The search for new physics in Fermilab’s Booster neutrino beamline

In this joint MiniBooNE-SciBooNE analysis, the experiments have excluded the area to the right of the 90 percent confidence level limit curve (blue) as a region where muon antineutrino disappearance may be occurring. This significantly narrows down the oscillation region for sterile neutrinos.

The conventional picture of neutrinos involves three types: the electron neutrino (and antineutrino), the muon (anti)neutrino, and the tau (anti)neutrino. As they zip through matter or space, the three types, or flavors, continually morph into each other in a phenomenon called neutrino oscillation. On the rare occasions that they interact with matter, they do so through the weak nuclear force.

But there is growing evidence for short-baseline neutrino anomalies that cannot be explained by the conventional picture. These anomalies include both an excess of events observed by Los Alamos National Laboratory’s Liquid Scintillator Neutrino Detector experiment and Fermilab’s MiniBooNE experiment and a deficit of events observed by reactor and radioactive-source experiments. If these anomalies are in fact due to neutrinos changing from one flavor to another, then there must be one or more new kinds of neutrinos. These additional neutrinos may be what are called “sterile neutrinos” because they do not interact by the weak nuclear force.

The MiniBooNE experiment, operating in Fermilab’s Booster neutrino beamline since 2002, can search for the appearance of electron (anti)neutrinos or the disappearance of muon (anti)neutrinos. MiniBooNE has reported on appearance signals consistent with the Los Alamos LSND experiment. If electron (anti)neutrino appearance in the Booster neutrino beamline is due to oscillations, with a sterile neutrino acting as an intermediary between electron flavor and muon flavor (anti)neutrinos, then observation of the muon (anti)neutrino disappearance would be a smoking-gun signal for the presence of these sterile neutrinos. With some back-of-the-envelope calculations, it can be shown that on the order of 10 percent of the muon (anti)neutrinos, at certain energies, should be transitioning to sterile neutrinos and hence be unobservable.

MiniBooNE performed a search for muon neutrino or antineutrino disappearance in 2009 and found no evidence for disappearance. However, the antineutrino analysis in particular was severely limited by systematic and statistical uncertainties. The experiment repeated the search for muon neutrino disappearance last year, with the inclusion of data from the SciBooNE detector. While MiniBooNE operated at a distance of 540 meters from the neutrino production target, SciBooNE was at a distance of 100 meters and helped to reduce some of the uncertainties in the number of neutrinos in the beam and the likelihood for neutrinos to interact in the detector. Once again, the results were consistent with no disappearance.

We have just completed an improved search for muon antineutrino disappearance using data from both the MiniBooNE and SciBooNE detectors. By bootstrapping off other muon neutrino measurements in MiniBooNE and SciBooNE, as well as by using a larger antineutrino data set and including improvements in the analysis methodology, the collaborations were able to significantly improve sensitivity for muon antineutrino disappearance. The results obtained are consistent with no short-baseline disappearance, and they have dramatically improved the excluded regions of the parameter space for short-baseline neutrino oscillations, pushing into the region of interest for sterile neutrino models. At Δm2 =1 eV2, the 90 percent confidence level limit is at sin2(2θ) = 0.12.

—Warren Huelsnitz

These physicists are responsible for this analysis.