Authorities are searching for a suspect in the killing of Nuno F.G. Loureiro, a prominent physics professor at the Massachusetts Institute of Technology who was shot at his home near Boston. Loureiro, a married 47-year-old from Portugal, was shot Monday night and died Tuesday at a local hospital. Authorities have not disclosed a possible motive, and no suspects were in custody as of Wednesday morning, the Norfolk District Attorney’s Office said.