arXiv subjects

D. O. Riska

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

At least 37 records · Page 2Linked to original sources

arXiv2004

Axial Transition Form Factors and Pion Decay of Baryon Resonances

The pion decay constants of the lowest orbitally excited states of the nucleon and the $Δ(1232)$ along with the corresponding axial transition form factors are calculated with Poincaré covariant constituent-quark models with instant, point and front forms of relativistic kinematics. The model wave functions are chosen such that the calculated electromagnetic and axial form factors of the nucleon represent the empirical values in all three forms of kinematics, when calculated with single-constituent currents. The pion decay widths calculated with the three forms of kinematics are smaller than the empirical values. Front and instant form kinematics provide a similar description, with a slight preference for front form, while the point form values are significantly smaller in the case of the lowest positive parity resonances.

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arXiv2004

Nuclei of Double-Charm Hyperons

The ground states of double-charm hyperons form a spin 1/2 isospin 1/2 multiplet analogous to that of nucleons. Their main strong interaction may be inferred directly from the corresponding nucleon-nucleon interaction by multiplication of the interaction components by the appropriate fractional difference between interaction strengths for pairs of light flavor quarks and pairs of triplets, e.g. nucleons, of light flavor quarks. By construction of the interaction between the recently discovered double-charm hyperons by this method from several realistic nucleon-nucleon interaction models it is shown that double-charm hyperons are likely to form bound (or possibly meta-stable) states akin to the deuteron in the spin triplet state. Double beauty baryons would form corresponding deeply bound states. Nucleons and double charm (beauty) hyperons will also form bound states. The existence of hypernuclei with double-charm and double-beauty hyperons, which are stable against the strong decay, is very likely.

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arXiv2004

Baryon Magnetic Moments in Relativistic Quark Models

It is shown that the phenomenological description of the baryon magnetic moments in the quark model carries over to the Poincaré covariant extension of the model. This applies to all the three common forms of relativistic kinematics with structureless constituent currents, which are covariant under the corresponding kinematic subgroups. In instant and front form kinematics the calculated magnetic moments depend strongly on the constituent masses, while in point form kinematics the magnetic moments are fairly insensitive to both the quark masses and the wave function model. The baryon charge radii and magnetic moments are determined in the different forms of kinematics for the light-flavor, strange and charm hyperons. The wave function model is determined by a fit to the electromagnetic form factor of the proton.

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arXiv2003

The Deuteron as a Canonically Quantized Biskyrmion

The ground state configurations of the solution to Skyrme's topological soliton model for systems with baryon number larger than 1 are well approximated with rational map ansätze, without individual baryon coordinates. Here canonical quantization of the baryon number 2 system, which represents the deuteron, is carried out in the rational map approximation. The solution, which is described by the 6 parameters of the chiral group SU(2)$\times$SU(2), is stabilized by the quantum corrections. The matter density of the variational quantized solution has the required exponential large distance falloff and the quantum numbers of the deuteron. Similarly to the axially symmetric semiclassical solution, the radius and the quadrupole moment are, however, only about half as large as the corresponding empirical values. The quantized deuteron solution is constructed for representations of arbitrary dimension of the chiral group.

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arXiv2003

Stable $uudd\bar s$ pentaquarks in the constituent quark model

The stability of strange pentaquarks $uudd\bar s$ is studied in a constituent quark model based on a flavor-spin hyperfine interaction between quarks. With this interaction model, which schematically represents the Goldstone boson exchange interaction between constituent quarks, the lowest lying strange pentaquark is a $p-$shell state with positive parity. The flavor-spin interaction lowers the energy of the lowest $p-$shell state below that of the lowest $s-$shell state, which has negative parity because of the negative parity of the strange antiquark. It is found that the strange pentaquark is stable against strong decay provided that the strange antiquark interacts by a fairly strong spin-spin interaction with $u$ and $d$ quarks. This interaction has a form that corresponds to $η$ meson exchange. Its strength may be inferred from the $π^0$ decay width of $D_s^*$ mesons.

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arXiv2003

Scaling of Hadronic Form Factors in Point Form Kinematics

The general features of baryon form factors calculated with point form kinematics are derived. With point form kinematics and spectator currents hadronic form factors are functions of $η:={1\over 4}(v_{out}-v_{in})^2$ and, over a range of $η$ values are insensitive to unitary scale transformations of the model wave functions when the extent of the wave function is small compared to the scale defined by the constituent mass, $ \ll 1/m^2$. The form factors are sensitive to the shape of such compact wave functions. Simple 3-quark proton wave functions are employed to illustrate these features. Rational and algebraic model wave functions lead to a reasonable representation of the empirical form factors, while Gaussian wave functions fail. For large values of $η$ point form kinematics with spectator currents leads to power law behavior of the wave functions.

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arXiv2002

The Coupling of eta Mesons to Quarks and Baryons from D_s^* -> D_s pi^0 Decay

The known ratio of the branching fractions for D_s^* --> D_s pi^0 and D_s^* --> D_s gamma may be used to extract the coupling of eta mesons to strange quarks once the value of the pi^0-eta mixing angle is known. This requires that realistic models for the spectra as well as the magnetic dipole (M1) decays of the heavy-light (Q qbar) mesons are available. The coupling of eta mesons to light quarks may then be estimated using SU(3) flavor symmetry. Applied to the quark model for the baryons, an eta NN pseudovector coupling constant of f_{eta NN} = 0.35^{+0.15}_{-0.25} is obtained. If the charm quark couples significantly to the eta meson, as is suggested by the decay mode psi' --> J/(psi eta), then somewhat larger values of f_{eta NN} can be obtained. These values are sufficiently small to be consistent with phenomenological analysis of photoproduction of the eta on the nucleon and the reaction pp --> pp eta.

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arXiv2002

Pion Rescattering in Two-Pion Decay of Heavy Quarkonia

The role of pion rescattering in $ππ$ decay of radially excited heavy quarkonia modeled in terms of a $Qππ$ coupling, is investigated within the framework of the covariant Blankenbecler-Sugar equation. The effects of pion rescattering (or pion exchange) are shown to be large, unless the coupling of the two-pion system to the heavy quarks is mediated by a fairly light scalar $σ$ meson, which couples to the gradients of the pion fields. The Hamiltonian model for the quarkonium states is formed of linear scalar confining, screened one-gluon exchange and instanton induced interaction terms. The widths and energy distributions of the basic decays $ψ'\to J/ψππ$ and $Υ'\to Υππ$ are shown to be satisfactorily described by this model. The implications of this model for the decays of the $Υ(3S)$ state are discussed.

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arXiv2002

Dynamical Interpretation of the Nucleon-Nucleon Interaction and Exchange Currents in the Large $N_C$ Limit

Expression of the nucleon-nucleon Interaction to order $1/N_C$ in terms of Fermi Invariants allows a dynamical interpretation of the interaction and leads to a consistent construction of the associated interaction currents to order $1/N_C$. The numerically significant components of 4 different modern realistic phenomenological interaction models are shown to admit very similar meson exchange interpretations in the large $N_C$ limit. Moreover the ratio of the volume integrals of the leading, next-to-leading and next-to-next leading order terms in these interaction models is roughly 300:5-10:0.1, which corresponds fairly well to the ratios of $1/N_C^2$ between the terms that would be suggested by the $1/N_C$ expansion if $N_C=3$. The $N_C$ dependence of the electromagnetic and axial interaction currents that are associated with these interaction components is derived and compared to that of the corresponding single nucleon currents.

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arXiv2001

Pi and PiPi Decays of Excited D Mesons

The $π$ and $ππ$ decay widths of the excited charm mesons are calculated using a Hamiltonian model within the framework of the covariant Blankenbecler-Sugar equation. The pion-light constituent quark coupling is described by the chiral pseudovector Lagrangian.

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arXiv2001

Low lying $qqqq\bar q$ 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 modelled by admixtures of sea-quark $qqq(q\bar q)^n$ 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 $qqqq\bar q$ 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 $qqqq\bar q$ admixtures.

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arXiv2001

Two-Pion Decay Widths of Excited Charm Mesons

The widths for $ππ$ decay of the L=1 charm mesons are calculated by describing the pion coupling to light constituents quarks by the lowest order chiral interaction. The wavefunctions of the charm mesons are obtained as solutions to the covariant Blankenbecler-Sugar equation. These solutions correspond to an interaction Hamiltonian modeled as the sum of a linear scalar confining and a screened one-gluon exchange (OGE) interaction. This interaction induces a two-quark contribution to the amplitude for two-pion decay, which is found to interfere destructively with the single quark amplitude. For the currently known L=1 $D$ mesons, the total $ππ$ decay widths are found to be $\sim 1$ MeV for the $D_1(2420)$ and $\sim 3$ MeV for the $D^*_2(2460)$ if the axial coupling of the constituent quark is taken to be $g_A^q=1$. The as yet undiscovered spin singlet $D_1^*$ state is predicted to have a larger width of 7 - 10 MeV for $ππ$ decay.

hep-ph↗

arXiv2000

Dibaryons as canonically quantized biskyrmions

The characteristic feature of the ground state configuration of the Skyrme model description of nuclei is the absence of recognizable individual nucleons. The ground state of the skyrmion with baryon number 2 is axially symmetric, and is well approximated by a simple rational map, which represents a direct generalization of Skyrme's hedgehog ansatz for the nucleon. If the Lagrangian density is canonically quantized this configuration may support excitations that lie close and possible below the threshold for pion decay, and therefore describe dibaryons. The quantum corrections stabilize these solutions, the mass density of which have the correct exponential fall off at large distances.

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arXiv2000

Pion Decay Widths of D mesons

The pionic decay rates of the excited $L=0,1$ $D$ mesons are calculated with a Hamiltonian model within the framework of the covariant Blankenbecler-Sugar {equation.} The interaction between the light quark and charm antiquark is described by a linear scalar confining and a screened one-gluon exchange interaction. The decay widths of the $D^*$ mesons obtain a contribution from the exchange current that is associated with the linear scalar confining interaction. If this contribution is taken into account along with the single quark approximation, the calculated decay rates of the charged $D^*$ mesons are readily below the current empirical upper limits if the axial coupling constant of the light constituent quarks is taken to be $g_A^q$ = 0.87, but reach the empirical upper limits if $g_A^q$ = 1. With the conventional values for $g_A^q$, the calculated widths of the $D_1$ and $D_2^*$ mesons fall somewhat below the experimental lower limits, leaving room for other decay modes as well, such as $ππ$ decay. The unrealistically large contribution from the axial charge operator to the calculated pion decay width of the $D_1$ meson is suppressed by taking into account the exchange charge effects that are associated with the scalar linear confining and vector one-gluon exchange interactions. The predicted values for the pionic widths of the hitherto undiscovered L=1 $D_1^*$ and $D_0^*$ mesons are found to be smaller than previous estimates.

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arXiv2000

Pion loop fluctuations of constituent quarks and baryons

The implications of the pion (meson) degrees of freedom for baryon properties, taken at the constituent quark and baryon levels are compared. It is shown that there is a dramatic qualitative difference between two approaches.

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arXiv2000

Nucleon form factors in the canonically quantized Skyrme model

The explicit expressions for the electric, magnetic, axial and induced pseudoscalar form factors of the nucleons are derived in the {\it ab initio} quantized Skyrme model. The canonical quantization procedure ensures the existence of stable soliton solutions with good quantum numbers. The form factors are derived for representations of arbitrary dimension of the SU(2) group. After fixing the two parameters of the model, $f_π$ and $e$, by the empirical mass and electric mean square radius of the proton, the calculated electric and magnetic form factors are fairly close to the empirical ones, whereas the the axial and induced pseudoscalar form factors fall off too slowly with momentum transfer.

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arXiv2000

The Reaction $pp\to ppη$ and the eta-Nucleon and Nucleon-Nucleon Interactions

The $π$, $η$ and $ρ$ exchange contributions to the cross-section for the reaction $pp\to ppη$ near threshold are calculated with a phenomenological description of the intermediate S11 (N(1535)) resonance for all exchange mechanisms. The final state interaction in the $pp$ system is described by realistic nucleon-nucleon interaction models. The sensitivity of the results to the phenomenological models for the $η$-nucleon transition amplitude and the nucleon-nucleon interaction is explored. The $η$ exchange mechanism is found to play a dominant role. The off-shell behavior of the $ηN \to ηN$ amplitude within the $η-$exchange amplitude leads to a result significantly different from that obtained with the constant scattering length approximation. The two-nucleon amplitudes that are associated with the short range components of the nucleon-nucleon interaction contribute significantly to the cross section.

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arXiv2000

Nucleon Resonance Transition Couplings to Vector Mesons

Recent heavy ion experiments indicate modifications of the $ρ-$meson mass in medium. In the CERES experiments $ρ-$mesons are produced at $\sim$ normal nuclear matter density, where hadrons are more appropriate constituents than quarks. A collective "nuclear $ρ$", in which every nucleon is excited to the N(1520) state, with equal amplitude, enters in this description. At the higher densities reached by future experiments constituent quarks become the appropriate variables. Here the $ρ$ and $ω$ transition couplings to the nucleon resonances up to 1700 MeV, including the N(1520), are derived by means of the chiral quark model. The relevant coupling constants are expressed in terms of the corresponding vector coupling constants to nucleons. The quality of the model relations is tested by a calculation of the corresponding pion-resonance coupling constants, which are known from the empirical pion decay widths.

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