  We propose a new global and fully inclusive variable \sqrt{s}_{min} for
determining the mass scale of new particles in events with missing energy at
hadron colliders. We define \sqrt{s}_{min} as the minimum center-of-mass parton
level energy consistent with the measured values of the total calorimeter
energy E and the total visible momentum \vec{P}. We prove that for an arbitrary
event, \sqrt{s}_{min} is simply given by the formula
\sqrt{s}_{min}=\sqrt{E^2-P_z^2}+\sqrt{\met^2+M_{inv}^2}, where M_{inv} is the
total mass of all invisible particles produced in the event. We use t\bar{t}
production and several supersymmetry examples to argue that the peak in the
\sqrt{s}_{min} distribution is correlated with the mass threshold of the parent
particles originally produced in the event. This conjecture allows a
determination of the heavy superpartner mass scale (as a function of the LSP
mass) in a completely general and model-independent way, and without the need
for any exclusive event reconstruction. In our SUSY examples of several
multijet plus missing energy signals, the accuracy of the mass measurement
based on \sqrt{s}_{min} is typically at the percent level, and never worse than
10%. After including the effects of initial state radiation and multiple parton
interactions, the precision gets worse, but for heavy SUSY mass spectra remains
10%.
