Search for electron antineutrino interactions with the Borexino counting test facility at Gran Sasso

Electron antineutrino interactions above the inverse beta decay energy of protons ($E_\bar{\nu}_e>$1.8) where looked for with the Borexino Counting Test Facility (CTF). One candidate event survived after rejection of background, which included muon-induced neutrons and random coincidences. An upper limit on the solar ν¯e flux, assumed having the 8B solar neutrino energy spectrum, of 1.1×105 cm−2~s−1 (90% C.L.) was set with a 7.8 ton × year exposure. This upper limit corresponds to a solar neutrino transition probability, νe→ν¯e, of 0.02 (90% C.L.). Predictions for antineutrino detection with Borexino, including geoneutrinos, are discussed on the basis of background measurements performed with the CTF.