  General relativistic magnetohydrodynamic (GRMHD) simulations of black hole
tilted disks -- where the angular momentum of the accretion flow at large
distances is misaligned with respect to the black hole spin -- commonly display
standing shocks, within a few to tens of gravitational radii from the black
hole. In GRMHD simulations of geometrically thick, optically thin accretion
flows, applicable to low-luminosity sources like Sgr A* and M87*, the shocks
have trans-relativistic speed, moderate plasma beta (the ratio of ion thermal
pressure to magnetic pressure is $\beta_\mathrm{pi1} \sim 1-8$), and low sonic
Mach number (the ratio of shock speed to sound speed is $M_s \sim 1-5$). We
study such shocks with two-dimensional particle-in-cell simulations and we
quantify the efficiency and mechanisms of electron heating, for the special
case of pre-shock magnetic fields perpendicular to the shock direction of
propagation. We find that the post-shock electron temperature $T_\mathrm{e2}$
exceeds the adiabatic expectation $T_\mathrm{e2,ad}$ by an amount
$T_\mathrm{e2}/T_\mathrm{e2,ad} - 1 \simeq 0.0016 M_s^{3.6}$, nearly
independent of the plasma beta and of the pre-shock electron-to-ion temperature
ratio $T_\mathrm{e1}/T_\mathrm{i1}$, which we vary from $0.1$ to unity. We
investigate the heating physics for $M_s \sim 5-6$ and find that electron
super-adiabatic heating is governed by magnetic pumping at
$T_\mathrm{e1}/T_\mathrm{i1}=1$, whereas heating by $B$-parallel electric
fields (i.e., parallel to the local magnetic field) dominates at
$T_\mathrm{e1}/T_\mathrm{i1}=0.1$. Our results provide physically-motivated
subgrid prescriptions for electron heating at the collisionless shocks seen in
GRMHD simulations of black hole accretion flows.
