Advances in Theoretical and Mathematical Physics

Volume 21 (2017)

Number 7

Special Issue: Proceedings of the Strings 2016 Conference in Beijing

Guest Editors: J. Maldacena (Institute for Advanced Study), H. Ooguri (California Institute of Technology), H. Babak (Harvard University), S. Li (Tsinghua University), W. Song (Tsinghua University), and H. Lin (Tsinghua University)

The hidden symmetry of the heterotic string

Pages: 1729 – 1745



Shamit Kachru (Stanford Institute for Theoretical Physics, Stanford University, Palo Alto, California, U.S.A.)

Arnav Tripathy (Department of Mathematics, Harvard University, Cambridge, Massachusetts, U.S.A.)


We propose that Borcherds’ Fake Monster Lie algebra is a broken symmetry of heterotic string theory compactified on $T^7 \times T^2$. As evidence, we study the fully flavored counting function for BPS instantons contributing to a certain loop amplitude. The result is controlled by $\Phi_{12}$, an automorphic form for $O (2, 26, \mathbb{Z})$. The degeneracies it encodes in its Fourier coefficients are graded dimensions of a second-quantized Fock space for this large symmetry algebra. This construction provides a concrete realization of Harvey and Moore’s proposed relationship between Generalized Kac–Moody symmetries and supersymmetric string vacua.

Published 19 March 2018