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Register a new userCan we measure $theta_{23}$ octant in 3+1 scheme?
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Current 3$ u$ global fits predict two degenerate solutions for $theta_{23}$: one lies in lower octant ($theta_{23} <pi/4$), and the other belongs to higher octant ($theta_{23} >pi/4$). Here, we study how the measurement of $theta_{23}$ octant would be affected in the upcoming Deep Underground Neutrino Experiment (DUNE) if there exist a light eV-scale sterile neutrino. We show that in 3+1 scheme, a new interference term in $ u_mu to u_e$ oscillation probability can spoil the chances of measuring $theta_{23}$ octant completely.
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Present global fits of world neutrino data hint towards non-maximal $theta_{23}$ with two nearly degenerate solutions, one in the lower octant ($theta_{23} <pi/4$), and the other in the higher octant ($theta_{23} >pi/4$). This octant ambiguity of $theta_{23}$ is one of the fundamental issues in the neutrino sector, and its resolution is a crucial goal of next-generation long-baseline (LBL) experiments. In this letter, we address for the first time, the impact of a light eV-scale sterile neutrino towards such a measurement, taking the Deep Underground Neutrino Experiment (DUNE) as a case study. In the so-called 3+1 scheme involving three active and one sterile neutrino, the $ u_mu to u_e$ transition probability probed in the LBL experiments acquires a new interference term via active-sterile oscillations. We find that this novel interference term can mimic a swap of the $theta_{23}$ octant, even if one uses the information from both neutrino and antineutrino channels. As a consequence, the sensitivity to the octant of $theta_{23}$ can be completely lost and this may have serious implications in our understanding of neutrinos from both the experimental and theoretical perspectives.
We study the possibility of determining the octant of the neutrino mixing angle $theta_{23}$, that is, whether $theta_{23}> 45^circ$ or $theta_{23}<45^circ$, in long baseline neutrino experiments. Here we numerically derived the sensitivity limits within which these experiments can determine, by measuring the probability of the $ u_{mu}to u_{e}$ transitions, the octant of $theta_{23}$ with a $5sigma$ certainty. The interference of the CP violation angle $delta$ with these limits, as well as the effects of the baseline length and the run-time ratio of neutrino and antineutrino modes of the beam have been analyzed.
We point out that leptonic weak-basis invariants are an important tool for the study of the properties of lepton flavour models. In particular, we show that appropriately chosen invariants can give a clear indication of whether a particular lepton flavour model favours normal or inverted hierarchy for neutrino masses and what is the octant of $theta_{23}$. These invariants can be evaluated in any conveniently chosen weak-basis and can also be expressed in terms of neutrino masses, charged lepton masses, mixing angles and CP violation phases.
We investigate the prospects for determining the octant of $theta_{23}$ in the future long baseline oscillation experiments. We present our results as contour plots on the ($theta_{23}-45^circ$, $delta$)--plane, where $delta$ is the CP phase, showing the true values of $theta_{23}$ for which the octant can be experimentally determined at 3$,sigma$, 2$,sigma$ and 1$,sigma$ confidence level, in particular, the impact of the non-unitarity of neutrino mixing.
In study of muon neutrino disappearance at 810 km, the NOvA experiment finds flavor mixing of the atmospheric sector to deviate from maximal ($sin^2theta_{23} = 0.5$) by 2.6 $sigma$. The result is in tension with the 295-km baseline measurements of T2K which are consistent with maximal mixing. We propose that $theta_{23}$ is in fact maximal, and that the disagreement is harbinger of environmentally-induced decoherence. The departure from maximal mixing can be accounted for by an energy-independent decoherence of strength $Gamma = (2.3 pm 1.1) times 10^{-23}$ GeV.
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