Maximal angular correlation in γ−γ\gamma-\gamma coincidences: a quantitative study

The measurement of the angular distribution of maximally correlated annihilation gamma rays radiated in coincidence, like those emitted from a $^{22}\mathrm{Na}$ source, is a classic experiment that is nowadays ordinarily performed in Nuclear Physics laboratory classes. For the first time we present an analytic expression for such angular distribution, which can be easily tested and confronted with the laboratory measurements.Comment: 12 pages, 3 figure

Dilatonic black holes in superstring gravity

We solve the dilaton field equation in the background of a spherically symmetric black hole in type II superstring theory with $\alpha'^3$ corrections in arbitrary $d$ spacetime dimensions. We then apply this result to obtain a spherically symmetric black hole solution with $\alpha'^3$ corrections in superstring theory compactified on a torus, coupled to such dilaton. For this black hole we obtain its mass, entropy, temperature, specific heat, and free energy.Comment: V2: some small changes in the presentation; references and discussion added; results unchanged. Published version. arXiv admin note: text overlap with arXiv:0912.305

Scattering of spherically symmetric d-dimensional alpha'-corrected black holes in string theory

We study scattering of minimally coupled massless scalar fields by non-extremal spherically symmetric black holes in d dimensions with string-theoretical alpha' corrections. We then obtain a formula for the low frequency absorption cross section for every black hole of this kind, which we apply to known black hole solutions. We compare the alpha' corrections for the absorption cross section with those for the black hole entropy, obtained through Wald's formula, in each case concluding that these corrections are different. We find a general covariant formula for the absorption cross section including alpha' corrections, in terms of the horizon area and temperature.Comment: 27 pages. v2: some clarifications; discussion on the scalar field equation improved; some parts moved from the main text to two appendices. v3: discussion on dependence under field redefinitions extended; new section with scheme-independent formulae for entropy and cross sectio

Absorption of scalars by extremal black holes in string theory

We show that the low frequency absorption cross section of minimally coupled test massless scalar fields by extremal spherically symmetric black holes in d dimensions is equal to the horizon area, even in the presence of string--theoretical $\alpha'$ corrections. Classically one has the relation $\sigma = 4 G S$ between that absorption cross section and the black hole entropy. By comparing in each case the values of the horizon area and Wald's entropy, we discuss the validity of such relation in the presence of higher derivative corrections for extremal black holes in many different contexts: in the presence of electric and magnetic charges, for nonsupersymmetric and supersymmetric black holes, in d=4 and d=5 dimensions. The examples we consider seem to indicate that this relation is not verified in the presence of $\alpha'$ corrections in general, although being valid in some specific cases (electrically charged maximally supersymmetric black holes in d=5). We argue that the relation $\sigma = 4 G S$ should in general be valid for the absorption cross section of scalar fields which, although being independent from the black hole solution, have their origin from string theory, and therefore are not minimally coupled.Comment: Published version with minor corrections. 23 page

Perturbation theory and stability analysis for string-corrected black holes in arbitrary dimensions

We develop the perturbation theory for R^2 string-corrected black hole solutions in d dimensions. After having obtained the master equation and the alpha'-corrected potential under tensorial perturbations of the metric, we study the stability of the Callan, Myers and Perry solution under these perturbations.Comment: Presented at 28th Spanish Relativity Meeting (ERE05): A Century of Relativity Physics, Oviedo, Asturias, Spain, 6-10 Sep 200

One loop superstring effective actions and N=8 supergravity

In a previous article we have shown the existence of a new independent R^4 term, at one loop, in the type IIA and heterotic effective actions, after reduction to four dimensions, besides the usual square of the Bel-Robinson tensor. It had been shown that such a term could not be directly supersymmetrized, but we showed that was possible after coupling to a scalar chiral multiplet. In this article we study the extended (N=8) supersymmetrization of this term, where no other coupling can be taken. We show that such supersymmetrization cannot be achieved at the linearized level. This is in conflict with the theory one gets after toroidal compactification of type II superstrings being N=8 supersymmetric. We interpret this result in face of the recent claim that perturbative supergravity cannot be decoupled from string theory in d>=4, and N=8, d=4 supergravity is in the swampland.Comment: 28 pages, no figure

Tensorial perturbations and stability of spherically symmetric d-dimensional black holes in string theory

We compute the tensorial perturbations to a general spherically symmetric metric in d dimensions with string–theoretical corrections quadratic in the Riemann tensor, from which we derive their respective potential. We use this result to study the stability of corresponding black hole solutions under such perturbations.Fundação para a Ciência e a Tecnologia (FCT

Dilatonic black holes in heterotic string theory

We obtain the dilaton solution in the background of a spherically symmetric black hole with curvature-squared corrections in arbitrary d spacetime dimensions and a spherically symmetric black hole solution with dilatonic charge and curvature-squared corrections in heterotic string theory compacti ed on a torus. For this black hole we obtain its entropy, temperature, speci c heat and mass.Fundação para a Ciência e a Tecnologia (FCT) - project PTDC/EIA/67661/2006 - CMAT - Financiamento plurianua