It was at the height of the Cold War, in the summer of 1950, when Bruno Pontecorvo mysteriously vanished behind the Iron Curtain. Who was he, and what caused him to disappear? Was he simply a physicist, or also a spy and communist radical? A prot g of Enrico Fermi, Pontecorvo was one of the most promising nuclear physicists in the world. He spent years hunting for the Higgs boson of his day--the neutrino--a nearly massless particle thought to be...