CAMBRIDGE – Forget the stock market and the low unemployment rate: the US economy isn’t working. Productivity growth, a key gauge of economic health, remains historically low. Median wages, an indicator of middle-class living standards, have barely grown in four decades. Inequality is high, and market power is increasingly concentrated in the hands of just a few companies. Americans used to dismiss Europe as the land of government-protected, noncompetitive firms. Now, European markets in most industries appear more competitive than those in the United States