Unbroken versus broken mirror world: a tale of two vacua

If the lagrangian of nature respects parity invariance then there are two d istinct possibilities: either parity is unbroken by the vacuum or it is spo ntaneously broken. We examine the two simplest phenomenologically consisten t gauge models which have unbroken and spontaneously broken parity symmetri es, respectively. These two models have a lagrangian of the same form, but a different parameter range is chosen in the Higgs potential. They both pre dict the existence of dark matter and can explain the MACHO events. However , the models predict quite different neutrino physics. Although both have l ight mirror (effectively sterile) neutrinos, the ordinary-mirror neutrino m ixing angles are unobservably tiny in the broken parity case. The minimal b roken parity model therefore cannot simultaneously explain the solar, atmos pheric and LSND data. By contrast, the unbroken parity version can explain all of the neutrino anomalies. Furthermore, we argue that the unbroken case provides the most natural explanation of the neutrino physics anomalies (i rrespective of whether evidence from the LSND experiment is included) becau se of its characteristic maximal mixing prediction.