Contemporary Laboratory Research Articles

Method for Measuring the Photon-Photon Total Cross Section

In the interaction of a high energy particle with a proton or nucleus, 9 the recoil spectrum of the target at very low momentum transfer is related simply. to the total cross section of the incident particle with the photon. Estimates of the size of the effect plus recent developments in low recoil techniques suggest that measurement of the photon-photon, as well as the 71 and K-photon cross sections, may be possible. (Submitted to Phys. Rev. Letters) * Work supported by the U. S. Atomic Energy Commission. The interaction of one photon with another remains one of the fundamental but unmeasured quantities in particle physics. There has been a considerable discussion, both theoretical and experimental on photon-photon elastic scattering, ’ y Iy-y -Iy, sometimes involving such Faustian methods as (two) simultaneous nuclear explosions. 2 A measurement, of the photon-photon total cross section, however, may in fact be within the scope of ‘more mundane contemporary laboratory techniques and accelerators. The imaginary part of the forward photon elastic scattering would then follow from the optical theorem,of course, leaving only the real part to be found from dispersion relations or other extreme means. Such a measurement may be possible by exploiting the phenomena of Coulomb production (Fig., 1) in which an incoming beam particle (k) strikes a virtual photon (q) in the Coulomb field of a charged target particle Z and makes a final state f. B the beam is highly relativistic and the energy of k is large compared to the t mass of k and the mass M* of the system f, then the reaction takes place with a very small momentum transfer and the virtual photon q may be thought of as a real photon. The process has a characteristic peak at very small q2, reflecting the very long range of the interaction. At high energy the very small value of q2 in the photon propagator may overcome the factor 01 = l/137 in the matrix element relative to processes taking place in the nuclear matter of the target. For the case where f is a single particle (“Primakoff Effect1t)3 the rate is related to the widths r(f k + ti4 and with incident photons has been used to find the widths r (n’--+v) and f (q”--. m) . 5 Generalizations of the effect to an arbitrary system f may be envisaged, such as that recently proposed6 where f is chosen to be the n-a system in order to examine the 8-a interaction outside the range of other hadrons. A basic result of the general analysis 42-67 is the following: If the process f -k + r exists for a real, q2 = 0 photon, then the Coulomb production process at

Textbook errors: Guest column- XIX. The relative reactivity of acetylenes and olefins toward bromine

Many contemporary textbooks and laboratory manuals in organic chemistry either state explicitly or imply that bromine ass much faster to acetylenes than to olefins.