Measurement of the helicity asymmetry $E$ in $\omega \to {\pi }^{+}{\pi }^{-}{\pi }^0$ photoproduction

The double-polarization observable $E$ was studied for the reaction $\gamma p\to p\omega$ 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, $\omega \to {\pi }^{+}{\pi }^{-}{\pi }^0$, using a circularly polarized tagged-photon beam with energies ranging from the $\omega$ 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 subthreshold $N\left(1720\right)3/2^{+}$ nucleon resonance. To describe the entire data set, which consisted of $\omega$ differential cross sections and a large variety of polarization observables, further contributions from other nucleon resonances were found to be necessary. With respect to nonresonant mechanisms, $\pi$ 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.

Figure 1

Degree of circular photon polarization as a function of photon energy for the two CEBAF energies of 1.645 GeV (blue) and 2.478 GeV (green).

Figure 2

The $z$-vertex distribution (axis along the beamline) in the FROST-g9a experiment based on about 30% of the total statistics for the full photon energy range. The three peaks for the different targets are clearly visible: butanol, carbon, and polyethylene (from left to right). Also visible are the exit window of the vacuum chamber and an enhancement to the left of the butanol peak where the target cup was attached to a stainless steel tube, which was used to insert the cup into the cryostat.

Figure 3

Left and middle: $\mathrm{\Delta }\beta =|{\beta }_c-{\beta }_m|$ distributions for protons and charged pions, respectively. The blue area indicates the $3\sigma$ cuts according to Eq. (4). Right: The distribution of ${\beta }_m$ versus particle momentum before the $3\sigma$ cuts.

Figure 4

The distribution of ${\beta }_m$ versus particle momentum after the $3\sigma$ cuts on $\mathrm{\Delta }\beta$ according to Eq. (4).

Figure 5

Confidence-level distribution for a one-constraint (1C) fit testing events for a missing ${\pi }^0$. The blue dashed line is based on raw events, whereas the black solid line is based on the final event sample after all corrections.

Figure 6

Examples of invariant ${\pi }^{+}{\pi }^{-}{\pi }^0$ mass distributions in the photon energy range $E_{\gamma }\in [1.5,1.6]$ GeV for events that were subjected to the $Q$-factor fitting (background subtraction). These events survived all kinematic cuts. The solid blue area indicates the background.

Figure 7

Measurement of the helicity asymmetry $E$ in the reaction $\gamma p\to p\omega$ using a circularly polarized photon beam and a longitudinally polarized target. The data are shown in 100-MeV-wide bins for the photon energy range $E_{\gamma }\in [1.1,2.3]$ GeV. The CLAS-FROST results (red circles $•$) are compared with results from the CBELSA/TAPS Collaboration [21], which used the radiative decay mode, $\omega \to {\pi }^0\gamma$ (blue squares $\blacksquare$). The black solid line represents the BnGa PWA solution. The data points include statistical uncertainties only; the total systematic uncertainty is given as bands at the bottom of each distribution.

Figure 8

Measurement of the helicity asymmetry $E$ in the reaction $\gamma p\to p\omega$ using a circularly polarized photon beam and a longitudinally polarized target. The data are shown in 0.25-wide-bins in $cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{\omega }$ and in 100-MeV-wide bins for the photon energy range $E_{\gamma }\in [1.1,2.3]$ GeV. The CLAS-FROST results (red circles $•$) are compared with results from the CBELSA/TAPS Collaboration [21], which used the radiative decay mode, $\omega \to {\pi }^0\gamma$ (blue squares $\blacksquare$). The black solid line represents the BnGa PWA solution. The data points include statistical uncertainties only; the total systematic uncertainty is given as bands at the bottom of each distribution.