Astroparticle Physics

6.2 Neutrino Astronomy 89ν e + e − → ν e + e − , (6.14)or produce electrons in neutrino–nucleon interactionsν e + N → e − + N ′ . (6.15)Muon neutrinos are detected in neutrino–nucleon interactionsaccording toν µ + N → µ − + N ′ . (6.16)Electron antineutrinos and muon antineutrinos produce correspondinglypositrons and positive muons. The chargedleptons (e + , e − , µ + , µ − ) can be detected via the Cherenkoveffect in water. The produced Cherenkov light is measuredwith 11 200 photomultipliers of 50 cm cathode diameter.In the GeV-range electrons initiate characteristic electromagneticcascades of short range while muons producelong straight tracks. This presents a basis for distinguishingelectron from muon neutrinos. On top of that, muonscan be identified by their decay in the detector thereby givingadditional evidence concerning the identity of the initiatingneutrino species. Figures 6.9 and 6.10 show an electronand muon event in Super-Kamiokande. Muons have awell-defined range and produce a clear Cherenkov patternwith sharp edges while electrons initiate electromagneticcascades thereby creating a fuzzy ring pattern.The result of the Super-Kamiokande experiment is thatthe number of electron-neutrino events corresponds to thetheoretical expectation while there is a clear deficit of eventsinitiated by muon neutrinos. The measured electron andmuon spectra compared to expectation are shown in Figs.6.11 and 6.12.Because of the different acceptance for electrons andmuons in the water Cherenkov detector, the ratio of muonsto electrons is compared to a Monte Carlo simulation. Forthe double ratio(N µ /N e ) dataR =(6.17)(N µ /N e ) Monte Carloone would expect the value R = 1 in agreement with thestandard interaction and propagation models. However, theSuper-Kamiokande experiment obtainsFig. 6.9Cherenkov pattern of an energeticelectron in the Super-KamiokandeDetector {9}distinguishingelectron and muon neutrinosdeficit of muon neutrinosFig. 6.10Cherenkov pattern for an energeticmuon in the Super-Kamiokandedetector {9}R = 0.69 ± 0.06 , (6.18)which represents a clear deviation from expectation.