A second-year mechanical engineering student, Zahra Khalkhali of Iran, who inadvertently sparked controversial new restrictions on Iranian students at the University of Massachusetts this month has become a cause célèbre on campus. She recently visited her family back home, but when she began making preparations to return, a routine immigration inquiry into her research — on fuel cells used to produce clean energy — prompted the university to drop its sponsorship over concerns her work could be seen as violating a 2012 law mean to prevent Iran from developing a nuclear weapon. UMass soon reversed its ban and now students who fought the policy want Khalkhali back. "The student did not do anything wrong, and we regret that she endured a difficult and trying period at the airport," UMass Chancellor Kumble R. Subbaswamy said Tuesday. Khalkhali heard about that, and it has given her hope.