3+1 and 3+2 Sterile Neutrino Fits

TL;DR

Problem: reconcile LSND and MiniBooNE short-baseline signals with other oscillation data using sterile-neutrino frameworks. Method: global fits of short-baseline data in 3+1 and 3+2 schemes, including mass splittings $\Delta m^2_{41}$, $\Delta m^2_{51}$ and CP phase $η$. Key results: in 3+1, $\chi^2_{\min}=100.2$ for 104 d.o.f. with GoF ≈ 59% and a very small $PGoF$ (~$6\times10^{-6}$); in 3+2, $\chi^2_{\min}=91.6$ for 100 d.o.f. with GoF ≈ 71% and $PGoF\approx5\times10^{-4}$, and two near-maximal minima for $η$ around $\frac{\pi}{2}$ and $\frac{3\pi}{2}$. Significance: the study shows that adding a second sterile state and CP-violating phase partially alleviates tensions between datasets and identifies testable parameter regions for near-future short-baseline experiments to probe $\Delta m^2_{41}$, $\Delta m^2_{51}$ and the CP phase.

Abstract

We present the results of fits of short-baseline neutrino oscillation data in 3+1 and 3+2 neutrino mixing schemes. In spite of the presence of a tension in the interpretation of the data, 3+1 neutrino mixing is attractive for its simplicity and for the natural correspondence of one new entity (a sterile neutrino) with a new effect (short-baseline oscillations). The allowed regions in the oscillation parameter space can be tested in near-future experiments. In the framework of 3+2 neutrino mixing there is less tension in the interpretation of the data, at the price of introducing a second sterile neutrino. Moreover, the improvement of the parameter goodness of fit is mainly a statistical effect due to an increase of the number of parameters. The CP violation in short-baseline experiments allowed in 3+2 neutrino mixing can explain the positive antinu_mu -> antinu_e signal and the negative nu_mu -> nu_e measurement in the MiniBooNE experiment. For the CP-violating phase we obtained two minima of the marginal chi^2 close to the two values where CP-violation is maximal.

Figures (13)

• Figure 1: Schematic description of the two possible 3+1 schemes that we are considering, taking into account that $|\Delta{m}^2_{21}| \ll |\Delta{m}^2_{31}| \ll |\Delta{m}^2_{41}|$.
• Figure 2: Exclusion curves obtained from the data of reactor $\bar{\nu}_{e}$ disappearance experiments (see Ref. 1101.2755).
• Figure 3: Exclusion curves obtained from the data of the CDHSW $\nu_{\mu}$ disappearance experiment Dydak:1984zq, and from atmospheric neutrino data (extracted from the analysis in Ref. 0705.0107).
• Figure 4: Exclusion curves in the $\sin^{2} 2\vartheta_{e\mu}$--$\Delta{m}^2_{41}$ plane obtained from the separate constraints in Figs. \ref{['rea-cnt']} and \ref{['dmu-sup']} (blue dashed line and green dotted line) and the combined constraint given by Eq. (\ref{['sem-exc']}) (red solid line) from disappearance experiments (Dis). The regions allowed by LSND and MiniBooNE antineutrino data are delimited by dark-blue long-dashed lines.
• Figure 5: Exclusion curve in the $\sin^{2} 2\vartheta_{e\mu}$--$\Delta{m}^2_{41}$ plane obtained with the addition to the disappearance constraint in Fig. \ref{['exc-dis']} of the constraints obtained from KARMEN hep-ex/0203021 (KAR), NOMAD hep-ex/0306037 (NOM) and MiniBooNE neutrino 0812.2243 (MB$\nu$) data (red solid line). The regions allowed by LSND and MiniBooNE antineutrino data are delimited by dark-blue long-dashed lines.