In an experiment performed at the Deutsches Elektronen-Synchrotron (DESY) 6.2-GeV electron synchrotron we have studied the production, at forward angles, of TT+TT" pairs from the reaction y+A~ A + TT +77(1)Measurements were made for various target nuclei A (Be, C, Al, Cu, Ag, and Pb), momenta of the 3.04, 3.5, and 4.5 GeV/c), and peak incident bremsstrahlung energies & max (4.35 and 6.02 GeV). The objects of the experiment were fourfold: first, to search for possible 0 + , 1~~, 2 + , • • • resonances by measuring the pair invariantmass distribution in the region 0.35-1.2 GeV/ c 2 ; second, to study the mechanism by which p° mesons are produced at high energy and low momentum transfers, where diffraction seems to dominate 1 ; third, to determine the p°-N cross section using the diffraction model; and fourth, to determine the mass and width of photoproduced p° mesons.The TT+TT-pairs were detected by a doublearm magnetic spectrometer which has been described in a previous publication. 2 Through scintillation-counter hodoscopes, the kinematics of each event was then defined within the following uncertainty: 6p/p = ±2%, 66/6 = ±3%, and 6m/m =±2%. The separation of TT + TT~" pairs from the background of e + e~ pairs was accomplished by four large-aperture threshold Cherenkov counters and two lead-Lucite shower counters; protons were rejected by time-offlight techniques.In the kinematical region accepted by our spectrometer (0 o = 4°-17.2°, p 0 = 1.2-2.25 GeV/ c, ^m ax = 4 -35 or 6.02 GeV), our measurements show that, for TT+TT-pair masses from 0.350 to 1.20 GeV/c 2 , Reaction (1) is dominated by p°-meson production. Within the statistical accuracy, 5%, no other TT + TT"~ enhancements were observed. Therefore, as described below, we have used Reaction (1) to study the differential p° production cross section and to gain information as to the production mechanism of p° mesons.To study the mechanism by which p° mesons are produced at high energy and low momentum transfer, we compare the data with the predictions of the diffraction models of Berman and Drell, Drell and Trefil, and Ross and Stodolsky, 3 in which the forward p° production cross section may be expressed (in the laboratory system) as
da/cm = C(A)p*f>(p)f T (R,t,G pN ),(where p is the momentum of the p° meson, R = r 0 A us is the nuclear radius, Op^ is the total p°-nucleon absorption cross section, and t is the square of the momentum transfer to the nucleus. C(A) is a normalization constant. The function/'(p) is proportional (following Berman and Drell) to p(p 2 + m 2 )~x /2 a 7l N 2 (p), where ^^jsjiP) is the total 7r-nucleon cross section.The function/j, describing the production and reabsorption of p° mesons by the target nucleus, is found as follows: The forward production amplitude/(0°) on each individual target nucleon is attenuated by a factor exp(-20pN® x ) where x = distance traveled by p° inside the nucleus after its production, and D = nucleon density function, a constant inside a sphere of radius R, zero elsewhere. Then, integrating over the nuc...