Low-energy Photoproduction Research Articles

Vector-meson electroproduction from generalized vector dominance

Including destructively interfering off-diagonal transitions of diffraction-dissociation type, we arrive at a formulation of GVD for exclusive vector-meson production in terms of a continuous spectral representation of dipole form. The transverse cross-section, σ T,γ ∗ p→V p , behaves asymptotically as 1/ Q 4, while R V≡σ L,γ ∗ p→V p /σ T,γ ∗ p→V p becomes asymptotically constant. Contributions violating s-channel helicity conservation stay at the level established in low-energy photoproduction and diffractive hadron–hadron interactions. The data on ρ 0-meson production from the Fermilab E665 Collaboration and on φ- and ρ 0-meson production from HERA are found to be in agreement with these predictions.

Meson photoproduction at threshold in three flavor soliton models

In Skyrme-type models, the leading term of the low-energy photoproduction amplitude is identical to the standard expression and independent of the number of flavors considered, but subleading terms are not.

Vector-meson-exchange and unitarity effects in low-energy photoproduction

Vector-meson-exchange effects are investigated for photoproduction in the $\ensuremath{\Delta}$ region. An improved analysis using a current-algebra model with unitarity corrections and the most recent multipole analysis indicates the necessity of vector-meson contributions. The required vector-meson coupling constants are consistent with quark-model values.

Strength ofΛpKcoupling by extrapolation of kaon photoproduction cross section

The $\ensuremath{\Lambda}\mathrm{pK}$ coupling constant has been determined by extrapolation from low-energy photoproduction differential scattering data. Data at the three energies, 1.06, 1.16, and 1.2 GeV, were analyzed by the optimal-polynomial-extrapolation method proposed by Cutkosky and Deo. In contrast to the very low value of this coupling constant obtained from multipole and other analyses, we find that $\frac{{{g}_{\ensuremath{\Lambda}\mathrm{pK}}}^{2}}{4\ensuremath{\pi}}={12.4}_{\ensuremath{-}0.5}^{+0.6}$, in good agreement with extrapolation results from the ${K}^{+}p$ elastic scattering data.

S-matrix and unitarity bounds for three-channel systems, with application to low-energy photoproduction of pions from nucleons

A parametrization of the S-matrix for three-channel systems is given, where we state explicitly the unitarity bounds on the free parameters. The free parameters are directly related to measurable quantities. The results are applied to low-energy photoproduction of pions from nucleons, and lower bounds for the electric dipole amplitudes E 0+ for π 0 production are obtained. These lower bounds follow without using the Fermi-Watson final-state theorem.

S-Channel Dominance and Charge Ratios in High-Energy Pion Photoproduction

It is shown that an explicit quark model, or duality diagrams, imply certain charge ratios of the squares of the imaginary parts of the amplitudes for $\ensuremath{\gamma}N\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}N$ and $\ensuremath{\gamma}N\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\Delta}$, which are observed to hold in experiments on the high-energy cross sections $\frac{d\ensuremath{\sigma}}{\mathrm{dt}}$ for $\ensuremath{-}0.2\ensuremath{\ge}t\ensuremath{\ge}\ensuremath{-}0.8$ Ge${\mathrm{V}}^{2}$. A fixed-$t$ dispersion-relation calculation, using knowledge of the low-energy photoproduction amplitudes, suggests that this may be due to cancellation of the contribution of the low- and medium-energy $s$- and $u$-channel resonances to the real parts of the amplitudes, so that the $s$ channel dominates the dispersion integral, and the quark-model relations also apply to the real parts of the amplitudes. In the extreme forward direction [${(\ensuremath{-}t)}^{\frac{1}{2}}\ensuremath{\lesssim}0.2$] the fixed-$t$ dispersion relations indicate that the $\ensuremath{\gamma}N\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}N$ amplitudes are predominantly real and dominated by the contributions to the dispersion relations of the Born approximation and other very-low-energy resonances. Some possible implications for purely hadronic reactions are discussed.

Some remarks on the interconnection between the Kroll-Ruderman theorem and the reliability of the Born approximation for the non-spin-flip p-wave photoproduction multipoles at low energies

A generalisation of the Kroll-Ruderman theorem is given for the non-spin-flip p-wave photoproduction multipoles near threshold. The reliability of the Born approximation for low-energy photoproduction p-waves is consequently discussed.

Low-energy electroproduction and equal-time commutators

We estimate low-energy photoproduction and electro-production of pions by using equal-time commutators between current densities and divergences and by including low-lying intermediate states. Predictions are presented for charged photoproductions- andp-wave multipoles, in agreement with experiments and dispersion theory, and for transversal and longitudinal electroproduction cross-sections near threshold. The dependence of the results on the axial vector form factorGA(t) is discussed.

Current algebra calculation of low-energy pion photoproduction for physical pions

Current and field commutators are used to derive Ward identities for the non-pole part of the pion photoproduction amplitude. The assumption that this part of the amplitude is as smoothly varying in the four-momenta as the Ward identities allow then leads directly to predictions for the low-energy photoproduction amplitudes for physical pions. The threshold values of the multipoles thus predicted are compared with the available data. The present calculation reduces to the earlier soft-pion result in the usual limit, but includes also explicit forms and magnitudes for the corrections to the soft-pion limit.

OnC-violation in photoproduction of pions and in proton Compton scattering

We investigate possibleC-violation in low-energy photo-production of pions and in proton Compton scattering. We introduceC-violation in the isobaric model and compare with experiment. Near theN* resonance region an assumption of maximalC-violation in photo-production fails badly, however at lower energies the violation can be important and still compatible with the experimental results on the angular distributions. The effects on Compton scattering, even under maximal violation, are very small to be detected with the present experimental possibilities.

Fixed angle dispersion relations for the low-energy photoproduction of pions on nucleons: Part I. Theory without pion-pion interaction

The dispersion approach to the low-energy photoproduction of pions on nucleons is re-examined in such a way as to avoid the contributions coming from the unphysical cosine region. Analytic representations for invariant amplitudes in the squared c.m. threemomentum plane are managed in such a way as to get rid of all but the dynamical singularities. As a first step of the study only the contributions from the direct and crossed photoproduction channels are retained, and relativistic dispersion relations for multipoles are deduced by integration over the scattering angle. Making use now of the unitarity condition in the “two intermediate particles” approximation, and considering the pion-nucleon phase shifts as quantities known from experiment, it is easy to transform the dispersion relations in question into a set of integral equations which are to be solved numerically. The numerical results and their comparison with experiment will be presented in a paper to follow.

Low-energy photoproduction of neutral mesons from complex nuclei

Angular distrihution measurements were made for tion /sup 0/ photoproduction from C, Al, Cu, Cd, and Pb, using 170Mev bremsstrahlung and two scintillation counter telescopes. The first peak in the distribution curves is fitted well by a Born approximation derived from electron scattering data, but the second peak does not agree with the approximation, the discrepancy being larger for the lighter elements and ascribed to interaction of the outgoing mesons with the recoil nucleus. An equation for 2/ spheres con ( taining 0.5% U ) divided by the Born approximation form factor, F/sup 2//sub Born/ sin/sup 2/ taining 0.5% U , in terms of A is constructed with an A/sup 1.85/ dependence, in fair agreement with the A/sup 2/ dependence predicted by the coherent prccess and indicating that nuclear absorption of outgoing mesons is small. (D.L.C.)