Discontinuous Decoherence Transition from Selective Channel Suppression

We identify a regime of open quantum system dynamics in which the observed coherence time exhibits a discontinuous transitionarising from selective loss of accessibility ofan effective decoherence channel. Within theGaussian influence-functional formalism, normalization of the Keldysh weight matrix leadsto a parameter-independent coherence-timeratioR≡ τpostτpre = 43 .The result follows entirely from standardopen-system theory and introduces no modification of quantum mechanics. The predicted transition differs fundamentally fromknowndynamical-decoupling fixed points, occurring in time at fixed control parametersand remaining independent of pulse number.Because the framework yields explicit falsification criteria, the predicted ratio providesa direct experimental probe of multi-channeldecoherence structure. The analysis suggeststhat R = 4/3 represents a structural invariant associated with the normalized geometryof the Keldysh representation.