According to interviews with half a dozen current and former Fed policymakers and staff, the concept that the economy can produce far lower levels of unemployment without stoking inflation is being built into Fed models and becoming increasingly entrenched in the central bank’s views. Fed policymakers’ December projections show most expect the Fed’s benchmark rate to rise to 2.5 percent or above by the end of 2016 from the 0-0.25 percent range now.