Is there a s trong enhancement of the sea inside the nucleus. Laboratoire de Physique Theorique, Universite de Nice, Pare Valrose, France (x)

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1 645 s there a s trong enhancement of the sea inside the nucleus Y. GABELLN Laboratoire de Physique Theorique Universite de Nice Pare Valrose France (x) ABSTRACT We show that we can extract from Drell-Yan processes some knowledge of the strength of the sea inside the nucleus. (%) Equipe de Recherche Associee au C.N.R. s.

2 646 We present briefly a possible and clean test of the strength of the! sea inside the nucleus ( ). n other words is the E.M. C. effect (2) a sea effect The answer can be given by measuring the ratio of Drell-Yan cross sections for proton nucleus e e and proton-deuterium e processes : R' (Q 2 y) P-N () dq2 dy where Q and y are the mass and rapidity o f the lepton pair and the cross sec tions have been normalized to one nucleon. ndeed let ' s compute first this ratio using a naive Drell-Yan model d G" naive dq 2 dy (2) where x 1 refers to the proton Bj erken variable x2 to the target one (nucleus or deuterium) ; we assume a constant K factor which is naive. We take for the kinematical range x 1 ) 2 which excludes any sea contribution from the proton structure functions and x2 <(. 2 ; due to the fact that the nucleus contains almost no u and d valence quarks we obtain for the structure functions products (3) where v stands for valence and s for sea. We get :. R ' na1ve s N (x2 ) (4) o -- s (x2 ) So R' appears to be a good quantity to test the strength of the sea inside the nucleus at least at the simplest level of the theory. Let ' s come now to a more realistic computation in which d () dq 2dy K( Q2 y) [ q (x - l ) q (x2 ) + (q... q ) ] (5)

3 647 The K factor is obtained by using the techni µes developed in ref ( 3 ) where the infrared singularities have been exponentiated and which lead to cross sections! in very good agreement with experimental data ( ). To compute R ' we use two models for the nucleon quarks densities )_ inside the nucleus - among all the models available (4 6 which fit the E.M. C. (2) and SLAC data. The first model will be the one presented by J. Szwed (5) referred as (6) KPS model which is a "weak sea" model in which (6) p where s (x) is the sea of the free proton ( ). t would lead to R' naive.. The second model is a parametrization a la Jaffe (8) where there is an enhancement of 40% for the sea of the nucleon in the nucleus : (7) and where the valence of the nucleus is shifted towards lower x (8) t would lead to R naive 1.4. F 2N (x) with these two models F2 D (x) for the nucleon structure functions are plotted on Fig.. We haven' t included the Fermi motion in the model a la Jaffe as we are interested by the low x2 values. The results of the computation of R We have plotted R' on fig.2a as a function of the rapidity at s 800 Gev2 for Q2 25 and 50 GeV2 using the two model s. We notice that this ratio is almost flat in y and Q for the KPS parametrization and is around as predicted by our crude analysi s whereas for the strong sea mode l the dependence in y and Q is much stronger and so is R' which is compatible with ( R' ). 1.4 for q 2 25 GeV2 and around. 7 at q 2 50 Gev2

4 648 y Fig.Zbshows the dependence on the mass of the lepton pair at fixed 0.6 which exhibits the same features as in fig.2a. Conclusion : Drell-Yan processes seem to allow a c1e.ar discrimination between weak sea and strong sea effects in the nucleus. REFERENCES. The results presented here are preliminary computations. For a more complete analysis see Y.GABELLN J.L.MEUNER and G.PLAUT Nice Preprint NTH 84/5. 2. J.J.AUBERT et al. Phys. Lett. 123B (1983) 275. R.G.ARNOLD et al. SLAG-PUB 3257 (1983) 3. P.CHAPPETTA T.GRANDOU M.LE BELLAC and J.L.MEUNER Nucl. Phys. B207 (1982) R.PESCHANSK these proceedings and F.E.CLOSE these proceedings and references therein. 5. J.SZWED these proceedings. 6. J.KUBAR G.PLAUT J.SZWED Nice preprint NTH 84/1. 7. J.J.AUBERT et al. Phys. Lett. 105B (1981) 315 and Phys. Lett. 123B (1981) R.L.JAFFE Phys. Rev. Lett. 50 (1903) 245.

5 649 R 0 ' 1.1 t } E MC S L AC x f.9 ' R N F --1. as a function of D F x at q 2 ' 2 25 Gev. Ful l l ine : 2 R ob tained with the KPS model. Dashed line : R obtained with. the model a la Jaf f e. Data from references ( 2 ).

6 650 R' / / " ' 6 P-N d o P-D as a function of y for s 800 GeV2 and R' - d 2 dq 2 dy dq dy GeV. Full lines : the KPS model predictions; upper curve : q2 25 Gev2 lower curve : q2 50 GeV2. Dashed lines : those of t e model a la Jaffe ; upper curve : q 2 50 GeV2 lower curve : Q 25 Gev2. R' z. " / a R ' as a function of Q for s 800 Gev2 J GeV y Full line : the KPS model predictions. Dashed lin1a : model a la Jaffe.