Exclusive $\pi ^{0}$ Electroproduction in the Resonance Region

Abstract

The exclusive electroproduction process $ep \rightarrow e'p'\pi ^{0}$ was measured in the range of the photon virtualities $Q^{2} = 0.4$–1.0 GeV$^{2}$, and the invariant mass range of the $p\pi ^{0}$ system $W = 1.1$–1.8 GeV. For the first time these kinematics are covered in exclusive $\pi ^{0}$ electroproduction off the proton with the nearly complete angular coverage in the $p\pi ^{0}$ center of mass system with high statistics. The cross section was measured and the structure functions $\sigma _T+\epsilon \sigma _L$, $\sigma _{TT}$ and $\sigma _{LT}$ were extracted via fitting the $\phi ^{*}_{\pi ^{0}}$ dependance. Analysis of these results has revealed the data sensitivity to the contribution from the nucleon resonances $N(1650)1/2^-$, $N(1685)5/2^{+}$, and $\varDelta (1700)3/2^{-}$. Combined studies of $\pi ^{+}n$ and $\pi ^{0}p$ electroproduction off proton data from CLAS at $W>1.6$ GeV will provide the first results on the high lying $N^{*}$ and $\varDelta$ electrocouplings at $Q^{2}$ < 1.0 GeV$^{2}$ for all excited nucleons with substantial decays to the $N\pi$ final states. These new experimental data will extend the insight into the complex interplay between the inner quark core and outer meson–baryon cloud in the structure of nucleon resonances with masses above 1.6 GeV.