arXiv subjects

E. S. Swanson

Publications and source records attributed to E. S. Swanson.

59 records · Page 4Linked to original sources

arXiv1994

Distinguishing Among Strong Decay Models

Two competing models for strong hadronic decays, the $^3P_0$ and $^3S_1$ models, are currently in use. Attempts to rule out one or the other have been hindered by a poor understanding of final state interactions and by ambiguities in the treatment of relativistic effects. In this article we study meson decays in both models, focussing on certain amplitude ratios for which the relativistic uncertainties largely cancel out (notably the $S/D$ ratios in $b_1\rightarrowπω$ and $a_1\rightarrowπρ$), and using a Quark Born Formalism to estimate the final state interactions. We find that the $^3P_0$ model is strongly favoured. In addition, we predict a $P/F$ amplitude ratio of $1.6\pm .2$ for the decay $π_2\rightarrowπρ$. We also study the parameter-dependence of some individual amplitudes (as opposed to amplitude ratios), in an attempt to identify a ``best'' version of the $^3P_0$ model.

hep-ph↗

arXiv1994

$I=0,1$ $ππ$ and $I=1/2$ $Kπ$ Scattering using Quark Born Diagrams

We extend the quark Born diagram formalism for hadron-hadron scattering to processes with valence $q\bar q$ annihilation, specifically $I=0$ and $I=1$ $ππ$ and $I=1/2$ $Kπ$ elastic scattering. This involves the $s$-channel hybrid annihilation process $q^2\bar q^2 \to q\bar q g \to q^2\bar q^2$ and conventional $s$-channel $q\bar q$ resonances, in addition to the $t$-channel gluon exchange treated previously using this formalism. The strength of the $t$-channel gluon amplitude is fixed by previous studies of $I=2$ $ππ$ and $I=3/2$ $Kπ$ scattering. The $s$-channel resonances $ρ(770)$, $K^*(892)$, $f_0(1400)$ and $K_0^*(1430)$ are incorporated as relativized Breit-Wigner amplitudes, with masses and energy-dependent widths fitted to experimental phase shifts. The strength of the $s$-channel gluon ``hybrid" annihilation diagrams is problematical since the perturbative massless-gluon energy denominator must be modified to account for the effect of confinement on the energy of the virtual hybrid state. Our naive expectation is that near threshold the hybrid diagrams are comparable in magnitude but opposite in sign to the contribution predicted using massless perturbative gluons. Fitting the strength of this amplitude to $I=0$ $ππ$ and $I=1/2$ $Kπ$ S-wave data gives a result consistent with this expectation. We find good agreement with experimental phase shifts from threshold to 0.9 GeV in $ππ$ and 1.6 GeV in $Kπ$ using this approach. We conclude that the most important contribution to low energy S-wave scattering in these channels arises from the nonresonant quark Born diagrams, but that the low-energy wings of the broad $s$-channel resonances $f_0(1400)$ and $K^*_0(1430)$ also give important contributions near threshold. The nonresonant contributions are found to be much smaller in $L\;

hep-ph↗

arXiv1993

NN Core Interactions and Differential Cross Sections from One Gluon Exchange

We derive nonstrange baryon-baryon scattering amplitudes in the nonrelativistic quark model using the ``quark Born diagram" formalism. This approach describes the scattering as a single interaction, here the one-gluon-exchange (OGE) spin-spin term followed by constituent interchange, with external nonrelativistic baryon wavefunctions attached to the scattering diagrams to incorporate higher-twist wavefunction effects. The short-range repulsive core in the NN interaction has previously been attributed to this spin-spin interaction in the literature; we find that these perturbative constituent-interchange diagrams do indeed predict repulsive interactions in all I,S channels of the nucleon-nucleon system, and we compare our results for the equivalent short-range potentials to the core potentials found by other authors using nonperturbative methods. We also apply our perturbative techniques to the N$Δ$ and $ΔΔ$ systems: Some $ΔΔ$ channels are found to have attractive core potentials and may accommodate ``molecular" bound states near threshold. Finally we use our Born formalism to calculate the NN differential cross section, which we compare with experimental results for unpolarised proton-proton elastic scattering. We find that several familiar features of the experimental differential cross section are reproduced by our Born-order result.

nucl-th↗

arXiv1992

Kaon-Nucleon Scattering Amplitudes and Z$^*$-Enhancements from Quark Born Diagrams

We derive closed form kaon-nucleon scattering amplitudes using the ``quark Born diagram" formalism, which describes the scattering as a single interaction (here the OGE spin-spin term) followed by quark line rearrangement. The low energy I=0 and I=1 S-wave KN phase shifts are in reasonably good agreement with experiment given conventional quark model parameters. For $k_{lab}> 0.7$ Gev however the I=1 elastic phase shift is larger than predicted by Gaussian wavefunctions, and we suggest possible reasons for this discrepancy. Equivalent low energy KN potentials for S-wave scattering are also derived. Finally we consider OGE forces in the related channels K$Δ$, K$^*$N and K$^*Δ$, and determine which have attractive interactions and might therefore exhibit strong threshold enhancements or ``Z$^*$-molecule" meson-baryon bound states. We find that the minimum-spin, minimum-isospin channels and two additional K$^*Δ$ channels are most conducive to the formation of bound states. Related interesting topics for future experimental and theoretical studies of KN interactions are also discussed.

nucl-th↗

arXiv1992

I=3/2 $K π$ Scattering in the Nonrelativisitic Quark Potential Model

We study $I=3/2$ elastic $Kπ$ scattering to Born order using nonrelativistic quark wavefunctions in a constituent-exchange model. This channel is ideal for the study of nonresonant meson-meson scattering amplitudes since s-channel resonances do not contribute significantly. Standard quark model parameters yield good agreement with the measured S- and P-wave phase shifts and with PCAC calculations of the scattering length. The P-wave phase shift is especially interesting because it is nonzero solely due to $SU(3)_f$ symmetry breaking effects, and is found to be in good agreement with experiment given conventional values for the strange and nonstrange constituent quark masses.

hep-ph↗