  Electron anti-neutrinos are commonly detected in liquid scintillator
experiments via inverse beta decay, by looking at the coincidence between the
reaction products, neutron and positron. Prior to positron annihilation, an
electron-positron pair may form an orthopositronium (o-Ps) state, with a mean
life of a few ns. Even if the o-Ps decay is speeded up by spin flip or pick off
effects, it may introduce distortions in the photon emission time distribution,
crucial for position reconstruction and pulse shape discrimination algorithms
in anti-neutrino experiments. Reversing the problem, the o-Ps induced time
distortion represents a new signature for tagging anti-neutrinos in liquid
scintillator.
  In this paper, we report the results of measurements of the o-Ps formation
probability and lifetime, for the most used solvents for organic liquid
scintillators in neutrino physics (pseudocumene, linear alkyl benzene,
phenylxylylethane, and dodecane). We characterize also a mixture of
pseudocumene +1.5 g/l of 2,5-diphenyloxazole, a fluor acting as wavelength
shifter.
  In the second part of the paper, we demonstrate that the o-Ps induced
distortion of the scintillation photon emission time distributions represent an
optimal signature for tagging positrons on an event by event basis, potentially
enhancing the anti-neutrino detection.
