Total Hadronic (γ,p) and (γ,d) Cross Sections from 4 TO 18 GeV

Abstract

Photoabsorption cross sections in hydrogen and deuterium have been measured from 3.7 to 17.9 GeV. The energy dependences are similar to those of strong-interaction total cross sections, as expected from the vector-meson-dominance model. The magnitude of (T T (yp) can be compared with data from yp~~p°p to determine a y-p coupling constant, y p 2 /47r = 0.37 ±0.03. This value disagrees with that obtained on the p mass shell, and hence there is only qualitative agreement with the vector-meson-dominance model.The total cross sections of hadrons on nucleons have provided important information about both the strong interactions and the particles involved. Similar data for photon-nucleon total cross sections has been lacking, however, because the interesting hadronic final states are two orders of magnitude less frequent than the background electromagnetic states. This problem has been overcome in the experiment reported here to the extent that total cross sections with systematic errors less than 2 % have been obtained for photoabsorption in hydrogen and deuterium over the energy range 3.7 to 17.9 GeV. The magnitude and energy dependence of the resulting (y,p) cross sections provide a check of vector-meson dominance. The following paper 1 describes the evaluation of the (y, n) cross section from the hydrogen and deuterium measurements and gives an interpretation of the data in terms of a Reggepole model.Previously we have reported 2 the dependence of the high-energy (7 to 18 GeV) photoabsorption cross sections on the size of the target nucleus, which showed that the photon does not behave in a purely electromagnetic manner, but rather is absorbed more like a strongly interacting particle in passing through a nucleus. Since the experimental arrangement has been described, 2 we shall give here only a brief outline of the setup used at the Stanford Linear Accelerator Center and discuss in addition those features of the experiment and its analysis which are particularly pertinent to obtaining reliable low-energy data.The experiment was performed with tagged photons. The tagged portion of the bremsstrahlung beam, which was produced by positrons of energy E QJ included four energy bins between 0.74£ 0 and 0.94E 0 . As shown in Fig. 1, wideangle photons were vetoed by the shower counter 609 A 0 , while a photon passing through the target also went through central holes in the hadron detectors S2a and S2b and produced a large pulse in the total-absorption anticounter, SI. The major background, pair production, had the same signature as a noninteracting photon, since pairs had a sufficiently small angle (typically 0.2 mrad or less) to pass through the 52 holes. Hadronic interactions were identified by a signal from one of the tagging channels in coincidence with a signal from the hadron detectors S2, with no large coincidence pulse from SI. The S2 counters, each a four-luyer sandwich of 2.5-cm lead followed by scintillator, were required to give either a four-fold coincidence (from S2a or S2b) or a total pulse height indicating...

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