ZURICH, Switzerland — UBS AG said Thursday that it would raise $15.5 billion in a rights issue at a 31 percent discount below the current share price. UBS, hard hit by its exposure to the U.S. mortgage market, said it will sell new shares at 21 francs ($20.09) each to existing shareholders, compared with the closing price of 30.64 francs ($29.31) on the Zurich exchange Wednesday.