High-energy muon-nucleus interactions at small four-momentum transfers

In an experiment carried out at CERN, inelastic scattering of muons with energy 280 GeV has been studied using targets of hydrogen, deuterium, carbon and calcium. The purpose of the experiment was to measure the cross section in the kinematic region of low momentum and large energy transfer from the muon to the target. This region was made accessible by using a specially designed trigger system capable of selecting muons scattered as little as 2 milliradian by the target. The nucleon structure function F2 was measured on deuterium in the range, 0.002 < x < 0.15, of the scaling variable and in the range 0.2 < Q2 < 8 (GeV/c)2, of the absolute value of the squared four-momentum transfer. No significant dependence of F2 on x is observed, while the dependence on Q2 behaves as has been observed in the so called deep inelastic scattering at higher Q2 ( Q2 > 7 (GeV/c)2 ). Comparing the inelastic cross sections, a very large energy transfer above 100 GeV shows a significant depletion of the cross section obtained on calcium, relative to those obtained on deuterium and on carbon. Comparison of the x dependences of the cross sections on calcium and on carbon or deuterium shows a more pronounced difference for data selected above Q2 = 1 (GeV(c)2 than below. This indicates that the observed differences are due to correlations between quarks in different nucleons rather than to the absorption of the hadronic components of the exchange photon by nucleons (shadowing). (author)