Nuclear Modification of Double Spin Asymmetries

We compute nuclear spin dependent structure functions using a dynamical model for bound nucleon densities and hence calculate nuclear modifications to asymmetries observed in recent doubly polarised deep inelastic scattering experiments. We conclude that while the individual densities are changed substantially by nuclear effects, the asymmetries themselves are largely insensitive to these changes. Recently a model was proposed [1] to explain the observed differences between free nucleon and bound nucleon structure functions in deep inelastic lepton nucleon scattering (DIS). This model used a dynamical approach, involving modifying a free nucleon input density distribution at a low input scale, Q = μ = 0.23 GeV, due to nuclear effects, and then evolving the resultant modified bound nucleon densities to the required Q scale of the experiment. The model gave satisfactory agreement with available data in a fairly broad Q range, from 0.5–30 GeV. It is interesting to ask how this model can be extended to a study of spin dependent bound-nucleon densities. The question is not merely academic as, in fact, data on the spin dependent deuteron and neutron structure functions have been obtained [2] from deuteron and He targets. Nuclear effects in deuteron are known to be small (though measureable), since the deuteron is a loosely bound nucleus. There have been a number of papers [3] dealing with nuclear modifications of spin asymmetries and structure functions in the case of the deuteron. We therefore confine our attention to possible nuclear effects on the double spin asymmetry measurements made with helium nuclei. In this case, it was pointed out by Woloshyn [4] that the protonic contribution to the asymmetry is negligible so that the He double spin asymmetry is sensitive to the spin dependent neutron structure function, g 1 (x,Q ). However, there may be additional modifications due to the presence of the nuclear medium, which we propose to study here. These are especially of importance for checking the validity of the Bjorken Sum rule. Our main conclusion is that the individual (spin independent as well as spin dependent) structure functions undergo substantial modifications due to nuclear effects; however, their ratio—the asymmetry—which is the measured quantity, is largely free from these and so gives hope that the neutron structure function may be unambiguously determined from such a measurement. e-mail: [email protected] Depending on the model, corrections due to nuclear effects in deuterium can be as large as 10%.