Here’s another reason for William Petersen to grumble. The ”CSI: Crime Scene Investigation” star has complained that CBS has diluted the franchise with spinoffs, even hinting that he’ll quit when his contract is up in a couple years. Now, according to Variety and The Hollywood Reporter, CBS is bringing to Las Vegas another handsome scientist who’ll probe people’s innards. It’s Rob Lowe, rebounding from his short-lived NBC drama ”The Lyon’s Den” to star in a new CBS pilot called ”Dr. Vegas.” He’ll play Dr. George Rivers, the staff medic at a casino. It may look like a comedown from serving in the White House on ”The West Wing,” but at least in ”Vegas,” he’ll be working at a place where the house always wins.