Small-Angle Photoproduction of Positive Pions from Hydrogen

The angular distribution of ${\pi }^{+}$ mesons produced in hydrogen by 225-Mev photons has been measured in the small-angle region. The mesons were produced in a liquid hydrogen target placed in the x-ray beam of the 300-Mev betatron. A focusing magnet was used to select mesons of the desired momentum. The large electron pair background was avoided by use of a very short x-ray pulse and a counter system which operated only after the pulse, counting the decays of ${\mu }^{+}$ mesons. Points have been obtained at center-of-mass angles of 10, 15, 20, 30, 45, 60, and 90 degrees. The observed angular distribution cannot be fitted by a quadratic in ${cos\theta }^{*}$, but is well fitted by the cross section calculated from the field theory of Chew, Goldberger, Low, and Nambu.