Interpreting the LSND anomaly: sterile neutrinos or CPT-violation or...?

TL;DR

The paper scrutinizes the LSND anomaly through two principal explanations: sterile neutrinos and CPT-violating neutrino masses. It shows that 2+2 sterile schemes are severely disfavoured by solar and atmospheric data, while 3+1 sterile scenarios struggle with disappearance constraints, leaving CPT-violation as a possible, though speculative, global fit. It also analyzes how KamLAND and WMAP data tighten these interpretations, increasingly challenging CPT-violating and 3+1 sterile solutions. The work outlines specific experimental signatures that could confirm or refute CPT-violating neutrinos and the sterile hypotheses, highlighting MiniBooNE, KamLAND, Borexino, and future long-baseline experiments as decisive tests. Overall, the LSND puzzle remains unresolved, but the viable explanatory frameworks are sharply constrained and testable by upcoming data.

Abstract

We first study how sterile neutrinos can fit the 5sigma bar-nu_mu --> bar-nu_e LSND anomaly: 2+2 solutions are strongly disfavoured by solar and atmospheric data, while 3+1 solutions can still give a poor fit (for a specific range of oscillation parameters, to be tested by MiniBooNE). If instead MiniBoone will find no nu_mu --> nu_e signal, we will have a hint for CPT violation. Already now, unlike sterile neutrinos, CPT-violating neutrino masses can nicely accomodate all safe and unsafe data. We study how much CPT must be conserved according to atmospheric and K2K data and list which CPT-violating signals could be discovered by forthcoming solar and long-baseline experiments.

Figures (5)

• Figure 1: The CPT-violating spectrum proposed in MY.
• Figure 2: $90\%$ CL regions from Karmen, CDHS, CCFR, Bugey, Chooz and LSND (shaded). The mixing angle $\theta$ on the horizontal axis is different for the different experiments.
• Figure 4: Best-fit regions at 90% and 99% CL (2 d.o.f.) for the LSND parameters assuming oscillations. Fig. \ref{['fig:best']}a assumes that the LSND anomaly is generated trough a sterile neutrino ('3+1' scheme). Fig. \ref{['fig:best']}b assumes that the LSND anomaly is generated by active neutrinos, while something else (e.g. neutrino decay, sterile neutrinos,…) generates either the atmospheric or the solar anomaly, without affecting LSND. The dotted lines show the regions suggested by only the LSND data. The dots show the best fit points.
• Figure 5: Fit of SK and K2K data for the neutrino and anti-neutrino atmospheric mass splitting at 68, 90, 99% CL (2 d.o.f.).
• Figure 7: Status of 3+1 oscillations including WMAP data. Fig. \ref{['fig:31MAP']}a: LSND favours the shaded region. Values of $\Delta m^2_{\rm LSND}$ above the horizontal dashed line are disfavoured by WMAP. The other dashed line shows the upper bound on $\theta$ from all other neutrino experiments. The continuous line shows the combination of the two previous constraints. All bounds are at $99\%$ CL for 2 dof. In fig. \ref{['fig:31MAP']}b we show the best fit 3+1 solution, including all data.