How might the Fed's large-scale asset purchases lower long-term interest rates?

Over the past two years the Federal Reserve has engaged in large-scale asset purchases (LSAPs), often grouped under the heading of "quantitative easing, " as an alternative means of stimulating the economy when policy rates are at their zero lower bound. Theoretical and empirical research shows how this policy may lower long-term interest rates, and financial market data suggest that the initial launch of LSAPs had an effect on expectations about future Fed policy.