Evidence of theta(13)>0 from global neutrino data analysis

TL;DR

The paper addresses whether the neutrino mixing angle $\theta_{13}$ is nonzero within the standard three-neutrino framework by performing a global oscillation analysis that updates prior hints with the latest T2K and MINOS appearance data and accounts for reactor-flux systematics. It finds strong evidence that $\sin^2\theta_{13}$ is nonzero, with best-fit values around $0.021$–$0.025$ and overall significance surpassing $3\sigma$, with consistency across datasets sensitive to $\delta m^2$ and $\Delta m^2$. The results are predominantly driven by accelerator appearance measurements (notably T2K) but are corroborated by solar, KamLAND, and atmospheric data. These findings have important implications for CP violation searches in the lepton sector, matter-effect studies to probe the mass hierarchy, and guide future reactor and accelerator experiments to refine $\sin^2\theta_{13}$ and the CP phase $\delta$.

Abstract

The neutrino mixing angle theta(13) is at the focus of current neutrino research. From a global analysis of the available oscillation data in a 3-neutrino framework, we previously reported [Phys. Rev. Lett. 101, 141801 (2008)] hints in favor of theta(13)>0 at the 90 % C.L. Such hints are consistent with the recent indications of nu(mu)-->nu(e) appearance in the T2K and MINOS long-baseline accelerator experiments. Our global analysis of all the available data currently provides >3 sigma evidence for nonzero theta(13), with 1-sigma ranges sin^2 theta(13) = 0.021+-0.007 or 0.025+-0.007, depending on reactor neutrino flux systematics. Updated ranges are also reported for the other 3-neutrino oscillation parameters (delta m^2, sin^2 theta(12)) and (Delta m^2, sin^2 theta(23)).

Figures (5)

• Figure 1: Global $3\nu$ oscillation analysis. Bounds on the mass-mixing oscillation parameters, in terms of standard deviations from the best fit. Solid (dashed) lines refer to the case with old (new) reactor neutrino fluxes. Note the $>3\sigma$ evidence for $\theta_{13}>0$.
• Figure 2: Breakdown of the evidence for $\theta_{13}>0$ from the global fit (ALL) into contributions coming from $\delta m^2$-sensitive data (Solar+KamLAND) and from $\Delta m^2$-sensitive data (ATM+LBL+CHOOZ). The left and right panels refer to old and new fluxes, respectively.
• Figure 3: Global $3\nu$ analysis. Preferred $\pm1\sigma$ ranges for the mixing parameter $\sin^2\theta_{13}$ from partial and global data sets. Solid and dashed error bars refer to old and new reactor neutrino fluxes, respectively.
• Figure 4: Global $3\nu$ analysis: Joint contours at 1, 2 and $3\sigma$ ($\Delta \chi^2=1$, 4 and 9) for couples of $\sin^2\theta_{ij}$ parameters, assuming old reactor neutrino fluxes. For new reactor fluxes, the best fits (and, to a large extent, also the contours) are shifted as indicated by the arrows.
• Figure 5: Standard deviations from the best fit in terms of the variable $\cos\delta\sin\theta_{13}$ for the two CP parities ($\cos\delta=\pm1)$ and for both normal hierarchy (NH) and inverted hierarchy (IH), using the ATM+LBL+CHOOZ data set. The curves refer to the analysis with old reactor neutrino fluxes; similar results (not shown) are obtained for new reactor fluxes.