Physics Letters B 272 (1991) 359-362 PHYSICS LETTERS 13 North-Holland The Gottfried sum rule and mesonic exchanges in the deuteron L.P. Kaptari Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Head Post Office, P.O. Box 79, SU-IOI O00Moscow, USSR and A.Yu. Umnikov Far Eastern State University, Sukhanov street 8, Vladivostok, USSR Received 11 July 1991; revised manuscript received 12 September 1991 Recent NMC data on the experimental value of the Gonfried sum are discussed. It is shown that the Gonfried sum is sensitive to the nuclear structure corrections, viz. the mesonic exchanges and binding effects. A new estimation of the Gottfried sum is given. The obtained result is close to the quark-parton prediction of -~. 1. Recently, the NMC data [1,2] on the ratio F~/F~ have been applied to derive the difference F~-F~ and to estimate the Gottfried sum (GS) Sc,=-f(F~-F~ )cLv/x [3] experimentally. Its value has been found to be below the quark-parton model expectation of -~, namely S~ =0.240_+0.016. (1) Serious theoretical speculations have appeared as a consequence of this discrepancy, e.g. the strong iso- spin violation in the proton sea-quark distribution [4] or the postponement of the onset of the Regge behavior to much smaller x values than have cur- rently been sampled experimentally [ 5 ]. Note that the experimental value of the GS is sen- sitive to the procedure of extraction of the ratio n p Fz/F 2 from the combined data on the deuteron and the proton. Since the deuteron is a more complicated system than a simple sum of two free nucleons, a number of structure factors may change the ratio F~/F~. At least one should be careful while consid- ering the influence of widely discussed nuclear ef- fects, such as Fermi motion, binding effects and me- sonic exchanges in nuclei. Though in the integral characteristics of nuclear structure functions (SF) these corrections are small, it is not evident that they can be neglected in the procedure of determination of the neutron SF F~(x) from the nuclear data. Moreover, the recent analysis of BCDMS data on the proton and the deuteron performed in ref. [6] has shown the noticeable influence of the deuteron struc- ture factors on the extracted neutron SF and the ratio F~/F~. It seems, the same corrections can also be ex- pected for the NMC data. The aim of this letter is to demonstrate that in the framework of the theoretical approach suggested in refs. [7] it is possible to extract the neutron SF so that the obtained value of the GS does not dramati- cally contradict the quark-parton predictions. It is shown that the nuclear corrections change the behav- ior of the difference F2-F2P n as x~0 as compared with the prediction of the NMC experimental data fit. 2. Since the SF have been measured not in the whole region of the scale variable x, it is useful to define the x-dependent Gottfried integral: l~(x,-x2) = ~i (F~-F~) dx, (2) J X xi and separately evaluate it in the measured and un- 0370-2693/91/$ 03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved. 359