Photoproduction of ${\varvec{\pi }}^{{\varvec{-}}}{\varvec{\varDelta }}^{{\varvec{++}}}$, ${\varvec{\pi }}^{{\varvec{+}}}{\varvec{\varDelta }}^\mathbf{0 }$, and ${\varvec{\pi }}^{{\varvec{+}}}{} \mathbf{n}$ on the Proton at ${\varvec{E}}_{{\varvec{\gamma }}}=\mathbf{1.5}$–3.0 GeV

Abstract

We carry out photoproduction experiments by using linearly polarized photon beams with energies of 1.5–3.0 GeV at SPring-8/LEPS. There is no distinct peak structure due to nucleon or $\varDelta$ resonances in the preliminary cross sections for the $\gamma p$ $\rightarrow$ $\pi ^{-}\varDelta ^{++}$, $\pi ^{+}\varDelta ^{0}$, and $\pi ^{+}n$ reactions for 0.7$<\cos \theta _{\pi }<$1. The cross section ratios $\sigma$($\pi ^{+}\varDelta ^{0}$)/$\sigma$($\pi ^{-}\varDelta ^{++}$) are close to 1/3, which is expected from the isospin = 1 exchange in the t-channel, at small $\pi$ angles and become larger than 1/3 at large $\pi$ angles. The photon beam asymmetries are small negative values, small positive values, and large positive values for the $\pi ^{-}\varDelta ^{++}$, $\pi ^{+}\varDelta ^{0}$, and $\pi ^{+}n$ reactions, respectively. The asymmetries are sensitive to reaction mechanisms and are found to be largely different among these reactions. The asymmetries are strong constraint in theoretical studies and expected to play an important role in searching for new nucleon resonances.