Octant of θ23 in Danger with a Light Sterile Neutrino

Abstract

Present global fits of world neutrino data hint towards non-maximal θ23 with two nearly degenerate solutions, one in the lower octant (θ23 < π/4), and the other in the higher octant (θ23 > π/4). This octant ambiguity of θ23 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 neutrinos, the νµ → νe transition probability probed in the LBL experiments acquires a new interference term via active-sterile oscillations. We find that this interference term can mimic a swap of the θ23 octant, even if one uses the information from both neutrino and antineutrino channels. As a consequence, the sensitivity to the octant of θ23 can be completely lost, and this may have serious implications for our understanding of neutrinos from both the experimental and theoretical perspectives.

Non-unitarity, sterile neutrinos, and non-standard neutrino interactions

“…Interesting correlations and degeneracies among the standard and new physics parameters can indeed take place in the averaged-out sterile neutrino scenario [55,[69][70][71]. In our results, even though the true values of the α parameters were set to zero, some very interesting correlations and degeneracies among θ 23 and the new physics parameters have been recovered.…”

Can we probe intrinsic CP and T violations and nonunitarity at long baseline accelerator experiments?

“…Going beyond the recent studies revealing how potential new physics scenarios can hinder the clean determination of this important parameter [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], we address the issue of clean separation of the intrinsic leptonic CP phase from the extrinsic contribution arising due to SI as well as new physics 7 . We also show the impact of new physics on testing non-unitarity.…”

“…We also probe deviations due to new physics scenarios. We go beyond the SM in two respects -one in which we introduce subdominant effects due to a possible source of new physics dubbed as NSI [17][18][19][20][21][22][23][24][25][26] and another where the presence of extra sterile state can lead to non-unitarity in the 3×3 part even though the overall mixing matrix is still unitary [27][28][29][30][31][32][33][34] and examine consequences relevant to long baseline experiments (For other new physics scenarios such as non-unitarity, see [35][36][37][38][39]). We highlight the regions in L − E space where the effects due to CP and T violation are drastically modified due to new physics.…”

Resolving the atmospheric octant by an improved measurement of the reactor angle

“…However it has also been shown recently that the octant sensitivity may completely change in the presence of the above non-standard features, i.e. non-unitarity [24], nonstandard interaction [25] or a light sterile neutrino [26].…”