Neutrino Matter Effect Invariants and the Observables of Neutrino   Oscillations
  We generalise our previous observation on the invariance of the Jarlskog determinant to matter effects in neutrino oscillations. Within the context of standard neutrino oscillation theory with matter effects, we present the complete set of (five) matter invariant observables for neutrino propagation in matter. We give some examples of their application. 
P.F. Harrison and W.G. Scott