The University of Massachusetts, Amherst, has backed down on a plan to bar Iranian students from science and engineering programs following a wave of protests. Last week, the university said it would ban Iranian nationals from graduate programs in chemical, computer and mechanical engineering, as well as natural sciences, in what it said was a move to comply with U.S. sanctions dating back to 2012. But after a chorus of outrage from both students and faculty, the university said Wednesday it would lift the ban and implement a less restrictive policy.