An absolute measurement of K^ photoproduction on the proton has been carried out in the threshold region (from 144.7 to 173 MeV) by use of tagged annihilation photons. The measured cross sections, differential in the recoiling-proton energy, are used to perform a multipole analysis which gives a value (-0.5±0.3)x 10""V' w^+ for the dipole amplitude EQ+, in disagreement with low-energy-theorem predictions. Total cross sections and coefficients of the K^ angular distribution are presented.PACS numbers: 13.60.LePion photoproduction amplitudes at threshold are fundamental quantities of jp-wave pion-nucleon physics. Low-energy theorems for soft photons and pions together with the PCAC (partial conservation of axial-vector current) hypothesis allow the expression of the pion photoproduction amplitude in terms of the pion-nucleon coupling constant and the static properties of the nucleon. ^ Charged-pion amplitudes are predicted up to first order in a mjM (the ratio of pion to nucleon mass) expansion. The corresponding experimental values are in good agreement with this approximation. From this comparison one can set limits for terms of higher order in the expansion. These terms are essentially dispersive corrections involving A^* resonances and vector-meson exchange.In the case of neutral-pion photoproduction, only the proton amplitude can be directly measured. Low-energy theorems show that this amplitude vanishes with mjM and they predict contributions up to second order. Within this framework, the electric dipole amplitude £o+(p;r^) is^ -2.4x 10~V/w^+ (these units will be used throughout the text). The mass difference of neutral and charged pions or nucleons induces, through the properties of unitarity and analyticity of the S matrix, an anomaly ("cusp") in the variation of EQ^ipn^) with energy.^'"* The coupling of n^p and n'^n channels (i) produces an imaginary part in the E^+ipn^) amplitude above K^ threshold (151.4 MeV), and (ii) enhances the real part of EQ+ipit^), as compared to its value when mass differences within multiplets are neglected, in the region between the it^ threshold (144.7 MeV) and the n'^ threshold. It is the value of this ideal isospinsymmetric amplitude at threshold which we will quote as EQ+ipjc^) and which is to be compared to the lowenergy-theorem predictions.Until now there has been no reliable experimental information on the threshold cross section, since all existing measurements have been performed with use of bremsstrahlung photon spectra with end-point energies exceeding the reaction threshold by more than 15 MeV. Extraction of the slope of the cross section at threshold from these experiments requires averaging and extrapolation procedures with uncertainties which are only partially accounted for in the quoted error. The current experimental value is EQ+(p7t^) '"-l.S±.0.6,^ in agreement with theoretical predictions.We report here an absolute measurement of neutralpion photoproduction on the proton using a tagged annihilation photon beam and covering the energy region from threshold ...

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