Nuclear Physics B316 (1989) 340-354 North-Holland, Amsterdam SHADOWING AND ENHANCEMENT OF QUARK DISTRIBUTION IN NUCLEI AT SMALL x L.L. FRANKFURT and M.I. STRIKMAN Leningrad Nuclear Physics Institute, Gatchina, Leningrad 188350. USSR Received 23 August 1988 We calculate absolute value, x, A dependencies of the nuclear shadowing phenomenon for sea- and valence-quark distributions at small x. The dominance of the enhancement of valence- quark distributions in nuclei is predicted for the EMC effect at larger x. 1. Introduction The study of nuclear shadowing in deep inelastic scattering has a long history (for a review of preQCD approaches see [1]), more recent attempts to account for current experience in QCD are reviewed in [2] (see also [3]). Yet for several years this phenomenon was ignored in most theoretical papers on the EMC effect. Now, with the experimental confirmation [4] of significant nuclear shadowing at rather large Q2 (i.e. in the leading twist) the theoretical discussion of nuclear effects in deep inelastic lepton-nuclei scattering should focus once again on the analysis of small-x physics and its implications for nuclear effects at larger x (x - 0.1). Besides this consideration helps to elucidate the role of characteristics for high energy collision phenomena such as the increase with 1/x of essential longitudinal dis- tances, the role of colour transparency of nuclei etc. In this paper we elaborate our analysis of nuclear shadowing at x-o 0 [2] to calculate the x and A dependencies of nuclear shadowing for singlet and nonsinglet channels. We also calculate the changes of the parton distributions at x < 0.15 resulting from nuclear shadowing at smaller x and find that these changes seem to be a major source of deviation of the nuclear structure function from additivity for x- 0.1. The paper is organized as follows. In sect. 2 we summarize some results of our analysis [2] necessary for the following discussion. In sect. 3 we calculate the x, A dependence of shadowing for the sea distribution and find the results to be in reasonable agreement with the EMC data [4] (sect. 5). We explain that nuclear shadowing effects in heavy quark Q (Q=a, b .... ) distributions in nuclei are suppressed by a factor l/m2Q as compared to the case of light quarks even at 0550-3213/89/$03.50:,¢ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)