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Off-shell pion structure function and flavor asymmetry in the nucleon sea

We study off-shell effects of the pion cloud on the sea quark distribution in the nucleon. The structure function of the off-mass-shell pion is obtained within the Nambu and Jona-Lasinio model, where the shape of the distribution function depends on the pion momentum. By using the momentum dependent pion structure function, the SU(2) flavor asymmetry in the nucleon see is reexamined within the pionic model, in which the sea quark distribution of the nucleon is given as a convolution of the off-shell pion structure function. We calculate experimentally observed quantities related to the SU(2) asymmetry of the nucleon sea such as the Gottfried sum and the Drell-Yan cross section ratios, and find these quantities are insensitive to the off-shell effects of the pions structure function. However, the $x$ dependence of $\bar{u}-\bar{d}$ calculated with the off-shell structure function shows a clear deviation from the result with the on-shell one. We evaluate also the cross section of semi-inclusive lepton-nucleon process with a slow nucleon production to demonstrate the off-shell effect to be checked experimentally.