Beam-Target Helicity Asymmetry for $\overrightarrow{\gamma }\overrightarrow{n}\to {\pi }^{-}p$ in the $N^{*}$ Resonance Region

We report the first beam-target double-polarization asymmetries in the $\gamma +n(p)\to {\pi }^{-}+p(p)$ reaction spanning the nucleon resonance region from invariant mass $W=1500$ to 2300 MeV. Circularly polarized photons and longitudinally polarized deuterons in solid hydrogen deuteride (HD) have been used with the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the $E$ polarization observable for an effective neutron target. These results have been incorporated into new partial wave analyses and have led to significant revisions for several $\gamma n{N}^{*}$ resonance photocouplings.

Figure 1

Reaction vertex position along the beam direction ($z$), reconstructed by tracking ${\pi }^{-}$ and $p$ in CLAS, shown for equivalent-flux data from full (solid blue) and empty (dotted red) targets. Beam entrance and exit windows generate peaks at $-11$ and $-5\mathrm{cm}$, respectively. A target-independent foil in the cryostat generates the peak at $+1\mathrm{cm}$. Regions included in the BKsub and in the KinFit and BDT analyses are indicated.

Figure 2

The $E$ asymmetry for $\gamma +\mathrm{D}\to {\pi }^{-}+p+(p_{\mathrm{miss}})$, averaged over all angles and energies, as determined in the BDT analysis. The uncertainties are statistical and are smallest near the peak of the $|p_{\mathrm{miss}}|$ distribution ($\sim 0.06\mathrm{GeV}/c$).

Figure 3

Angular distributions of $E$ in the $\gamma n\to {\pi }^{-}p$ center of mass (c.m.) for two different $\pm 20\mathrm{MeV}$ invariant mass bins, as determined from BKsub (black open circles), KinFit (red squares), and BDT (green crosses) analyses, respectively. KinFit and BDT points are shifted slightly in angle for clarity.

Figure 4

$E$ asymmetries for $\overrightarrow{\gamma }\overrightarrow{n}\to {\pi }^{-}p$ (blue squares), grouped in $\pm 20\mathrm{MeV}$ invariant mass ($W$) bins, shown with recent PWA fits that include these data: solid red curves from SAID [24], with shaded bands indicating variations across the energy bin, solid black lines from BnGa [25]. Also plotted at three $W$ values (1580, 1900, and 2220 MeV) are previous PWA solutions that did not include the present data set in the multipole search: red-dotted curves from SAID [CM12], based on all data up to 2012 [26], red-dashed curves from SAID [AS25], including all previously published data; grey dot-dashed curves from BnGa [2014-02], based on all data up to 2014 [27], black short-dashed curves from a BnGa PWA using all previously published data.

Figure 5

Argand plots of the $P_{13}nM$ (top) and $G_{17}nM$ (bottom) multipoles from the $\pi$ threshold to $W=2300\mathrm{MeV}$. Solid arrows indicate increasing $W$. SAID and BnGa PWA are shown in the left and right columns, respectively. As in the legend, red diamonds are on-line versions [24, 25, 26, 27], light green crossed squares are fitted to all previously published data, and dark green circles augment these with the new $E$ asymmetries.