Most Federal Reserve policymakers expect the U.S. jobless rate will stop plunging and stabilize right around its long-term normal level, a risky forecast given that this apparently hasn’t happened in at least a half century. Across decades of economic records as the economy healed after recessions, the rate dipped well below what analysts consider normal, the so-called “natural rate” of unemployment. Fed policymakers are betting history will unfold differently this time.