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

A hard-pion current-algebra model is employed to derive expressions for the pion electro- and photoproduction amplitudes. The low-energy predictions for the multipole amplitudes and differential cross sections of photoproduction are analysed and compared with the experimental data.     

Compton scattering on the pion and radiative pion photoproduction from the proton

The contribution of the pion polarizabilities to radiative pion photoproduction has been investigated. It has been shown how an extrapolation of future experimental data on radiative pion photoproduction to the pion pole can give information on the polarizabilities of pion.     

Radiative corrections to pion nucleus scattering

The radiative corrections to elastic pion-nucleus scattering are evaluated for kinematical conditions pertaining to the pion spectrometer facilities at the newer meson factories, and are found to be experimentally relevant. A reasonably simple, experimentally useful approximation to the exact expression for the radiative correction is given.     

Radiative charged pion photoproduction and pion polarizabilities: An analysis within heavy baryon chiral perturbation theory

We analyze the amplitude for radiative charged pion photoproduction within the framework of heavy baryon chiral perturbation theory and discuss the best experimental setup for the extraction of the pion polarizabilities from the differential cross section. Correspondence: C.-W. Kao, Department of Physics, Chung Yuan Christian University, Chung-Li 320, Taiwan     

Resonances of any spin in pion photoproduction and nucleon Compton scattering

Describing the vertices by form factors and the propagators by their spectral representations, we calculate the contribution of the exchange diagramsγπ→M ^j→ ¯N N andγN→N ^j+1/2→γN to the invariant and helicity amplitudes, for meson (M _j) and baryon (N ^j+1/2) resonances of any spin (j integer ≧ 1) and both parities ((?)^j and (?)^j+1).     

Backward pion photoproduction

We present a systematic analysis of backward pion photoproduction for the reactions p p and p n . Regge phenomenology is applied at invariant collision energies above 3GeV in order to fix the reaction amplitude. A comparision with older data on - and -photoproduction at = 180^° indicates that the high-energy limit as given by the Regge calculation could be reached possibly at energies of around ≃ 3 GeV. In the energy region of 2.5 GeV, covered by the new measurements of p p differential cross-sections at large angles at ELSA, JLab, and LEPS, we see no clear signal for a convergence towards the Regge results. The baryon trajectories obtained in our analysis are in good agreement with those given by the spectrum of excited baryons.     

Electron scattering and charged pion photoproduction on12C and13C

Electron scattering and charged pion photoproduction at 200 MeV on¹²C and¹³C have been studied by the nuclear shell model to investigateM1 andE2 nuclear form factors of the isovector type. Starting with the Op-shell model of Hauge-Maripuu, the effects of core polarization by theM 3Y force and the one-pion-exchange current are taken into account. The effect of core polarization is found to be essential in both electron scattering and pion photoproduction, while the meson-exchange current, which also improves the calculated results, gives contributions of about 10%–15%.     

Partial-wave decomposition of pion and photoproduction amplitudes

Partial-wave amplitudes for production and decay of baryon resonances are constructed in the framework of the operator expansion method. The approach is fully relativistically invariant and allows us to perform combined analyses of different reactions imposing directly analyticity and unitarity constraints. All formulas are given explicitly in the form used by the Crystal Barrel Collaboration in the (partly forthcoming) analyses of the electro-, photo- and pion-induced meson production data.     

Elastic photoproduction of neutral pions from helium

Zeitschrift für Physik A Hadrons and nuclei - Differential cross sections are calculated for the elastic photoproduction of neutral pions from He4 under the impulse approximation. We use a...     

Deep inelastic Compton scattering and large-p T pion photoproduction in QCD hard collision model

Inclusive photon(pion) production at large-p _T in γp collisions is studied in the framework of QCD perturbation theory, taking into account the effects due to parton transverse momenta. The contributions from various components of photons are equally important at relatively smallp _T (e.g.,p _T?4 Ge V/c at \(\sqrt s = 17GeV\) ). At largep _T, however, the contributions from the subprocesses with direct photon coupling are most important; quark Compton scattering is the dominant subprocess in inelastic Compton scattering.     

Nonlocal theory of single pion photoproduction

A nonlocal interaction theory is formulated for photoproduction processes by introducing a form function. An effective nonlocal, Lorentz invariant and gauge invariant Lagrangian density for the four-field interaction that gives rise to a production amplitude is constructed. The general structure of the form function is investigated by using some restrictions of the form function. For low energy ⁰ photoproduction an explicit form of the Fourier component of the form function is obtained. The physical model of the present formalism is to assume, similar to the strong absorption model, that the system in intermediate states is confined in a finite domain of space. For ⁰ production the linear dimension of this domain is obtained to be ₀ = 3·88F. It is important to observe that the extent of electromagnetic distribution in a nucleon is also nearly the size of ₀. This is believed to be the reason that in low energy pion photoproduction the effects of electromagnetic structure of a nucleon are irrelevant. The unpolarized differential and total cross sections are calculated for ⁰ production in helicity representation and the predictions are found to be in good agreement with experiments.     

Polarisation parameters in neutral pion photoproduction

The polarisation parameters Σ, P and T have been measured for the process γp→π⁰p in the photon energy range 1300–2100 MeV and c.m. angles between 30° and 110°, in an experiment with a polarised beam and polarised target. The results are compared with a recent theoretical analysis which fits data from threshold to 16 GeV. The new data are in general agreement with the analysis, but with some significant discrepancies in detail.     

Neutral pion photoproduction off nucleons revisited

We investigate threshold pion photoproduction in the framework of heavy baryon chiral perturbation theory. We give the expansion of the electric dipole amplitude E ₀₊ to three orders in μ the ratio of the pion to nucleon mass, and show that it is slowly converging. We argue that this observable is not a good testing ground for the chiral dynamics of QCD. In contrast, we exhibit new and fastly converging low-energy theorems in the P-waves which should be used to constrain the data analysis. We also discuss the importance of polarization observables to accurately pin down certain multipoles and give predictions for the reaction γ n → π⁰ n.     

Charged pion photoproduction from light nuclei

The photoproduction of 0–150-MeV charged pions from light nuclei is studied from a distorted wave impulse approximation (DWIA) approach. The final nuclear states are restricted to a finite set of isospin analogs of excited states of the target nucleus. The final state interactions of the pion with the residual nucleus are incorporated via optical potentials. The elementary photoproduction operator used is that of Blomqvist and Laget which is derived in a general reference frame. To gain insight into the predictive power of this DWIA approach, total and differential cross sections for π^± production from ⁶Li, ⁷Li, ¹⁰B, ¹²C, and ¹⁴N are calculated and compared with available data. It is found that, with a few exceptions, reasonable agreement is obtained between theory and experiment as long as the nuclear wave functions are constrained to fit other electromagnetic and weak processes and the optical potentials are constrained to fit pion-nucleus elastic scattering data. We conclude that, at this stage, using the Blomqvist-Laget operator in a DWIA calculation adequately describes the dynamics of charged pion photoproduction from complex nuclei. We illustrate how this reaction can be used to obtain information on the short range nature of the pion wave function and on nuclear wave functions. Shortcomings of and improvements on this calculation are also suggested.     

Double pion photoproduction off 40Ca

The photoproduction of π ⁰ π ⁰ and π ⁰ π ^± pairs off ⁴⁰Ca has been investigated with the TAPS detector using the Glasgow photon tagging spectrometer at the Mainz MAMI accelerator. Data have been taken for incident photon energies in the energy range from 200-820MeV. Total cross-sections have been extracted from threshold up to the maximum photon energy and invariant-mass distributions of the pion pairs have been obtained for incident photon energies between 400-500MeV and 500-550MeV. The double π ⁰ invariant-mass distributions show some relative enhancement with respect to the mixed charge channel at small invariant masses. The effects are smaller than previously observed for lead nuclei and the distributions do not significantly deviate from carbon data. The data are in good agreement with the results of recent calculations in the framework of the BUU model, with careful treatment of final-state interaction effects but without an explicit in-medium modification of scalar, iso-scalar pion pairs. This means that for Ca most of the experimentally observed effect can be explained by final-state interactions. Only at low incident photon energies there is still a small low mass enhancement of the double π ⁰ data over the BUU results.     

Neutral pion photoproduction at high energies

A Regge model with absorptive corrections is employed in a global analysis of the world data on the reactions p p and n n for photon energies from 3 to 18GeV. In this region resonance contributions are expected to be negligible so that the available experimental information on differential cross-sections and single and double polarization observables at - t 2 GeV²allows us to determine the reaction amplitude reliably. The model amplitude is then used to predict observables for photon energies below 3GeV. A detailed comparison with recent data from the CLAS and CB-ELSA Collaborations in that energy region is presented. Furthermore, the prospects for determining the radiative decay width via the Primakoff effect from the reaction p p are explored.     

Polarisation parameters in positive pion photoproduction

Measurements have been made of the polarisation parameters Σ, T and P for the process γp → π⁺n in the photon energy range 520–2250 MeV at c.m. angles between 30° and 120°. These data were obtained in a double polarisation experiment, using the polarised photon beam from the Daresbury electron synchrotron incident on a polarised proton target. The data are compared with predictions from current theoretical analyses.     

Threshold photoproduction of charged pions on14N

Pion photoproduction processes¹⁴N_gs(γ, π ⁺)¹⁴C and¹⁴N_gs(γ, π ^?)¹⁴O have been studied in the threshold region. These processes provide an excellent tool to study the corrections to soft pion theorems and Kroll-Ruderman limit as applied to nuclear processes. The agreement with the available experimental data for these processes is better with the empirical wave functions while the shell-model wave functions predict a much higher value. Detailed experimental studies of these reactions at threshold, it is shown, are expected to lead to a better understanding of the shell-model inputs and radial distributions in the 1p state.     

Radiative tau decay into pion

The possibility of the experimental investigation of the radiative tau decay into pion is discussed including background conditions. The decay width is estimated to be Br(τ→v _τ π γ)=0.15% (at the minimal registered photon energy 50 MeV). The contribution of the structural component is 10% of the total radiation. The main background is produced by photons from the τ→v _τ π ^? π ⁰ decay. We investigate the angular and energy spectra of the decay products and analyse the background conditions. The kinematic region was found where the structural radiation dominates over background.