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Ashoke Sen

Publications and source records attributed to Ashoke Sen.

At least 19 recordsLinked to original sources
arXiv2026

A New Term in Type II Effective Action

We show that the one loop effective action of type IIA and IIB string theories in ten dimensions have terms proportional to the dilaton times the ten dimensional Euler density, in apparent violation of the soft dilaton theorem. This can be traced to the existence of the zero modes of the world-sheet superconformal ghost fields and resolves a puzzle that arose recently in the analysis of black hole index when we compactify these theories on Calabi-Yau manifolds of non-zero Euler number.

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arXiv2026

Extended Supergravity Needs String Scale Cut-off

Many string compactifications down to four non-compact space-time dimensions with N=8, N=6 and N=4 supersymmetry have BPS black holes carrying pure D-brane charges and preserving four supersymmetries. The string coupling does not flow in these backgrounds and can be set to any arbitrary value. Therefore the supersymmetric index of these black holes must be independent of the string coupling. On the other hand, explicit computation of one loop correction to the index from gravitational path integral is sensitive to the choice of ultraviolet cut-off. We show that if the cut-off scale is chosen to be the string scale in accordance with the rules of string theory, then the dependence of the index on the string coupling disappears in accordance with the expectation from supersymmetry. Similar results are obtained for type II string theories compactified on Calabi-Yau manifolds with zero Euler number. For non-zero Euler number we encounter a puzzle that we discuss but do not fully resolve.

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arXiv2026

Instanton-Induced Closed-String Amplitudes in Minimal Superstring Theory at Subleading Order

We compute the disk one-point function, the disk two-point function, and the annulus one-point function of the cosmological constant operator in the type 0A and type 0B minimal superstring theories with (1,1) ZZ instanton boundary conditions. The moduli-space integrals appearing in the disk two-point function and the annulus one-point function have divergences associated with open-string-channel degenerations, which must be regulated using open-closed string field theory. The definition of the string field theory interaction vertices requires a choice of locations for the picture-changing operators, which we specify in detail. After carefully taking into account all contributions, including those from vertical integration, we find that the results precisely match the expectations from DDK-KPZ scaling. Our technical results on the detailed construction of interaction vertices are a first step toward understanding the analogous quantities in the ten-dimensional type IIB superstring, where one also needs to understand how to treat the bosonic and fermionic collective coordinates at subleading order.

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arXiv2026

Logarithm of charge ratio in black hole entropy

Logarithmic correction to BPS black hole entropy, computed from microscopic description, often contains terms involving large ratios of charges, besides the logarithmic terms involving the overall scale of the charges. If the electric charges are much larger than the magnetic charges, then the attractor value of the string coupling is small and one might hope to use weakly coupled string theory to compute logarithmic corrections involving ratios of charges from the macroscopic side. We compute these for black holes in flat space-time, preserving four supercharges, in $\mathcal{N} = 2$, $\mathcal{N}=4$ and $\mathcal{N}=8$ supersymmetric string compactifications in four dimensions. We find perfect agreement with the microscopic results in $\mathcal{N}=4$ and $\mathcal{N}=8$ theories, for which the microscopic results are known. Various stringy and statistical mechanical effects become important in this analysis, including 1) use of the correct ultra-violet cut-off (string scale instead of Planck scale), 2) correct path integral measure (ultra-local measure with appropriate dilaton dependent metric), 3) use of the correct path integral variable (Kalb-Ramond 2-form instead of the dual axion) and 4) change of ensemble (from grand canonical to microcanonical). We also verify that the measure we use is consistent with what follows from the BV formalism of string field theory.

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arXiv2026

How to Expose a Black Hole

According to the correspondence principle of Horowitz and Polchinski, many black holes in string theory are continuously deformed to usual quantum systems involving D-branes and fundamental strings when the string coupling becomes sufficiently small. Therefore if we consider a configuration in space-time where the dilaton varies over an appropriate range, then a black hole moving in such a background will smoothly transition from the black hole state to a normal quantum state whose microstates are not hidden behind an event horizon. The possible obstruction to this mechanism comes from the fact that if the dilaton varies too fast then the adiabatic approximation may break down and / or the ambient space-time itself may collapse to a black hole and get hidden from the asymptotic observer. On the other hand, if the dilaton varies too slowly then the time that it takes for the black hole to travel the required distance will exceed the evaporation time of the black hole. We show that by choosing the background appropriately these obstructions can be avoided and a gentle motion towards the weak coupling region will convert the black hole into a normal quantum state without an event horizon.

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arXiv2025

Exotic Branes and Symmetries of String Theory

Are duality transformations symmetries of string theory? For AdS space-time the answer is no for generic asymptotic values of the moduli, since the duality symmetry is broken explicitly in the dual conformal field theory. In contrast, in string theory in flat space-time, monodromies around codimension two exotic branes show that duality transformations are spontaneously broken discrete gauge symmetries with observable consequences, provided macroscopic loops of these branes are not hidden behind an event horizon. We discuss how this can be achieved and how the situation in flat space-time differs from that in AdS space-time. We also discuss observability of codimension two non-BPS branes, codimension one BPS and non-BPS branes and higher codimension branes of infinite tension.

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arXiv2025

Generating Function of Single Centered Black Hole Index from the Igusa Cusp Form

We introduce manifestly duality invariant generating function of the index of single centered black holes in the heterotic string theory compactified on a six dimensional torus. This function is obtained by subtracting, from the inverse of the Igusa cusp form, the generating function of the index of two centered black holes constructed from the Dedekind eta function. We also study the analytic properties of this function in the Siegel upper half plane.

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arXiv2025

Scattering of D0-branes and Strings

It has been known for about thirty years that a scattering amplitude involving D0-branes and closed strings suffers from infrared divergences beyond tree level. These divergences arise because the conventional world-sheet approach cannot account for the difference between the D0-brane's momentum before and after scattering. We show that, by using string field theory, the divergence can be removed and the amplitude rendered finite and unambiguous. We illustrate this using the simplest possible example in bosonic string theory: a three-point function with one incoming and one outgoing D0-brane and an incoming or outgoing closed string tachyon.

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arXiv2025

Decorating Asymptotically Flat Space-Time with the Moduli Space of String Theory

N=2, 4 and 8 supersymmetric string theories in four dimensional flat space-time have moduli space of vacua. We argue that starting from a theory where the moduli approach a particular moduli space point A at infinity, we can construct a classical solution that contains an arbitrarily large space-time region where the moduli take values corresponding to any other moduli space point B of our choice to any desired accuracy. Therefore the observables of a theory with a given set of asymptotic values of the moduli will have complete information on the observables for any other asymptotic values of the moduli. Also it is physically impossible for any experiment, performed over a finite time, to determine the asymptotic values of the moduli. We point out the difference between asymptotically flat space-time and asymptotically AdS space-time in this regard and discuss the possible implication of these results for holographic duals of string theories in flat space-time. For N=2 supersymmetric theories, A and B could correspond to compactifications on topologically distinct Calabi-Yau manifolds related by flop or conifold transitions.

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arXiv2025

How to Create a Flat Ten or Eleven Dimensional Space-time in the Interior of an Asymptotically Flat Four Dimensional String Theory

By taking large mass and charge limit of a black hole in string theory we can create arbitrarily large regions where the space-time is approximately flat, but the moduli fields take values different from their asymptotic values. In this paper we describe a special case of this where black hole solutions in a four dimensional string theory, in the large mass and charge limit, can have an arbitrarily large region outside the horizon where a local observer will experience type IIA string theory in flat ten dimensional space-time. The curvature and other field strengths remain small everywhere between the asymptotic four dimensional observer and the ten dimensional region. By going to a different region of space, we can also get a large region where a local observer experiences M-theory in flat eleven dimensional space-time. By taking another solution in the same theory, one can create an arbitrarily large region where a local observer will experience type IIB string theory in flat ten dimensional space-time.

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arXiv2025

Are Moduli Vacuum Expectation Values or Parameters?

Banks has argued that the moduli of string theory are not vacuum expectation values but parameters. We offer a different perspective on this question. Given two different points P and Q in the moduli space, we shall regard them as different vacua of the same underlying theory if in a theory where the asymptotic values of the moduli correspond to the point P, we can perform an experiment that can determine the spectrum and S-matrix of a theory where the asymptotic values of the moduli correspond to the point Q. We argue that in asymptotically flat space-time, this can be achieved by taking a charged black hole in the limit of large mass and charges. In this limit the local geometry at any point outside the horizon is indistinguishable from flat space-time. However the moduli vary slowly over the entire region so that their values at faraway points can differ by order unity. Therefore, by sending out experimental teams to different regions outside the horizon, an asymptotic observer can measure the spectrum and S-matrix for different values of the moduli.

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arXiv2025

All Order Classical Electromagnetic Soft Theorems

If a set of charged objects collide in space and the fragments disperse, then this process will emit electromagnetic waves. Classical soft photon theorem determines the constant term and the leading power law fall-off of the wave-form at late and early times in terms of only the momenta and charges of the incoming and outgoing objects. In this paper we determine an infinite set of subleading terms in the late and early time expansion of the wave-form, which also depend only on the momenta and charges of the incoming and outgoing particles. For two-particle scattering, we derive a resummed low-frequency electromagnetic wave-form, as well as the resummed wave-form at early and late times. In this analysis we ignore the effect of long range gravitational interaction, but our result is unaffected by any other short range interactions among the objects.

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arXiv2024

Gravitational Wave Tails from Soft Theorem: A Short Review

If a set of massive objects collide in space and the fragments disperse, possibly forming black holes, then this process will emit gravitational waves. Computing the detailed gravitational wave-form associated with this process is a complicated problem, not only due to the non-linearity of gravity but also due to the fact that during the collision and subsequent fragmentation the objects could undergo complicated non-gravitational interactions. Nevertheless the classical soft graviton theorem determines the power law fall-off of the wave-form at late and early times, including logarithmic corrections, in terms of only the momenta of the incoming and outgoing objects without any reference to what transpired during the collision. In this short review I shall explain the results and very briefly outline the derivation of these results.

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arXiv2024

D-instanton Induced Effective Action and its Gauge Invariance

The effect of D-instantons on closed string scattering amplitudes may be encoded into an effective action obtained by integrating out the (transient) open string fields. In order that this respects the gauge invariance of the theory, the sum of the perturbative closed string field theory action and the D-instanton induced effective action must satisfy the quantum Batalin-Vilkovisky master equation. In a previous paper this was proved for the effective action induced by up to two instantons. We generalize the proof for arbitrary number of D-instantons.

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arXiv2024

String Field Theory: A Review

As of today there exist consistent, gauge-invariant string field theories describing all string theories: bosonic open and closed strings, open superstrings, heterotic strings and type II strings. The construction of these theories require algebraic ingredients, such as $A_\infty$ and $L_\infty$ homotopy algebras, geometric ingredients, relevant to the building of moduli spaces of Riemann surfaces and the distribution of picture changing operators, and field-theoretic ingredients, involving two-dimensional CFT's and BCFT's and Batalin-Vilkovisky quantization. Applications of string field theory include the description of non-perturbative phenomena such as tachyon condensation and classical solutions, and the resolution of a number of ambiguities that bedevil the world-sheet formulation of perturbative string theory. It also allows, given a proper definition of contours of integration for momenta, for a proof of unitarity and a clear understanding of the ultraviolet finiteness of the theory. In this article we review these developments.

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arXiv2024

On the normalization of open-closed string amplitudes

We use the factorization constraints of open-closed string field theory to determine the signs and normalizations of general string amplitudes with both open and closed string external states. The normalization of all amplitudes is controlled by the genus, the number of boundaries, the number of open and closed string insertions, the string coupling and the D-brane tension. The challenge with signs arises because the relevant moduli spaces are not complex manifolds and have no obvious orientation. We deal with this by fixing a specific convention for the sign of the integration measure over the moduli space and adopting a consistent prescription for the ordering of operators and ghost insertions inside correlators.

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arXiv2024

Supersymmetric Index for Small Black Holes

Supersymmetric elementary string states in the compactified heterotic string theory are described by small black holes that have zero area event horizon. In this paper we compute the supersymmetric index of such elementary string states using gravitational path integral. The dominant contribution to the path integral comes from an Euclidean rotating black hole solution of the supergravity theory with a finite area event horizon, but the logarithm of the index, computed from the saddle point, vanishes. Nevertheless we show that the solution is singular on certain subspaces of the horizon where higher derivative corrections can be important, and once the higher derivative corrections are taken into account the solution could yield a finite result for the logarithm of the index whose form agrees with the microscopic results up to an overall numerical constant. While the numerical constant is not determined in our analysis, we show that it is independent of the details of the compactification and even the number of non-compact dimensions, in agreement with the microscopic results.

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