Measurement of the helicity asymmetry $E$ in $\ddot{I}‰\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}\ddot{I}{€}^0$ photoproduction

Abstract

The double-polarization observable $E$ was studied for the reaction ${\hat{I}}^{3}p\to p\ddot{I}‰$ using the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B at the Thomas Jefferson National Accelerator Facility and the longitudinally-polarized frozen-spin target (FROST). The observable was measured from the charged decay mode of the meson, $\ddot{I}‰\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}\ddot{I}{€}^0$, using a circularly-polarized tagged-photon beam with energies ranging from the $\ddot{I}‰$ threshold at 1.1 to 2.3 GeV. A partial-wave analysis within the Bonn-Gatchina framework found dominant contributions from the $3/2^{+}$ partial wave near threshold, which is identified with the sub-threshold $N(1720)3/2^{+}$ nucleon resonance. To describe the entire data set, which consisted of $\ddot{I}‰$ differential cross sections and a large variety of polarization observables, further contributions from other nucleon resonances were found to be necessary. With respect to non-resonant mechanisms, $\ddot{I}€$ exchange in the $t$-channel was found to remain small across the analyzed energy range, while pomeron $t$-channel exchange gradually grew from the reaction threshold to dominate all other contributions above $W\approx 2$ GeV.

12 pages, 8 figures