Measurement of the beam asymmetry $\mathrm{\Sigma }$ for ${\pi }^0$ and $\eta$ photoproduction on the proton at $E_{\gamma }=9$ GeV

We report measurements of the photon beam asymmetry $\mathrm{\Sigma }$ for the reactions $\vec{\gamma }p\to p{\pi }^0$ and $\vec{\gamma }p\to p\eta$ from the GlueX experiment using a 9 GeV linearly polarized, tagged photon beam incident on a liquid hydrogen target in Jefferson Lab's Hall D. The asymmetries, measured as a function of the proton momentum transfer, possess greater precision than previous ${\pi }^0$ measurements and are the first $\eta$ measurements in this energy regime. The results are compared with theoretical predictions based on $t$-channel, quasiparticle exchange and constrain the axial-vector component of the neutral meson production mechanism in these models.

Figure 1

(a) Photon beam intensity versus energy as measured by the pair spectrometer (not corrected for instrumental acceptance). (b) Photon beam polarization as a function of beam energy, as measured by the triplet polarimeter, with data points offset horizontally by $\pm 0.015\mathrm{GeV}$ for clarity.

Figure 2

(a) Energy loss $dE/dx$ versus positively charged track momentum and (b) the spectrum of missing mass squared for the reaction $\vec{\gamma }p\to p\gamma \gamma$.

Figure 3

$\gamma \gamma$ invariant mass distribution with clear peaks at the ${\pi }^0$ and $\eta$ masses, superimposed with background estimated from $\vec{\gamma }p\to p\omega$, $\omega \to {\pi }^0\gamma$ simulation.

Figure 4

Candidate event yield as a function of the proton momentum transfer $-t$ for (a) $\vec{\gamma }p\to p{\pi }^0$ and (b) $\vec{\gamma }p\to p\eta$, without corrections for instrumental acceptance. The acceptance functions (red dashed), determined from MC simulation, are shown for comparison.

Figure 5

$\vec{\gamma }p\to p{\pi }^0$ yield (statistical errors only) versus ${\varphi }_p$ integrated over $-t$ for (a) PERP and (b) PARA. (c) The yield asymmetry, fit with Eq. (4) to extract $\mathrm{\Sigma }$.

Figure 6

Beam asymmetry $\mathrm{\Sigma }$ for (a) $\vec{\gamma }p\to p{\pi }^0$ and (b) $\vec{\gamma }p\to p\eta$ (black filled circles). Uncorrelated systematic errors are indicated by gray bars and combined statistical and systematic uncertainties are given by the black error bars. The previous SLAC results [19] at ${\overline{E}}_{\gamma }=10$ GeV (blue open circles) are also shown along with various Regge theory calculations.