Separated longitudinal and transverse cross sections for charged pion electroproduction from ${}^1\mathrm{H}$, ${}^2\mathrm{H}$, and ${}^3\mathrm{He}$ were measured at $Q^2=0.4\mathrm{}(\mathrm{GeV}/c{)}^2$ for two values of the invariant mass, $W=1.15\mathrm{}\mathrm{GeV}$ and $W=1.60\mathrm{}\mathrm{GeV}$, in a search for a mass dependence which would signal the effect of nuclear pions. This is the first such study that includes recoil momenta significantly above the Fermi surface. The longitudinal cross section, if dominated by the pion-pole process, should be sensitive to nuclear pion currents. Comparisons of the longitudinal cross section target ratios to a quasifree calculation reveal a significant suppression in ${}^3\mathrm{He}$ at $W=1.60\mathrm{}\mathrm{GeV}$. The $W=1.15\mathrm{}\mathrm{GeV}$ results are consistent with simple estimates of the effect of nuclear pion currents, but are also consistent with pure quasifree production.

• Received 25 May 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.202301