  The radiative efficiency of super-Eddington accreting black holes (BHs) is
explored for magnetically-arrested disks (MADs), where magnetic flux builds-up
to saturation near the BH. Our three-dimensional general relativistic radiation
magnetohydrodynamic (GRRMHD) simulation of a spinning BH (spin $a/M=0.8$)
accreting at $\sim 50$ times Eddington shows a total efficiency $\sim 50\%$
when time-averaged and total efficiency $\gtrsim 100\%$ in moments. Magnetic
compression by the magnetic flux near the rotating BH leads to a thin disk,
whose radiation escapes via advection by a magnetized wind and via transport
through a low-density channel created by a Blandford-Znajek (BZ) jet. The BZ
efficiency is sub-optimal due to inertial loading of field lines by optically
thick radiation, leading to BZ efficiency $\sim 40\%$ on the horizon and BZ
efficiency $\sim 5\%$ by $r\sim 400r_g$ (gravitational radii) via absorption by
the wind. Importantly, radiation escapes at $r\sim 400r_g$ with efficiency
$\eta\approx 15\%$ (luminosity $L\sim 50L_{\rm Edd}$), similar to $\eta\approx
12\%$ for a Novikov-Thorne thin disk and beyond $\eta\lesssim 1\%$ seen in
prior GRRMHD simulations or slim disk theory. Our simulations show how BH spin,
magnetic field, and jet mass-loading affect the radiative and jet efficiencies
of super-Eddington accretion.
