Fifty years ago, Arthur Mitchell, an African American star at the New York City Ballet, responded to the assassination of Martin Luther King, Jr., by founding a new ballet company in a garage in Harlem. The Dance Theatre of Harlem was instantly embraced, toured the world, and maintained a pantheon of ballerinas many of us have never forgotten. Celebrating its 50th birthday at the Guggenheim’s up-to-the-minute Works & Process series, DTH performs works from the repertory, including the first three themes of George Balanchine’s The Four Temperaments. —L.J.