On the Sudakov form factor, and a factor of two

TL;DR

This paper investigates why the Sudakov form factor doubles when computed for off-shell fermions versus on-shell, revealing insights into infrared and collinear singularities and emphasizing the significance of factors of two.

Contribution

The paper provides an explanation for the factor of two difference in the Sudakov form factor between off-shell and on-shell fermion calculations.

Findings

The Sudakov form factor is larger by a factor of two for off-shell fermions.

Infrared and collinear singularities play a crucial role in this difference.

Factors of two are essential in understanding the form factor behavior.

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.