems is discussed with emphasis on the combinatorial aspects. The numberN of quarks and the numberN _c of colours are the relevant variables. In onedimension and in the meson and baryon sector the problem is completely soluble. When multiquark hadrons are included an iterative procedure still allows a complete solution. Higher dimensions are difficult to treat but global hadronization, without dimensionality constraints, is again soluble. As a general result the baryon/meson ratio is, for large quark densities, rather large, ?1/3 and may grow without limit asN→∞.     

The Λ(1405) state in a chiral unitary approach with off-shell corrections to dimensional regularized loop functions

The Bethe-Salpeter equation is solved in the framework of the unitary coupled-channel approximation by using the pseudoscalar meson-baryon octet interaction. The loop function of the intermediate meson and baryon is deduced in a dimensional regularization scheme, where the relativistic kinetic effect and off-shell corrections are taken into account. Based on the experimental data at the K~-p threshold, the subtraction constants in the loop function are determined. The squared amplitude is suppressed strongly and only one Λ(1405) state is generated dynamically in the strangeness S =-1 and isospin I =0 sector.     

SU(2N f) ⊗O(3) light diquark symmetry and current-induced heavy baryon transition form factors

We study the current-induced bottom baryon to charm baryon transitions in the Heavy Quark Symmetry limit asm _q→∞. Our discussion involvess-wave tos-wave as well ass-wave top-wave transitions. Using a constituent quark model picture for the light diquark system with an underlyingSU(2N _f)?O(3) symmetry and the heavy quark symmetry we arrive at a number of new predictions for the reduced form factors that describe these transitions.     

Low energy theorems in a nonlocal chiral quark model at finite temperature

We solve the Dyson equation and the Bethe-Salpeter equation for a nonlocal effective quark interaction kernel which is instantaneous and separable. The momentum-dependent dynamical quark mass, the scalar and pseudoscalar meson masses, the pion decay constant and the quark meson coupling constant are calculated at finite temperature in the Hartree approximation for the quark self energy. We obtain relations between these quantities, which coincide to leading order in the current quark mass (m ₀/Δm) with the basic low energy theorems: the Goldstone theorem, the Gell-Mann-Oakes-Renner relation and the Goldberger-Treimann relation at finite temperature. A formula for the σ?π mass gap is obtained which exhibits an additional contribution from the momentum dependence of the quark mass.     

Spectrum of lattice gauge theories with fermions from a 1/d expansion at strong coupling

We study the strong coupling limit of U(N) or SU(N) gauge theories with fermions on a lattice. The integration over the gauge and fermion degrees of freedom is performed by analytic methods, leading to a partition function in terms of localized meson and baryon fields. A method for deriving a systematic expansion in the inverse of the space-time dimension of the corresponding Green functions is developed. It is applied to the study of spontaneous breakdown of chiral symmetry, which occurs for any U(N) or SU(N) theory with fermions in the fundamental representation. Meson and baryon spectra are then computed, and found to be in close agreement with those obtained by numerical methods at finite coupling. The pion decay constant is estimated.     

Meson cloud and SU(3) symmetry breaking in parton distributions

We apply the meson cloud model to the calculation of non-singlet parton distributions in the nucleon sea, including the octet and the decuplet cloud baryon contributions. We give special attention to the differences between non-strange and strange sea quarks, trying to identify possible sources of SU(3) flavor breaking. An analysis in terms of the parameter is presented, and we find that the existing SU(3) flavor asymmetry in the nucleon sea can be quantitatively explained by the meson cloud. We also consider the baryon, finding similar conclusions. Received: 28 July 2000 / Published online: 23 October 2000