Spin dependent parton distributions in a bound nucleon

The determination of the spin distribution functions for the neutron requir es an understanding of the changes induced by a nuclear medium. We present the first estimates of the changes induced in the spin-dependent, valence p arton distributions of the proton and neutron bound in He-3 and Li-6. For H e-3 the result is quite reassuring, in that the off-shell correction to g(1 n) is very small. On the other hand, in Li-6 the corrections to the spin di stributions in the bound proton are sufficiently large that they should be taken into account if one aims for a precision of better than 10%. We provi de a simple parametrization of the off-shell changes in the valence spin di stributions of He-3 and Li-6, which can be used with any conventional estim ate of nuclear binding and Fermi motion corrections. (C) 1999 Published by Elsevier Science B.V. All rights reserved.