Neutral Pion Photoproduction Research Articles

Neutral pion photoproduction on deuterons in the resonance energy region

The gamma d to d pi 0 process asymmetries for linearly polarized photons Sigma and vector polarized deuterons T were calculated in terms of impulse approximation (IA). The IA is based on the factorization of the isovector gamma N to N pi 0 amplitudes and the deuteron form factors. Several IA versions which include different gamma N to N pi amplitudes, different deuteron wavefunctions and different Fermi momentum choices were used in the calculations. The available experimental data on the asymmetry Sigma can be described in terms of the IA with a certain Fermi momentum choice. The asymmetry T energy and angular dependences are also sensitive to the Fermi momentum choice. Both asymmetries Sigma and T are found to be rather large in the energy range considered, Egamma <800 MeV.

Neutral pion photoproduction at threshold

We evaluate the neutral pion photoproduction amplitude at threshold to one loop in chiral perturbation theory. Virtual pions generated infrared singularities in the scattering amplitude and modify the familiar low-energy expansion of the electric dipole amplitude E o+ at next-to-leading order.

ON THRESHOLD PION PHOTOPRODUCTION AND LOW-ENERGY THEOREMS

We show that low-energy data from the neutral-pion photoproduction reaction can be adequately described without resort to exotic mechanisms. An apparent discrepancy, which has been linked to recent measurements, appears to be due to assumptions regarding the rescattering correction and coupling constants. Consequences of this result are discussed.

Erratum: Pion-nucleon interaction and neutral-pion photoproduction on the proton near threshold

Neutral-pion photoproduction on the proton near threshold is treated in a Lippmann-Schwinger--type formalism. Rescattering effects are found to be important and depend strongly on the final-state {pi}{ital N} {ital S}{sub 11} interaction.

Low-energy theorem for neutral pion photoproduction on a proton.

The low-energy theorem (LET) prediction for neutral pion photoproduction on the proton remains valid even if isospin symmetry is broken. All rescattering effects are implicitly taken into account. Therefore, the data should not be corrected for final-state interactions in order to compare with the LET. As a consequence of such a correction, the previously quoted discrepancy between the LET and the experiment is overestimated.

Neutral pion photoproduction at threshold and explicit chiral symmetry breaking

We investigate effects of explicit chiral symmetry breaking on the ( γ, π 0) amplitude at threshold (the photoproduction “sigma”- term) using a chiral bag model of the nucleon. We point out a strong connection between this term and the flavor singlet axial coupling constant g A 0 which measures the spin fraction carried by the quarks inside the nucleon. A comparison is made with recent experimental data which show deviations from the standard low energy theorem for photoproduction. The possible origin of this discrepancy is discussed on the basis of our calculations.

Pion photoproduction on nucleons from threshold tO Eγ = 230 MeV

Single-pion photoproduction at low energy is investigated on the basis of the current algebra phenomenological lagrangian modified to provide an agreement of multipole amplitudes of the γp → π 0p reaction at the threshold with experiments. There are five free parameters (coupling constants) which the amplitudes of pion photoproduction processes depend on. Only one of them survives after we assume that the unmodified model gives a reasonably good description of π + photoproduction. It has been found from the threshold value of the E 0+ suπ 0 multipole. It appears that the single successful interpretation of the neutral pion photoproduction reaction is impossible throughout the photon energy region from threshold to 230 MeV. More data are needed on π° photoproduction for backward and (especially) forward directions to decide definitely whether current algebra predictions of pion photoproduction can be compatible with experimental data.

Off-shell form factors and low energy theorems for pion photoproduction.

The derivation of the low energy theorem for threshold photoproduction of neutral pions on a nucleon is reconsidered. Instead of considering explicitly the Born terms, as done in earlier derivations, a more general class of diagrams is separated from the total amplitude. Gauge invariance is imposed on the operator level and the partial conservation of the axial-vector current constraint is incorporated. Besides recovering the older results in this approach, relations for the off-shell behavior of the strong and electromagnetic vertices are derived.

Pi 0 photoproduction on the neutron near threshold.

Neutral pion photoproduction on the neutron near threshold is investigated with the dynamical model of Nozawa, Blankleider, and Lee. It is shown that the final state interaction amplitude derived from the dynamical model could involve large cancellations between the different pion photoproduction mechanisms. We find that the final state interaction plays an essential role in this channel because of the small Born amplitude. At threshold, we obtain {ital E}{sub 0+}({pi}{sup 0}{ital n}) =0.21{times}10{sup {minus}3}/{ital m}{sub {pi}}{sup +}.

Neutral pion photoproduction on the nucleon near threshold

Neutral pion photoproduction on the nucleon near threshold is investigated using a dynamical model. It is shown that the commonly used procedure, based on an analytical continuation of the K-matrix to the unphysical region, is not compatible to a dynamical approach. We show that the final state interaction ( FSI) amplitude derived from a dynamical model could involve large cancellation between the different pion photoproduction mechanisms. This indicates that the FSI due to the intermediate π 0 p state can be as important as that due to the π + n intermediate state. At threshold, we obtain E 0+ = −1.92 × 10 −3 m π + . This number is close to the measured value of E 0+ = −1.5 × 10 −3 m π + . No violation of the low energy theorem is required to obtain good agreement between the calculated total cross sections and experimental data from threshold to about 400 MeV incident photon energy.

Photoproduction of neutral pions and low energy theorems

Photoproduction of neutral pions and low energy theorems

Neutral pion photoproduction on the nucleon near threshold.

Neutral pion photoproduction on the nucleon near threshold is investigated using a dynamical model. Analytic properties of the final-state interaction amplitude in the energy region near {pi}{sup +}n production threshold are examined in detail. It is shown that the commonly used procedure, based on an analytical continuation of the {ital K} matrix to the unphysical region, is not compatible to an approach incorporating full off-energy-shell dynamics. Our calculation indicates that the final-state interaction amplitude derived from a dynamical model could involve large cancellation between the different pion photoproduction mechanisms. This leads to the surprising result that the final-state interaction effect due to the intermediate {pi}{sup 0}p state can be as important as that due to the {pi}{sup +}n intermediate state. At threshold, we obtain {ital E}{sub 0+}={minus}1.92{times}10{sup {minus}3}/{ital m}{sub {pi}{sup +}}. This number is close to the measured value of {ital E}{sub 0+}={minus}1.5{times}10{sup {minus}3}/m{sub {pi}{sup +}}. No violation of the low-energy theorem is required to obtain good agreement between the calculated total cross sections and experimental data from threshold to about 400-MeV incident photon energy.

Neutral pion production at threshold and low-energy theorems

Recent experiments of neutral pion photoproduction show a strong discrepancy between experimental data and the predictions of low-energy theorems (LET). The basic ingredients of these theorems are the conservation of the electromagnetic current and the hypothesis of a partially conserved axial current (PCAC). Both, anomalies in the t-channel, and rescattering in a simple R-matrix formalism, lead to small corrections of the order of 10 – 20 %. However, the investigation of the effects of chiral symmetry breaking and isospin symmetry breaking at the quark level in a current algebraic approach leads to substantial contributions of the right order of magnitude.

Pion-nucleon interaction and neutral-pion photoproduction on the proton near threshold.

Neutral-pion photoproduction on the proton near threshold is treated in a Lippmann-Schwinger--type formalism. Rescattering effects are found to be important and depend strongly on the final-state \ensuremath{\pi}N ${S}_{11}$ interaction.

Inelastic production of neutral pions in the4He(?, ??p)3H reaction

The yield of the4He(γ, π‡p)3H reaction has been studied in a bremsstrahlung beam of photons with a maximum energy of 450 MeV. The measurements were made on an experimental setup consisting of a total absorption Cerenkov gamma spectrometer and a helium streamer chamber. A kinematic analysis is made of the angular distributions of the disintegration products of the4He nucleus to study the mechanism of inelastic photoproduction of neutral pions. The relatively high event density in the region of small proton-triton separation angles is attributable to the quasielastic mechanism of the reaction in which pion photoproduction is accompanied by excitation of the4He nucleus.

Photoproduction of pions off nucleons and nuclei

Present status of pion photoproduction on nucleons and nuclei is discussed. Transitions on bound nuclear states in charged pion photoproduction and the coherent processes in neutral pion photoproduction are examined from the point of view of self-consistent description of all theory ingredients.

Differential cross sections for coherent and incoherent neutral-pion photoproduction from calcium.

Differential and absolute cross sections for the coherent reaction $^{40}\mathrm{Ca}$(\ensuremath{\gamma},${\ensuremath{\pi}}^{0}$${)}^{40}$Ca g.s., as well as for the incoherent reaction $^{40}\mathrm{Ca}$(\ensuremath{\gamma},${\mathrm{\ensuremath{\pi}}}^{0}$${)}^{40}$Ca(g.s.), have been measured with monochromatic photons in the threshold region (${E}_{\ensuremath{\gamma}}$=157--169 MeV). The differential cross section for the coherent process exhibits a diffraction minimum due to the nuclear mass form factor while the incoherent cross section is rather structureless. The cross sections are in good agreement with recent distorted-wave impulse-approximation calculations.

Higher order effects in neutral pion photoproduction at threshold

We estimate the absorptive part of the rescattering effect in the E0 multipole amplitude for γp→π0p at threshold. Our result is contradictory to recent experimental analyses.

Neutral pion photoproduction on the proton near threshold and the chiral symmetry.

The recent precise measurements of the neutral pion photoproduction on the proton near threshold seems to be inconsistent with the predictions of the low-energy theorems on photoproduction of pions. In this note, we have sought to resolve this problem, in the framework of an approximate SU(2)\ifmmode\times\else\texttimes\fi{}SU(2) symmetry of the pion-nucleon dynamics, by incorporating the contribution from the nucleon matrix elements of the equal-time commutator between the divergence of the axial-vector current and the electromagnetic current, which was introduced earlier by Furlan et al. We find that a mean value of the nonstrange quark mass m^=1/2(${m}_{u}$+${m}_{d}$)=(7\ifmmode\pm\else\textpm\fi{}2) MeV, leads to an estimate of the symmetry breaking terms compatible with experiment.

A spectrometer for near threshold photoproduction of neutral pions

A π 0 spectrometer is described which has been installed at the Glasgow-Mainz tagging spectrometer at the Mainz Microtron. It consists of 176 lead glass detectors arranged in two blocks of 8 × 11 detectors. It is employed to investigate neutral pion production off the proton and off complex nuclei with monochromatic photons between threshold and ≈175 MeV.