Just the law, ma'am. Back in 1998, the Second Circuit, in Halligan v. Piper Jaffray, Inc., held that an arbitration award could be vacated if the arbitrator demonstrated manifest disregard of the law and the facts. However, in Wallace v. Buttar, the Court has reversed course and held that manifest disregard of the facts is not a basis for vacating an arbitration award. The Court stated: "To the extent that a federal court may look upon the evidentiary record of an arbitration proceeding at all, it may do so only for the purpose of discerning whether a colorable basis exists for the panel's award so as to assure that the award cannot be said to be the result of the panel's manifest disregard of the law."
The decision can be found here.