  We propose to determine the magnetic dipole polarizability of the proton
directly from the beam asymmetry of low-energy Compton scattering based on the
fact that the leading non-Born contribution to the asymmetry is due to the
magnetic polarizability alone; the electric polarizability cancels out. The
beam asymmetry thus provides the simplest observable with a clean separation of
the magnetic polarizability from the electric one. Introducing polarizabilities
in a Lorentz-invariant fashion we compute the higher-order (recoil) effects of
polarizabilities on beam asymmetry and show that these effects are most
suppressed in forward kinematics. With the prospects of precision Compton
experiments at the MAMI and HIGS facilities in mind, we argue why the beam
asymmetry could be the best way to measure the elusive magnetic polarizability
of the proton.
