Short-baseline Neutrinos: Recent Results and Future Prospects

The evidence is compelling that neutrinos undergo flavor change as they propagate. In recent years, experiments have observed the phenomenon of neutrino oscillations using disparate neutrino sources: the sun, fission reactors, accelerators, and secondary cosmic rays. The standard model of particle physics needs only simple extensions – neutrino masses and mixing – to accommodate all neutrino oscillation results to date, save one. The 3.8σ-significant νe excess reported by the LSND collaboration [1] is consistent with νμ→ νe oscillations with a mass-squared splitting of ∆m 2 ∼ 1 eV. This signal, which has not been independently verified, is inconsistent with other oscillation evidence unless more invasive standard model extensions (e.g., sterile neutrinos) are considered. The Mini Booster Neutrino Experiment (MiniBooNE) is designed to search for νμ→ νe oscillations with sufficient sensitivity to confirm or refute the LSND signal. MiniBooNE uses the Fermilab Booster neutrino beam, which begins with 8 GeV protons impinging on a beryllium target. The positively charged secondary mesons (mostly π, but some K) produced in the target are magnetically focused forward into a 50 m air-filled decay region. Their subsequent decay chains lead to the high intensity ∼1 GeV neutrino source. The neutrinos are predominantly νμ, but K and μ decays lead to a 0.6% νe contamination that represents a large irreducible background to the νμ→ νe search (as the best-fit LSND oscillation probability is ∼0.3%). The MiniBooNE detector sits 541 m downstream of the proton target, with most of the space in between occupied by earth. The detector is a 6.1 m radius spherical steel tank filled with 800 tons of mineral oil. An opaque shell of diameter 5.75 m, concentric with the steel tank, divides the oil into two optically isolated regions. The thin outer region is instrumented with 240 8-inch PMTs and serves as a veto shield for incoming cosmic rays and for partially contained neutrino events. The inner main region is viewed by 1280 8-inch PMTs. The Cherenkov (and, at a lower level,