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D. O. Riska

Publications and source records attributed to D. O. Riska.

At least 55 records · Page 3Linked to original sources
arXiv2000

Mesonic Anapole Form Factors of the Nucleons

The chiral quark model posits pseudoscalar and vector meson couplings to constituent quarks. The parity violating meson-quark couplings lead to anapole moments and form factors of the nucleons. These arise both as parity violating meson loop fluctuations as well as exchange currents and polarization currents that are induced by the parity violating interaction between quarks. Because of cancellations between the different contributions the magnitude of the calculated anapole moments is only of the order $\sim 10^{-8}$ and is determined mainly by magnitude of the parity-violating meson-nucleon coupling constants.

hep-ph↗
arXiv2000

Spectra and M1 Decay Widths of Heavy-Light Mesons

The M1 decay widths of charm and bottom mesons and their excited states are sensitive to the relativistic aspect of the quark current operators and obtain a significant contribution from the interaction current that is associated with the scalar confining interaction between heavy quarks ($Q$) and light antiquarks ($\bar q$). Consequently they provide direct evidence on the latter. The spectra and the M1 transition widths of the $D$, $D_s$ and $B$, $B_s$ mesons and their orbital excitations are calculated here within the framework of the Blankenbecler-Sugar equation, which allows a covariant treatment, while retaining a formal similarity to the nonrelativistic approach. The hyperfine interaction in the $Q\bar q$ mesons is modeled by screened gluon exchange, which shares many features with the instanton induced interaction. The quality of the model is tested by calculation of the spectra and M1 decay widths of charmonium.

hep-ph↗
arXiv2000

Strangeness Form Factors of the Proton in the Chiral Quark Model

The chiral quark model describes the strangeness components of the light quarks as fluctuations into strange mesons and quarks. The single strange pseudoscalar and vector meson loop fluctuations of the constituent $u$- and $d$-quarks give rise to only very small strangeness form factors for the proton. This result is in line with recent experimental results, given their wide uncertainty range.

hep-ph↗
arXiv1999

The Confining Interaction and Radiative Decays of Heavy Quarkonia

The radiative spin-flip transition rates of heavy quarkonium states depend sensitively on the matrix elements of the effective confining interaction through the associated two-quark exchange current operator. The Hamiltonian model based on a scalar linear confining interaction with a single gluon exchange hyperfine term is shown to provide an adequate description of the $J/ψ\to η_cγ$ and $ψ(2S)\to η_cγ$ decay widths once the relativistic single quark magnetic moment operator is treated without approximation and the exchange current is taken into account. Predictions are given for the radiative spin-flip decay widths of the 1S,2S and 3S states of the $c\bar c$, $b\bar b$ and $B_c^+$ systems. In the B_c^+ system the gluon exchange current also contributes to the spin-flip transition rates.

hep-ph↗
arXiv1999

The Strangeness Magnetic Moment of the Proton in the Chiral Quark Model

The strangeness magnetic moment of the proton is shown to be small in the chiral quark model. The dominant loop contribution is due to kaons. The K^* loop contributions are proportional to the difference between the strange and light constituent quark masses or $m_{K^*}^{-2}$ and therefore small. The loop fluctuations that involve radiative transitions between K^* mesons and kaons are small, when the cut-off scale in the loops is taken to be close to the chiral symmetry restoration scale. The net loop amplitude contribution to the strangeness magnetic moment of the proton is about -0.05, which falls within the uncertainty range of the experimental value.

hep-ph↗
arXiv1999

Baryons and Nuclei in the Large $N_c$ Limit

The relation between the Skyrme model and the constituent quark model, which appears in the large $N_c$ limit is described. Examples of similarity in the predicted phenomenology for baryons are shown. Finally the application to nuclei is discussed.

nucl-th↗
arXiv1999

Nucleon Resonance Effects in $pp\to ppπ^0$ near Threshold

The role of the low lying nucleon resonances beyond the $Δ(1232)$ in the reaction $pp\to ppπ^0$ near threshold is shown to be numerically significant by a calculation, which takes into account the pion re-scattering contribution described by chiral perturbation theory and the short-range mechanisms that are implied by the nucleon-nucleon interaction model. The intermediate N(1440) (P$_{11}$) resonance is excited by the short-range exchange mechanisms, while the N(1535) ($S_{11}$) and N(1520) ($D_{13}$) resonances are excited by $η$ and $ρ$ meson exchange, respectively. The $P_{11}$ increases the calculated cross section, whereas the $S_{11}$ and $D_{13}$ resonances decrease it. The calculation takes full account of the initial and final state interactions.

nucl-th↗
arXiv1999

Two-Pion Exchange Interaction Between Constituent Quarks

The two-pion exchange interaction between constituent quarks is shown to enhance the effect of the the isospin dependent spin-spin component of the one-pion exchange interaction, and to cancel out its tensor component. It therefore provides a partial explanation for the phenomenological observation that the hyperfine interaction between constituent quarks is well described by a flavor dependent spin-spin interaction, which is attractive at short and repulsive at long range. The spin-orbit component of the two-pion exchange interaction is stronger than and has the opposite sign from that associated with the linear confining interaction in the $P-$shell multiplets.

hep-ph↗
arXiv1999

Pionic Fluctuations of Constituent Quarks

Pion loop fluctuations of the constituent u and d quarks are shown to give only a minute contribution to the intrinsic charge radius of the neutron, under the assumption that the pion-quark coupling constant has its conventional value, with a cut-off scale of the order of 1.0 GeV. The contribution from the pion loops to the anomalous magnetic moment of the neutron represents a small ($\sim$ 12-14%) increase over of the static quark model value.

hep-ph↗
arXiv1998

Electromagnetic Structure of the Trinucleons

The electromagnetic form factors of the trinucleons 3H and 3He are calculated with wave functions obtained with the Argonne AV18 two-nucleon and Urbana IX three-nucleon interactions. Full account is taken of the two-body currents required by current conservation with the AV18 interaction as well as those associated with N-Delta transition currents and the currents of Delta resonance components in the wave functions. Explicit three-nucleon current operators associated with the two-pion exchange three-nucleon interaction arising from irreducible S-wave pion-nucleon scattering is constructed and shown to have very little effect on the calculated magnetic form factors. The calculated magnetic form factor of 3H, and charge form factors of both 3H and 3He are in satisfactory agreement with the experimental data. However, the position of the zero in the magnetic form factor of 3He is slightly underpredicted.

nucl-th↗
arXiv1998

The $π-$Gluon Exchange Interaction Between Constituent Quarks

The interaction mediated by irreducible pion and gluon exchange between constituent quarks is calculated and shown to have a strong tensor component, which tends to cancel the pion exchange tensor interaction between quarks. Its spin-spin component is somewhat weaker than the pion exchange spin-spin interaction, while its central and spin-orbit components are small in comparison to the corresponding single gluon exchange interactions. The combination of the $π-$gluon exchange interaction with the single pion exchange interaction and a weak gluon exchange interaction between constituent quarks has the qualitative features required for understanding the hyperfine splittings of the spectra of the nucleon and the $Δ$ resonances.

hep-ph↗
arXiv1997

Stability and Representation Dependence of the Quantum Skyrmion

A constructive realization of Skyrme's conjecture that an effective pion mass ``may arise as a self consistent quantal effect'' based on an ab initio quantum treatment of the Skyrme model is presented. In this quantum mechanical Skyrme model the spectrum of states with $I=J$, which appears in the collective quantization, terminates without any infinite tower of unphysical states. The termination point depends on the model parameters and the dimension of the SU(2) representation. Representations, in which the nucleon and $Δ_{33}$ resonance are the only stable states, exist. The model is developed for both irreducible and reducible representations of general dimension. States with spin larger than 1/2 are shown to be deformed. The representation dependence of the baryon observables is illustrated numerically.

nucl-th↗
arXiv1997

Covariant Quark Model for the Baryons

A family of simply solvable covariant quark models for the baryons is presented. With optimal parameter choices the models reproduce the empirical spectra of the baryons in all flavor sectors to an accuracy of a few percent. Complete spectra are obtained for all states of the strange, charm and beauty hyperons with $L \leq 2$. The magnetic moments and axial coupling constants of the ground state baryons correspond to those of conventional quark models. We construct current-density operators that are consistent with empirical nucleon form factors at low and medium energies.

hep-ph↗
arXiv1997

Poincare Covariant Quark Models of Baryon Form Factors

Poincare covariant quark models of the the nucleon, the $Δ$ resonance and their excitations are explored. The baryon states are represented by eigenfunctions of the four-velocity and a confining mass operator, which reproduces the empirical spectrum up to $\sim$ 1700 MeV to an accuracy of $\sim 6%$. Models of constituent quark currents provide the relations between ground-state properties and transition amplitudes.

hep-ph↗
arXiv1997

The Charm and Bottom Hyperons in a Chiral Quark Model

The spectrum of the C=1 hyperons is well described by the constituent quark model, if the fine structure interaction between the light and strange quarks is mediated by the $SU(3)_F$ octet of light pseudoscalar mesons, which are the Goldstone bosons of the hidden approximate chiral symmetry of QCD. With the addition of a phenomenological flavor exchange interaction of the same form between the light and the charm quarks to describe the $Σ_c-Σ_c^*$ and $Ξ_c^s-Ξ_c^*$ splittings, the splittings between the C=1 states fall within 10-30 MeV of the empirical values. Predictions are presented for the lowest negative parity excited states and the magnetic moments as well. Corresponding predictions for the B=-1 hyperon states are also given.

hep-ph↗
arXiv1996

Baryon Magnetic Moments and Axial Coupling Constants with Relativistic and Exchange Current Effects

The large relativistic corrections to the constituent quark current operators improve the predictions for the axial couplings of the baryons, but worsen those for their magnetic moments. The exchange current corrections that are associated with flavor and spin dependent hyperfine interactions between the quarks with a form suggested by pseudoscalar meson exchange can compensate the relativistic corrections to the baryon magnetic moments. This is demonstrated by a calculation of the magnetic moments of the non-strange and strange baryons using wave functions and exchange current operators, which correspond to a recent phenomenological spin- and flavor dependent interquark interaction model with a linear confining interaction, which yields a spectrum close to the empirical one. The possibility that part of the flavor and spin dependent interaction could be due to vector and axial-vector exchange is explored.

hep-ph↗