  The observation of new physics events with large missing transverse energy at
the LHC would potentially serve as evidence for the direct production of dark
matter. A crucial step toward verifying such evidence is the measurement of the
would-be dark matter mass. If, for example, the invisible particles are found
to have masses consistent with zero, it may prove very challenging to ascertain
whether light dark matter or neutrinos are being observed. We assume that new
invisible particles are pair-produced in a ttbar-like topology and use two
MT2-based methods to measure the masses of the particles associated with the
missing energy. Instead of simulating events and backgrounds, we estimate the
uncertainty associated with measuring the mass of the invisible particle by
assuming a fixed value of the uncertainty associated with the location of the
MT2 endpoint. We find that if this uncertainty associated with measuring the
MT2 endpoints is, quite optimistically, O(1 GeV), the invisible particles must
have masses greater than O(10 GeV) so they can be distinguished from massless
ones at 95% CL. If the results from the CoGeNT, DAMA/LIBRA, and CRESST
experiments have indeed revealed the existence of light dark matter with mass
O(10 GeV), our results suggest that it may be difficult for the LHC to
distinguish dark matter from neutrinos solely via mass measurements.
