LSND versus MiniBooNE: Sterile neutrinos with energy dependent masses and mixing?

Standard active--sterile neutrino oscillations do not provide a satisfactory description of the LSND evidence for neutrino oscillations together with the constraints from MiniBooNE and other null-result short-baseline oscillation experiments. However, if the mass or the mixing of the sterile neutrino depends in an exotic way on its energy all data become consistent. I explore the phenomenological consequences of the assumption that either the mass or the mixing scales with the neutrino energy as $1/E_\nu^r (r > 0)$. Since the neutrino energy in LSND is about 40 MeV, whereas MiniBooNE operates at around 1 GeV, oscillations get suppressed in MiniBooNE and the two results become fully compatible for $r\gtrsim 0.2$. Furthermore, also the global fit of all relevant data improves significantly by exploring the different energy regimes of the various experiments. The best fit $\chi^2$ decreases by 12.7 (14.1) units with respect to standard sterile neutrino oscillations if the mass (mixing) scales with energy.Comment: 18 pages, 3 figures, discussion extended, version to appear in JHE

4-Neutrino mass schemes and the likelihood of (3+1)-mass spectra

We examine the (3+1)-class of 4-neutrino mass spectra within a rigorous statistical analysis based on the Bayesian approach to probability. The data of the Bugey, CDHS and KARMEN experiments are combined by using a likelihood function. Our statistical approach allows us to incorporate solar and atmospheric neutrino data and also the result of the CHOOZ experiment via inequalities which involve elements of the neutrino mixing matrix and are derived from these data. For any short-baseline $\Delta m^2$ we calculate a bound on the LSND transition amplitude $A_{\mu;e}$ and find that, in the $\Delta m^2$--$A_{\mu;e}$ plane, there is no overlap between the 99% CL region allowed by the latest LSND analysis and the region allowed by our bound on $A_{\mu;e}$ at 95% CL; there are some small overlap regions if we take the bound at 99% CL. Therefore, we conclude that, with the existing data, the (3+1)-neutrino mass spectra are not very likely. However, treating the (2+2)-spectra with our method, we find that they are well compatible with all data.Comment: Latex, 22 pages, 3 figure

Is the effect of the Sun's gravitational potential on dark matter particles observable?

We consider the effect of the Sun's gravitational potential on the local phase space distribution of dark matter particles, focusing on its implication for the annual modulation signal in direct detection experiments. We perform a fit to the modulation signal observed in DAMA/LIBRA and show that the allowed region shrinks if Solar gravitational focusing (GF) is included compared to the one without GF. Furthermore, we consider a possible signal in a generic future direct detection experiment, irrespective of the DAMA/LIBRA signal. Even for scattering cross sections close to the current bound and a large exposure of a xenon target with 270 ton yr it will be hard to establish the presence of GF from data. In the region of dark matter masses below 40 GeV an annual modulation signal can be established for our assumed experimental setup, however GF is negligible for low masses. In the high mass region, where GF is more important, the significance of annual modulation itself is very low. We obtain similar results for lighter targets such as Ge and Ar. We comment also on inelastic scattering, noting that GF becomes somewhat more important for exothermic scattering compared to the elastic case.Comment: 19 pages, 20 figures, 1 table, v2: added Fig. 3 for DAMA, and comments on light target

Sterile neutrino oscillations after first MiniBooNE results

In view of the recent results from the MiniBooNE experiment we revisit the global neutrino oscillation fit to short-baseline neutrino data by adding one or two sterile neutrinos with eV-scale masses to the three Standard Model neutrinos, and for the first time we consider also the global fit with three sterile neutrinos. Four-neutrino oscillations of the (3+1) type have been only marginally allowed before the recent MiniBooNE results, and become even more disfavored with the new data (at the level of $4\sigma$). In the framework of so-called (3+2) five-neutrino mass schemes we find severe tension between appearance and disappearance experiments at the level of more than $3\sigma$, and hence no satistfactory fit to the global data is possible in (3+2) schemes. This tension remains also when a third sterile neutrino is added, and the quality of the global fit does not improve significantly in a (3+3) scheme. It should be noted, however, that in models with more than one sterile neutrino the MiniBooNE results are in perfect agreement with the LSND appearance evidence, thanks to the possibility of CP violation available in such oscillation schemes. Furthermore, if disappearance data are not taken into account (3+2) oscillations provide an excellent fit to the full MiniBooNE spectrum including the event excess at low energies.Comment: 30 pages, 12 figures, minor improvements of text and abstract, summary table added, matches version to be published in Phys. Rev.

On the determination of the leptonic CP phase

The combination of data from long-baseline and reactor oscillation experiments leads to a preference of the leptonic CP phase $\delta_{\rm CP}$ in the range between $\pi$ and $2\pi$. We study the statistical significance of this hint by performing a Monte Carlo simulation of the relevant data. We find that the distribution of the standard test statistic used to derive confidence intervals for $\delta_{\rm CP}$ is highly non-Gaussian and depends on the unknown true values of $\theta_{23}$ and the neutrino mass ordering. Values of $\delta_{\rm CP}$ around $\pi/2$ are disfavored at between $2\sigma$ and $3\sigma$, depending on the unknown true values of $\theta_{23}$ and the mass ordering. Typically the standard $\chi^2$ approximation leads to over-coverage of the confidence intervals for $\delta_{\rm CP}$. For the 2-dimensional confidence region in the ($\delta_{\rm CP},\theta_{23}$) plane the usual $\chi^2$ approximation is better justified. The 2-dimensional region does not include the value $\delta_{\rm CP} = \pi/2$ up to the 86.3\% (89.2\%)~CL assuming a true normal (inverted) mass ordering. Furthermore, we study the sensitivity to $\delta_{\rm CP}$ and $\theta_{23}$ of an increased exposure of the T2K experiment, roughly a factor 12 larger than the current exposure and including also anti-neutrino data. Also in this case deviations from Gaussianity may be significant, especially if the mass ordering is unknown.Comment: 25 pages, 12 figures. Matches version which is to appear in JHEP. New appendix with the first anti-neutrino results from T2K is adde

Determination of the neutrino mass ordering by combining PINGU and Daya Bay II

The relatively large measured value of theta_13 has opened various possiblities to determine the neutrino mass ordering, among them using PINGU, the low-energy extention of the IceCube neutrino telescope, to observe matter effects in atmospheric neutrinos, or a high statistics measurment of the neutrino energy spectrum at a reactor neutrino experiment with a baseline of around 60 km, such as the Daya Bay II project. In this work we point out a synergy between these two approaches based on the fact that when data are analysed with the wrong neutrino mass ordering the best fit occurs at different values of |Delta m^2_31| for PINGU and Daya Bay II. Hence, the wrong mass ordering can be excluded by a mismatch of the values inferred for |Delta m^2_31|, thanks to the excellent accuracy for Delta m^2_31 of both experiments. We perform numerical studies of PINGU and Daya Bay II sensitivities and show that the synergy effect may lead to a high significance determination of the mass ordering even in situations where the individual experiments obtain only poor sensitivity.Comment: 23 pages, 7 figures (15 eps files), typos and minor mistakes corrected, final version to appear in JHE