What is the test for an arbitrator to vacate an arbitration award for failing to disclose their relationship with the opposing party's counsel?

. . . If an arbitrator 'failed to disclose within the time required for disclosure a ground for disqualification of which the arbitrator was then aware,' the trial court must vacate the arbitration award. ( 1286.2, subd. (a)(6)(A).)" (Haworth v. Superior Court (2010) 50 Cal.4th 372, 381.) The party petitioning to vacate the award is not required to show that the nondisclosure prejudiced him. (Id., at p. 383.) Because the material facts here are not in dispute, "[w]hether [Saxe] was required to disclose the [relationships between her and respondents' counsel] is a mixed question of fact and law that should be reviewed de novo." (Id., at p. 385.)

"The question is not whether [Saxe] actually was biased or even whether [s]he was likely to be impartial . . . . The question here is how an objective, reasonable person would view [her] ability to be impartial. [Citations.]" (Haworth v. Superior Court, supra, 50 Cal.4th at pp 385-386.) "'Impartiality' entails the 'absence of bias or prejudice in favor of, or against, particular parties or classes of parties, as well as maintenance of an open mind.' [Citation.]" (Id., at p. 389.)