  Carrier mobility in solids is generally limited by electron-impurity or
electron-phonon scattering depending on the most frequently occurring event.
Three body collisions between carriers and both phonons and impurities are
rare; they are denoted supercollisions (SCs). Elusive in electronic transport
they should emerge in relaxation processes as they allow for large energy
transfers. As pointed out in Ref. \onlinecite{Song2012PRL}, this is the case in
undoped graphene where the small Fermi surface drastically restricts the
allowed phonon energy in ordinary collisions. Using electrical heating and
sensitive noise thermometry we report on SC-cooling in diffusive monolayer
graphene. At low carrier density and high phonon temperature the Joule power
$P$ obeys a $P\propto T_e^3$ law as a function of electronic temperature $T_e$.
It overrules the linear law expected for ordinary collisions which has recently
been observed in resistivity measurements. The cubic law is characteristic of
SCs and departs from the $T_e^4$ dependence recently reported for metallic
graphene below the Bloch-Gr\"{u}neisen temperature. These supercollisions are
important for applications of graphene in bolometry and photo-detection.
