On the Sudakov form factor, and a factor of two

Abstract

I answer a question that Roman Jackiw asked me, and I draw some lessons from the answer. The question is: why is the Sudakov form factor larger by a factor of two, if computed for off-shell fermions, in comparison to the on-shell case? The answer sheds some light on the interplay between infrared and collinear singularities -- and the importance of factors of two.

19 pages, 2 figures. Contribution to the volume "Roman Jackiw -- 80th Birthday Festschrift". Note added with important extra references and some historical remarks; some typos corrected. Two more typos corrected and a note added. Final version, to be published in the Jackiw Festschrift volume