M-theory PP-waves, Penrose Limits and Supernumerary Supersymmetries

TL;DR

This paper explores supersymmetric pp-wave solutions in M-theory and related string theories, highlighting the conditions for supernumerary supersymmetries and their implications for solvable string models.

Contribution

It classifies a broad class of pp-waves, including Penrose limits and non-limit solutions, analyzing supersymmetry properties and coordinate dependencies of Killing spinors.

Findings

Identification of pp-waves with supernumerary supersymmetries.

Conditions for Killing spinors to be coordinate-independent.

Implications for solvable string and matrix models.

Abstract

We study supersymmetric pp-waves in M-theory, their dimensional reduction to D0-branes or pp-waves in type IIA, and their T-dualisation to solutions in the type IIB theory. The general class of pp-waves that we consider encompass the Penrose limits of AdS_p\times S^q with (p,q)=(4,7), (7,4), (3,3), (3,2), (2,3), (2,2), but includes also many other examples that can again lead to exactly-solvable massive strings, but which do not arise from Penrose limits. All the pp-waves in D=11 have 16 ``standard'' Killing spinors, but in certain cases one finds additional, or ``supernumerary,'' Killing spinors too. These give rise to linearly-realised supersymmetries in the string or matrix models. A focus of our investigation is on the circumstances when the Killing spinors are independent of particular coordinates (x^+ or transverse-space coordinates), since these will survive at the field-theory level in dimensional reduction or T-dualisation.