  Light incident upon materials can induce changes in their electrical
conductivity, a phenomenon referred to as photoresistance. In semiconductors,
the photoresistance is negative, as light-induced promotion of electrons across
the band gap enhances the number of charge carriers participating in transport.
In superconductors, the photoresistance is positive because of the destruction
of the superconducting state, whereas in normal metals it is vanishing. Here we
report a qualitative deviation from the standard behavior in metallic graphene.
We show that Dirac electrons exposed to continuous wave (CW) terahertz (THz)
radiation can be thermally decoupled from the lattice by 50~K which activates
hydrodynamic electron transport. In this regime, the resistance of graphene
constrictions experiences a decrease caused by the THz-driven superballistic
flow of correlated electrons. We analyze the dependencies of the negative
photoresistance on the carrier density, and the radiation power and show that
our superballistic devices operate as sensitive phonon-cooled bolometers and
can thus offer a picosecond-scale response time. Beyond their fundamental
implications, our findings underscore the practicality of electron
hydrodynamics in designing ultra-fast THz sensors and electron thermometers.
