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Register a new userNucleon Helicity in Pion Photoproduction
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Pion-photoproduction data is examined to check for the nucleon-helicity conservation predicted by asymptotic QCD. The differential cross section shows agreement with constituent-counting rules, and polarization data is not in disagreement with conservation of nucleon helicity. However large uncertainties in the polarization measurements do not allow a conclusive statement. The helicity amplitudes from a partial-wave analysis are also examined for helicity conservation. While the amplitudes become small as $s$ increases, the $s$ dependence of the helicity-conserving amplitudes is similar to the dependence of the non-conserving amplitudes.
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Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz in the three isospin channels $vec{gamma}pto pi^{+}pi^0n$, $vec{gamma}pto pi^{0}pi^0p$ and $vec{gamma}pto pi^{+}pi^{-}p$ . The circularly polarized photons, produced from bremsstrahlung of longitudinally polarized electrons, were tagged with the Glasgow magnetic spectrometer. Charged pions and the decay photons of $pi^0$ mesons were detected in a $~4pi$ electromagnetic calorimeter which combined the Crystal Ball detector with the TAPS detector. The precisely measured asymmetries are very sensitive to details of the production processes and are thus key observables in the modeling of the reaction dynamics.
We present a pion photoproduction model on the free nucleon based on an Effective Lagrangian Approach (ELA) which includes the nucleon resonances ($Delta(1232)$, N(1440), N(1520), N(1535), $Delta (1620)$, N(1650), and $Delta (1700)$), in addition to Born and vector meson exchange terms. The model incorporates a new theoretical treatment of spin-3/2 resonances, first introduced by Pascalutsa, avoiding pathologies present in previous models. Other main features of the model are chiral symmetry, gauge invariance, and crossing symmetry. We use the model combined with modern optimization techniques to assess the parameters of the nucleon resonances on the basis of world data on electromagnetic multipoles. We present results for electromagnetic multipoles, differential cross sections, asymmetries, and total cross sections for all one pion photoproduction processes on free nucleons. We find overall agreement with data from threshold up to 1 GeV in laboratory frame.
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 allow 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.
We study the helicity distributions of light flavor quark-antiquark ($q bar{q}$) pairs in the nucleon sea. The valence quarks are handled by adopting the light-cone SU(6) quark-spectator-diquark model and the sea $q bar{q}$ pairs are treated from statistical consideration by introducing the helicity suppression factors $l_q(x)$ and $bar l_q(x)$ to parametrize the helicity distributions of q-flavor sea quark and antiquark respectively, while $Delta l_q(x)=l_q(x)-bar l_q(x)$ represents a combined effect of helicity contribution due to sea $q bar{q}$ pairs. From fitting the nucleon polarization asymmetries $A^N_1$ in inclusive deep inelastic scattering processes and the single-spin asymmetries $A^{W^{pm}}_L$ in Drell-Yan type processes, we find a significant asymmetry between the quark and antiquark helicity distributions of the nucleon sea. Therefore the quark-antiquark asymmetry of helicity distributions of nucleon sea $q bar{q}$ pairs, i.e., $Delta q_s(x) eq Delta bar q_s(x)$, plays an important role for a comprehensive understanding of the nucleon spin content.
Dielectron production in the $pi N$ interaction at not large energies is studied. The dominant contribution of the $Delta$-isobar creation in the intermediate state at incident pion momenta of about 0.3-0.4 GeV$/$c is shown. The experimental distributions over the angle and effective mass $M_{e^+e_-}$ of the $e^+e^-$ pair are described satisfactorily. This stimulated us to present theoretical predictions for the $M_{e^+e_-}$ distribution in the process $pi^- prightarrow ne^+e^-$ at different incident momenta, which could be verified, for example, by the HADES experiments.
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