Jeff Larson is back at work, which is saying something. In July 2007, the Boston hedge fund manager lost $1.5 billion in a matter of days, and it seemed he might never sell a security again. Larson’s crucial misstep: He bought corporate bonds on margin, meaning he borrowed a lot of money in an effort to buy even more bonds. But the bonds tanked when the credit market froze last year, and though Larson’s move didn’t wipe out his investors, he did have to close his ﬁrm, Sowood Capital; ﬁre 90 employees; and give up his Back Bay ofﬁces. “A loss of this magnitude is as devastating to us as it is to you,” he said in an apologetic conference call with his clients, which included Harvard University, the Boston Foundation, and the state pension system.