Beam-Helicity Asymmetries in Double-Charged-Pion Photoproduction on the Proton

Beam-helicity asymmetries for the two-pion-photoproduction reaction $\overrightarrow{\gamma }p\to p{\pi }^{+}{\pi }^{-}$ have been studied for the first time in the resonance region for center-of-mass energies between 1.35 and 2.30 GeV. The experiment was performed at Jefferson Lab with the CEBAF Large Acceptance Spectrometer using circularly polarized tagged photons incident on an unpolarized hydrogen target. Beam-helicity-dependent angular distributions of the final-state particles were measured. The large cross-section asymmetries exhibit strong sensitivity to the kinematics and dynamics of the reaction. The data are compared with the results of various phenomenological model calculations, and show that these models currently do not provide an adequate description for the behavior of this new observable.

Figure 1

Angle definitions for the circularly polarized real-photon reaction $\overrightarrow{\gamma }p\to p{\pi }^{+}{\pi }^{-}$; ${\theta }_{\mathrm{cm}}$ is defined in the overall center-of-mass frame, and $\theta$ and $\varphi$ are defined as the ${\pi }^{+}$ polar and azimuthal angles in the rest frame of the ${\pi }^{+}{\pi }^{-}$ system with the $z$ direction along the total momentum of the ${\pi }^{+}{\pi }^{-}$ system (helicity frame).

Figure 2

Angular distributions for selected center-of-mass energy bins (each with $\Delta W=50\mathrm{MeV}$) of the cross-section asymmetry for the $\overrightarrow{\gamma }p\to p{\pi }^{+}{\pi }^{-}$ reaction. The data are integrated over the detector acceptance. The statistical uncertainties are mostly smaller than the symbol size. The solid and dotted curves are the results from model calculations by Mokeev et al. [19, 20, 21] (for $1.45\mathrm{GeV}\le W\le 1.80\mathrm{GeV}$) with relative phases of 0 and $\pi$ between the background- and $\pi \Delta$-subchannel amplitudes, respectively. The dashed curves show results of calculations by Fix and Arenhövel [23] (for $W\le 1.70\mathrm{GeV}$).

Figure 3

Helicity asymmetries at $W=1.50\mathrm{GeV}$ for nine bins of the invariant mass $M(p{\pi }^{+})$, as indicated. The solid curves are the results of Mokeev et al. [19, 20, 21]. The dashed curves show results of calculations by Fix and Arenhövel [23].

Figure 4

Helicity asymmetry as a function of the invariant mass $M(p{\pi }^{-})$ for $W=1.55\mathrm{GeV}$ (filled circles) and 1.95 GeV (open triangles) and a 30°-wide $\varphi$-angle range centered at $\varphi =105°$. The curves are the results of Mokeev et al. [19, 20, 21] (solid line) and Fix and Arenhövel [23] (dashed line) for $W=1.55\mathrm{GeV}$ only. Note that the result of Fix and Arenhövel is in phase with the data (filled circles) and that of Mokeev et al. is not. The vertical lines indicate the masses of the known $N$ and $\Delta$ resonances.