The $\widehat{\mathrm{\Gamma }}$-genus and a regularization of an $S^1$-equivariant Euler class

We show that a new multiplicative genus, in the sense of Hirzebruch, can be obtained by generalizing a calculation due to Atiyah and Witten. We introduce this as the $\widehat{\mathrm{\Gamma }}$-genus, compute its value for some examples and highlight some of its interesting properties. We also indicate a connection with the study of multiple zeta values, which gives an algebraic interpretation for our proposed regularization procedure.