Publications

2021

• Topological Bifurcations and Reconstruction of Travelling Waves
  
  • Oblak Blagoje
  Phys.Fluids, 2021, 33, pp.027107. This paper is devoted to periodic travelling waves solving Lie-Poisson equations based on the Virasoro group. We show that the reconstruction of any such solution can be carried out exactly, regardless of the underlying Hamiltonian (which need not be quadratic), provided the wave belongs to the coadjoint orbit of a uniform profile. Equivalently, the corresponding "fluid particle motion" is integrable. Applying this result to the Camassa-Holm equation, we express the drift of particles in terms of parameters labelling periodic peakons and exhibit orbital bifurcations: points in parameter space where the drift velocity varies discontinuously, reflecting a sudden change in the topology of Virasoro orbits. (10.1063/5.0041269)
  DOI : 10.1063/5.0041269
• Plasmas créés par laser: Généralités et applications choisies
  
  • Mora Patrick
  , 2021.
• Comparing light-front quantization with instant-time quantization
  
  • Mannheim Philip D.
  • Lowdon Peter
  • Brodsky Stanley J.
  Physics Reports, Elsevier, 2021, 891, pp.1-65. In this paper we compare light-front quantization and instant-time quantization both at the level of operators and at the level of their Feynman diagram matrix elements. At the level of operators light-front quantization and instant-time quantization lead to equal light-front time commutation (or anticommutation) relations that appear to be quite different from equal instant-time commutation (or anticommutation) relations. Despite this we show that at unequal times instant-time and light-front commutation (or anticommutation) relations actually can be transformed into each other, with it only being the restriction to equal times that makes the commutation (or anticommutation) relations appear to be so different. While our results are valid for both bosons and fermions, for fermions there are subtleties associated with tip of the light cone contributions that need to be taken care of. At the level of Feynman diagrams we show for non-vacuum Feynman diagrams that the pole terms in four-dimensional light-front Feynman diagrams reproduce the widely used three-dimensional light-front on-shell Hamiltonian Fock space formulation in which the light-front energy and light-front momentum are on shell. Moreover, we show that the contributions of pole terms in non-vacuum instant-time and non-vacuum light-front Feynman diagrams are equal. However, because of circle at infinity contributions we show that this equivalence of pole terms fails for four-dimensional light-front vacuum tadpole diagrams. Then, and precisely because of these circle at infinity contributions, we show that light-front vacuum tadpole diagrams are not only nonzero, they quite remarkably are actually equal to the pure pole term instant-time vacuum tadpole diagrams. Light-front vacuum diagrams are not correctly describable by the on-shell Hamiltonian formalism, and thus not by the closely related infinite momentum frame prescription either. Thus for the light-front vacuum sector we must use the off-shell Feynman formalism as it contains information that is not accessible in the on-shell Hamiltonian Fock space approach. We show that light-front quantization is intrinsically nonlocal, and that for fermions this nonlocality is present in Ward identities. One can project fermion spinors into so-called good and bad components, and both of these components contribute in Ward identities. Central to our analysis is that the transformation from instant-time coordinates and fields to light-front coordinates and fields is a unitary, spacetime-dependent translation. Consequently, not only are instant-time quantization and light-front quantization equivalent, because of general coordinate invariance they are unitarily equivalent. (10.1016/j.physrep.2020.09.001)
  DOI : 10.1016/j.physrep.2020.09.001
• Extracting color octet NRQCD matrix elements from $J/\psi$ production at the EIC
  
  • Boer Daniël
  • Pisano Cristian
  • Taels Pieter
  Physical Review D, American Physical Society, 2021, 103 (7), pp.074012. Recently unpolarized and polarized J/ψ(ϒ) production at the Electron-Ion Collider (EIC) has been proposed as a new way to extract two poorly known color-octet NRQCD long-distance matrix elements: ⟨0|O8J/ψ(S01)|0⟩ and ⟨0|O8J/ψ(P03)|0⟩. The proposed method is based on a comparison to open heavy-quark pair production ideally performed at the same kinematics. In this paper we analyze this proposal in more detail and provide predictions for the EIC based on the available determinations of the color-octet matrix elements. We also propose two additional methods that do not require comparison to open heavy-quark pair production. (10.1103/PhysRevD.103.074012)
  DOI : 10.1103/PhysRevD.103.074012
• Double Copy for Celestial Amplitudes
  
  • Casali Eduardo
  • Puhm Andrea
  Physical Review Letters, American Physical Society, 2021, 126 (10), pp.101602. Celestial amplitudes which use conformal primary wave functions rather than plane waves as external states offer a novel opportunity to study properties of amplitudes with manifest conformal covariance and give insight into a potential holographic celestial conformal field theory at the null boundary of asymptotically flat space. Since translation invariance is obscured in the conformal basis, features of amplitudes that heavily rely on it appear to be lost. Among these are the remarkable relations between gauge theory and gravity amplitudes known as the double copy. Nevertheless, properties of amplitudes reflecting fundamental aspects of the perturbative regime of quantum field theory are expected to survive a change of basis. Here we show that there exists a well-defined procedure for a celestial double copy. This requires a generalization of the usual squaring of numerators which entails first promoting them to generalized differential operators acting on external wave functions and then squaring them. We demonstrate this procedure for three- and four-point celestial amplitudes and give an argument for its validity to all multiplicities. (10.1103/PhysRevLett.126.101602)
  DOI : 10.1103/PhysRevLett.126.101602
• Celestial diamonds: conformal multiplets in celestial CFT
  
  • Pasterski Sabrina
  • Puhm Andrea
  • Trevisani Emilio
  Journal of High Energy Physics, Springer, 2021, 11, pp.072. We examine the structure of global conformal multiplets in 2D celestial CFT. For a 4D bulk theory containing massless particles of spin s = $ \left\{0,\frac{1}{2},1,\frac{3}{2},2\right\} $ we classify and construct all SL(2,ℂ) primary descendants which are organized into ‘celestial diamonds’. This explicit construction is achieved using a wavefunction-based approach that allows us to map 4D scattering amplitudes to celestial CFT correlators of operators with SL(2,ℂ) conformal dimension ∆ and spin J. Radiative conformal primary wavefunctions have J = ±s and give rise to conformally soft theorems for special values of ∆ ∈ $ \frac{1}{2}\mathbb{Z} $. They are located either at the top of celestial diamonds, where they descend to trivial null primaries, or at the left and right corners, where they descend both to and from generalized conformal primary wavefunctions which have |J| ≤ s. Celestial diamonds naturally incorporate degeneracies of opposite helicity particles via the 2D shadow transform relating radiative primaries and account for the global and asymptotic symmetries in gauge theory and gravity. (10.1007/JHEP11(2021)072)
  DOI : 10.1007/JHEP11(2021)072
• Revisiting the conformally soft sector with celestial diamonds
  
  • Pasterski Sabrina
  • Puhm Andrea
  • Trevisani Emilio
  Journal of High Energy Physics, Springer, 2021, 11, pp.143. Celestial diamonds encode the structure of global conformal multiplets in 2D celestial CFT and offer a natural language for describing the conformally soft sector. The operators appearing at their left and right corners give rise to conformally soft factorization theorems, the bottom corners correspond to conserved charges, and the top corners to conformal dressings. We show that conformally soft charges can be expressed in terms of light ray integrals that select modes of the appropriate conformal weights. They reside at the bottom corners of memory diamonds, and ascend to generalized currents. We then identify the top corners of the associated Goldstone diamonds with conformal Faddeev-Kulish dressings and compute the sub-leading conformally soft dressings in gauge theory and gravity which are important for finding nontrivial central extensions. Finally, we combine these ingredients to speculate on 2D effective descriptions for the conformally soft sector of celestial CFT. (10.1007/JHEP11(2021)143)
  DOI : 10.1007/JHEP11(2021)143
• Functional renormalization group and 2PI effective action formalism
  
  • Blaizot Jean-Paul
  • Pawlowski Jan M.
  • Reinosa Urko
  Annals of Physics, Elsevier Masson, 2021, 431, pp.168549. We combine two non-perturbative approaches, one based on the two-particle-irreducible (2PI) action, the other on the functional renormalization group (fRG), in an effort to develop new non-perturbative approximations for the field theoretical description of strongly coupled systems. In particular, we exploit the exact 2PI relations between the two-point and four-point functions in order to truncate the infinite hierarchy of equations of the functional renormalization group. The truncation is ”exact” in two ways. First, the solution of the resulting flow equation is independent of the choice of the regulator. Second, this solution coincides with that of the 2PI equations for the two-point and the four-point functions, for any selection of two-skeleton diagrams characterizing a so-called <math display="inline" id="d1e1205" altimg="si322.svg"><mi>Φ</mi></math>-derivable approximation. The transformation of the equations of the 2PI formalism into flow equations offers new ways to solve these equations in practice, and provides new insight on certain aspects of their renormalization. It also opens the possibility to develop approximation schemes going beyond the strict <math display="inline" id="d1e1210" altimg="si322.svg"><mi>Φ</mi></math>-derivable ones, as well as new truncation schemes for the fRG hierarchy. (10.1016/j.aop.2021.168549)
  DOI : 10.1016/j.aop.2021.168549
• Quantum Entangled Fractional Topology and Curvatures
  
  • Hutchinson Joel
  • Hur Karyn Le
  Commun.Phys., 2021, 4 (1), pp.144. We propose a two-spin quantum-mechanical model with applied magnetic fields acting on the Poincaré-Bloch sphere, to reveal a new class of topological energy bands with Chern number one half for each spin-1/2. The mechanism behind this fractional topology is a two-spin product state at the north pole and a maximally entangled state close to the south pole. The fractional Chern number of each spin can be measured through the magnetizations at the poles. We study a precise protocol where the spin dynamics in time reflects the Landau-Zener physics associated with energy band crossing effects. We show a correspondence between the two-spin system and topological bilayer models on a honeycomb lattice. These models describe semimetals with a nodal ring surrounding the region of entanglement. (10.1038/s42005-021-00641-0)
  DOI : 10.1038/s42005-021-00641-0
• Incoherent hydrodynamics of density waves in magnetic fields
  
  • Donos Aristomenis
  • Pantelidou Christiana
  • Ziogas Vaios
  Journal of High Energy Physics, Springer, 2021, 05, pp.270. We use holography to derive effective theories of fluctuations in spontaneously broken phases of systems with finite temperature, chemical potential, magnetic field and momentum relaxation in which the order parameters break translations. We analytically construct the hydrodynamic modes corresponding to the coupled thermoelectric and density wave fluctuations and all of them turn out to be purely diffusive for our system. Upon introducing pinning for the density waves, some of these modes acquire not only a gap, but also a finite resonance due to the magnetic field. Finally, we study the optical properties and perform numerical checks of our analytical results. A crucial byproduct of our analysis is the identification of the correct current which describes the transport of heat in our system. (10.1007/JHEP05(2021)270)
  DOI : 10.1007/JHEP05(2021)270
• Ergodic edge modes in the 4D quantum Hall effect
  
  • Stéphan Jean-Marie
  • Estienne Benoit
  • Oblak Blagoje
  SciPost Physics, SciPost Foundation, 2021, 11 (1). The gapless modes on the edge of four-dimensional (4D) quantum Hall droplets are known to be anisotropic: they only propagate in one direction, foliating the 3D boundary into independent 1D conduction channels. This foliation is extremely sensitive to the confining potential and generically yields chaotic flows. Here we study the quantum correlations and entanglement of such edge modes in 4D droplets confined by harmonic traps, whose boundary is a squashed three-sphere. Commensurable trapping frequencies lead to periodic trajectories of electronic guiding centers; the corresponding edge modes propagate independently along S^1 S 1 fibers, forming a bundle of 1D conformal field theories over a 2D base space. By contrast, incommensurable frequencies produce quasi-periodic, ergodic trajectories, each of which covers its invariant torus densely; the corresponding correlation function of edge modes has fractal features. This wealth of behaviors highlights the sharp differences between 4D Hall droplets and their 2D peers; it also exhibits the dependence of 4D edge modes on the choice of trap, suggesting the existence of observable bifurcations due to droplet deformations. (10.21468/SciPostPhys.11.1.016)
  DOI : 10.21468/SciPostPhys.11.1.016
• Wavefunction of the universe: Reparametrization invariance and field redefinitions of the minisuperspace path integral
  
  • Partouche Hervé
  • Toumbas Nicolaos
  • de Vaulchier Balthazar
  Nuclear Physics B, North-Holland ; Elsevier [1967-....], 2021, 973, pp.115600. We consider the Hartle–Hawking wavefunction of the universe defined as a Euclidean path integral that satisfies the “no-boundary proposal.” We focus on the simplest minisuperspace model that comprises a single scale factor degree of freedom and a positive cosmological constant. The model can be seen as a non-linear σ-model with a line-segment base. We reduce the path integral over the lapse function to an integral over the proper length of the base and use diffeomorphism-invariant measures for the ghosts and the scale factor. As a result, the gauge-fixed path integral is independent of the gauge. However, we point out that all field redefinitions of the scale factor degree of freedom yield different choices of gauge-invariant path-integral measures. For each prescription, we compute the wavefunction at the semi-classical level and find a different result. We resolve in each case the ambiguity in the form of the Wheeler–DeWitt equation at this level of approximation. By imposing that the Hamiltonians associated with these possibly distinct quantum theories are Hermitian, we determine the inner products of the corresponding Hilbert spaces and find that they lead to a universal norm, at least semi-classically. Quantum predictions are thus independent of the prescription at this level of approximation. Finally, all wavefunctions of the Hilbert spaces of the minisuperspace model we consider turn out to be non-normalizable, including the no-boundary states. (10.1016/j.nuclphysb.2021.115600)
  DOI : 10.1016/j.nuclphysb.2021.115600
• On overlapping Feynman (sub)graphs
  
  • Reinosa Urko
  Physical Review D, American Physical Society, 2021, 103, pp.105015. We discuss, on general grounds, how two subgraphs of a given Feynman graph can overlap with each other. For this, we use the notion of connecting and returning lines that describe how any subgraph is inserted within the original graph. This, in turn, allows us to derive "non-overlap" theorems for one-particle-irreducible subgraphs with $2$, $3$ and $4$ external legs. As an application, we provide a simple justification of the skeleton expansion for vertex functions with more than five legs, in the case of scalar field theories. We also discuss how the skeleton expansion can be extended to other classes of graphs. (10.1103/PhysRevD.103.105015)
  DOI : 10.1103/PhysRevD.103.105015
• Quantum system dynamics with a weakly nonlinear Josephson junction bath
  
  • Yang Jing
  • Jussiau Étienne
  • Elouard Cyril
  • Le Hur Karyn
  • Jordan Andrew N.
  Physical Review B, American Physical Society, 2021, 103 (8), pp.085402. We investigate the influence of a weakly nonlinear Josephson bath consisting of a chain of Josephson junctions on the dynamics of a small quantum system (LC oscillator). Focusing on the regime where the charging energy is the largest energy scale, we perturbatively calculate the correlation function of the Josephson bath to the leading order in the Josephson energy divided by the charging energy while keeping the cosine potential exactly. When the variation of the charging energy along the chain ensures fast decay of the bath correlation function, the dynamics of the LC oscillator that is weakly and capacitively coupled to the Josephson bath can be solved through the Markovian master equation. We establish a duality relation for the Josephson bath between the regimes of large charging and Josephson energies, respectively. The results can be applied to cases where the charging energy either is nonuniformly engineered or disordered in the chain. Furthermore, we find that the Josephson bath may become non-Markovian when the temperature is increased beyond the zero-temperature limit in that the bath correlation function gets shifted by a constant and does not decay with time. (10.1103/PhysRevB.103.085402)
  DOI : 10.1103/PhysRevB.103.085402
• Spreading of Correlations and Entanglement in the Long-Range Transverse Ising Chain
  
  • Schneider J.T.
  • Despres J.
  • Thomson S.J.
  • Tagliacozzo L.
  • Sanchez-Palencia L.
  Physical Review Research, American Physical Society, 2021, 3 (1), pp.L012022. Whether long-range interactions allow for a form of causality in nonrelativistic quantum models remains an open question with far-reaching implications for the propagation of information and thermalization processes. Here, we study the out-of-equilibrium dynamics of the one-dimensional transverse Ising model with algebraic long-range exchange coupling. Using a state of the art tensor-network approach, complemented by analytic calculations and considering various observables, we show that a weak form of causality emerges, characterized by nonuniversal dynamical exponents. While the local spin and spin correlation causal edges are sub-ballistic, the causal region has a rich internal structure, which, depending on the observable, displays ballistic or superballistic features. In contrast, the causal region of entanglement entropy is featureless and its edge is always ballistic, irrespective of the interaction range. Our results shed light on the propagation of information in long-range interacting lattice models and pave the way to future experiments, which are discussed. (10.1103/PhysRevResearch.3.L012022)
  DOI : 10.1103/PhysRevResearch.3.L012022
• Observation and modelling of Stimulated Raman Scattering driven by an optically smoothed laser beam in experimental conditions relevant for Shock Ignition
  
  • Cristoforetti G.
  • Hüller Stefan
  • Koester P.
  • Antonelli L.
  • Atzeni S.
  • Baffigi F.
  • Batani D.
  • Baird C.
  • Booth N.
  • Galimberti M.
  • Glize K.
  • Héron Anne
  • Khan M.
  • Loiseau P.
  • Mancelli D.
  • Notley M.
  • Oliveira P.
  • Renner O.
  • Smid M.
  • Schiavi A.
  • Tran G.
  • Woolsey N. C.
  • Gizzi L. A.
  High Power Laser Science and Engineering, Cambridge University Press, 2021, 9, pp.e60. We report results and modelling of an experiment performed at the TAW Vulcan laser facility, aimed at investigating laser-plasma interaction in conditions which are of interest for the Shock Ignition scheme to Inertial Confinement Fusion, i.e. laser intensity higher than 10^16 W/cm2 impinging on a hot (T > 1 keV), inhomogeneous and long scalelength preformed plasma. Measurements show a significant SRS backscattering (4 - 20% of laser energy) driven at low plasma densities and no signatures of TPD/SRS driven at the quarter critical density region. Results are satisfactorily reproduced by an analytical model accounting for the convective SRS growth in independent laser speckles, in conditions where the reflectivity is dominated by the contribution from the most intense speckles, where SRS gets saturated. Analytical and kinetic simulations well reproduce the onset of SRS at low plasma densities in a regime strongly affected by non linear Landau damping and by filamentation of the most intense laser speckles. The absence of TPD/SRS at higher densities is explained by pump depletion and plasma smoothing driven by filamentation. The prevalence of laser coupling in the low density profile justifies the low temperature measured for hot electrons (7 - 12 keV), well reproduced by numerical simulations.