Quark Model Predictions Research Articles

Spin splittings among charmed hadrons

The mass differences between spin-1/2 and spin-3/2 baryons are compared to the mass differences between spin-0 and spin-1 mesons. Results of simulations for charmed hadrons in the quenched approximation from a tadpole-improved anisotropic action are discussed in the context of other lattice calculations, quark model predictions, heavy quark symmetry predictions and experimental data.

Electromagnetic Transition Form Factors and Dilepton Decay Rates of Nucleon Resonances

Relativistic, kinematically complete phenomenological expressions for the dilepton decay rates of nucleon resonances with arbitrary spin and parity are derived in terms of the magnetic, electric, and Coulomb transition form factors. The dilepton decay rates of the nucleon resonances with masses below 2 GeV are estimated using the extended vector meson dominance (VMD) model for the transition form factors. The model provides a unified description of the photo- and electroproduction data, γ(γ*)N→N*, the vector meson decays, N*→Nϕ(ω), and the dilepton decays, N*→Nℓ+ℓ−. The constraints on the transition form factors from the quark counting rules are taken into account. The parameters of the model are fixed by fitting the available photo- and electroproduction data and using results of the multichannel partial-wave analysis of the π N scattering. Where experimental data are not available, predictions of the non-relativistic quark models are used as an input. The vector meson coupling constants of the magnetic, electric, and Coulomb types are determined. The dilepton widths and the dilepton spectra from decays of nucleon resonances with masses below 2 GeV are calculated.

Properties of theΛ(1670)12−Resonance

Recently the Crystal Ball Collaboration measured precise new data for the near-threshold reaction K(-)p-->etaLambda, which is dominated by formation of the Lambda(1670)1 / 2(-). In this Letter, we present results of a unitary, multichannel analysis that incorporates the new Crystal Ball data. For our preferred fit, we obtain mass M = 1673+/-2 MeV, width Gamma = 23+/-6 MeV, and elasticity x = 0.37+/-0.07. This elasticity is significantly larger than previously recognized. Resonance parameters of our preferred fit are in striking agreement with the quark-model predictions of Koniuk and Isgur.

Low-lying [formula omitted] states in the baryon spectrum

The coupling to light mesons leads to large widths and shifts of the energy of the excited states in the baryon spectrum from the predictions of the constituent quark model with three valence quarks. This coupling may be modeled by admixtures of sea-quark qqq(q q ̄ ) n configurations in the resonances. A schematic flavor- and spin-dependent interaction model, similar to the one which reproduces the low-lying part of the experimental baryon spectrum in the valence quark model, is shown to bring the lowest qqqq q ̄ states with L=1 and positive parity below the states with L=0. Because of the partial overlap with the corresponding qqq states this suggests that most of the low-lying states in the baryon-resonance spectrum have substantial qqqq q ̄ admixtures.

Radiative leptonicBdecays in the instantaneous Bethe-Salpeter approach

Rare radiative leptonic decay $B\to l\bar \nu_l \gamma$ is studied in the instantaneous Bethe-Salpeter approach. The results are compared to other relativistic quark model predictions.

Quark model study of the η photo-production:

An extensive and systematic study of the recent eta photoproduction data up to 1.2 GeV is presented within a chiral constituent quark model. A model embodying all known nucleonic resonances shows clear need for a yet undiscovered third S11 resonance in the second resonance region, for which we determine the mass (1.729 GeV) and the total width (183 MeV). Furthermore, we extract the configuration mixing angles, an important property of the quark-quark interaction in the quark model, for the resonances S11(1535) and S11(1650), as well as for the resonances D13(1520) and D13(1700). Our results agree well with the quark model predictions. In addition, the partial eta N decay widths and/or the photo-excitation helicity amplitudes for the nucleonic resonances S11(1535), S11(1650), P11(1710), P13(1720), D13(1520), D13(1700), D15(1675), and F15(1680) are also obtained in this approach.

Quark model and production of neutral strange secondaries in neutrino and antineutrino beams

The experimental data on $K^0$ and $\Lambda$ production by $\nu$ and $\bar{\nu}$ beams are compared with the predictions of quark model assuming that the direct production of secondaries dominates. Disagreement of these predictions with the data allows one to suppose that there exists considerable resonance decay contribution to the multiplicities of produced secondaries.

Orbital angular momentum parton distributions in light-front dynamics

We study the quark angular momentum distribution in the nucleon within a light-front covariant quark model. Special emphasis is put into the orbital angular momentum: a quantity which is very sensitive to the relativistic treatment of the spin in a light-front dynamical approach. Discrepancies with the predictions of the low-energy traditional quark models where relativistic spin effects are neglected, are visible also after perturbative evolution to higher momentum scales. Orbital angular momentum distributions and their contribution to the spin sum rule are calculated for different phenomenological mass operators and compared with the results of the MIT bag model.

Charge symmetry violation in the parton distributions of the nucleon

We point out that charge symmetry violation in both the valence and sea quark distributions of the nucleon has a non-perturbative source. We calculate this non-perturbative charge symmetry violation using the meson cloud model, which has earlier been successfully applied to both the study of SU(2) flavour asymmetry in the nucleon sea and quark-antiquark asymmetry in the nucleon. We find that the charge symmetry violation in the valence quark distribution is well below 1%, which is consistent with most low energy tests but significantly smaller than the quark model prediction about 5%-10%. Our prediction for the charge symmetry violation in the sea quark distribution is also much smaller than the quark model calculation.

Determining the quantum numbers of excited heavy mesons

We discuss the decays X ∗→Xe +e − (“Dalitz decays”) of excited heavy mesons into their ground states and an electron–positron pair. We argue that the measurement of the invariant mass spectrum of the lepton pair gives clear indication on the quantum numbers of the excited meson and thus provides an experimental test of the quark model predictions.

Hadron electric dipole moments from CP-odd operators of dimension five via QCD sum rules: the vector meson

We present a complete analysis of the electric dipole moment (EDM) of the ρ-meson induced by CP violating operators of dimension 4 and 5 within the QCD sum rules approach. The set of CP-odd operators includes the theta term and the electric and chromoelectric dipole moments of the three light quarks. We find that the ρ-meson EDM induced purely by the EDMs of quarks is smaller than, but still in reasonable agreement with, the predictions of a naive constituent quark model. However, the chromoelectric dipole moments, including that of the strange quark, give comparable and sometimes larger contributions. We also consider the effect on the hadronic EDM of the existence of Peccei-Quinn symmetry. When this symmetry is active, chromoelectric dipole moments induce a linear term in the axion potential which leads to a numerically important vacuum contribution to the hadronic EDM.

Weak magnetism in chiral quark models

We discuss symmetry breaking in the weak magnetism form factors for the semileptonic octet baryon decays. In the chiral quark model, the symmetry breaking can be accounted for in the masses and the quark spin polarizations can take on more general values due to Goldstone boson depolarization. Here we clarify some features of the chiral quark model prediction for the weak magnetism and compare to the corresponding result of the chiral quark soliton model.

Masses and internal structure of mesons in the string quark model

The relativistic quantum string quark model, proposed earlier, is applied to all mesons, from pion to $\Upsilon$, lying on the leading Regge trajectories (i.e., to the lowest radial excitations in terms of the potential quark models). The model describes the meson mass spectrum, and comparison with measured meson masses allows one to determine the parameters of the model: current quark masses, universal string tension, and phenomenological constants describing nonstring short-range interaction. The meson Regge trajectories are in general nonlinear; practically linear are only trajectories for light-quark mesons with non-zero lowest spins. The model predicts masses of many new higher-spin mesons. A new $K^*(1^-)$ meson is predicted with mass 1910 Mev. In some cases the masses of new low-spin mesons are predicted by extrapolation of the phenomenological short-range parameters in the quark masses. In this way the model predicts the mass of $\eta_b(1S)(0^{-+})$ to be $9500\pm 30$ MeV, and the mass of $B_c(0^-)$ to be $6400\pm 30$ MeV (the potential model predictions are 100 Mev lower). The relativistic wave functions of the composite mesons allow one to calculate the energy and spin structure of mesons. The average quark-spin projections in polarized $\rho$-meson are twice as small as the nonrelativistic quark model predictions. The spin structure of $K^*$ reveals an 80% violation of the flavour SU(3). These results may be relevant to understanding the ``spin crises'' for nucleons.

Light-meson spectroscopy

We survey the current status of light meson spectroscopy. We begin with a general introduction to meson spectroscopy and and its importance in understanding the physical states of Quantum Chromo Dynamics (QCD). Phemenological models of hadron spectroscopy are described with particular emphasis on the constituent quark model and the qualitative features it predicts for the meson spectrum. We next discuss expectations for hadrons lying outside the quark model, such as hadron states with excited gluonic degrees of freedom. These states include so-called hybrids glueballs}, as well as multiquark states. The established meson states are compared to the quark model predictions and we find that most meson states are well described by the quark model. However, a number of states in the light-quark sector do not fit in well, suggesting the existence of hadronic states with additional degrees of freedom. We end with a brief description of future directions in meson spectroscopy.

BB intermeson potentials in the quark model

In this paper we derive quark model results for scattering amplitudes and equivalent low energy potentials for heavy meson pairs, in which each meson contains a heavy quark. This "BB" system is an attractive theoretical laboratory for the study of the nuclear force between color singlets; the hadronic system is relatively simple, and there are lattice gauge theory (LGT) results for V_BB(r) which may be compared to phenomenological models. We find that the quark model potential (after lattice smearing) has qualitative similarities to the LGT potential in the two B*B* channels in which direct comparison is possible, although there is evidence of a difference in length scales. The quark model prediction of equal magnitude but opposite sign for I=0 and I=1 potentials also appears similar to LGT results at intermediate r. There may however be a discrepancy between the LGT and quark model I=1 BB potentials. A numerical study of the two-meson Schrodinger equations in the (bqbar)(bqbar) and (cqbar)(cqbar) sectors with the quark model potentials finds a single "molecule", in the I=0 BB* sector. Binding in other channels might occur if the quark model forces are augmented by pion exchange.

Proton polarization shifts in electronic and muonic hydrogen

The contribution of virtual excitations to the energy levels of electronic and muonic hydrogen is investigated combining a model-independent approach for the main part with quark model predictions for the remaining corrections. Precise values for the polarization shifts are obtained in the long-wavelength dipole approximation by numerically integrating over measured total photoabsorption cross sections. These unretarded results are considerably reduced by including retardation effects in an approximate way since the average momentum transfer (together with the mean excitation energy) turns out to be larger than usually assumed. Transverse and seagull contributions are estimated in a simple harmonic oscillator quark model and found to be non-negligible. Possible uncertainties and improvements of the final results are discussed.

Valence QCD: Connecting QCD to the quark model

A valence QCD theory is developed to study the valence quark properties of hadrons. To keep only the valence degrees of freedom, the pair creation through the Z graphs is deleted in the connected insertions, whereas the sea quarks are eliminated in the disconnected insertions. This is achieved with a new ``valence QCD'' Lagrangian where the action in the time direction is modified so that the particle and antiparticle decouple. It is shown in this valence version of QCD that the ratios of isovector to isoscalar matrix elements (e.g., ${F}_{A}{/D}_{A}$ and ${F}_{S}{/D}_{S}$ ratios) in the nucleon reproduce the SU(6) quark model predictions in a lattice QCD calculation. We also consider how the hadron masses are affected on the lattice and discover new insights into the origin of dynamical mass generation. It is found that, within statistical errors, the nucleon and the $\ensuremath{\Delta}$ become degenerate for the quark masses we have studied (ranging from 1 to 4 times the strange mass). The $\ensuremath{\pi}$ and $\ensuremath{\rho}$ become nearly degenerate in this range. It is shown that valence QCD has the C, P, T symmetries. The lattice version is reflection positive. It also has the vector and axial symmetries. The latter leads to a modified partially conserved axial Ward identity. As a result, the theory has a $\mathrm{U}{(2N}_{F})$ symmetry in the particle-antiparticle space. Through lattice simulation, it appears that this is dynamically broken down to ${\mathrm{U}}_{q}{(N}_{F})\ifmmode\times\else\texttimes\fi{}{\mathrm{U}}_{\overline{q}}{(N}_{F})$. Furthermore, the lattice simulation reveals spin degeneracy in the hadron masses and various matrix elements. This leads to an approximate ${\mathrm{U}}_{q}{(2N}_{F})\ifmmode\times\else\texttimes\fi{}{\mathrm{U}}_{\overline{q}}{(2N}_{F})$ symmetry which is the basis for the valence quark model. In addition, we find that the masses of N, $\ensuremath{\Delta},\ensuremath{\rho},\ensuremath{\pi}{,a}_{1}$, and ${a}_{0}$ all drop precipitously compared to their counterparts in the quenched QCD calculation. This is interpreted as due to the disappearance of the ``constituent'' quark mass which is dynamically generated through tadpole diagrams. The origin of the hyperfine splitting in the baryon is largely attributed to the Goldstone boson exchanges between the quarks. Both of these are the consequences of the lack of chiral symmetry in valence QCD. We discuss its implications concerning the models of hadrons.

Is the resonance D(2637) really a radial excitation?

We consider various possible identifications of the quantum numbers of the resonance D(2637) recently observed by DELPHI in the D ∗ππ channel. We argue that in spite of a good agreement of the measured mass with the quark-model prediction for the radial excitation, a total width as small as ≤15 MeV makes problematic its identification as a radial charm excitation. The J P =2 −,3 − orbitally excited mesons with such a mass could have widths of the observed order of magnitude. However in this case one would expect two neighbouring states with the mass difference of about 30–50 MeV corresponding to the nearly degenerate components of the heavy-meson multiplet with light-quanta angular momentum j=5/2, and moreover, according to the quark-model predictions the mass of the orbital excitation should be more than 50 MeV larger than 2637 MeV. Thus we conclude that, at present, we find no fully convincing understanding of the quantum numbers of the observed resonance.

Study of the baryon resonances structure via η photoproduction

Within a quark model the process γp → ηp, for E γ lab < 1.2 GeV, is investigated. We find that the recent data from GRAAL on the polarized beam asymmetry require significant contributions from the resonances D 13(1520), F 15(1680), and to a lesser extent from P 13(1720) in addition to those from the dominant S 11(1535) resonance, implied by the Mainz differential cross-section data. The contributions from the resonances S 11(1650), D 13(1700) and D 15(1675) are found consistently small but sensitive to the polarized target asymmetry measured recently at ELSA. These results are shown to be consistent with the quark model predictions.

Formula omitted] decay in light cone QCD

The radiative B c→ℓ ν ̄ γ decay is investigated in the Standard Model in framework of the light cone QCD. The transition form factors and decay width are calculated. A comparison of light cone QCD and constituent quark model predictions on these quantities is presented.