Zhang-Kawazumi Invariants and Superstring Amplitudes

TL;DR

This paper explores the connection between Zhang-Kawazumi invariants, number theory, and superstring amplitudes, revealing how higher-genus invariants relate to string corrections and proposing a way to compare theoretical predictions with string theory results.

Contribution

It introduces a novel link between Zhang-Kawazumi invariants and superstring amplitude corrections, extending the understanding of modular forms in string theory at higher genus.

Findings

The D^6 R^4 term at two loops is proportional to a Zhang-Kawazumi invariant.

Higher invariants generalize Zhang-Kawazumi invariants for genus two.

An explicit formula for the higher invariant associated with D^8 R^4 is derived.

Abstract

Invariance of Type IIB superstring theory under SL(2,Z) or S-duality implies dependence on the complex coupling T through real and complex modular forms in T. Their structure may be understood explicitly in an expansion of superstring corrections to Einstein's equations of gravity, in powers of derivatives D and curvature R. The perturbative loop expansion in the string coupling for the 4-string amplitude governs corrections of the form D^{2p} R^4 for all p. We show that, at two-loop order, the D^6 R^4 term is proportional to the integral of a modular invariant introduced by Zhang and Kawazumi in number theory and related to the Faltings delta-invariant studied for genus-two by Bost. The structure of two-loop superstring amplitudes for p>3 leads to higher invariants, which generalize Zhang--Kawazumi invariants at genus two. An explicit formula is derived for the unique higher invariant associated with order D^8 R^4. In an attempt to compare the prediction for the D^6 R^4 correction from superstring perturbation theory with the one produced by S-duality and supersymmetry of Type IIB, various reformulations of the invariant are given. This comparison with string theory leads to a predicted value for the integral of the Zhang-Kawazumi invariant over the moduli space of genus-two surfaces.