Measurement of $\mathit{ep}\to \mathit{ep}{\pi }^0$ beam spin asymmetries above the resonance region

The beam spin asymmetry (BSA) in the exclusive reaction $\overrightarrow{e}p\to \mathit{ep}{\pi }^0$ was measured with the CEBAF 5.77 GeV polarized electron beam and Large Acceptance Spectrometer (CLAS). The $x_B,Q^2,t$, and ϕ dependences of the ${\pi }^0$ BSA are presented in the deep inelastic regime. The asymmetries are fitted with a $\sin \varphi$ function and their amplitudes are extracted. Overall, they are of the order of 0.04–0.11 and roughly independent of $t$. This is the signature of a nonzero longitudinal-transverse interference. The implications concerning the applicability of a formalism based on generalized parton distributions, as well as the extension of a Regge formalism at high photon virtualities, are discussed.

Figure 1

Schematic representation of the handbag diagram for neutral pion production. The symbol $g$ stands for a gluon exchange between quark lines.

Figure 2

Distribution of squared missing mass for the $\mathit{ep}\to \mathit{epX}$ reaction before (black line) and after (thick red line) all cuts on other variables are applied. The arrow points to the pion mass, while the shaded green area corresponds to the selected events.

Figure 3

Distributions of the two-photon invariant mass, after the application of all cuts described in the text, for the four configurations IC-IC, IC-EC, EC-IC, and EC-EC, from left to right. The shaded areas correspond to the selected peaks (in green) and to the side-bands used in the background subtraction (in red). Note the change of scale for the last three configurations.

Figure 4

(Left) Kinematic coverage and binning in the ($x_B,Q^2$) plane. (Right) $A(\varphi )$ for one of the 13 ($x_B,Q^2$) bins and one of the 5 bins in $t$, corresponding to $\langle x_B\rangle =0.249,\langle Q^2\rangle =1.95$ GeV${}^2$, and $\langle t\rangle =-0.29$ GeV${}^2$; the black dashed curve corresponds to a fit with $A\simeq \alpha \sin \varphi$ and the red solid curve to the JML model discussed in the text.

Figure 5

Fit parameter α, as extracted from $A\simeq \alpha \sin \varphi$, as a function of $-t$. The location of each individual plot corresponds to the approximate coverage in $(x_B,Q^2)$, except the upper left one (an enlargement of the lower left one), which indicates the scales common to all plots. The grey areas indicate the maximal size of systematic uncertainties. For selected kinematics, the red curves correspond to the JML model discussed in the text.

Figure 6

Diagrams describing the neutral pion production in the JML model. (a) Pole terms. (b) Box diagram with elastic ${\pi }^0$ rescattering. (c) Box diagram with charge exchange (${\pi }^{+}N,{\pi }^{+}{\Delta }^0$, and ${\pi }^{-}{\Delta }^{++}$ are the three intermediate states considered). The exchanged mesons are to be understood as the corresponding Regge trajectories, and $\mathbf{P}$ stands for the Pomeron.