3-flavor and 4-flavor implications of the latest T2K and NO$\nu$A electron (anti-)neutrino appearance results

Abstract

The two long-baseline experiments T2K and NO$\nu$A have recently presented new findings. T2K has shown the first $\bar \nu_e$ appearance data while NO$\nu$A has released the first $\nu_e$ appearance results. These data are of particular importance because they allow us to probe for the first time in a direct (or manifest) way the leptonic CP-violation. In fact, it is the first time that a hint of CP-violation arises from the comparison of the observations of neutrinos and antineutrinos. We consider the implications of such new results both for the standard 3-flavor framework and for the non-standard 3+1 scheme involving one sterile neutrino species. The 3-flavor analysis shows a consolidation of the previous trends, namely a slight preference for $\sin \delta <0$, disfavoring CP conservation ($\delta =0, \pi$) with a statistical significance close to $90\%$ C.L., and a mild preference (at more than 68\% C.L.) for the normal hierarchy. In a 3+1 framework, the data constrain two CP-phases ($\delta_{13} \equiv \delta$ and $\delta_{14}$), which exhibit a slight preference for the common value $\delta_{13} \simeq \delta_{14} \simeq -\pi/2 $. Interestingly, in the enlarged four neutrino scheme the preference for the normal hierarchy found within the 3-flavor framework completely disappears. This indicates that light sterile neutrinos may constitute a potential source of fragility in the capability of the two LBL experiments of discriminating the neutrino mass hierarchy.