Status of four-neutrino mass schemes: a global and unified approach to current neutrino oscillation data

TL;DR

This work presents a global, statistically coherent analysis of four-neutrino mass schemes (3+1) and (2+2) to reconcile solar, atmospheric, and LSND data via a unified parameter framework that includes a light sterile neutrino. By separately modeling solar, atmospheric, and short-baseline data and then combining them, the authors find that solar+atmospheric data favor (3+1) while atmospheric+SBL data favor (2+2); overall, the global fit places the best support on (3+1), with (2+2) as a close competitor. Importantly, the LSND result strongly disfavors the pure three-active-neutrino scenario, making the four-neutrino hypothesis appealing, though LSND alone cannot decisively resolve between (3+1) and (2+2). The study highlights the need for new experiments sensitive to sterile components (e.g., neutral-current atmospheric measurements or more precise SBL disappearance data) to break the residual degeneracy between the two schemes. The results underscore LSND's pivotal role and guide future experimental directions in the search for sterile neutrinos.

Abstract

We present a unified global analysis of neutrino oscillation data within the framework of the four-neutrino mass schemes (3+1) and (2+2). We include all data from solar and atmospheric neutrino experiments, as well as information from short-baseline experiments including LSND. If we combine only solar and atmospheric neutrino data, (3+1) schemes are clearly preferred, whereas short-baseline data in combination with atmospheric data prefers (2+2) models. When combining all data in a global analysis the (3+1) mass scheme gives a slightly better fit than the (2+2) case, though all four-neutrino schemes are presently acceptable. The LSND result disfavors the three-active neutrino scenario with only $Δm^2_{sol}$ and $Δm^2_{atm}$ at 99.9% CL with respect to the four-neutrino best fit model. We perform a detailed analysis of the goodness of fit to identify which sub-set of the data is in disagreement with the best fit solution in a given mass scheme.

Figures (8)

• Figure 1: The six types of four-neutrino mass spectra. The different distances between the masses on the vertical axes symbolize the different scales of mass-squared differences required to explain solar, atmospheric and LSND data with neutrino oscillations.
• Figure 2: $\Delta\chi^2_\mathsc{sol}$ as a function of $\eta_s$ for the different solutions to the solar neutrino problem, as presented in Fig. 3 of Ref. Concha:2001zi.
• Figure 3: (a) 90% and 99% C.L. allowed regions for the parameters $d_s = c_{24}^2 c_{34}^2$ (ordinate) and $d_\mu = s_{24}^2$ (abscissa) from atmospheric neutrino data. The best fit point is marked with a star. (b) $\Delta\chi^2_\mathsc{atm}$ as a function of $d_s$ using old macroOld and new macroNew MACRO data. Also shown are the $\Delta\chi^2$-values corresponding to 90% and 99% C.L. for 1 d.o.f..
• Figure 4: Parameter dependence of the three data sets solar, atmospheric and SBL. Exact definitions of the parameters are given in Secs. \ref{['sec:SBL']}, \ref{['sec:solar']} and \ref{['sec:atmospheric']}.
• Figure 5: $\Delta\chi^2_\mathsc{atm}$ as a function of the fraction of the sterile neutrino in solar oscillations $\eta_s$ for all four-neutrino mass schemes, (3+1)$_a$, (3+1)$_b$ and (2+2).