Some open topics in the field of low energy photon-nucleon interactions are discussed--which in my opinion are of interest both for obtaining new information on nucleon structure as well as for precision tests of our theoretical understanding of chiral dynamics. In particular, I discuss p-wave multipoles in charged pion photoproduction off protons as well as the role of spin-dependent effects in (unpolarized) Compton scattering off nucleons. There the concept of dynamical spin polarizabilities is found to be essential for the understanding of differential Compton cross sections above 120 MeV cms photon energy.
We consider multipole amplitudes for low-energy pion photoproduction, constructed with minimal model dependence, at single energies. Comparisons with fits to the full resonance region are made. Explanations are suggested for the discrepancies and further experiments are motivated.
The scattering amplitude of the photoproduction of $π^{+}$ is calculated using the helicity formalism. The pseudovector coupling $π$-N interaction with the non-perturbative term is applied to the perturbative expansion. The parameters for the self-energy are taken from the results of the $π$-N scattering in our previous study.
The possibilities of a model-independent partial wave analysis for pion, eta or kaon photoproduction are discussed in the context of complete experiments. It is shown that the helicity amplitudes obtained from at least 8 polarization observables including beam, target and recoil polarization can not be used to analyze nucleon resonances. However, a truncated partial wave analysis, which requires only 5 observables will be possible with minimal model assumptions.
Chiral O(Q^4) two-body operators for s-wave pion photoproduction on the NN system
利用手征微扰理论，导出NN系统中s波π介子光生的两体流为O（Q^4）。对于有趣的情况下，3S1 <->1S0过渡，我们表明，轴向等矢量两核子接触项连接的短距离物理的π介子光生π介子生产和几个重要的电弱反应。我们还发现，标准的手征拉格朗日给出了一个伽玛π π N N顶点，在以前的文献中没有明确提到。本文给出了相应的费曼规则，并简要地讨论了它在某些过程中的重要作用。
We explore the photoproduction mechanisms for charged and neutral pion pairs off a heavy nucleus at threshold. We calculate the production of charged pairs in the Coulomb field of the nucleus in the Born approximation using explicit expressions for the differential cross sections and their connection with the total cross-section. The production of sigma mesons at threshold and their subsequent decay into pions is an important mechanism and is investigated quantitatively in a framework that can be used for both charged and neutral pions. Sigma mesons can be produced via electromagnetic (photon …
A gauge invariant unitary theory for pion photoproduction
Ward-Takahashi恒等式对于光生振幅的规范不变性至关重要。在这里，我们证明了酉性，特别是包含的$πN$和$γπN$阈值平等的基础上产生的光生振幅，满足两体酉性和广义沃德高桥身份。最终振幅是反应$πN\rightarrowπN$和$γN\rightarrowπN$的一组耦合通道积分方程的解。%数据可根据要求从作者处获得，%电子邮件：phchv@pippin.cc.flinders.edu.au %提交至Physical Review C
The photoproduction of pion off nucleon is investigated within a dynamical coupled-channels approach based on the Juelich pi-N model, which has been quite successful in the description of pi-N to pi-N scattering for center-of-mass energies up to 1.9 GeV. The full pion photoproduction amplitude is constructed to satisfy the generalized Ward-Takahashi identity and hence, it is fully gauge invariant. The calculated differential cross sections and photon spin asymmetries up to 1.65 GeV center-of-mass energy for the reactions gamma p to pi^+ n, gamma p to pi^0 p and gamma n to pi^- p are in good ag…
Pion photoproduction reactions are investigated in a dynamical coupled-channel approach based on the Juelich pi-N model, which presently includes the hadronic pi-N and eta-N stable channels as well as the pi-Delta, sigma-N and rho-N effective channels. This model has been quite successful in the description of pi-N to pi-N scattering for center-of-mass energies up to 1.9 GeV. The full pion photoproduction amplitude is constructed to satisfy the generalized Ward-Takahashi identity and hence, it is fully gauge invariant. The calculated differential cross sections and photon spin asymmetries up t…
We present an ongoing project to assess the importance of D-waves and the $Δ(1232)$ resonance for descriptions of neutral pion photoproduction in Heavy Baryon Chiral Perturbation Theory. This research has been motivated by data published by the A2 and CB-TAPS collaborations at MAMI [1]. This data has reached unprecedented levels of accuracy from threshold through to the $Δ$ resonance. Accompanying the experimental work, there has also been a series of publications studying the theory that show that, to go beyond an energy of $E_γ=170$ MeV, it is necessary to include other aspects, in particula…
Unitarity constraint for threshold coherent pion photoproduction on the deuteron and chiral perturbation theory
利用么正性约束计算了两步过程gamma + d -> p + n -> pi 0 + d对氘核相干π介子产生振幅虚部的贡献。结果表明，这种吸收过程是不可忽略的，在从阈区γ + d -> pi 0 + d截面中提取基元中子产生幅度时，必须考虑这种吸收过程。此外，还指出在次主导阶以上的重子手征微扰理论中，gamma + d -> pi 0 + d的一致计算需要包含这个吸收过程.
Model dependence of multipole analysis has been explored through energy-dependent and single-energy fits to pion photoproduction data. The MAID energy-dependent solution has been used as input for an event generator producing realistic pseudo data. These were fitted using the SAID parametrization approach to determine single-energy and energy-dependent solutions over a range of lab photon energies from 200 to 1200 MeV. The resulting solutions were found to be consistent with the input amplitudes from MAID. Fits with a $χ$-squared per datum of unity or less were generally achieved. We discuss e…
A model for the pion photoproduction on nuclei in the $A\left({γ,\, πNN} \right)B$ reaction is presented. This is an extension of our recent model for the $A\left({γ,\, πN} \right)B$ reaction. In this approach we have moved beyond the standard shell-model considering $Δ$\textit{N} correlations in the nuclear wave functions, which are caused by the virtual transitions $NN \to ΔN \to NN$ in the ground state of the nucleus. The main ingredients of the model are the two- and three-particle density matrices and the transition operators $γΔ\to Nπ$ and $γN \to Nπ$. The direct and exchange reaction me…
