D. Knauss, K. Scharnhorst: Two-loop renormalization of non-smooth string operators in Yang-Mills theory. Annalen der Physik (Leipzig), 7. Series, 41 (496 of all series altogether):4(1984)331-344 (DOI: 10.1002/andp.19844960413). [INSPIRE record]
Abstract: The renormalization constants for string operators both with smooth and non-smooth non-self-intersecting open contour have been evaluated up to the order g4.