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# String/Black Hole Transition

In String Theory, General Relativity is naturally contained in the eﬀective theory describing the low energy dynamics of the string massless modes. It is then reasonable to hope that String Theory can replace the general relativistic thermodynamic description of black holes by a truly fundamental one, explaining the nature of the black hole degrees of freedom and their dynamics. There have been many important steps in this direction. The general idea is to consider a system made up of objects that exist in the theory, such as fundamental strings or D-branes. At zero string coupling one has a detailed and satisfying description of the system in ﬂat spacetime. Then, as the coupling increases, the system starts to self interact curving spacetime around it, and at some point it becomes a black hole. One can then use the curved geometry thermodynamic description of the system. Matching properties, specially the entropy, in some particular cases, in these two descriptions has been one of the greatest successes of String Theory.

In a recent paper we gave evidence in favor of a String/Black Hole transition in the case of BPS fundamental string states of the Heterotic string. We computed the absorption cross section for several ﬁelds both in the black hole and in the perturbative string phases. At zero frequency, these cross sections can be seen as order parameters for the transition. In particular, for the scalars ﬁxed at the horizon (attractor mechanism) the cross section evolves to zero when the horizon is formed. In the future, we would like to go on exploring this line of research:

- The String/Black Hole transition seems to be accessible in the classical limit where the dynamics is described by the α –corrected equations of motion.

- The absorption cross section determined, at tree level, in the string phase was independent of the massless particle polarization. It would be very interesting to see if this universal behavior persists at 1–loop.

- It would also be important to study non–supersymmetric cases using a similar approach. In particular, one could be able to describe Hawking radiation of near extremal black holes.

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