Flavor asymmetry of the nucleon
  The flavor asymmetry of the nucleon sea is discussed in an unquenched quark model for baryons in which the effects of quark-antiquark pairs (u anti-u, d anti-d and s anti-s) are taken into account in an explicit form. The inclusion of q anti-q pairs leads automatically to an excess of anti-d over anti-u quarks in the proton, in agreement with experimental data. 
R. Bijker (ICN-UNAM, Mexico) and E. Santopinto (INFN, Genova)