arXiv subjects

D. O. Riska

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

70 records · Page 4Linked to original sources

arXiv1996

Meson Exchange and Pion Rescattering Contributions to the Cross Section for $pp\to ppπ^0$

Short range meson-exchange mechanisms such as $ρ\toωπ^0$ contribute significantly to the amplitude for $pp\rightarrow ppπ^0$ near threshold in addition to pion rescattering. The uncertainty of the latter contribution, which depends on poorly determined parameters, is estimated. Allowing for this uncertainty and taking the short range meson-exchange contributions into account makes it possible to reproduce the measured cross section.

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arXiv1996

The Quantum Skyrmion in Representations of General Dimension

The representations of general dimension are constructed for the $SU(2)$ Skyrme model, treated quantum mechanically {\it ab initio. } This quantum Skyrme model has a negative mass term correction, that is not present in the classical Hamiltonian. The magnitude of the quantum mechanical mass correction increases with the dimension of the representation of the $SU(2)$ group. In the case of a 5-dimensional representation it is possible to obtain satisfactory predictions for the nucleon mass with the empirical value for the pion decay constant.

hep-ph↗

arXiv1996

The Breathing Modes of the $B=2$ Skyrmion and the Spin-Orbit Interaction

The coupling of the breathing and rotational modes of the skyrmion-skyrmion system leads to a nucleon-nucleon spin-orbit interaction of short range, as well as to spin-orbit potentials for the transitions $NN \to N(1440)N$, $NN \to NN(1440)$ and $NN \to N(1440)N(1440)$. The longest range behaviour of these spin-orbit potentials is calculated in closed form.

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arXiv1995

Quark Model Explanation of the $N^*\to Nη$ Branching Ratios

The constituent quark model can explain the strong selectivity of the $Nη$ decay branching ratios of the nucleon resonances if the fine structure interaction between the constituent quarks is described in terms of Goldstone boson exchange. This chiral quark model predicts that the resonances $N(1535)$, $N(1710)$, $Λ(1670)$, $Σ(1750)$, which have mixed flavor and spin symmetry $[21]_{FS} [21]_F [21]_S$ wavefunctions in lowest order, should have large $Nη$ branching ratios, while $Nη$ decay of the other resonances that have different flavor-spin symmetry should be strongly suppressed in agreement with the experimental branching ratios.

hep-ph↗

arXiv1995

The Spectrum of the Nucleons and the Strange Hyperons and Chiral Dynamics

The spectra of the nucleons, $Δ$ resonances and the strange hyperons are well described by the constituent quark model if in addition to the harmonic confinement potential the quarks are assumed to interact by exchange of the $SU(3)_F$ octet of pseudoscalar mesons, which are the Goldstone bosons associated with the hidden approximate chiral symmetry of QCD. In its $SU(3)_F$ invariant approximation the pseudoscalar exchange interaction splits the multiplets of $SU(6)_{FS}\times U(6)_{conf}$ in the spectrum to multiplets of $SU(3)_F\times SU(2)_S\times U(6)_{conf}$. The position of these multiplets differ in the baryon sectors with different strangeness because of the mass splitting of the pseudoscalar octet and the different constituent masses of the u,d and s quarks that breaks $SU(3)_F$ flavor symmetry. A description of the whole spectrum, to an accuracy of $\simeq$ 4\% or better, is achieved if one matrix element of the boson interaction for each oscillator shell is extracted from the empirical mass splittings. The ordering of the positive and negative parity states moreover agrees with the empirical one in all sectors of the spectrum. A discussion of the conceptual basis of the model and its various phenomenological ramifications is presented.

hep-ph↗

arXiv1995

Repulsive Short Range Three-Nucleon Interaction

The three nucleon interaction that arises from pion and the effective scalar and vector meson exchange components of the nucleon-nucleon interaction is constructed and shown to be repulsive. Using several wavefunction models we show that this interaction reduces the calculated binding energy of the trinucleons by about 200 keV. The contributions of intermediate $N$(1440) resonances to this three nucleon interaction and of the $D$-state component in the trinucleon model are estimated, and shown to be small.

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arXiv1995

Systematics of the Light and Strange Baryons and the Symmetries of QCD

The spectra of the nucleons and the strange hyperons are well described by a harmonic confinement potential for the constituent quarks and an SU(3) flavor-symmetric interaction mediated by the pseudoscalar octet that is associated with the hidden approximate chiral symmetry of QCD. The spectrum is formed of multiplets of $SU(6)_{FS}\times U(6)_{conf}$, which are mixed by the octet interaction. With 4 matrix elements of the octet mediated interaction determined by the lowest mass splittings in the nucleon spectrum the resulting mass formula predicts the whole spectrum with an accuracy of $\sim$ 5\%.

hep-ph↗

arXiv1995

THE COUPLING OF THE $f_1(1285)$ MESON TO THE ISOSCALAR AXIAL CURRENT OF THE NUCLEON

The weak decay as well as the strong nucleon coupling constants of the isoscalar axial vector meson $f_1 (1285)$ are estimated within the octet quark model. It is shown that the empirical value for the coupling of $\overline{s}s$ quarkonium to nucleons can be understood by attributing the $f_1(1285)$-nucleon coupling to the $a_0πN$ triangle diagram.

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arXiv1994

The Baryon Spectrum and Chiral Dynamics

The fine structure of the low energy part of the nucleon and strange hyperon spectra, which are formed of single states without parity doublets, may be understood in terms of an $SU(3)$ flavor-symmetric quark-quark interaction that describes chiral pseudoscalar boson exchange. The model predicts the fine structure splittings within 10-30\% of their empirical values and provides an explanation of the reversed ordering of the lowest positive and negative parity resonances in the nucleon and the $Λ$ hyperon spectra.

hep-ph↗

arXiv1994

The Anharmonic Correction in the Soliton Model for the Hyperons

We derive the anharmonic correction to the hyperon energy in the bound state version of the topological soliton model for the hyperons, and show that it represents a negative correction of at most 10% to the energy of the bound heavy flavour two-meson system in the case of cascade hyperons. The main anharmonic correction arises from the mass term in the Lagrangian density. For large meson masses the consistency of the model requires that the anharmonic correction decreases as the inverse square root of the mass of the heavy flavour meson.

hep-ph↗

arXiv1994

Two-body Pion Absorption on $^3He$ at Threshold

It is shown that a satisfactory explanation of the ratio of the rates of the reactions $^3He(π^-,nn)$ and $^3He(π^-,np)$ for stopped pions is obtained once the effect of the short range two-nucleon components of the axial charge operator for the nuclear system is taken into account. By employing realistic models for the nucleon-nucleon interaction in the construction of these components of the axial charge operator, the predicted ratios agree with the empirical value to within 10-20\%.

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arXiv1994

Representations of General Dimension for the Skyrme Model

We construct the representations of general dimension for the soliton solution to the $SU(2)$ Skyrme model, and show that at the classical level the dependence on the dimension of the representation ($2j+1$) appears only as an overall factor $j(j+1)(2j+1)$ in the Lagrangian density, which may be absorbed by a rescaling of the parameters. Alternate stabilizing terms in the model will in general have a different $j$-dependence and have to be rescaled accordingly in order to achieve representation independent predictions. In contrast the quantum corrections do depend on the dimension of the representation and in general differ from those obtained in the fundamental representation of $SU(2)$.

hep-ph↗

arXiv1994

Higher Resonance Contributions to the Adler-Weisberger Sum Rule in the Large N_c Limit

We determine the $N_c$--dependence of the resonance contributions to the Adler--Weisberger sum rule for the inverse square $1/g_A^2$ of the axial charge coupling constant and show that in the large $N_c$ limit the contributions of the Roper-like excitations scale as $O(1/N_c)$. Consistency with the $1/N_c^2$ scaling of the $1/g_A^2$ term in the sum rule requires these contributions to cancel against each other.

hep-ph↗

arXiv1993

The $πσ$-Axial Exchange Current

The axial exchange current operator that arises from the coupling of the axial field to the pion and effective scalar field in nuclei is constructed. The spatial dependence of this exchange current operator may be determined directly from the nucleon-nucleon interaction. This "$πσ$"-axial exchange current contributes about 5\% to the quenching of the nucleon axial current coupling constant $g_A^{GT}$ in heavy nuclei.

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arXiv1993

Electromagnetic Currents and the Blankenbecler-Sugar Equation

The effective electromagnetic current density for a two-nucleon system that is described by the Blankenbecler-Sugar equation is derived. In addition to the single nucleon currents there are exchange currents of two different origins. The first is the exchange current that is required to compensate for the violation of the continuity equation in the impulse approximation. The second is an exchange current, which arises in the quasipotential reduction from the Bethe-Salpeter equation, and which represents effects of suppressed degrees of freedom. Explicit general expressions are given for both of these exchange currents. The results are applicable to both elastic and inelastic processes.

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arXiv1992

Dilatons in the topological soliton model for the hyperons

We show that the predicted hyperon masses in the topological soliton model are very sensitive to the value of the gluon condensate parameter that appears when the scale invariance and trace anomaly of QCD are taken into account by introduction of a dilaton field. This contrasts with the insensitivity of the soliton properties to the dilaton coupling. In order that the predicted strange and charmed hyperon spectra agree with the empirical ones the gluon condensate parameter has to be about (400 MeV)$^4$, which agrees with the result obtained from QCD sum rules. This implies that the bag formed by the scalar field must be very shallow.

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