March 2000 Maximal νe oscillations, Borexino and smoking guns

We examine the maximal ν e → ν s and ν e → ν µ,τ oscillation solutions to the solar neutrino problem. These solutions lead to roughly a 50% solar flux reduction for the large parameter range 3 × 10 −10 < ∼ δm 2 /eV 2 < ∼ 10 −3. It is known that the earth regeneration effect may cause a potentially large night-day effect even for maximal neutrino oscillations. We investigate the night-day asymmetry predictions for the forthcoming Borexino measurement of the 7 Be neutrinos for both maximal ν e → ν s and ν e → ν µ,τ oscillations. If y × 10 −8 < ∼ δm 2 /eV 2 < ∼ 2y × 10 −5 (with y ≃ 0.5 for ν e → ν s case and y ≃ 1 for the ν e → ν µ,τ case) then the maximal neutrino oscillations will lead to observable night-day asymmetries in Borexino and/or superKamiokande. With Kamland covering the high mass range, 10 −5 < ∼ δm 2 /eV 2 < ∼ 10 −3 and Borexino/SuperK covering the low mass range, 3 × 10 −10 < ∼ δm 2 /eV 2 < ∼ 5 × 10 −9 (" just so " region), essentially all of the δm 2 parameter space will soon be scrutinized. 1 Maximal oscillations occupy a special point in parameter space. Neutral Kaons and B-mesons both oscillate maximally with their antiparticle partners. Interestingly there is now strong evidence from solar [1] and atmospheric [2] neutrino experiments that electron and muon neutrinos also oscillate maximally with some as yet unidentified partner. Identifying these states is one of the most pressing issues in particle physics. One possibility is that each of the three known neutrinos oscillates maximally with an approximately sterile partner. This behaviour is expected to occur if parity is an unbroken symmetry of nature [3,4]. In this theory, the sterile flavour maximally mixing with the ν e is identified with the mirror electron neutrino. The characteristic maximal mixing feature occurs because of the underlying exact parity symmetry between the ordinary and mirror sectors. The maximal mixing observed for atmospheric muon neutrinos is nicely in accord with this framework (see e.g. [5]), which has the atmospheric neutrino problem resolved through 'ν µ → mirror partner' oscillations. Alternatively, it has also been suggested [6] that each of the known neutrinos are pseudo-Dirac fermions [7] which has each …