Pi NN form factor for explaining sea-quark distributions in the nucleon.

Abstract

We reinvestigate a soft \ensuremath{\pi}NN form factor with a monopole cutoff parameter ${\mathrm{\ensuremath{\Lambda}}}_{1}$0.5 GeV obtained by Frankfurt, Mankiewicz, and Strikman for explaining sea-quark distributions in the nucleon. It is found that their estimate is small in comparison to the limit ${\mathrm{\ensuremath{\Lambda}}}_{1}$0.65--0.7 GeV estimated in this investigation. Studying processes involving \ensuremath{\pi}NN and \ensuremath{\pi}N\ensuremath{\Delta} vertices, we find that the limit for the dipole form factor is ${\mathrm{\ensuremath{\Lambda}}}_{2}$0.95 GeV (which corresponds to ${\mathrm{\ensuremath{\Lambda}}}_{1}$0.6 GeV). Therefore, a typical \ensuremath{\pi}NN form factor with ${\mathrm{\ensuremath{\Lambda}}}_{1}$\ensuremath{\sim}0.6 GeV in quark models could be consistent with deep-inelastic experimental data at this stage. However, it is softer than a \ensuremath{\pi}NN form factor with ${\mathrm{\ensuremath{\Lambda}}}_{1}$\ensuremath{\sim}1 GeV widely used in nuclear physics. Using the cutoff ${\mathrm{\ensuremath{\Lambda}}}_{2}$=0.95 GeV, we investigate pionic contributions to a SU(2${)}_{\mathit{f}}$-breaking distribution u\ifmmode\bar\else\textasciimacron\fi{}(x)-d\ifmmode\bar\else\textasciimacron\fi{}(x) in the nucleon. We find that the pionic contributions to u\ifmmode\bar\else\textasciimacron\fi{}(x)-d\ifmmode\bar\else\textasciimacron\fi{}(x) are negative and a contribution to the deviation from the Gottfried sum rule is estimated to be -0.041, which could explain a part of the discrepancies indicated by recent New Muon Collaboration experiments.