The hypermultiplet moduli space in Type IIA string theory compactified on a rigid Calabi-Yau threefold X , corresponding to the "universal hypermultiplet", is described at tree-level by the symmetric space SU (2, 1)/(SU (2) × U (1)). To determine the quantum corrections to this metric, we posit that a discrete subgroup of the continuous tree-level isometry group SU (2, 1), namely the Picard modular group SU (2, 1; Z[i]), must remain unbroken in the exact metric -including all perturbative and non-perturbative quantum corrections. This assumption is expected to be valid when X admits complex multiplication by Z[i]. Based on this hypothesis, we construct an SU (2, 1; Z[i])-invariant, non-holomorphic Eisenstein series, and tentatively propose that this Eisenstein series provides the exact contact potential on the twistor space over the universal hypermultiplet moduli space. We analyze its non-Abelian Fourier expansion, and show that the Abelian and non-Abelian Fourier coefficients take the required form for instanton corrections due to Euclidean D2branes wrapping special Lagrangian submanifolds, and to Euclidean NS5-branes wrapping the entire Calabi-Yau threefold, respectively. While this tentative proposal fails to reproduce the correct one-loop correction, the consistency of the Fourier expansion with physics expectations provides strong support for the usefulness of the Picard modular group in constraining the quantum moduli space. E SL(2,Z) 3/2 as a function of the "axio-dilaton" C (0) + ie −φ , valued on the fundamental domain M = SL(2, Z)\SL(2, R)/SO(2) of the Poincaré upper half plane. This proposal reproduced the known tree-level and one-loop corrections [6,7], predicted the absence of higher loop corrections, later verified by an explicit two-loop computation [8], and suggested the exact form of D(-1)-instanton contributions, later corroborated by explicit matrix model computations [9,10]. From the mathematical point of view, perturbative corrections and instanton contributions correspond, respectively, to the constant terms and Fourier coefficients of the automorphic form E SL(2,Z) 3/2 . This work was extended to toroidal compactifications of Mtheory, where the R 4 -type corrections were argued to be given by Eisenstein series of the respective U-duality group [11][12][13], predicting the contributions of Euclidean Dp-brane instantons, and, when n ≥ 6, NS5-branes. Unfortunately, extracting the constant terms and Fourier coefficients of Eisenstein series is not an easy task, and it has been difficult to put the conjecture to the test. Part of our motivation is to develop the understanding of Eisenstein series beyond the relatively well understood case of G(Z) = SL(n, Z).
The Hypermultiplet Moduli Space of N = 2 SupergravityCompactifications with fewer unbroken supersymmetries (N ≤ 2 in D = 4) lead to moduli spaces which are generically not symmetric spaces. An interesting example is type IIA string theory compactified on a Calabi-Yau threefold X , leading to N = 2 supergravity in four dimensions coupled to ...