  Exclusive $\pi^0$ electroproduction at a beam energy of 5.75 GeV has been
measured with the Jefferson Lab CLAS spectrometer. Differential cross sections
were measured at more than 1800 kinematic values in $Q^2$, $x_B$, $t$, and
$\phi_\pi$, in the $Q^2$ range from 1.0 to 4.6 GeV$^2$,\ $-t$ up to 2 GeV$^2$,
and $x_B$ from 0.1 to 0.58. Structure functions $\sigma_T +\epsilon \sigma_L,
\sigma_{TT}$ and $\sigma_{LT}$ were extracted as functions of $t$ for each of
17 combinations of $Q^2$ and $x_B$. The data were compared directly with two
handbag-based calculations including both longitudinal and transversity GPDs.
Inclusion of only longitudinal GPDs very strongly underestimates $\sigma_T
+\epsilon \sigma_L$ and fails to account for $\sigma_{TT}$ and $\sigma_{LT}$,
while inclusion of transversity GPDs brings the calculations into substantially
better agreement with the data. There is very strong sensitivity to the
relative contributions of nucleon helicity flip and helicity non-flip
processes. The results confirm that exclusive $\pi^0$ electroproduction offers
direct experimental access to the transversity GPDs.
