Electroproduction of Rho and Phi Mesons

We report measurements of p” and $ electroproduction at 19.5 GeV in a wide-aperture spectrometer which detected the scattered electron and the decay products of the vector mesons. As Iq 2 I increases, the rho mass spectrum shape changes, the momentum transfer distribution broadens, and the ratio of the rho cross section to the total cross sec,“i tion decreases. The ratio of longitudinally to transversely polarized . . po mesons is .45 T : :i at lq21 = rni, and the interference between longitudinal and transverse amplitudes is almost maximal. The relative + meson cross section also decreases as lq21 increases. (Submitted to Phys. Rev. Letters) *Work supported by the U. S. Atomic Energy Commission. **Present address: Birmingham Radiation Centre, University of Birmingham, P. 0. Box 363, Birmingham B15 2TT, England. TPresent address: Vanderbilt University, Nashville, Tennessee 37203. t”fPresent address: Rutherford High Energy Laboratory, Chilton, Didcot, Berkshire, England. Many photoproduction processes can be understood by assuming that the photon couples directly to vector meson states. By studying vector meson electroproduction we seek to determine how this coupling evolves as the photon becomes space-like and its polarization has longitudinal as well as transverse components. We report here a measurement of rho electroproduction which combines high virtual photon energies and the ability to study rho production and decay angular correlations. 1 The rho electroproduction reaction +ep ep n r (1) can be regarded as an inelastic electron scatter (e ey*) followed by the virtual photoproduction of a rho (y*p pp) followed by the rho decay b --r $n-). Quantities describing the electron scatter are q’, the photon mass squared, E , the photon polarization, and s, the c. m. energy squared in the y*p collision. The rho production is characterized by t’, the four momentum transfer squared to the proton less its smallest possible value (tmin), and $,, the azimuthal angle between the electron scatter plane and the rho production plane. The final ~‘rsystem is described by its invariant mass, rnrr, @, the azimuthal angle between the rho production plane and the rho decay plane, and 0, the rho decay polar angle. The angle $ s $e+ $ is the angle between the electron scatter plane and the rho decay plane in the limit t’ -0. We use the same angle conventions as Dieterle . 2 The experimental apparatus, which has been described elsewhere, ’ consisted of a 19.5 GeV/c electron beam incident on’s hydrogen target followed by a large-aperture magnetic spectrometer. The optical spark chambers were triggered by an array of shower counters each time an incident electron scattered more than 30 mrad from the beam with an energy E1 2 4 GeV.