U.S. stocks closed flat Tuesday, amid a sharp sell-off in Hewlett-Packard shares and renewed concerns about Europe. The Dow Jones industrial average slipped 7.45, or 0.06 percent, to 12,788.51. The broader Standard & Poor's 500 index gained 0.93, or 0.07 percent, to 1,387.82. The technology-heavy Nasdaq composite index rose 0.61, or 0.02 percent, to 2,916.68.