The neutrino is a pesky critter: nearly impossible to detect and changing flavor at the drop of a hat. Enrico Fermi and Bruno Pontecorvo were the first to realize that neutrinos must exist (based on conservation of spin), but it was to be a long time from those first conjectures to the first definitive detection of a neutrino. There are actually three flavors of neutrino (corresponding to the three leptons: electrons, muons, and tau particles), plus their anti-neutrino counterparts. It turns out that any given neutrino detection experiment can’t just detect any neutrino: if you are looking for a muon neutrino and an electon neutrino happens by, you won’t see it. In fact, you won’t see many muon neutrinos either, because their chance of interacting with matter is about the same as your chance of winning the lottery 3 times in a row. Neutrinos that formed shortly after the big bang are mostly still zipping through space, completely unaware that anything exists in the universe.
Making things more difficult for the experimenter is that neutrinos change flavor in flight: solar electon neutrinos spontaneously become muon neutrinos on their journey, and change back with a now known frequency, resulting in 2/3 fewer electron neutrinos reaching the earth than were predicted.
Frank Close does a good job telling this story. I would have liked to see more diagrams and maybe an equation or two, and photographs of the gigantic neutrino detectors would have been nice, but he gets the story across even without the visual aids. So, kudos.