  Quantum materials represent one of the most promising frontiers in the quest
for faster, lightweight, energy efficient technologies. However, their inherent
complexity and rich phase landscape make them challenging to understand or
manipulate in useful ways. Here we present a new ultrafast electron calorimetry
technique that can systematically uncover new phases of quantum matter. Using
time- and angle-resolved photoemission spectroscopy, we measure the dynamic
electron temperature, band structure and heat capacity. We then show that this
is a very sensitive probe of phase changes in materials, because electrons
react very quickly, and moreover generally are the smallest component of the
total heat capacity. This allows us to uncover a new long-lived metastable
state in the charge density wave material 1T-TaSe$_2$, that is distinct from
all of the known equilibrium phases: it is characterized by a significantly
reduced effective heat capacity that is only 30% of the normal value, due to
selective electron-phonon coupling to a subset of phonon modes. As a result,
significantly less energy is required to melt the charge order and transform
the state of the material than under thermal equilibrium conditions.
