Joseph R. Biden Jr. is looking for a potential vice president in one of the most tumultuous moments in modern American history. His selection committee is attempting to winnow an exceptionally diverse field. So who’s on the list? Guest: Alexander Burns, who covers national politics for The New York Times. For more information on today’s episode, visit nytimes.com/thedaily  Background reading: This is where the top candidates stand in Mr. Biden’s search for a running mate.