BKM Lie superalgebra for the Z_5 orbifolded CHL string

TL;DR

This paper constructs a hyperbolic BKM Lie superalgebra related to the Z_5 orbifolded CHL string, analyzes its modular forms, and explores the structure of walls of marginal stability for various orbifold models.

Contribution

It introduces the first hyperbolic BKM Lie superalgebra associated with the Z_5 orbifolded CHL string and extends the understanding of walls of marginal stability to N=4,5,6 models.

Findings

Constructed the modular form tilde{Phi}_2 for Z_5 orbifolded CHL string.

Identified the BKM Lie superalgebra tilde{G}_5 as hyperbolic.

Extended the arithmetic structure of walls of marginal stability to N=4,5,6 models.

Abstract

We study the Z_5-orbifolding of the CHL string theory by explicitly constructing the modular form tilde{Phi}_2 generating the degeneracies of the 1/4-BPS states in the theory. Since the additive seed for the sum form is a weak Jacobi form in this case, a mismatch is found between the modular forms generated from the additive lift and the product form derived from threshold corrections. We also construct the BKM Lie superalgebra, tilde{G}_5, corresponding to the modular form tilde{Delta}_1 (Z) = tilde{Phi}_2 (Z)^{1/2} which happens to be a hyperbolic algebra. This is the first occurrence of a hyperbolic BKM Lie superalgebra. We also study the walls of marginal stability of this theory in detail, and extend the arithmetic structure found by Cheng and Dabholkar for the N=1,2,3 orbifoldings to the N=4,5 and 6 models, all of which have an infinite number of walls in the fundamental domain. We find that analogous to the Stern-Brocot tree, which generated the intercepts of the walls on the real line, the intercepts for the N >3 cases are generated by linear recurrence relations. Using the correspondence between the walls of marginal stability and the walls of the Weyl chamber of the corresponding BKM Lie superalgebra, we propose the Cartan matrices for the BKM Lie superalgebras corresponding to the N=5 and 6 models.