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Register a new userIndirect searches for sterile neutrinos at a high-luminosity Z-factory
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A future high-luminosity $Z$-factory has the potential to investigate lepton flavour violation. Rare decays such as $Z to ell_1^mp ell_2^pm$ can be complementary to low-energy (high-intensity) observables of lepton flavour violation. Here we consider two extensions of the Standard Model which add to its particle content one or more sterile neutrinos. We address the impact of the sterile fermions on lepton flavour violating $Z$ decays, focusing on potential searches at FCC-ee (TLEP), and taking into account experimental and observational constraints. We show that sterile neutrinos can give rise to contributions to BR($Z to ell_1^mp ell_2^pm$) within reach of the FCC-ee. We discuss the complementarity between a high-luminosity $Z$-factory and low-energy charged lepton flavour violation facilities.
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We study the optimization of a green-field, two-baseline reactor experiment with respect to the sensitivity for electron antineutrino disappearance in search of a light sterile neutrino. We consider both commercial and research reactors and identify as key factors the distance of closest approach and detector energy resolution. We find that a total of 5 tons of detectors deployed at a commercial reactor with a closest approach of 25 m can probe the mixing angle $sin^22theta$ down to $sim5times10^{-3}$ around $Delta m^2sim 1$ eV$^2$. The same detector mass deployed at a research reactor can be sensitive up to $Delta m^2sim20-30$ eV$^2$ assuming a closest approach of 3 m and excellent energy resolution, such as that projected for the Taishan Antineutrino Observatory (TAO). We also find that lithium doping of the reactor could be effective in increasing the sensitivity for higher $Delta m^2$ values.
Neutrino physics is nowadays receiving more and more attention as a possible source of information for the long-standing problem of new physics beyond the Standard Model. The recent measurement of the mixing angle $theta_{13}$ in the standard mixing oscillation scenario encourages us to pursue the still missing results on leptonic CP violation and absolute neutrino masses. However, puzzling measurements exist that deserve an exhaustive evaluation. The NESSiE Collaboration has been setup to undertake conclusive experiments to clarify the muon-neutrino disappearance measurements at small $L/E$, which will be able to put severe constraints to models with more than the three-standard neutrinos, or even to robustly measure the presence of a new kind of neutrino oscillation for the first time. To this aim the use of the current FNAL-Booster neutrino beam for a Short-Baseline experiment has been carefully evaluated. Its recent proposal refers to the use of magnetic spectrometers at two different sites, Near and Far ones. Their positions have been extensively studied, together with the possible performances of two OPERA-like spectrometers. The proposal is constrained by availability of existing hardware and a time-schedule compatible with the undergoing project of a multi-site Liquid-Argon detectors at FNAL. The experiment to be possibly setup at Booster will allow to definitively clarify the current $ u_{mu}$ disappearance tension with $ u_{e}$ appearance and disappearance at the eV mass scale.
For a long time there were 3 main experimental indications in favor of the existence of sterile neutrinos: $bar{ u_e}$ appearance in the $bar{ u_mu}$ beam in the LSND experiment, $bar{ u_e}$ flux deficit in comparison with theoretical expectations in reactor experiments, and $ u_e$ deficit in calibration runs with radioactive sources in the Ga solar neutrino experiments SAGE and GALEX. All three problems can be explained by the existence of sterile neutrinos with the mass square difference in the ballpark of $1~mathrm{eV^2}$. Recently the MiniBooNE collaboration observed electron (anti)neutrino appearance in the muon (anti)neutrino beams. The significance of the effect reaches 6.0$sigma$ level when combined with the LSND result. Even more recently the NEUTRINO-4 collaboration claimed the observation of $bar{ u_e}$ oscillations to sterile neutrinos with a significance slightly higher than 3$sigma$. If these results are confirmed, New Physics beyond the Standard Model would be required. More than 10 experiments are devoted to searches of sterile neutrinos. Six very short baseline reactor experiments are taking data just now. We review the present results and perspectives of these experiments.
The IceCube neutrino telescope at the South Pole has measured the atmospheric muon neutrino spectrum as a function of zenith angle and energy in the approximate 320 GeV to 20 TeV range, to search for the oscillation signatures of light sterile neutrinos. No evidence for anomalous $ u_mu$ or $bar{ u}_mu$ disappearance is observed in either of two independently developed analyses, each using one year of atmospheric neutrino data. New exclusion limits are placed on the parameter space of the 3+1 model, in which muon antineutrinos would experience a strong MSW-resonant oscillation. The exclusion limits extend to $mathrm{sin}^2 2theta_{24} leq$ 0.02 at $Delta m^2 sim$ 0.3 $mathrm{eV}^2$ at the 90% confidence level. The allowed region from global analysis of appearance experiments, including LSND and MiniBooNE, is excluded at approximately the 99% confidence level for the global best fit value of $|$U$_{e4}|^2$.
We study the potential of the CNGS beam in constraining the parameter space of a model with one sterile neutrino separated from three active ones by an $mathcal{O}(eVq)$ mass-squared difference, $Dmq_Sbl$. We perform our analysis using the OPERA detector as a reference (our analysis can be upgraded including a detailed simulation of the ICARUS detector). We point out that the channel with the largest potential to constrain the sterile neutrino parameter space at the CNGS beam is $ u_mu to u_tau$. The reason for that is twofold: first, the active-sterile mixing angle that governs this oscillation is the less constrained by present experiments; second, this is the signal for which both OPERA and ICARUS have been designed, and thus benefits from an extremely low background. In our analysis we also took into account $ u_mu to u_e$ oscillations. We find that the CNGS potential to look for sterile neutrinos is limited with nominal intensity of the beam, but it is significantly enhanced with a factor 2 to 10 increase in the neutrino flux. Data from both channels allow us, in this case, to constrain further the four-neutrino model parameter space. Our results hold for any value of $Dmq_Sbl gtrsim 0.1 eVq$, textit{i.e.} when oscillations driven by this mass-squared difference are averaged. We have also checked that the bound on $theta_{13}$ that can be put at the CNGS is not affected by the possible existence of sterile neutrinos.
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