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Abstract
The $\sp4$He to D structure function ratio was measured in deep inelastic muon-nucleus scattering at an incident muon momentum of 200 GeV. The kinematic range 0.0035 $<$ x $<$ 0.65 and 0.5 $<$ Q$\sp2$ $<$ 90 GeV$\sp2$ was covered, where x is the Bjorken scaling variable and $-$Q$\sp2$ the mass of the virtual photon squared. The experiment was carried out by the New Muon Collaboration (NMC) at CERN, the European Laboratory for Particle Physics.
At small x the ratio is significantly smaller than unity; the size of the depletion grows with decreasing x and amounts to 7% at x = 0.0035. At intermediate x, x $\approx$ 0.1, the ratio shows some enhancement above unity (by about 1%). The results do not depend significantly on Q$\sp2$.
An extensive review of the experimental and theoretical status in the field of nuclear effects in deep inelastic scattering is given. The present results, as well as those for the ratios of the carbon and calcium to deuterium structure functions, also measured by the NMC, are compared with the available theoretical predictions.
