A Jet Break in the X-ray Light Curve of Short GRB 111020A: Implications for Energetics and Rates par Fong, W, Berger, E, Margutti, R, Zauderer, B, Troja, E, Czekala, I, Chornock, R, Gehrels, N, Sakamoto, T, Fox, D, Podsiadlowski, P