Target and beam-target spin asymmetries in exclusive pion electroproduction for $Q^2>1 {\mathrm{GeV}}^2$. II. $ep\to e{\pi }^{0}p$

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive ${\pi }^0$ electroproduction reaction ${\gamma }^{*}p\to p{\pi }^0$, expanding an analysis of the ${\gamma }^{*}p\to n{\pi }^{+}$ reaction from the same experiment. The results were obtained from scattering of 6-GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The kinematic ranges covered are $1.1<W<3$ GeV and $1<Q^2<6 {\mathrm{GeV}}^2$. Results were obtained for about 5700 bins in $W, Q^2, cos\left({\theta }^{*}\right)$, and ${\varphi }^{*}$. The beam-target asymmetries were found to generally be greater than zero, with relatively modest ${\varphi }^{*}$ dependence. The target asymmetries exhibit very strong ${\varphi }^{*}$ dependence, with a change in sign occurring between results at low $W$ and high $W$, in contrast to ${\pi }^{+}$ electroproduction. Reasonable agreement is found with phenomenological fits to previous data for $W<1.6$ GeV, but significant differences are seen at higher $W$. When combined with cross-sectional measurements, as well as ${\pi }^{+}$ observables, the present results will provide powerful constraints on nucleon resonance amplitudes at moderate and large values of $Q^2$, for resonances with masses as high as 2.4 GeV.

Figure 1

Two-photon invariant mass distributions for (a) $ep\to e\gamma \gamma p$ and (b) $ep\to e\gamma \gamma \left(p\right)$, with all relevant exclusivity cuts applied. The vertical dashed lines show the cuts used for $ep\to e\gamma \gamma p$. The solid (dashed) curve in the lower panel is with (without) the application of the ${\chi }^2$ cut discussed in the text.

Figure 2

Electron-pion missing mass spectra for the topologies (a) $ep\to e\gamma \gamma \left(p\right)$ and (b) $ep\to e\gamma \gamma p$. Counts from the ammonia target are shown as the solid circles and counts from the carbon target (scaled by the ratio of integrated luminosities on bound nucleons) are shown as the open circles. All other applicable exclusivity cuts have been applied. The vertical dashed lines indicate the cuts used.

Figure 3

Distribution of ${\left(M_x^{eN}\right)}^2$ for (a) the topology $ep\to e\gamma \gamma p$ and (b) the topology $ep\to e\gamma \left(\gamma \right)p$. Symbols are as in Fig. 2. The vertical dashed lines show the cuts used. All other relevant exclusivity cuts have been applied.

Figure 4

Distribution of the in-plane (out-of-plane) angular difference in the predicted and observed proton direction cosines for the topology $ep\to e\gamma \gamma p$ are shown in the upper (lower) panel. The solid black points are for the ammonia target, while the open circles are from the carbon target, scaled by integrated luminosity. The vertical dashed lines indicate the cuts used in the analysis. All other relevant exclusivity cuts have been applied.

Figure 5

Distributions of electron-proton-photon missing mass squared for the $ep\to e\gamma \left(\gamma \right)p$ topology for (a) $1.1<W<1.45$ GeV and (b) $2.15<W<2.5$ GeV. Symbols are as in Fig. 2. The vertical dashed lines show the cuts used. All other relevant exclusivity cuts have been applied.

Figure 6

Distributions of angular difference between the predicted and measured photon (horizontal axis) vs electron-proton-photon missing energy (vertical axis) for the $ep\to e\gamma \left(\gamma \right)p$ topology. Panel (a) is for photons with energy greater than 2 GeV, with the remainder of the events in panel (b). The dashed lines indicate the two cuts used in the analysis. All other exclusivity cuts have been applied.

Figure 7

Distribution of $M_{\gamma \left(\gamma \right)}$ for the topology $ep\to e\gamma \left(\gamma \right)p$. Symbols are as in Fig. 2. The vertical dashed lines show the cuts used. All other relevant exclusivity cuts have been applied.

Figure 8

Dilution factors as a function of $W$ for the $ep\to e\gamma \gamma p$ topology (solid curves), the $ep\to e\gamma \left(\gamma \right)p$ topology (long dashed curves), and the $ep\to e\gamma \gamma \left(p\right)$ topology (short dashed curves) for the four $Q^2$ bins of this experiment and a typical bin in $cos\left({\theta }^{*}\right)$. For the two sets of dashed curves, smaller values of $f$ correspond to higher values of $Q^2$.

Figure 9

Beam-target double spin asymmetry $A_{LL}$ for the reaction $ep\to e{\pi }^{0}p$ as a function of ${\varphi }^{*}$ in seven bins in $W$ (columns) and six $cos\left({\theta }^{*}\right)$ bins (rows). The results are from the two lower $Q^2$ bins of this analysis. The column headings include the average value of virtual photon polarization $\epsilon$. The error bars reflect statistical uncertainties only. The solid red curves are from the MAID 2007 fit [13] and the blue dashed curves are from a JANR fit [14].

Figure 10

Same as Fig. 9, except for the two larger $Q^2$ bins of this analysis.

Figure 11

Target single-spin asymmetry $A_{UL}$ for the reaction $ep\to e{\pi }^{0}p$ as a function of ${\varphi }^{*}$ in seven bins in $W$ (columns) and six $cos\left({\theta }^{*}\right)$ bins (rows). The results are from the two lower $Q^2$ bins of this analysis. The error bars reflect statistical uncertainties only. The solid red curves are from the MAID 2007 fit [13] and the blue dashed curves are from a JANR fit [14].

Figure 12

Same as Fig. 11, except for the two larger $Q^2$ bins of this analysis.