Proton Form Factor Ratio, $\mu_p G_E^p/G_M^p$ from Double Spin Asymmetry

TL;DR

This paper measures the proton's electric to magnetic form factor ratio at high momentum transfer using double spin asymmetry, confirming previous discrepancies and extending the highest $Q^2$ measurement with this technique.

Contribution

It provides a new measurement of the proton form factor ratio at high $Q^2$ using double spin asymmetry, offering an independent check of previous methods and extending the kinematic range.

Findings

Results agree with previous polarization data at similar $Q^2$.

Confirms discrepancy between Rosenbluth and polarization-transfer methods.

Achieves highest $Q^2$ measurement with double spin asymmetry technique.

Abstract

The ratio of the electric and magnetic form factor of the proton, $\mu_p G_E^p/G_M^p$, has been measured for elastic electron-proton scattering with polarized beam and target up to four-momentum transfer squared, $Q^2=5.66$ (GeV/c)$^2$ using the double spin asymmetry for target spin orientation aligned nearly perpendicular to the beam momentum direction. This measurement of $\mu_p G_E^p/G_M^p$ agrees with the $Q^2$ dependence of previous recoil polarization data and reconfirms the discrepancy at high $Q^2$ between the Rosenbluth and the polarization-transfer method with a different measurement technique and systematic uncertainties uncorrelated to those of the recoil-polarization measurements. The form factor ratio at $Q^2$=2.06 (GeV/c)$^2$ has been measured as $\mu_p G_E^p/G_M^p = 0.720 \pm 0.176_{stat} \pm 0.039_{sys}$, which is in agreement with an earlier measurement with the polarized target technique at similar kinematics. The form factor ratio at $Q^2$=5.66 (GeV/c)$^2$ has been determined as $\mu_p G_E^p/G_M^p=0.244\pm0.353_{stat}\pm0.013_{sys}$, which represents the highest $Q^2$ reach with the double spin asymmetry with polarized target to date.