Publications

2020

• Hints of unitarity at large $N$ in the $O(N)^3$ tensor field theory
  
  • Benedetti Dario
  • Gurau Razvan
  • Harribey Sabine
  • Suzuki Kenta
  Journal of High Energy Physics, Springer, 2020, 2020 (2), pp.72. We compute the OPE coefficients of the bosonic tensor model of \cite{Benedetti:2019eyl} for three point functions with two fields and a bilinear with zero and non-zero spin. We find that all the OPE coefficients are real in the case of an imaginary tetrahedral coupling constant, while one of them is not real in the case of a real coupling. We also discuss the operator spectrum of the free theory based on the character decomposition of the partition function. (10.1007/JHEP02(2020)072)
  DOI : 10.1007/JHEP02(2020)072
• Probing spin correlations using angle-resolved photoemission in a coupled metallic/Mott insulator system
  
  • Sunko Veronika
  • Mazzola Federico
  • Kitamura Sota
  • Khim Seunghyun
  • Kushwaha Pallavi
  • Clark Oliver J
  • Watson Matthew D
  • Marković Igor
  • Biswas Deepnarayan
  • Pourovskii Leonid
  • Kim Timur
  • Lee Tien-Lin
  • Thakur Pardeep Kumar
  • Rosner Helge
  • Georges Antoine
  • Moessner Roderich
  • Oka Takashi
  • Mackenzie Andrew
  • King Philip
  Science Advances, American Association for the Advancement of Science (AAAS), 2020, 6 (6), pp.eaaz0611. A nearly free electron metal and a Mott insulating state can be thought of as opposite ends of the spectrum of possibilities for the motion of electrons in a solid. Understanding their interaction lies at the heart of the correlated electron problem. In the magnetic oxide metal PdCrO2, nearly free and Mott-localized electrons exist in alternating layers, forming natural heterostructures. Using angle-resolved photoemission spectroscopy, quantitatively supported by a strong coupling analysis, we show that the coupling between these layers leads to an “intertwined” excitation that is a convolution of the charge spectrum of the metallic layer and the spin susceptibility of the Mott layer. Our findings establish PdCrO2 as a model system in which to probe Kondo lattice physics and also open new routes to use the a priori nonmagnetic probe of photoemission to gain insights into the spin susceptibility of correlated electron materials. (10.1126/sciadv.aaz0611)
  DOI : 10.1126/sciadv.aaz0611
• Finite-temperature density matrix embedding theory
  
  • Sun Chong
  • Ray Ushnish
  • Cui Zhi-Hao
  • Stoudenmire Miles
  • Ferrero M.
  • Chan Garnet Kin-Lic
  Physical Review B, American Physical Society, 2020, 101 (7), pp.075131. We describe a formulation of the density matrix embedding theory at finite temperature. We present a generalization of the ground-state bath orbital construction that embeds a mean-field finite-temperature density matrix up to a given order in the Hamiltonian, or the Hamiltonian up to a given order in the density matrix. We assess the performance of the finite-temperature density matrix embedding on the one-dimensional Hubbard model both at half-filling and away from it, and the two-dimensional Hubbard model at half-filling, comparing to exact data where available, as well as results from finite-temperature density matrix renormalization group, dynamical mean-field theory, and dynamical cluster approximations. The accuracy of finite-temperature density matrix embedding appears comparable to that of the ground-state theory, with, at most, a modest increase in bath size, and competitive with that of cluster dynamical mean-field theory. (10.1103/PhysRevB.101.075131)
  DOI : 10.1103/PhysRevB.101.075131
• Real-frequency diagrammatic Monte Carlo at finite temperature
  
  • Vučičević J.
  • Ferrero M.
  Physical Review B, American Physical Society, 2020, 101 (7), pp.075113. Diagrammatic expansions are a central tool for treating correlated electron systems. At thermal equilibrium, they are most naturally defined within the Matsubara formalism. However, extracting any dynamic response function from a Matsubara calculation ultimately requires the ill-defined analytical continuation from the imaginary- to the real-frequency domain. It was recently proposed [Phys. Rev. B 99, 035120 (2019)] that the internal Matsubara summations of any interaction-expansion diagram can be performed analytically by using symbolic algebra algorithms. The result of the summations is then an analytical function of the complex frequency rather than Matsubara frequency. Here we apply this principle and develop a diagrammatic Monte Carlo technique which yields results directly on the real-frequency axis. We present results for the self-energy Σ(ω) of the doped 32x32 cyclic square-lattice Hubbard model in a non-trivial parameter regime, where signatures of the pseudogap appear close to the antinode. We discuss the behavior of the perturbation series on the real-frequency axis and in particular show that one must be very careful when using the maximum entropy method on truncated perturbation series. Our approach holds great promise for future application in cases when analytical continuation is difficult and moderate-order perturbation theory may be sufficient to converge the result. (10.1103/PhysRevB.101.075113)
  DOI : 10.1103/PhysRevB.101.075113
• Data-driven study of timelike Compton scattering
  
  • Grocholski O.
  • Moutarde H.
  • Pire B.
  • Sznajder P.
  • Wagner J.
  European Physical Journal C: Particles and Fields, Springer Verlag (Germany), 2020, 80, pp.171. In the framework of collinear QCD factorization, the leading twist scattering amplitudes for deeply virtual Compton scattering (DVCS) and timelike Compton scattering (TCS) are intimately related thanks to analytic properties of leading and next-to-leading order amplitudes. We exploit this welcome feature to make data-driven predictions for TCS observables to be measured in near future experiments. Using a recent extraction of DVCS Compton form factors from most of the existing experimental data for that process, we derive TCS amplitudes and calculate TCS observables only assuming leading-twist dominance. Artificial neural network techniques are used for an essential reduction of model dependency, while a careful propagation of experimental uncertainties is achieved with replica methods. Our analysis allows for stringent tests of the leading twist dominance of DVCS and TCS amplitudes. Moreover, this study helps to understand quantitatively the complementarity of DVCS and TCS measurements to test the universality of generalized parton distributions, which is crucial e.g. to perform the nucleon tomography. (10.1140/epjc/s10052-020-7700-9)
  DOI : 10.1140/epjc/s10052-020-7700-9
• Mean field flow equations and asymptotically free scalar fields
  
  • Kopper Christoph
  , 2020. The flow equations of the renormalisation group permit to analyse the perturbative $n$-point functions of renormalisable quantum field theories. Rigorous bounds implying renormalisablility allow to control large momentum behaviour, infrared singularities and large order behaviour in the number of loops and the number of arguments $n\,$. Gauge symmetry which is broken by the flow in momentum or position space, can be shown to be restored in the renormalised theory. In this paper we want to do a first but important step towards a rigorous nonperturbative analysis of the flow equations (FEs). We restrict to massive scalar fields and analyse the {\it mean field limit} where the Schwinger or 1PI functions are considered to be momentum independent or, otherwise stated, are replaced by their zero momentum values. We regard smooth solutions of the system of FEs for the $n$-point functions for different sets of boundary conditions. We will realise that allowing for non\-vanishing irrelevant terms permits to construct {\it asymptotically free} and thus nontrivial {\it scalar field theories} in the mean field approximation. We will also analyse the so-called trivial solution so far generally believed to exhaust four-dimensional scalar field theory. The method paves the way to a study of the system of FEs beyond the mean field limit.
• Dark-Matter Spontaneous Freeze Out
  
  • Partouche Hervé
  , 2020. We consider the possibility that thermalized dark-matter particles acquire their mass thanks to the spontaneous breaking of a symmetry below some critical temperature. We describe the regime where a freeze out mechanism takes place shortly after the onset of the phase transition, while the dark-matter mass has not yet reached its final constant value. For such a “spontaneous freeze out” to yield the correct relic density, the present-time cross section of annihilation of the dark matter into Standard-Model states has to be one or two orders of magnitude larger than in the case of a constant dark-matter mass.
• Super-statistical description of thermo-magnetic properties of a system of 2D GaAs quantum dots with gaussian confinement and Rashba spin–orbit interaction
  
  • Castaño-Yepes Jorge David
  • Amor-Quiroz D.A.
  Physica A: Statistical Mechanics and its Applications, Elsevier, 2020, 548, pp.123871. We examine the effect of non-equilibrium processes modeled by the introduction of a generalized Boltzmann factor on the thermal and magnetic properties of an array of two-dimensional GaAs quantum dots in the presence of an external uniform and constant magnetic field. The model consists of a single-electron subject to a confining Gaussian potential with a spin–orbit interaction in the Rashba approach. We compute the specific heat and the magnetic susceptibility within the formalism of χ2 -superstatistics from the exact solution of the Schrödinger equation. Furthermore, an analytic solution for the partition function allows a study of the impact of the number of subsystems on the superstatistical corrections and confirms that the ordinary thermo-magnetic properties are recovered whenever the thermal distribution can be approximated by a Dirac delta. Also, we found a progressive disappearance of the Schottky anomaly with decreasing number of subsystems, while the specific heat ceases to be a monotonically increasing function with respect to the average temperature when the χ2 -distribution is spread over a large range of temperatures. Remarkably, the introduction of fluctuations in the temperature is found to suppress the paramagnetic phase transition that would otherwise appear at low temperatures. Finally, we emphasize that an appropriate construction of the definition of physical observables is crucial for obtaining a correct description of the physics derived from a non-extensive construction of the entropy. (10.1016/j.physa.2019.123871)
  DOI : 10.1016/j.physa.2019.123871
• Topological invariants and the definition of energy in quadratic gravity theory
  
  • Giribet Gaston
  • Miskovic Olivera
  • Olea Rodrigo
  • Rivera-Betancour David
  Physical Review D, American Physical Society, 2020, 101 (6), pp.064046. We provide a simple method to compute the energy in higher curvature gravity in asymptotically AdS spacetimes in even dimensions. It follows from the combined use of topological terms added to the gravity action, and the Wald charges derived from the augmented action functional. No additional boundary terms are needed. As a consistency check, we show that the formula for the conserved quantities derived in this way yields the correct result for the mass of asymptotically AdS black holes. (10.1103/PhysRevD.101.064046)
  DOI : 10.1103/PhysRevD.101.064046
• Scalar sunset diagram at finite temperature with imaginary square masses
  
  • van Egmond Duifje Maria
  • Reinosa Urko
  Physical Review D, American Physical Society, 2020, 102 (3), pp.036013. We evaluate the finite temperature scalar sunset diagram with imaginary square masses, that appears in the Gribov-Zwanziger approach to Yang-Mills (YM) theory beyond one-loop order. Since YM theory at finite temperature is governed by center-symmetry and the Polyakov loop, we also include the possibility of a constant temporal background gauge field in the form of color-dependent imaginary chemical potentials. (10.1103/PhysRevD.102.036013)
  DOI : 10.1103/PhysRevD.102.036013
• Swampland, Gradient Flow and Infinite Distance
  
  • Kehagias Alex
  • Lüst Dieter
  • Lüst Severin
  Journal of High Energy Physics, Springer, 2020, 04, pp.170. In the first part of this paper we will work out a close and so far not yet noticed correspondence between the swampland approach in quantum gravity and geometric flow equations in general relativity, most notably the Ricci flow. We conjecture that following the gradient flow towards a fixed point, which is at infinite distance in the space of background metrics, is accompanied by an infinite tower of states in quantum gravity. In case of the Ricci flow, this conjecture is in accordance with the generalized distance and AdS distance conjectures, which were recently discussed in the literature, but it should also hold for more general background spaces. We argue that the entropy functionals of gradient flows provide a useful definition of the generalized distance in the space of background fields. In particular we give evidence that for the Ricci flow the distance ∆ can be defined in terms of the mean scalar curvature of the manifold, ∆ ∼ log $ \overline{R} $. For a more general gradient flow, the distance functional also depends on the string coupling constant.In the second part of the paper we will apply the generalized distance conjecture to gravity theories with higher curvature interactions, like higher derivative R$^{2}$ and W$^{2}$ terms. We will show that going to the weak coupling limit of the higher derivative terms corresponds to the infinite distance limit in metric space and hence this limit must be accompanied by an infinite tower of light states. For the case of the R$^{2}$ or W$^{2}$ couplings, this limit corresponds to the limit of a small cosmological constant or, respectively, to a light additional spin-two field in gravity. In general we see that the limit of small higher curvature couplings belongs to the swampland in quantum gravity, just like the limit of a small U(1) gauge coupling belongs to the swampland as well. (10.1007/JHEP04(2020)170)
  DOI : 10.1007/JHEP04(2020)170
• Shape dependence of renormalized holographic entanglement entropy
  
  • Anastasiou Giorgos
  • Moreno Javier
  • Olea Rodrigo
  • Rivera-Betancour David
  Journal of High Energy Physics, Springer, 2020, 09, pp.173. We study the holographic entanglement entropy of deformed entangling regions in three-dimensional CFTs dual to Einstein-AdS gravity, using a renormalization scheme based on the addition of extrinsic counterterms. In this prescription, when even- dimensional manifolds are considered, the universal contribution to the entanglement entropy is identified as the renormalized volume of the Ryu-Takayanagi hypersurface, which is written as the sum of a topological and a curvature term. It is shown that the change in the renormalized entanglement entropy due to the deformation of the entangling surface is encoded purely in the curvature contribution. In turn, as the topological part is given by the Euler characteristic of the Ryu-Takayanagi surface, it remains shape independent. Exploiting the covariant character of the extrinsic counterterms, we apply the renormalization scheme for the case of deformed entangling regions in AdS$_{4}$/CFT$_{3}$, recovering the results found in the literature. Finally, we provide a derivation of the relation between renormalized entanglement entropy and Willmore energy. The presence of a lower bound of the latter makes manifest the relation between the AdS curvature of the Ryu-Takayanagi surface and the strong subadditivity property. (10.1007/JHEP09(2020)173)
  DOI : 10.1007/JHEP09(2020)173
• Collectivity of heavy mesons in proton-nucleus collisions
  
  • Zhang Cheng
  • Marquet Cyrille
  • Qin Guang-You
  • Shi Yu
  • Wang Lei
  • Wei Shu-Yi
  • Xiao Bo-Wen
  Physical Review D, American Physical Society, 2020, 102 (3), pp.034010. Using a model based on the color glass condensate framework and the dilute-dense factorization, we systematically study the azimuthal angular correlations between a heavy flavor meson and a light reference particle in proton-nucleus collisions. The obtained second-harmonic coefficients (also known as the elliptic flows) for J/ψ and D0 agree with recent experimental data from the LHC. We also provide predictions for the elliptic flows of ϒ and B mesons, which can be measured in the near future at the LHC. This work can shed light on the physical origin of the collectivity phenomenon in the collisions of small systems. (10.1103/PhysRevD.102.034010)
  DOI : 10.1103/PhysRevD.102.034010
• Crossed beam energy transfer in the presence of laser speckle ponderomotive self-focusing and nonlinear sound waves
  
  • Hüller Stefan
  • Raj Gaurav
  • Rozmus Wojciech
  • Pesme Denis
  Physics of Plasmas, American Institute of Physics, 2020, 27 (022703). Crossed beam energy transfer, CBET, is investigated by taking into account the speckle structure of smoothed laser beams that overlap in a plasma with an inhomogeneous flow profile. Using the two-dimensional simulation code HARMONY, it is shown how ponderomotive self-focusing of laser speckles in crossing beams can significantly affect the transfer of energy from one beam to the other. The role of plasma flow on speckle self-focusing is investigated and revisited, in particular its consequences in terms of redirection and increasing angular spread of the laser beams due to beam bending and plasma-induced smoothing, respectively. At close-to-sonic flow the onset of self-focusing in the beam speckle structure occurs at considerably lower beam intensities than expected for the case without flow. CBET and speckle self-focusing can hence occur together when two crossed beams with equal frequency will resonantly exchange energy via their ponderomotively driven density perturbations flowing with sound speed. From the simulations it is found that consequences of ponderomotive self-focusing can be expected above an average intensity threshold scaling as I L ∼ 2 × 10 14 W cm −2 (λ 0 /1µm) −2 (T e /keV) with impact on the spatial and temporal coherence of the transmitted light. The density perturbations due to the ponderomotive force of the crossing beams can locally be enhanced in self-focusing speckles, partly leading to shock-like structures. These structures eventually increase the effect of plasma-induced smoothing and are at the origin of the stronger angular spread. (10.1063/1.5125759)
  DOI : 10.1063/1.5125759
• Conformally Soft Theorem in Gravity
  
  • Puhm Andrea
  Journal of High Energy Physics, Springer, 2020, 09, pp.130. A central feature of scattering amplitudes in gravity or gauge theory is the existence of a variety of energetically soft theorems which put constraints on the amplitudes. Celestial amplitudes which are obtained from momentum-space amplitudes by a Mellin transform over the external particle energies cannot obey the usual energetically soft theorems. Instead, the symmetries of the celestial sphere imply that the scattering of conformally soft particles whose conformal weights under the 4D Lorentz group SL(2, ℂ) are taken to zero obey special relations. Such conformally soft theorems have recently been found for gauge theory. Here, I show conformally soft factorization of celestial amplitudes for gravity and identify it as the celestial analogue of Weinberg’s soft graviton theorem. (10.1007/JHEP09(2020)130)
  DOI : 10.1007/JHEP09(2020)130
• Sextic tensor field theories in rank $3$ and $5$
  
  • Benedetti Dario
  • Delporte Nicolas
  • Harribey Sabine
  • Sinha Ritam
  Journal of High Energy Physics, Springer, 2020, 06, pp.065. We study bosonic tensor field theories with sextic interactions in d < 3 dimensions. We consider two models, with rank-3 and rank-5 tensors, and U(N)$^{3}$ and O(N)$^{5}$ symmetry, respectively. For both of them we consider two variations: one with standard short-range free propagator, and one with critical long-range propagator, such that the sextic interactions are marginal in any d < 3. We derive the set of beta functions at large N , compute them explicitly at four loops, and identify the respective fixed points. We find that only the rank-3 models admit melonic interacting fixed points, with real couplings and critical exponents: for the short-range model, we have a Wilson-Fisher fixed point with couplings of order $ \sqrt{\epsilon } $, in d = 3 − ϵ; for the long-range model, instead we have for any d < 3 a line of fixed points, parametrized by a real coupling g$_{1}$ (associated to the so-called wheel interaction). By standard conformal field theory methods, we then study the spectrum of bilinear operators associated to such interacting fixed points, and we find a real spectrum for small ϵ or small g$_{1}$. (10.1007/JHEP06(2020)065)
  DOI : 10.1007/JHEP06(2020)065
• Confronting same-sign W-boson production with parton correlations
  
  • Cotogno Sabrina
  • Kasemets Tomas
  • Myska Miroslav
  Journal of High Energy Physics, Springer, 2020, 10, pp.214. The future runs of LHC offer a unique opportunity to measure correlations between two partons inside the proton, which have never been experimentally detected. The process of interest is the production of two positively charged W-bosons decaying in the muon channel. We present a detailed analysis of proton-proton collisions at $ \sqrt{s} $ = 13 TeV, where we combine Monte Carlo event generators with our calculations of parton correlations. We carefully compare double parton scattering to relevant background processes and trace a path towards a clean signal sample. Several observables are constructed to demonstrate the effect of parton correlations with respect to clear benchmark values for uncorrelated scatterings. We find that especially spin correlations can be responsible for large effects in the variables we study, because of their direct relation with the parton angular momentum and, therefore, the directions of the muon momenta. We estimate the significance of the measurements as a function of the integrated luminosity and conclude that the LHC has the potential to detect, or put strong limits on, parton correlations in the near future. (10.1007/JHEP10(2020)214)
  DOI : 10.1007/JHEP10(2020)214
• Infinite Black Hole Entropies at Infinite Distances and Tower of States
  
  • Bonnefoy Quentin
  • Ciambelli Luca
  • Lüst Dieter
  • Lüst Severin
  Nuclear Physics B, North-Holland ; Elsevier [1967-....], 2020, 958, pp.115112. The aim of this paper is to elucidate a close connection between the black hole area law and the infinite distance conjecture in the context of the swampland. We consider families of black hole geometries, parametrized by their event horizon areas or by the values of their entropies, and show that the infinite entropy limit is always at infinite distance in the space of black hole geometries. It then follows from the infinite distance conjecture that there must be a tower of states in the infinite entropy limit, and that ignoring these towers on the horizon of the black hole would invalidate the effective theory when the entropy becomes large. We call this the black hole entropy distance conjecture. We then study two candidates for the tower of states. The first are the Kaluza-Klein modes of the internal geometry of extremal N=2 black holes in string theory, whose masses on the horizon are fixed by the N=2 attractor formalism, and given in terms of the black hole charges similarly to the entropy. However, we observe that it is possible to decouple their masses from the entropy, so that they cannot generically play the role of the tower. We thus consider a second kind of states: inspired by N-portrait quantum models of non-extremal black holes, we argue that the Goldstone-like modes that interpolate among the black hole microstates behave like the expected light tower of states. (10.1016/j.nuclphysb.2020.115112)
  DOI : 10.1016/j.nuclphysb.2020.115112
• Asymptotic Symmetries and Celestial CFT
  
  • Donnay Laura
  • Pasterski Sabrina
  • Puhm Andrea
  Journal of High Energy Physics, Springer, 2020, 09, pp.176. We provide a unified treatment of conformally soft Goldstone modes which arise when spin-one or spin-two conformal primary wavefunctions become pure gauge for certain integer values of the conformal dimension ∆. This effort lands us at the crossroads of two ongoing debates about what the appropriate conformal basis for celestial CFT is and what the asymptotic symmetry group of Einstein gravity at null infinity should be. Finite energy wavefunctions are captured by the principal continuous series ∆ ∈ 1 + iℝ and form a complete basis. We show that conformal primaries with analytically continued conformal dimension can be understood as certain contour integrals on the principal series. This clarifies how conformally soft Goldstone modes fit in but do not augment this basis. Conformally soft gravitons of dimension two and zero which are related by a shadow transform are shown to generate superrotations and non-meromorphic diffeomorphisms of the celestial sphere which we refer to as shadow superrotations. This dovetails the Virasoro and Diff(S$^{2}$) asymptotic symmetry proposals and puts on equal footing the discussion of their associated soft charges, which correspond to the stress tensor and its shadow in the two-dimensional celestial CFT. (10.1007/JHEP09(2020)176)
  DOI : 10.1007/JHEP09(2020)176
• Non-vanishing zero-temperature normal density in holographic superfluids
  
  • Goutéraux Blaise
  • Mefford Eric
  Journal of High Energy Physics, Springer, 2020, 11, pp.091. The low energy and finite temperature excitations of a d + 1-dimensional system exhibiting superfluidity are well described by a hydrodynamic model with two fluid flows: a normal flow and a superfluid flow. In the vicinity of a quantum critical point, thermodynamics and transport in the system are expected to be controlled by the critical exponents and by the spectrum of irrelevant deformations away from the quantum critical point. Here, using gauge-gravity duality, we present the low temperature dependence of thermodynamic and charge transport coefficients at first order in the hydrodynamic derivative expansion in terms of the critical exponents. Special attention will be paid to the behavior of the charge density of the normal flow in systems with emergent infrared conformal and Lifshitz symmetries, parameterized by a Lifshitz dynamical exponent z > 1. When 1 ≤ z < d + 2, we recover (z = 1) and extend (z > 1) previous results obtained by relativistic effective field theory techniques. Instead, when z > d + 2, we show that the normal charge density becomes non-vanishing at zero temperature. An extended appendix generalizes these results to systems that violate hyperscaling as well as systems with generalized photon masses. Our results clarify previous work in the holographic literature and have relevance to recent experimental measurements of the superfluid density on cuprate superconductors. (10.1007/JHEP11(2020)091)
  DOI : 10.1007/JHEP11(2020)091
• Bootstrapping boundary-localized interactions
  
  • Behan Connor
  • Di Pietro Lorenzo
  • Lauria Edoardo
  • van Rees Balt C.
  Journal of High Energy Physics, Springer, 2020, 2020 (12), pp.182. We study conformal boundary conditions for the theory of a single real scalar to investigate whether the known Dirichlet and Neumann conditions are the only possibilities. For this free bulk theory there are strong restrictions on the possible boundary dynamics. In particular, we find that the bulk-to-boundary operator expansion of the bulk field involves at most a ‘shadow pair’ of boundary fields, irrespective of the conformal boundary condition. We numerically analyze the four-point crossing equations for this shadow pair in the case of a three-dimensional boundary (so a four-dimensional scalar field) and find that large ranges of parameter space are excluded. However a ‘kink’ in the numerical bounds obeys all our consistency checks and might be an indication of a new conformal boundary condition. (10.1007/JHEP12(2020)182)
  DOI : 10.1007/JHEP12(2020)182
• Landau diagrams in AdS and S-matrices from conformal correlators
  
  • Komatsu Shota
  • Paulos Miguel F.
  • van Rees Balt C.
  • Zhao Xiang
  Journal of High Energy Physics, Springer, 2020, 2020 (46), pp.046. Quantum field theories in AdS generate conformal correlation functions on the boundary, and in the limit where AdS is nearly flat one should be able to extract an S-matrix from such correlators. We discuss a particularly simple position-space procedure to do so. It features a direct map from boundary positions to (on-shell) momenta and thereby relates cross ratios to Mandelstam invariants. This recipe succeeds in several examples, includes the momentum-conserving delta functions, and can be shown to imply the two proposals in [1] based on Mellin space and on the OPE data. Interestingly the procedure does not always work: the Landau singularities of a Feynman diagram are shown to be part of larger regions, to be called ‘bad regions’, where the flat-space limit of the Witten diagram diverges. To capture these divergences we introduce the notion of Landau diagrams in AdS. As in flat space, these describe on-shell particles propagating over large distances in a complexified space, with a form of momentum conservation holding at each bulk vertex. As an application we recover the anomalous threshold of the four-point triangle diagram at the boundary of a bad region. (10.1007/JHEP11(2020)046)
  DOI : 10.1007/JHEP11(2020)046
• On large mass gamma gamma and gamma-meson photoproduction
  
  • Szymanowski L.
  • Pire B.
  • Wallon S.
  , 2020, pp.12-15. Enlarging the set of hard exclusive reactions to be studied in the framework of QCD collinear factorization opens new possibilities to access generalized parton distributions (GPDs). We studied the photoproduction of a large invariant mass photon-photon or photon-meson pair in the generalized Bjorken regime which may be accessible both at JLab and at the EIC.
• String Defects, Supersymmetry and the Swampland
  
  • Angelantonj Carlo
  • Bonnefoy Quentin
  • Condeescu Cezar
  • Dudas Emilian
  Journal of High Energy Physics, Springer, 2020, 11, pp.125. Recently, Kim, Shiu and Vafa proposed general consistency conditions for six dimensional supergravity theories with minimal supersymmetry coming from couplings to strings. We test them in explicit perturbative orientifold models in order to unravel the microscopic origin of these constraints. Based on the perturbative data, we conjecture the existence of null charges $Q \cdot Q=0 $ for any six-dimensional theory with at least one tensor multiplet, coupling to string defects of charge $Q$. We then include the new constraint to exclude some six-dimensional supersymmetric anomaly-free examples that have currently no string or F-theory realization. We also investigate the constraints from the couplings to string defects in case where supersymmetry is broken in tachyon free vacua, containing non-BPS configurations of brane supersymmetry breaking type, where the breaking is localized on antibranes. In this case, some conditions have naturally to be changed or relaxed whenever the string defects experience supersymmetry breaking, whereas the constraints are still valid if they are geometrically separated from the supersymmetry breaking source.
• Gaussian concentration and uniqueness of equilibrium states in lattice systems
  
  • Chazottes J.-R
  • Moles J
  • Redig F
  • Ugalde E
  Journal of Statistical Physics, Springer Verlag, 2020, 181. We consider equilibrium states (that is, shift-invariant Gibbs measures) on the configuration space $S^{\mathbb{Z}^d}$ where d ≥ 1 and S is a finite set. We prove that if an equilibrium state for a shift-invariant uniformly summable potential satisfies a Gaussian concentration bound, then it is unique. Equivalently, if there exist several equilibrium states for a potential, none of them can satisfy such a bound. (10.1007/s10955-020-02658-1)
  DOI : 10.1007/s10955-020-02658-1