An experimental drug can sharply increase levels of H.D.L., the good cholesterol, potentially offering an entirely new way to help prevent heart attacks, researchers have found. In a preliminary study, researchers at the University of Pennsylvania and Tufts University found that the drug doubled H.D.L., or high density lipoprotein, in people with worrisomely low levels of the cholesterol. The drug, called torcetrapib, also reduced low density lipoprotein, or L.D.L., the bad cholesterol. Until now, doctors have concentrated largely on lowering bad cholesterol by giving patients statin drugs, which are used by millions of Americans and reduce heart attacks by about...