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

Abstract:

We report measurements of the photon beam asymmetry $\Sigma$ 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, quasi-particle exchange and constrain the axial-vector component of the neutral meson production mechanism in these models.

arXiv: arXiv:1701.08123

 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 GeV for clarity. Figure 2: (a) Energy loss $dE/dx$ versus postively 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 $\phi_p$ integrated over $-t$ for (a) PERP and (b) PARA. (c) The yield asymmetry, fit with Eq. (4) to extract $\Sigma$. Figure 6: Beam asymmetry $\Sigma$ 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.