Measurement of Shadowing in Photon-Nucleus Total Cross Sections from 20 to 185 GeV

Abstract

We have measured total hadronic photoproduction cross sections on carbon, copper, and lead. Tagged-photon energies ranged from 20 to 185 GeV for copper and from 45 to 82 GeV for carbon and lead. The energy and A dependence of shadowing were computed by comparing these results to the hydrogen cross section as measured nearly simultaneously with the same apparatus. We observed somewhat more shadowing than did most experiments at lower photon energies.The photoproduction cross section from complex nuclei should be less than the sum of individual nucleon cross sections because (naively) some nucleons "shadow" others by absorbing out the hadronic part of the photon beam. This effect has been observed in photoproduction by real photons of up to 18 GeV. 1 " 6 Although the results of Ref. 1 disagreed with the vector-meson-dominance (VMD) model used in that paper, it is possible to find models 7 that do give reasonable agreement with the shadowing observed in photoproduction. VMD models do, however, have difficulty accounting for the rapid decrease of shadowing when the photons become slightly spacelike. 8 More data will be useful for suggesting the direction in which models must be elaborated.We have measured the dependence of the total photoproduction cross section on A, the atomic weight of the target nucleus. One reason for doing so was to get a more accurate measurement of shadowing than has been hitherto possible. The cross section is easier to measure at high energies and our apparatus was designed to achieve the very high precision needed for detecting the small energy dependence of the hydrogen cross section. 9 Another reason for doing this measurement was to extend the energy range of shadowing data. Such an extension could, for example, show effects of higher-mass states. The higher the mass of a vector state, the higher the photon energy must be in order that the state contribute to shadowing. In fact, neutron and K L cross sections on nuclei do show an increase in the amount of shadowing with increasing energy,, 10 " 12 This increase can be interpreted as an effect of "inelastic screening" 13 of the forward scattering amplitude, in which an incident hadron diffractively dissociates (into a possibly higher-mass state) at one point within the nucleus and recombines at another. In VMD calculations of shadowing, this would correspond to including off-diagonal terms.The measurement was performed in the Fermilab tagged-photon beam. Tagged photons were produced from copper radiators of 6 and 15 mils (0.0107 and 0.0267 radiation lengths). The beam, detection apparatus, and trigger were the same as used in a measurement of the total photoproduction cross section on hydrogen. 9 However, the hydrogen target was replaced by carbon, copper, and lead targets-each a rectangle larger than the beam and each of approximately 0.1 radiation length in thickness. These targets were mounted in a vacuum box so that each could be rotated into the beam. An empty target slot permitted measurement and subtraction of the rate of hadronic...

Precise Measurement of the Nuclear Dependence of Structure Functions in Light Nuclei

“…Their results were consistent with the results from the EMC collaboration which were taken at much higher Q 2 . Also, the low-xBj data were consistent with earlier photoproduction data from [24].…”

The A dependence of the nuclear structure function ratios

“…Fig. 5 also shows the results for real (Q2 = 0 GeV 2) photons of 60 GeV [24,25]. These are available, however, only for deuterium, carbon, copper and lead; we therefore used a power-law interpolation (o-c< A '~) to obtain the values shown.…”

Compton scattering from nuclei and photo-absorption sum rules

“…. Data are from [27] for the proton and the deuteron, and from [22][23][24] for heavier nuclei. The Regge plus Pomeron curves are shown by dashed lines.…”

“…[25] for a comprehensive review of GDR data and theory). As an example, the 207 Pb data in the nuclear range are plotted along with the higher energy [20]; data in the hadronic and high energy range 0.2 GeV≤ ν ≤100 GeV are from [21][22][23][24]. Nuclear deformations are responsible for the giant resonance that saturates the cross section for ν < ∼ 100 MeV (region I).…”