Dunbar, D., Bidder, S. J., Bjerrum-Bohr, N., & Perkins, W. B. (2005). Twistor space structure of the box coefficients of one-loop amplitudes. Physics Letters B, 608(1-2), pp. 151-163. doi:10.1016/j.physletb.2004.12.030

Dunbar, David, Steven J. Bidder, N.E.J Bjerrum-Bohr, and Warren B. Perkins. "Twistor Space Structure of the Box Coefficients of One-loop Amplitudes." Physics Letters B 608, no. 1-2 (2005): 151-163.

Dunbar, David, Steven J. Bidder, N.E.J Bjerrum-Bohr, and Warren B. Perkins. "Twistor Space Structure of the Box Coefficients of One-loop Amplitudes." Physics Letters B 608.1-2 (2005): 151-163.