Threshold Pion Photoproduction Research Articles

Measurement of the proton scalar polarizabilities at MAMI

The electric (αE1) and magnetic (βM1) scalar polarizabilities describe the response of the nucleon to an applied electric or magnetic field. They are not only fundamental properties related to the internal structure and dynamics of the nucleon, but they are important also in other areas of physics, such as atomic structure. The values of αE1 and βM1 quoted by the Particle Data Group were determined using data on the unpolarized differential cross-section of the Compton scattering $\gamma p \to \gamma p$. The measurement of the beam asymmetry Σ3, provides an alternative approach to the extraction of the scalar polarizabilities, with different sensitivity and systematics compared to the unpolarized crosssection. This asymmetry was measured recently for the first time below the pion photoproduction threshold by the A2 Collaboration with the Crystal Ball/TAPS experiment at MAMI (Mainz, Germany). A new high precision measurement of both a unpolarized cross-section and beam asymmetry Σ3a is ongoing at MAMI and the polarizabilities αE1 and βM1 will be extracted with unprecedented precision.

Determination of the scalar polarizabilities of the proton using beam asymmetry $\Sigma_{3}$ Σ 3 in Compton scattering

The scalar dipole polarizabilities, $\alpha_{E1}$ and $\beta_{M1}$, are fundamental properties related to the internal dynamics of the nucleon. The currently accepted values of the proton polarizabilities were determined by fitting to unpolarized proton Compton scattering cross section data. The measurement of the beam asymmetry $\Sigma_{3}$ in a certain kinematical range provides an alternative approach to the extraction of the scalar polarizabilities. At the Mainz Microtron (MAMI) the beam asymmetry was measured for Compton scattering below pion photoproduction threshold for the first time. The results are compared with model calculations and the influence of the experimental data on the extraction of the scalar polarizabilities is determined.

Experiments with compton back scattered gamma beams (on GRAAL collaboration results)

Experimental study of photonuclear reactions using Compton back scattered gamma beams at intermediate energies (from pion photoproduction threshold up to a few GeV) is reviewed. The last review of this subject was published about 10 years ago. Since then, new fundamental data on nucleon spin structure and photonuclear excitation mechanisms have appeared in the literature. Principal attention in the present review is paid to the GRAAL (Grenoble Accelerateur Anneau Laser) experiment conducted at ESRF (European Synchrotron Radiation Facility). It is shown that, in addition to the principal polarization experimental program based on high degree of gamma beam polarization, new results on different applications have been obtained. For example, total photoabsorption and partial meson photoproduction cross sections were measured with high accuracy, and a new method for studying an interaction of unstable short-lived mesons with nuclear media (tagged mesons) was proposed. New results on anisotropy of the speed of light with respect to the dipole of cosmic microwave background radiation were evaluated. It is noted that new opportunities arise due to unique conditions appearing while conducting the experiments with the Compton back scattering technique, which provides the hard photon spectrum, high degree of beam polarization, and low backgrounds.

Photofission and total photoabsorption cross sections in the energy range of shadowing effects

The energy dependence of photofission cross section for heavy nuclei has recently been well described in terms of a Monte Carlo calculation at energies from the pion photoproduction threshold up to 1 GeV [see, for instance, A. Deppman et al., Phys. Rev. Lett. 87 (2001) 182701]. Recent experimental data from CLAS (CEBAF Large Angle Spectrometer) collaboration have extended the measured photofission cross section up to 3.5 GeV for actinide and preactinide nuclei. In this work we address the calculation of photoabsorption and photofission cross sections for actinide and preactinide nuclei above 1 GeV, a region where the shadowing effect plays an important role in the nuclear photoabsorption process.

Photofissility at 1 GeV for nuclei throughout the periodic table

A new approach to evaluate nuclear photofissilities at incident photon energies above the pion photoproduction threshold has been recently developed and proved to describe successfully the fissilities of natPb and 232Th target nuclei at energies ~0.2-4.0 GeV. The method is merely a simple, semiempirical description of the photofission reactions in which fissility, f, is governed by two basic quantities, namely, i) the first-chance fission probability, $\bar{f}_1$, for the average cascade residual, and ii) the slope, $\bar{s}$, of the chance-fission probability associated with the average evaporative sequence of fissionable residuals. In the present work we have extended this approach to analyse photofissity data that have been accumulated over the past fourty years or so, measured at 1 GeV, for nearly fourty target nuclei extending from Ti up to Np. Results have shown that the variation of fissility with Z²/A could be described quite satisfactorily by the proposed model.

High energy neutrinos from radio-quiet active galactic nuclei

Most active galactic nuclei (AGN) lack prominent jets, and show modest radio emission and significant x-ray emission which arises mainly from the galactic core, very near the central black hole. We use a quantitative scenario of such core-dominated radio-quiet AGN, which attributes a substantial fraction of the x-ray emission to the presence of abortive jets involving the collision of gas blobs in the core. Here we investigate the consequences of the acceleration of protons in the shocks from such collisions. We find that protons will be accelerated up to energies above the pion photoproduction threshold on both the x rays and the UV photons from the accretion disk. The secondary charged pions decay, producing neutrinos. We predict significant fluxes of TeV-PeV neutrinos, and show that the AMANDA II detector is already constraining several important astrophysical parameters of these sources. Larger cubic kilometer detectors such as IceCube will be able to detect such neutrinos in less than one year of operation, or otherwise rule out this scenario.

New approach to nuclear photofission reactions above 0.15 GeV

A simple approach to evaluate nuclear photofissilities at energies above the pion photoproduction threshold has been developed. It is based on the current, two-step model for intermediate-energy photonuclear reactions, i.e. a photon-induced intranuclear cascade followed by a fission-evaporation competition process for the excited, post-cascade residual nucleus. The calculation method (semiempirical by nature) shows that fissility (i.e., total fission probability) is governed by two basic quantities, namely, the first-chance fission probability for the average cascade residual, and a parameter which defines an evaporative sequence of residuals in which the average, equivalent chance-fission probabilities of nuclides belonging to the same generation are located. The natPb photofissility data measured recently in the range ~ 0.2 - 3.8 GeV at the Thomas Jefferson Laboratory could be explained very satisfactorily by the present approach.

Nuclear photofissility of natPb and 232Th at energies above the pion photoproduction threshold

A new, simple approach based on the intranuclear cascade plus the evaporation–fission competition mechanisms of photonuclear reactions has been developed to calculate fissility values at energies above ∼200 MeV. This approach has been successfully used in explaining fissility data for natPb and 232Th target nuclei at energies up to 3.8 GeV recently obtained at the Thomas Jefferson Laboratory. Predictions of photofissility up to 5–6 GeV for these nuclei are also given.

Prospects for measurements of parity-violatingphotoproduction of $\pi^\pm$

I address the experimental feasibility of two measurements related to the photoproduction of pions, in the context of the G0 experiment. The first involves an extraction of the parity-violating pionnucleon coupling constant h;π N N (1) via a measurement of parity-violating threshold pion photoproduction. The second involves an extraction of d Δ, which parameterizes parity-violating photoproduction of pions on the Δ-resonance. The second process might have an anomalously large asymmetry, from a model based on hyperon decays.

Deuteron Compton scattering below pion photoproduction threshold

Deuteron Compton scattering below pion photoproduction threshold is considered in the framework of the nonrelativistic diagrammatic approach with the Bonn OBE potential. A complete gauge-invariant set of diagrams is taken into account which includes resonance diagrams without and with NN-rescattering and diagrams with one- and two-body seagulls. The seagull operators are analyzed in detail, and their relations with free- and bound-nucleon polarizabilities are discussed. It is found that both dipole and higher-order polarizabilities of the nucleon are required for a quantitative description of recent experimental data. An estimate of the isospin-averaged dipole electromagnetic polarizabilities of the nucleon and the polarizabilities of the neutron is obtained from the data.

Determination of the πNN coupling constant in the VPI/GWU πN → πN partial-wave and dispersion relation analysis

Our extraction of the charged pion-nucleon coupling constant from πp elastic scattering data is outlined. A partial wave analysis (Tπ < MeV) is performed simultaneously with a fixed-t dispersion relation (DR) analysis (< 800 MeV). The πNN coupling constant g2/4π and other DR parameters are searched to find the best fit. The result 13.73 ±0.01 ±0.07 (first error statistical, second systematic) is found to be insensitive to database changes and Coulomb barrier corrections. This value satisfies important elements of low energy QCD like the Goldberger-Treiman discrepancy, the Dashen-Weinstein sum rule, and chiral predictions of threshold pion photoproduction.

Hadron structure in the non-perturbative regime of QCD: Isospin symmetry and its violation

I discuss recent progress made in calculating electromagnetic corrections in the framework of the effective field theory of QCD. In the case of elastic pion-pion scattering, strong interaction predictions have been worked out to two loop accuracy. I present first results for the electromagnetic corrections in the case of neutral pions. Here, the only sizeable effect comes from the charged to neutral pion mass difference. In the presence of nucleons, isospin violation can be measured in threshold pion photoproduction. I review the present status of the theoretical predictions and the experimental data. I argue that a deeper understanding of isospin violation based on a more precise study of such reactions can be achieved.

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.

Deuteron Photodisintegration in the Diagrammatic Approach

Photodisintegration of the deuteron below the pion-photoproduction threshold is considered in the framework of the diagrammatic approach. The deuteron wave function, the final-state N–N interaction and meson-exchange currents are constructed from the nonrelativistic versions of the Bonn OBE potential. A detailed comparison with the existing data is made. It is shown that the model gives a satisfactory agreement with the experimental data. Possible sources for the remaining disagreements are discussed.

Threshold pion photoproduction of nucleons in the chiral quark model.

In this paper, we show that the low energy theorem (LET) of the threshold pion-photoproduction can be fully recovered in the quark model. An essential result of this investigation is that the quark-pion operators are obtained from the effective chiral Lagrangian, and the low energy theorem does not require the constraints on the internal structures of the nucleon. The pseudoscalar quark-pion coupling generates an additional term at order $\mu=m_{\pi}/M$ only in the isospin amplitude $A^{(-)}$. The role of the transitions between the nucleon and the resonance $P_{33}(1232)$ and P-wave baryons are also discussed, we find that the leading contributions to the isospin amplitudes at $O(\mu^2)$ are from the transition between the P-wave baryons and the nucleon and the charge radius of the nucleon. The leading contribution from the P-wave baryons only affects the neutral pion production, and improve the agreement with data significantly. The transition between the resonance $P_{33}(1232)$ and the nucleon only gives an order $\mu^3$ corrections to $A^{(-)}$.

Threshold pion photoproduction in chiral perturbation theory

We analyze threshold pion photoproduction in the framework of baryon chiral perturbation theory. In case of neutral pion photoproduction, virtual pions generate infrared singularities which modify the familiar form of the low-energy theorem for the electric dipole amplitude E 0+. In particular, we predict E 0+ π 0 p ( s th )=−(1.33±0.09)× −3/ M π+ . We also find a satisfactory description of the recently measured total cross section for γp→ π 0p in the threshold region. The process γn→ π 0n is entirely given by the loop and counterterm contributions. It would be interesting to have an experimental determination of the corresponding cross section in the threshold region. In case of the charged channels, the loop corrections are much more tame.

Threshold pion photoproduction and chiral models of the nucleon

We present calculations of the chiral and isospin symmetry-breaking corrections to the threshold pion photoproduction amplitude E 0+. The calculations are based on two chiral models of the nucleon: the chiral bag model and the non-topological soliton model. We find that ΔE 0+ from explicit chiral-symmetry breaking is substantial, but not sufficient to explain the discrepancy between the measured γp → π 0p threshold amplitude and its predicted value based on the chiral low-energy theorem.

Threshold pion photoproduction in a light-cone quark model

The instantaneous and seagull graphs are calculated for pion photoproduction in a relativistic light-cone model of the nucleon. In both pseudoscalar and pseudovector coupling we find the ratios A (−): A (0): A (+) = 1: ( −1 2 μ):( −9 5 μ) in the nonrelativistic limit. These results correspond to the sum of seagull and Z-graph in the nonrelativistic quark model. In pseudovector coupling also the numerical results for realistic-model parameters are close to those values.

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.

Pion photoproduction in the skyrme model and low-energy theorems

Threshold pion photoproduction has been calculated for the nucleon, within the framework of the Skyrme model. The calculation involves two different types of coupling between the pion and the nucleon, either as a small amplitude fluctuation or as a chiral perturbation about the nucleon soliton solution. It is shown that a correct determination of the axial coupling constant in the chiral limit is a compulsory prerequisite in order to reproduce the leading term of the charged production (Kroll-Ruderman theorem). Both methods fail to predict the higher order terms of low-energy theorems, which are of particular importance for the neutral pion production. This shortcoming is not due to the adiabatic treatment of the rotation matrices, but is a consequence of the approximations for the pion-nucleon coupling mechanism and of the neglect of the Dirac sea in the theory.