It is shown that the supersymmetry-preserving automorphisms of any non-linear σ-model on K3 generate a subgroup of the Conway group Co1. This is the stringy generalisation of the classical theorem, due to Mukai and Kondo, showing that the symplectic automorphisms of any K3 manifold form a subgroup of the Mathieu group M23. The Conway group Co1 contains the Mathieu group M24 (and therefore in particular M23) as a subgroup. We confirm the predictions of the Theorem with three explicit CFT realisations of K3: the T 4 /Z2 orbifold at the self-dual point, and the two Gepner models (2) 4 and (1) 6 . In each case we demonstrate that their symmetries do not form a subgroup of M24, but lie inside Co1 as predicted by our

http://maths.dur.ac.uk/lms/103/talks/0701gab.pdf

Symmetries of K3 sigma models

Unravelling Mathieu Moonshine

Single centered supersymmetric black holes in four dimensions have spherically symmetric horizon and hence carry zero angular momentum. This leads to a specific sign of the helicity trace index associated with these black holes. Since the latter are given by the Fourier expansion coefficients of appropriate meromorphic modular forms of Sp(2,Z) or its subgroup, we are led to a specific prediction for the signs of a subset of these Fourier coefficients which represent contributions from single centered black holes only. We explicitly test these predictions for the modular forms which compute the index of quarter BPS black holes in heterotic string theory on T^6, as well as in Z_N CHL models for N=2,3,5,7.

How Do Black Holes Predict the Sign of the Fourier Coefficients of Siegel Modular Forms

Since euclidean global AdS_2 space represented as a strip has two boundaries, the state / operator correspondence in the dual CFT_1 reduces to the standard map from the operators acting on a single copy of the Hilbert space to states in the tensor product of two copies of the Hilbert space. Using this picture we argue that the corresponding states in the dual string theory living on AdS_2 x K are described by twisted version of the Hartle-Hawking states, the twists being generated by a large unitary group of symmetries that this string theory must possess. This formalism makes natural the dual interpretation of the black hole entropy, -- as the logarithm of the degeneracy of ground states of the quantum mechanics describing the low energy dynamics of the black hole, and also as an entanglement entropy between the two copies of the same quantum theory living on the two boundaries of global AdS_2 separated by the event horizon.

State Operator Correspondence and Entanglement in AdS_2/CFT_1

We consider four dimensional CHL models with sixteen spacetime supersymmetries obtained from orbifolds of type IIA superstring on K3 x T^2 by a Z_N symmetry acting (possibly) non-geometrically on K3. We show that most of these models (in particular, for geometric symmetries) are self-dual under a weak-strong duality acting on the heterotic axio-dilaton modulus S by a "Fricke involution" S --> -1/NS. This is a novel symmetry of CHL models that lies outside of the standard SL(2,Z)-symmetry of the parent theory, heterotic strings on T^6. For self-dual models this implies that the lattice of purely electric charges is N-modular, i.e. isometric to its dual up to a rescaling of its quadratic form by N. We verify this prediction by determining the lattices of electric and magnetic charges in all relevant examples. We also calculate certain BPS-saturated couplings and verify that they are invariant under the Fricke S-duality. For CHL models that are not self-dual, the strong coupling limit is dual to type IIA compactified on T^6/Z_N, for some Z_N-symmetry preserving half of the spacetime supersymmetries.

/pdf/fricke-s-duality-in-chl-models-4m7t4d2xyw.pdf

Fricke S-duality in CHL models

BKM Lie superalgebras from counting twisted CHL dyons - II

Two moonshines for L2(11) but none for M12

A second-quantized version of Mathieu moonshine leads to product formulae for functions that are potentially genus-two Siegel Modular Forms analogous to the Igusa Cusp Form. The modularity of these functions do not follow in an obvious manner. For some conjugacy classes, but not all, they match known modular forms. In this paper, we express the product formulae for all conjugacy classes of $M_{24}$ in terms of products of standard modular forms. This provides a new proof of their modularity.

/pdf/mathieu-moonshine-and-siegel-modular-forms-14th6kvjnu.pdf

Mathieu Moonshine and Siegel Modular Forms

We construct a semi-holographic effective theory in which the electron of a two-dimensional band hybridizes with a fermionic operator of a critical holographic sector, while also interacting with other bands that preserve quasiparticle characteristics. Besides the scaling dimension $\nu$ of the fermionic operator in the holographic sector, the effective theory has two {dimensionless} couplings $\alpha$ and $\gamma$ determining the holographic and Fermi-liquid-type contributions to the self-energy respectively. When $\sim 0.67 < \nu < \sim 0.8$, the DC resistivity is linear in $T$ to a remarkably good approximation in a narrow range of $\alpha/\gamma$ and over a range of temperatures extending from about $0.3\,E_F$ to about $20-40 \, E_F$ for small couplings. For higher values of $\nu$ lesser than unity which realizes a marginal Fermi liquid with a specific temperature dependence of the self-energy, we also find linear-in-T resistivity to reasonably good approximation in an even wider range of temperatures that extends to arbitrarily small values.

Linear-in-T resistivity from semiholographic non-Fermi liquid models

A second-quantized version of Mathieu moonshine leads to product formulae for functions that are potentially genus-two Siegel Modular Forms analogous to the Igusa Cusp Form. The modularity of these functions do not follow in an obvious manner. For some conjugacy classes, but not all, they match known modular forms. In this paper, we express the product formulae for all conjugacy classes of M24 in terms of products of standard modular forms. This provides a new proof of their modularity.

/pdf/mathieu-moonshine-and-siegel-modular-forms-1unhr504qn.pdf

Sutapa Samanta

Papers

BKM Lie superalgebras from counting twisted CHL dyons - II

Two moonshines for L2(11) but none for M12

Mathieu Moonshine and Siegel Modular Forms

Linear-in-T resistivity from semiholographic non-Fermi liquid models

Mathieu moonshine and Siegel Modular Forms