arXiv subjects
Publications and source records attributed to D. O. Riska.
The development of the chiral dynamics based description of nuclear electroweak currents is reviewed. Gerald E. (Gerry) Brown's role in basing theoretical nuclear physics on chiral Lagrangians is emphasized. Illustrative examples of the successful description of electroweak observables of light nuclei obtained from chiral effective field theory are presented.
Using the bound state version of the topological soliton model for the baryons we show that the existence of a bound (or quasi-bound) $\bar D$-soliton state leads to the possibility of having hidden charm pentaquarks with quantum numbers and masses, which are compatible with those of the candidates recently reported by the LHCb experiment. The implications of heavy quark symmetry are elaborated.
There is a natural cancellation between the contributions of the $qqq$ and $qqqq\bar q$ components to the axial charge of the N(1535) resonance. While the probability of the former is larger than that of the latter, its coefficient in the axial charge expression is exceptionally small. The magnitude of two of the corresponding coefficients of the $qqqq\bar q$ components are in contrast large and have the opposite sign. This result provides a phenomenological illustration of the recent unquenched lattice calculation result that the axial charge of the N(1535) resonance is very small, if not vanishing \cite{takah}. The result sets an upper limit on the magnitude of the probability of $qqqq\bar q$ components as well.
It is shown that the data on the pion charge form factor admit the possibility for a substantial sea-quark components in the pion wave function. If the charge form factor is calculated with instant form kinematics in a constituent quark model that is extended to include explicit $(q\bar q)^2$ components in the pion wave function, that component will give the dominant contribution to the calculated $π^+$ charge form factor at large values of momentum transfer. The present experimental values $Q^2$ can be described well with $(q\bar q)^2$ component admixtures of up to 50%. The sensitivity of the calculated $π^+$ charge form factor to whether one of the quarks or one of the antiquarks is taken to be in the P-state is small.
The electromagnetic and axial form factors of the nucleon and its lowest positive parity excitations, the Delta(1232) and the N*(1440), are calculated with constituent-quark models that are specified by simple algebraic representations of the mass-operator eigenstates. Poincaré covariant current operators are generated by the dynamics from single-quark currents that are covariant under a kinematic subgroup. The dependence of the calculated form factors on the choice of kinematics and on the gross features of the wave functions is illustrated for instant-form, point-form, and front-form kinematics. A simple algebraic form of the orbital ground state wave function, which depends on two parameters, allows a fair description of all the form factors over the empirically accessible range, although with widely different choices of the parameters, which determine the range and shape of the orbital wave function. The neutron electric form factor requires additional features, for instance the presence of mixed symmetry S-state component with 1 -- 2 % probability in the ground state wave function. Instant and front form kinematics demand a spatially extended wave function, whereas in point form kinematics the form factors may be described with a quite compact wave function.
The covariant quark model is shown to allow a phenomenological description of the neutron electric form factor, G_E^n(Q^2), in the impulse approximation, provided that the wave function contains minor (~ 3 %) admixtures of the lowest sea-quark configurations. While that form factor is not very sensitive to whether the \bar q in the qqqq\bar q component is in the P-state or in the S-state, the calculated nucleon magnetic form factors are much closer to the empirical values in the case of the former configuration. In the case of the electric form factor of the proton, G_E^p(Q^2), a zero appears in the impulse approximation close to 9 GeV^2, when the \bar q is in the P-state. That configuration, which may be interpreted as a pion loop ("cloud") fluctuation, also leads to a clearly better description of the nucleon magnetic moments. When the amplitude of the sea-quark admixtures are set so as to describe the electric form factor of the neutron, the qqqq\bar q admixtures have the phenomenologically desirable feature, that the electric form factor of the proton falls at a more rapid rate with momentum transfer than the magnetic form factor.
The contributions from the $qqqq\bar q$ components to the magnetic moments of the octet as well as the $Δ^{++}$ and $Ω^-$ decuplet baryons are calculated for the configurations that are expected to have the lowest energy if the hyperfine interaction depends both on spin and flavor. The contributions from the $u\bar u$, $d\bar d$ and $s\bar s$ components are given separately. It is shown that addition of $qqqq\bar q$ admixtures to the ground state baryons can improve the overall description of the magnetic moments of the baryon octet and decuplet in the quark model without SU(3) flavor symmetry breaking, beyond that of the different constituent masses of the strange and light-flavor quarks. The explicit flavor (and spin) wave functions for all the possible configurations of the $qqqq\bar q$ components with light and strange $q\bar q$ pairs are given for the baryon and octet and decuplet. Admixtures of ~ 10% of the $qqqq\bar q$ configuration where the flavor-spin symmetry is $[4]_{FS}[22]_F[22]_S$, which is likely to have the lowest energy, in particular reduces the deviation from the empirical values of the magnetic moments of the $Σ^ -$ and the $Ξ^0$ compared with the quark model.
The role of $q\bar q$ components in the nucleon and the N(1440) resonance is studied by explicit coupling of the lowest positive parity $qqqq\bar q$ state to the $qqq$ components in the harmonic oscillator quark model. The lowest energy $qqqq\bar q$ component, where the 4-quark subsystem has the flavor-spin symmetry $[4]_{FS}[22]_F[22]_S$, is close in energy to the lowest positive parity excitation of the nucleon in the $qqq$ quark model. The confining interaction leads to a strong mixing of the $qqqq\bar q$ system and the positive parity excited state of the $qqq$ system. This result is in line with the phenomenological indications for a two-component structure of the N(1440) resonance. The presence of substantial $q\bar q$ components in the N(1440) can bring about a reconciliation of the constituent quark model with the large empirical decay width of the N(1440).
The rational map approximation to the solution to the SU(2) Skyrme model with baryon number B=4 is canonically quantized. The quantization procedure leads to anomalous breaking of the chiral symmetry, and exponential falloff of the energy density of the soliton at large distances. The model is extended to SU(2) representations of arbitrary dimension. These soliton solutions capture the double node feature of the empirical $α$ particle charge form factor, but as expected lead to a too compact matter distribution. Comparison to phenomenology indicates a preference for the fundamental representation.
The role of 5-quark components in the pion and electromagnetic decays and transition form factors of the N(1440) is explored. The $qqqq\bar q$ components, where the 4-quark subsystem has the flavor-spin symmetries $[4]_{FS}[22]_F[22]_S$ and $[4]_{FS}[31]_F[31]_S$, which are expected to have the lowest energy of all $qqqq\bar q$ configurations, are considered in detail with a nonrelativistic quark model. The matrix elements between the 5-quark components of the N(1440) and the nucleon, $qqqq\bar q\to qqqq\bar q$, play a minor role in these decays, while the transition matrix elements $qqqq\bar q\to qqq$ and $qqq\to qqqq\bar q$ that involve quark antiquark annihilation are very significant. Both for the electromagnetic and strong decay the change from the valence quark model value is dominated by the confinement triggered $q\bar q$ annihilation transitions. In the case of pion decay the calculated decay width is enhanced substantially both by the direct $q\bar q \to π$ and also by the confinement triggered $q\bar q\to π$ transitions. Agreement with the empirical value for the pion decay width may be reached with a $\sim$ 30% $qqqq\bar q$ component in the N(1440).
Five-quark $qqqq\bar q$ components in the $Δ(1232)$ are shown to contribute significantly to $Δ(1232)\to Nπ$ decay through quark-antiquark annihilation transitions. These involve the overlap between the $qqq$ and $qqqq\bar q$ components and may be triggered by the confining interaction between the quarks. With a $\sim$ 10% admixture of five-quark components in the $Δ(1232)$ the decay width can be larger by factors 2 - 3 over that calculated in the quark model with 3 valence quarks, depending on the details of the confining interaction. The effect of transitions between the $qqqq\bar q$ components themselves on the calculated decay width is however small. The large contribution of the quark-antiquark annihilation transitions thus may compensate the underprediction of the width of the $Δ(1232)$ by the valence quark model, once the $Δ(1232)$ contains $qqqq\bar q$ components with $\sim$ 10% probability.
The present empirical information on the strangeness form factors indicates that the corresponding $uuds\bar s$ component in the proton is such that the $uuds$ subsystem has the flavor spin symmetry $[4]_{FS}[22]_F[22]_S$ and mixed orbital symmetry $[31]_X$. This $uuds\bar s$ configuration leads to the empirical signs of all the form factors $G_E^s$, $G_M^s$ and $G_A^s$. An analysis with simple quark model wave functions for the preferred configuration shows that the qualitative features of the empirical strangeness form factors may be described with a $\sim$ 15% admixture of $uuds\bar s$ with a compact wave function in the proton. Transition matrix elements between the $uud$ and the $uuds\bar s$ components give significant contributions.
An admixture of 10-20 % of qqqq\bar q components in the Delta(1232) resonance is shown to reduce the well known underprediction for the decay width for Delta(1232)->N gamma decay by about half and that of the corresponding helicity amplitudes from a factor ~ 1.7 to ~ 1.5. The main effect is due to the quark-antiquark annihilation transitions qqqq\bar q -> qqq gamma, the consideration of which brings the ratio A_{3/2}/A_{1/2} and consequently the E2/M1 ratio R_{EM} into agreement with the empirical value. Transitions between qqqq\bar q components in the resonance and the nucleon qqqq\bar q->qqqq\bar q gamma are shown to enhance the calculated width by only a few percent, as long as the probability of the qqqq\bar q component of the Delta(1232) and the proton is at most ~ 20 %. The transitions qqqq\bar q->qqqq\bar q gamma between the qqqq\bar q components in the Delta(1232) and the proton do not lead to a nonzero value for R_{EM}.
The implications of the empirical signatures for the positivity of the strangeness magnetic moment $μ_s$, and the negativity of the strangeness contribution to the proton spin $Δ_s$, on the possible $uuds\bar s$ configurations of five quarks in the proton are analyzed. The empirical signs for the values of these two observables can only be obtained in configurations where the $uuds$ system is orbitally excited and the $\bar s$ quark is in the ground state. The configurations, in which the $\bar s$ is orbitally excited, which include the conventional $K^+Λ^0$ congfiguration, with the exception of that, in which the $uuds$ component has spin 2, yield negative values for $μ_s$. Here the strangeness spin $Δ_s$, the strangeness magnetic moment $μ_s$ and the axial coupling constant $G_A^s$ are calculated for all possible configurations of the $uuds\bar s$ component of the proton. In the configuration with $[4]_{FS}[22]_F[22]_S$ flavor-spin symmetry, which is likely to have the lowest energy, $μ_s$ is positive and $Δ_s\simeq G_A^s\simeq -1/3μ_s$.
A complete analysis is given of the implications of the empirical indications for a positive strangeness magnetic moment $μ_s$ of the proton on the possible configurations of the $uuds\bar s$ component of the proton. A positive value for $μ_s$ is obtained in the $s\bar s$ configuration where the $uuds$ subsystem is in an orbitally excited state with $[4]_{[FS]}[22]_F[22]_S$ flavor-spin symmetry, which is likely to have the lowest energy. The configurations in which the $\bar s$ is orbitally excited, which include the conventional $K^+Λ^0$ configuration, with exception of that in which the $uuds$ component has spin 2, yield negative values for $μ_s$. The hidden strangeness analogues of recently proposed quark cluster models for the $θ^+$ pentaquark give differing signs for $μ_s$.
A complete canonical quantization of the SU(3) Skyrme model performed in the collective coordinate formalism in general irreducible representations. In the case of SU(3) the model differs qualitatively in different representations. The Wess-Zumino-Witten term vanishes in all self-adjoint representations in the collective coordinate method for separation of space and time variables. The canonical quantization generates representation dependent quantum mass corrections, which can stabilize the soliton solution. The standard symmetry breaking mass term, which in general leads to representation mixing, degenerates to the SU(2) form in all self-adjoint representations.
Several realistic phenomenological nucleon-nucleon interaction models are employed to investigate the possibility of bound deuteron-like states of such heavy flavor hyperons and nucleons, for which the interaction between the light flavor quark components is expected to be the most significant interaction. The results indicate that deuteron-like bound states are likely to form between nucleons and the $Ξ_c^{'}$ and $Ξ_{cc}$ charm hyperons as well as between $Ξ$ hyperons and double-charm hyperons. Bound states between two $Σ_c$ hyperons are also likely. In the case of beauty hyperons the corresponding states are likely to be deeply bound.
The $Δ-N$ electromagnetic transition form factors are calculated in the Poincaré covariant quark model in three forms of relativistic kinematics. Addition of $D-$state components to pure $S-$state model wave functions, chosen so as to reproduce the empirical elastic electromagnetic nucleon form factors with single constituent currents, brings the calculated $R_{EM}$ ratio for the $Δ(1232)\to Nγ$ transition closer to the empirical values in instant and point form kinematics. The calculated $R_{SM}$ ratio is insensitive to the $D-$state component. In front form kinematics the substantial violation of the angular condition for the spin 3/2 resonance transition amplitude in the impulse approximation prevents a unique determination of $R_{EM}$ and $R_{SM}$, both of which are very sensitive to $D-$state components. In no form of kinematics do $D-$state deformations of the rest frame baryon wave functions alone suffice for a description of the empirical values of these ratios.