Valeant Pharmaceuticals, parent company of Bausch & Lomb, has initiated the search for a new chief executive following the resignation of J. Michael Pearson, who returned to the job after a two-month medical leave. He will remain on duty until the appointment of his successor. Meanwhile, Valeant announced the nomination of a new director, William A. Ackman, to its board. He heads up an investment company, Pershing Square Capital Management, that owns 9.0 percent of Valeant's shares and led a failed attempt by the group to take over Allergan last year.