Citigroup Inc. named Vikram Pandit, the head of its investment banking business, as chief executive after searching five weeks for someone to restore the bank's profitability and reputation. The banking company also on Tuesday named Sir Win Bischoff, who has been Citi's acting CEO, as its chairman. He replaces Robert E. Rubin, who had stepped into the role when former CEO and chairman Charles Prince was ousted last month.