arXiv subjects

Eduardo Conde

Publications and source records attributed to Eduardo Conde.

At least 19 recordsLinked to original sources

arXiv2019

An optimization model for line planning and timetabling in automated urban metro subway networks

In this paper we present a Mixed Integer Nonlinear Programming model that we developed as part of a pilot study requested by the R&D company Metrolab in order to design tools for finding solutions for line planning and timetable situations in automated urban metro subway networks. Our model incorporates important factors in public transportation systems from both, a cost-oriented and a passenger-oriented perspective, as time-dependent demands, interchange stations, short-turns and technical features of the trains in use. The incoming flows of passengers are modeled by means of piecewise linear demand functions which are parameterized in terms of arrival rates and bulk arrivals. Decisions about frequencies, train capacities, short-turning and timetables for a given planning horizon are jointly integrated to be optimized in our model. Finally, a novel Math-Heuristic approach is proposed to solve the problem. The results of extensive computational experiments are reported to show its applicability and effectiveness to handle real-world subway networks

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arXiv2017

BMS Supertranslations and Not So Soft Gravitons

In a previous article, we have argued that Low's sub-leading soft photon theorem can be recovered as a Ward identity associated to the same large gauge transformations that control the leading piece of the theorem. The key for that was to link the energy expansion displayed in the soft theorem to a $\frac{1}{r}$ expansion that we can perform in the associated asymptotic charge. We expect this idea to be valid in general, and here we provide compelling evidence for it by showing how the same method works in the case of Einstein-Hilbert gravity. More precisely, we are able to derive the three orders of the tree-level soft graviton theorem simply from the BMS supertranslation charge, known to give rise to the leading soft graviton theorem. In particular, we do not need to invoke superrotations (nor extended superrotations) at any point of the argument.

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arXiv2017

Remarks on Asymptotic Symmetries and the Sub-leading Soft Photon Theorem

A deep connection has been recently established between soft theorems and symmetries at null infinity in gravity and gauge theories, recasting the former as Ward identities of the latter. In particular, different orders (in the frequency of the soft particle) in the soft theorems are believed to be controlled by different asymptotic symmetries. In this paper we argue that this needs not be the case by focusing on the soft photon theorem. We argue that the sub-leading soft factor follows from the same symmetry responsible for the leading one, namely certain residual (large) gauge transformations of the gauge theory. In particular, expanding the associated charge in inverse powers of the radial coordinate, the (sub-)leading charge yields the (sub-)leading soft factor.

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arXiv2016

Lorentz Constraints on Massive Three-Point Amplitudes

Using the helicity-spinor language we explore the non-perturbative constraints that Lorentz symmetry imposes on three-point amplitudes where the asymptotic states can be massive. As it is well known, in the case of only massless states the three-point amplitude is fixed up to a coupling constant by these constraints plus some physical requirements. We find that a similar statement can be made when some of the particles have mass. We derive the generic functional form of the three-point amplitude by virtue of Lorentz symmetry, which displays several functional structures accompanied by arbitrary constants. These constants can be related to the coupling constants of the theory, but in an unambiguous fashion only in the case of one massive particle. Constraints on these constants are obtained by imposing that in the UV limit the massive amplitude matches the massless one. In particular, there is a certain Lorentz frame, which corresponds to projecting all the massive momenta along the same null momentum, where the three-point massive amplitude is fully fixed, and has a universal form.

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arXiv2016

D3-D5 theories with unquenched flavors

We construct the string duals of the defect theories generated when N_f flavor D5-branes intersect N_c color D3-branes along a 2+1 dimensional subspace. We work in the Veneziano limit in which N_c and N_f are large and N_f/N_c is fixed. By smearing the D5-branes, we find supergravity solutions that take into account the backreaction of the flavor branes and preserve two supercharges. When the flavors are massless the resulting metric displays an anisotropic Lifshitz-like scale invariance. The case of massive quarks is also considered.

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arXiv2016

Spinor-Helicity Three-Point Amplitudes from Local Cubic Interactions

We make an explicit link between the cubic interactions of off-shell fields and the on-shell three-point amplitudes in four dimensions. Both the cubic interactions and the on-shell three-point amplitudes had been independently classified in the literature, but their relation has not been made explicit. The aim of this note is to provide such a relation and discuss similarities and differences of their constructions. For the completeness of our analysis, we also derive the covariant form of all parity-odd massless vertices.

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arXiv2014

On the Regularization of On-Shell Diagrams

In this letter we discuss a regularization scheme for the integration of generic on-shell forms. The basic idea is to extend the three-particle amplitudes to the space of unphysical helicities keeping the dimension of the related coupling constant fixed, and construct on-shell forms out of them. We briefly discuss the analytic structure of the extended on-shell diagrams, both at tree level and one loop. Furthermore, we propose an integration contour which, applied to the relevant on-shell forms, allows to extract the four-particle amplitudes in Lorentz signature at one loop. With this contour at hand, we explicitly apply our procedure to this case obtaining the IR divergences as poles in the deformation parameter space, as well as the correct functional form for the finite term. This procedure provides a natural regularization for generic on-shell diagrams.

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arXiv2014

D-instanton probe and the enhançon mechanism from a quiver gauge theory

We study the $\mathcal N=2$ field theory realized by D3-branes on the ${\mathbb C}^2/{\mathbb Z}_2$ orbifold. The dual supergravity solution exhibits a repulson singularity cured by the enhancon mechanism. By comparing the open and closed string descriptions of a probe D-instanton, we can compute the exact non-perturbative profile of the supergravity twisted field, which determines the supergravity background. We then show how the non-trivial IR physics of the field theory translates into the stringy effects that give rise to the enhancon mechanism and the associated excision procedure.

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arXiv2013

Unquenched massive flavors and flows in Chern-Simons matter theories

We construct a holographic dual to the three-dimensional ABJM Chern-Simons matter theory with unquenched massive flavors. The flavor degrees of freedom are introduced by means of D6-branes extended along the gauge theory directions and delocalized in the internal space. To find the solution we have to solve the supergravity equations of motion with the source terms introduced by the D6-branes. The background we get is a running solution representing the renormalization group flow between two fixed points, at the IR and the UV, in both of which the geometry is of the form AdS_4 x M_6, where M_6 is a six-dimensional compact manifold. Along the flow, N=1 supersymmetry is preserved and the flavor group is Abelian. The flow is generated by changing the quark mass m_q. When m_q is very large we recover the original unflavored ABJM solution, while for m_q very small our solution becomes asymptotically equivalent to the one found recently for massless smeared flavors. We study the effects of the dynamical quarks as their mass is varied on different observables, such as the holographic entanglement entropy, the quark-antiquark potential, the two-point functions of high dimension bulk operators, and the mass spectrum of mesons.

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arXiv2012

The Twelve-Graviton Next-to-MHV Amplitude from Risager's Construction

The MHV or CSW expansion of tree-level Yang-Mills amplitudes provides an elegant and simple way of obtaining analytic formulas for S-matrix elements. Inspired by the BCFW technique, a systematic approach to obtain the MHV expansion was introduced by Risager, using a particular complex deformation. Although it works for Yang-Mills amplitudes, Risager's technique fails to provide an MHV expansion already for Next-to-MHV gravity amplitudes with more than eleven particles, as shown by Bianchi, Elvang and Freedman in 2008. This fact implies that in this sector there is a contribution at infinity starting at n = 12. In this note we determine the explicit analytic form of this residue at infinity for n = 12. Together with the terms of the Risager MHV expansion, the residue at infinity completes the first full CSW-like analytic expression of the twelve-graviton NMHV amplitude. Our technique can also be used to compute the residue at infinity for higher points.

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arXiv2012

On the gravity dual of Chern-Simons-matter theories with unquenched flavor

We find solutions of type IIA supergravity which are dual to three-dimensional Chern-Simons-matter theories with unquenched fields in the fundamental representation of the gauge group (flavors). In the holographic dual the addition of flavor is performed by means of D6-branes that are extended along the Minkowski gauge theory directions and are delocalized in the internal space in such a way that the system is N=1 supersymmetric and the flavor group is abelian. For massless flavors the corresponding geometry has the form of a product space AdS_4x M_6, where M_6 is a six-dimensional compact manifold whose metric is obtained by squashing the Fubini-Study metric of CP^3 with suitable constant factors which depend on the number of flavors. We compute the effect of dynamical quarks in several observables and, in some cases, we compare our results with the ones corresponding to the N=3 supergravity solutions generated by localized flavor branes. We also show how to generalize our results to include massive flavors.

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arXiv2012

Towards the string dual of tumbling and cascading gauge theories

We build type IIB backgrounds that can be interpreted as the dual description of field theories in which the dynamics shows many non-trivial phenomena, generalizing the baryonic branch of the Klebanov-Strassler system. We illustrate the steps of the explicit construction with a particularly interesting example. The dual field theory exhibits the expected behavior of an N=1 supersymmetric gauge theory which, over different ranges of the radial direction, is undergoing a cascade of Seiberg dualities, a period of running, a cascade of Higgsings (tumbling) and finally gaugino condensation.

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arXiv2012

A tale of two cascades: Higgsing and Seiberg-duality cascades from type IIB string theory

We construct explicitly new solutions of type IIB supergravity with brane sources, the duals of which are N = 1 supersymmetric field theories exhibiting two very interesting phenomena. The far UV dynamics is controlled by a cascade of Seiberg dualities analogous to the Klebanov- Strassler backgrounds. At intermediate scales a cascade of Higgsing appears, in the sense that the gauge group undergoes a sequence of spontaneous symmetry breaking steps which reduces its rank. Deep in the IR, the theory confines, and the gravity background has a non-singular end of space. We explain in detail how to generate such solutions, discuss some of the Physics associated with them and briefly comment on the possible applications.

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arXiv2011

Exploring the S-Matrix of Massless Particles

We use the recently proposed generalised on-shell representation for scattering amplitudes and a consistency test to explore the space of tree-level consistent couplings in four-dimensional Minkowski spacetime. The extension of the constructible notion implied by the generalised on-shell representation, i.e. the possibility to reconstruct at tree level all the scattering amplitudes from the three-particle ones, together with the imposition of the consistency conditions at four-particle level, allow to rediscover all the known theories and their algebra structure, if any. Interestingly, this analysis seems to leave room for high-spin couplings, provided that at least the requirement of locality is weakened. We do not claim to have found tree-level consistent high-spin theories, but rather that our methods show signatures of them and very likely, with a suitable modification, they can be a good framework to perform a systematic search.

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arXiv2011

On the holographic dual of N=1 SQCD with massive flavors

We construct holographic duals to N=1 SQCD with a quartic superpotential and unquenched massive flavors. Our backgrounds are generated by D5-branes wrapping two-dimensional submanifolds of an internal space. The flavor degrees of freedom are introduced by means of D5-branes extended along two-dimensional calibrated surfaces, and act as sources of the different supergravity fields. The backgrounds we get include the backreaction of the flavor branes and generalize the geometries obtained so far to the case in which the fundamental matter is massive. The supergravity solutions we find are regular everywhere and depend on a radial function which can be determined from the distribution of flavor branes used as sources. We also work out the holomorphic structure of the model and explore some of its observable consequences.

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arXiv2011

On the Tree-Level Structure of Scattering Amplitudes of Massless Particles

We provide a new set of on-shell recursion relations for tree-level scattering amplitudes, which are valid for any non-trivial theory of massless particles. In particular, we reconstruct the scattering amplitudes from (a subset of) their poles and zeroes. The latter determine the boundary term arising in the BCFW-representation when the amplitudes do not vanish as some momenta are taken to infinity along some complex direction. Specifically, such a boundary term can be expressed as a sum of products of two on-shell amplitudes with fewer external states and a factor dependent on the location of the relevant zeroes and poles. This allows us to recast the amplitudes to have the standard BCFW-structure, weighted by a simple factor dependent on a subset of zeroes and poles of the amplitudes. We further comment on the physical interpretation of the zeroes as a particular kinematic limit in the complexified momentum space. The main implication of the existence of such recursion relations is that the tree-level approximation of any consistent theory of massless particles can be fully determined just by the knowledge of the corresponding three-particle amplitudes.

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arXiv2011

Kutasov-like duality from D5-branes wrapping hyperbolic cycles

We study the addition of N_f flavor D5-branes to supergravity solutions describing D5-branes wrapping two-cycles of genus g>1 inside a six-dimensional space equipped with an SU(3)-structure. The non-zero genus g on the gravity side is dual to the existence of massless adjoint chiral superfields. Three types of internal manifolds are considered, each involving one of the following fibered products: H_2 x SL_2, S^2 x SL_2 or H_2 x S^3, where SL_2 stands for the universal cover of SL(2,R). For the first one, we investigate the dual field theories. We show that some of the solutions with N_f non-zero are dual to four-dimensional N = 1 field theories exhibiting a Kutasov-like duality taking N_c to k N_f - N_c, and keeping N_f fixed. Computed from the supergravity picture, k is in general a rational number, which can be made integer to fit the expectation from the field theory side. We finally study some other properties of those field theories.

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arXiv2010

Holographic duals of SQCD models in low dimensions

We obtain gravity duals to supersymmetric gauge theories in two and three spacetime dimensions with unquenched flavor. The supergravity solutions are generated by a set of color branes wrapping a compact cycle in a Calabi-Yau threefold, together with another set of flavor branes extended along the directions orthogonal to the cycle wrapped by the color branes. We construct supergravity backgrounds which include the backreaction induced by a smeared set of flavor branes, which act as delocalized dynamical sources of the different supergravity fields.

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