Fitting high-energy Littlest Seesaw parameters using low-energy neutrino data and leptogenesis

We show that the four high-energy Littlest Seesaw parameters in the flavour basis, namely two real Yukawa couplings plus the two right-handed neutrino masses, can be determined by an excellent fit to the seven currently constrained observables of low-energy neutrino data and leptogenesis. Taking into account renormalisation group corrections, we estimate χ2 ≃ 1.5-2.6 for the three d.o.f., depending on the high-energy scale and the type of non-supersymmetric Littlest Seesaw model. We extract allowed ranges of neutrino parameters from our fit data, including the approximate mu-tau symmetric predictions θ23 = 45° ± 1° and δ = −90° ± 5°, which, together with a normal mass ordering with m1 = 0, will enable Littlest Seesaw models to be tested in future neutrino experiments.