Seminars

Abstract: Pure gravity in AdS_3 is a theory of boundary excitations, most simply expressed as a constrained free scalar with an improved stress tensor that is needed to match the Brown--Henneaux central charge. Excising a finite part of AdS gives rise to a static gauge Nambu--Goto action for the boundary graviton. This is the T\bar{T} deformation of the infinite volume theory, as the effect of the improvement term on the deformed action can be absorbed into a field redefinition. I will show that the classical gravitational stress tensor is reproduced order by order using the T\bar{T} trace equation. I will also argue that the trace equation imposes sufficient constraints on the ordering ambiguities and allows to determine the energy spectrum. The correlation functions, however, are not completely fixed by the trace equation. I will show that both the gravitational action and the T\bar{T} deformation allow for finite improvement terms, which can be matched with total derivative terms in Zamolodchikov's point splitting definition of the T\bar{T} operator.

Abstract: In many models of charged holographic matter at zero temperature, there exists a sound mode in the spectrum of linearized fluctuations. I will explain recent work connecting this sound mode to an emergent higher form symmetry that has a mixed 't Hooft anomaly with the global U(1) symmetry. At finite temperature, the higher form symmetry is explicitly broken by an emergent photon that couples the charged matter to a quantum critical bath. This eventually destroys the sound mode and gives rise to the familiar charge diffusion mode and a parametrically long-lived mode. The long-lived mode leads to a sharp Drude-like electrical conductivity whose DC limit scales with a power of T that is controlled by a dangerously irrelevant deformation of the quantum critical point.

Abstract: The entropy of the three-charge NS5-F1-P black hole in Type IIA string theory comes from the breaking of N1 F1 strings into N1N5 little strings, which become independent momentum carriers. In M theory, the little strings correspond to strips of M2 brane that connect pairs of parallel M5 branes separated along the M-theory direction. I will show that if one takes into account the backreaction of the M-theory little strings on the M5 branes one obtains a maze-like structure, to which one can add momentum waves. This gives rise to a momentum-carrying brane configuration – a super-maze – which locally preserves 16 supercharges. Then, using the process of local supersymmetry enhancement, I will argue that themelia, which are extended objects in string theory that have 16 supersymmetries locally, play a central role in the description of black-hole microstructure.

Abstract: In this talk I will present a rigorous construction of examples of black hole formation, starting from regular initial data for a self-gravitating charged scalar field, which settle down to extremal Reissner--Nordström in finite advanced time. In particular, our result can be viewed as a definitive disproof of the "third law of black hole thermodynamics.’’ This is joint work with Christoph Kehle (ETH Zürich). 

Abstract: Despite the many successes of QFT, we still have very few tools for directly computing strongly-coupled dynamics, and even fewer means of studying QFTs at finite temperature. I will discuss a new approach for accomplishing this goal, called conformal truncation, which uses data from conformal field theories to compute observables in more general QFTs. After presenting the general approach, I will discuss its application to 1+1d scalar field theory, in particular the calculation of finite-temperature observables and the signatures of chaotic dynamics at general coupling.

Abstract: The smallness of the cosmological constant Λ, together with swampland ideas, lead to the recent Dark Dimension proposal, which postulates that there exists a single mesoscopic extra dimension with size of a few micrometers. In particular, a single tower of light states should appear, with masses scaling like m ~ Λ^{1/4}. In this talk, I will review salient features of the Dark Dimension proposal and explain how a strongly warped throat with its redshifted KK tower provides a natural string-theoretical mechanism realizing this aforementioned scaling, with the dark dimension being the one along the throat. I will point out challenges that may arise when considering such a setup, in particular concerning the masses of other KK towers.

Abstract: Infinite distance limits in theories with gravity are expected to unravel secrets of the UV quantum gravity. In this talk we will focus on supergravity theories with 16 supercharges and study their BPS and Non-BPS infinite distance limits and show that they mainly lead to decompactifications. We will then show that a particular type of BPS infinite distance  limits provides strong constraints on the rank of the gauge group in d dimensions given by r<=26-d. We will then focus on the Non-BPS limits in 9 dimensions and show that they lead to a decompactification to Type IIA supergravity on an interval. Without relying on string theory, we will provide a  bottom-up explanations for various UV features of the theory, such as the physics near the orientifold branes and the worldvolume theories of different stacks of non-perturbative 8-branes.

Abstract: Hydrodynamics can be thought of as an effective field theory (EFT) of thermalized matter, where the effective degrees of freedom are the fluid’s thermodynamic parameters, e.g. its temperature. As any local EFT, hydrodynamics admits a derivative expansion in local operators. Truncating this expansion to a finite order allows identifying operators differing by sub-leading terms. Which inequivalent operators one uses to write down hydrodynamics, defines the so-called hydrodynamic frame. The choice of hydrodynamic frame can affect the thermodynamic and even the causal properties of a fluid, however it should always leave its transport coefficients intact. In this talk, I will show that previously discovered results in anomalous hydrodynamics are in fact frame-dependent. Namely, I will show that the non-zero frequency, thermoelectric conductivities of (3+1)-dimensional hydrodynamics with a U(1) chiral anomaly and an O(∂) magnetic field B, are explicitly frame-dependent when truncated to first order in derivatives. We resolve this issue by incorporating B into the fluid's equilibrium state.

Abstract: Effective-field-theory Wilson coefficients can be bounded by applying basic principles of the underlying theory, including unitarity, causality and Lorentz invariance. We first discuss the derivation of such bounds; in particular, we constrain the space of allowed corrections to Einstein gravity. Then, we present evidence that much stronger bounds should exist for physically sensible theories. We obtain this evidence by explicitly constructing perturbative and non-perturbative graviton amplitudes.

Abstract: I will argue on the possibility that the smallness of some physical parameters signal a universe corresponding to a large distance corner in the string landscape of vacua. Such parameters can be the scales of dark energy and supersymmetry breaking, leading to a generalization of the dark dimension proposal. I will discuss the theoretical framework and some of its main physical implications to particle physics and cosmology.

Abstract: Love numbers parameterize the linear response of a compact body when subjected to perturbing external fields. Besides being useful observables probing the internal structure of celestial bodies in binary systems through the radiation waveforms detectable by interferometric apparatuses, they provide an exceptional opportunity to test the naturalness dogma. In particular, asymptotically flat general-relativistic black holes in four spacetime dimensions have the property of having exactly vanishing static Love numbers. This vanishing is translated to the absence of certain Wilson coefficients in the point-particle effective description of compact bodies and cannot be explained by any background symmetries forbidding their presence. In this talk, I will present the emergence of an enhanced globally defined SL(2,R) (``Love’’) symmetry that acts on black hole perturbations. This is an approximate symmetry manifesting itself in the near-zone region and precisely offers selection rules forbidding the presence of these Wilson coefficients via representation theory arguments. I will further discuss various properties of the Love symmetry including a more rigorous geometric statement of its approximate nature and the prospects of its existence in modified theories of gravity. Most importantly, the Love symmetry appears to be related to another well-known exact SL(2,R) structure associated with black holes: the enhanced isometries of the near-horizon throat of extremal black holes.

Abstract: I will describe a class of cosmological models with negative cosmological constant and a potential holographic interpretation, and I will argue that such models can generically exhibit a phase of accelerated expansion. I will also comment on the microscopic structure of these models and the new perspectives they may offer on various puzzles in cosmology. 

Abstract: My research touches upon several aspects of the strong interaction via studies of heavy-ion and pp collisions, which I will discuss during the seminar (namely the system size and lifetime of the QGP phase, the hadronization process as well as the interactions between final-state hadrons before they reach the detector). I will introduce the basic concepts and methods that I use, that is two-particle correlations in both the momentum space (referred to as femtoscopy) and in the angular space. Finally, I will also show the connection of this research to other areas of science such as astrophysics and neutron stars.

Moreover, I will show my contribution to the technical aspects of the experiment, which is done in collaboration with computer scientists. Two such projects are currently ongoing, one related to improving the particle identification capabilities of ALICE by providing Machine Learning-based solutions for general use by the Collaboration, and the second one related to outreach purposes, enhancing the collision visualization software of ALICE which is deployed in the ALICE Control Room and also used in the ALICE MasterClass tool. The latter one is used every year across the World by hundreds of high-school children during the MasterClass hands-on sessions.

Abstract: I will discuss some novel aspects related to the chaotic behaviour of highly excited string scattering amplitudes. In particular I will review the DDF construction of HES states and I will discuss the connection between chaos and thermal effects that emerges in the general decay of HES states. Finally I will discuss  the possibility of exploring the size and the shape of arbitrarily excited string states. 

Abstract: Taking string theory off shell requires breaking Weyl invariance on the worldsheet, i.e. the worldsheet theory is now a QFT instead of a CFT. I will explain Tseytlin’s approach to taking the worldsheet theory off-shell in a consistent manner, with a particular emphasis on the subtleties involved in calculating the sphere amplitude. This approach allows for the derivation of a classical string action which gives rise to the correct equations of motion and S-matrix, to all orders in perturbation theory.   I will also briefly describe the Susskind-Uglum derivation of the black hole entropy S = A/4G from the off-shell string worldsheet.  Based on arXiv:2211.08607 and 2211.16448.

Abstract: The original singularity theorems of Penrose and Hawking have, in their hypotheses, pointwise energy conditions violated by some classical and all quantum fields. If we want to extend their validity to semiclassical gravity, these conditions have to be replaced by weaker ones. In this talk I will first discuss recent results for singularity theorems with weakened energy conditions, some of which are obeyed by quantum fields. Then I will argue for the need of singularity theorems with worldvolume averaged energy conditions both in the timelike and the null case. For each case I will present progress and open questions.

Abstract: One of the main achievements in quantum physics is the discovery of methods of 'classical simulation', i.e. efficient algorithms for the simulation of equilibrium states and dynamics of quantum many-body models in classical (non-quantum) computers. These methods have revolutionised research in condensed matter and atomic physics, enabling precise numerical experiments that have led to important theoretical advances. Despite their success, these methods have been so far limited to lattice models. In this talk, I will present a new method for the classical simulation of relativistic QFTs, demonstrating its capabilities and discussing its limitations.

Abstract: I will first talk about the superradiant and Gregory-Laflamme instabilities of the equally-spinning Myers-Perry black string in 6 dimensions. Then, I will construct backreacted black strings, named black resonator strings and helical black strings, that branch from the onset of the superradiant instability and have fewer isometries. I will explain that, among the isometries of the Myers-Perry black string, each of the time translation, shifts along the string, and rotation is individually broken by the superradiant perturbation, but some of their linear combinations are preserved in the two new solutions.

Abstract:  100 years ago, Friedmann and Kasner discovered the first exact cosmological solutions to Einstein’s field equations, revealing the presence of a striking new phenomenon, namely, the Big Bang singularity. Since then, it has been the object of study in a great deal of research on general relativity. However, the nature of the ‘generic’ Big Bang singularity still remains a mystery. Rivaling scenarios are abound (monotonicity, chaos, spikes) that make the classification of all solutions a very intricate problem. I will give a historic overview of the subject and describe recent progress that confirms a small part of the conjectural picture.

Abstract: We  will discuss   some aspects of the classically scale-invariant renormalizable 4-derivative scalar field theory. Similar models appear  in the context of conformal supergravity or in the description of the crystalline phase of membranes. We will suggest how to define Poincare-invariant scattering amplitudes by assuming that only massless non-growing  modes appear as external states. In  shift-symmetric interacting theory there are no IR divergences in loop diagrams despite $1/p^4$ propagator. We  will  demonstrate  how  non-unitarity of this theory manifests itself at the level of  one-loop massless scattering amplitude.

Abstract: We analyse models of Matrix Quantum Mechanics in the double scaling limit that contain non-singlet states. The finite temperature partition function of such systems contains non-trivial winding modes (vortices) and is expressed in terms of a group theoretic sum over representations. We then focus in the case when the first winding mode is dominant (model of Kazakov-Kostov-Kutasov). In the limit of large representations (continuous Young diagrams), and depending on the values of the parameters of the model such as the compactification radius and the string coupling, the dual geometric background corresponds to that of a long string (winding mode) condensate or a 2d2d (non-supersymmetric) Black Hole. In the matrix model we can tune these parameters and explore various phases and regimes. Our construction allows us to identify the origin of the microstates of these backgrounds, arising from non trivial representations, and paves the way for computing various observables on them.

Abstract: Quantum chaos addresses the quantum mechanics of systems that are chaotic in the classical limit. Application fields include condensed matter physics, the physics of cold atomic and optical systems, particle physics, and gravity. Classical chaos manifests itself in the properties of phase space trajectories, quantum chaos  in strong correlations of spectra. In this talk I will review how these two structures are related to each other. We will then discuss how the bridge between classical and quantum chaos has become relevant as a guiding principle in the construction of a two-dimensional holographic correspondence.

Abstract: In this talk, we will study the effects of an anomalous Z' boson on the anomalous magnetic moment of the muon (g-2), and especially the impact of its axial coupling. We mainly evaluate the negative contribution to (g-2) of such couplings at one-loop and look at the anomalous couplings generated at two loops. We find areas of the parameter space, where the anomalous contribution becomes comparable and even dominant compared to the one-loop contribution. We show that in such cases, the cutoff of the theory is sufficiently low, so that new charged fermions can be found in the next round of collider experiments. 

Abstract: I will attempt to present in a pedagogical way two recent results on the structure of vacuum 4d asymptotically flat spacetimes. First, I will derive their complete set of conserved ``celestial’’ charges in the linear theory within the Bondi framework and prove their equivalence to the holomorphic basis of Freidel et al. where it was shown that the charges form the Lw_{1+\infty} algebra. I will then review elements of the post-Minkowskian formalism and explain the dictionary between the basic ingredients of the formalism, the canonical multipole moments, and the Bondi data. I will then use this dictionary to derive the complete set of conserved charges of non-radiating spacetimes, which consist of the celestial charges excluding the Newman-Penrose charges. I will show that this complete set can be alternatively described as the set of Geroch-Hansen multipole moments, the generalized BMS charges and dual supermomenta, and finally additional subleading celestial charges related to subleading memory effects.

Abstract: n this talk, I will discuss anomalies of fermions with spacetime-dependent masses. Using Fujikawa's method, it is found that the anomalies can be described in terms of superconnections introduced by a mathematician D. Quillen in 1985. I will explain some of the applications of our anomaly formulas to the systems with interfaces and spacetime boundaries. This talk is based on our recent paper arXvi:2106.01591, which was written in collaboration with Hayato Kanno.

Abstract: We construct new static solutions to gauged supergravity that, via the AdS/CFT correspondence are dual to thermal phases in N=4 SYM at finite chemical potential. These solutions dominate the micro-canonical ensemble and are required to ultimately reproduce the microscopic entropy of AdS black holes. These are constructed in two distinct truncations of gauged supergravity and can be uplifted to solutions of type IIB supergravity. Together with the known phases of the truncation with three equal charges, our findings permit a good understanding of the full phase space of SYM thermal states with three arbitrary chemical potentials.

Abstract: We consider a crossing symmetric dispersion relation (CSDR) for CFT four point correlation with identical scalar operators, which is manifestly symmetric under the cross-ratios u,v interchange. This representation has several features in common with the CSDR for quantum field theories. It enables a study of the expansion of the correlation function around u=v=1/4, which is used in the numerical conformal bootstrap program. We elucidate several remarkable features of the dispersive representation using the four point correlation function of Phi_{1,2} operators in 2d minimal models as a test-bed. When the dimension of the external scalar operator (Delta_sigma) is less than 1/2, the CSDR gets contribution from only a single tower of global primary operators with the second tower being projected out. We find that there is a notion of low twist dominance (LTD) which, as a function of Delta_sigma, is maximized near the 2d Ising model as well as the non-unitary Yang-Lee model. The CSDR and LTD further explain positivity of the Taylor expansion coefficients of the correlation function around the crossing symmetric point and lead to universal predictions for specific ratios of these coefficients. These results carry over to the epsilon expansion in 4-epsilon dimensions. We also conduct a preliminary investigation of geometric function theory ideas, namely the Bieberbach-Rogosinski bounds.

Abstract: The AdS/CFT correspondence provides a map between data appearing in AdS gravity and CFT sources and one-point functions in the presence of sources. This data appears in a near-boundary expansion for asymptotically AdS spacetimes, the Fefferman-Graham expansion. In this talk I will discuss the large-order behaviour of the Fefferman-Graham expansion for black holes and holographic RG-flows. I will identify the physical features limiting its radius of convergence and discuss the causal structure appearing over multiple sheets of the complex Fefferman-Graham plane

Abstract: I will describe recent work on the boundary interpretation of orbits around an AdS black hole. When the orbits are far away from the black hole, these orbits describe heavy-light double-twist operators in the boundary CFT. I will discuss how the dimensions of these operators can be computed exactly in terms of quasinormal modes in the bulk, using techniques from supersymmetric gauge theory. I will also explain how these results are related to the concept of quantum scars, which are eigenstates that do not obey the eigenstate thermalisation hypothesis.

Abstract: TBA

Abstract: Using the analytical conformal bootstrap, supersymmetric localization and AdS/CFT correspondence, we determine the complete four-point graviton scattering amplitude in the maximally supersymmetric AdS solution of M-theory upto a certain order in 1/c expansion, c being the central charge of the dual superconformal field theory. The talk will be based on the results from the recent paper 2207.11138 which builds upon several previous works including 2107.10274 and 2005.07175.

Abstract: In large-N gauge theories, evidence has emerged recently that between confined and deconfined phases a partially-deconfined phase can appear, in which only a subset of colours deconfine. The existence of such a phase has implications for the map between degrees of freedom under gauge/gravity duality and black hole phase diagrams, where a counterpart to the partially-deconfined saddle should be present. We investigate properties of partial deconfinement on the field theory side, first considering the partially-deconfined saddle of large-N pure Yang-Mills theory. Here, the colour degrees of freedom split into confined and deconfined sectors. We argue with the use of numerical simulations that a linear confinement potential is generated in the confined sector, implying the formation of flux tubes, whereas the potential is screened in the deconfined sector and behaves instead according to the perimeter law. Furthermore, we find that the onset of partial deconfinement coincides with the breaking of chiral symmetry breaking, providing an order parameter for the partially-deconfined phase. We conjecture that partial deconfinement is accompanied by a unique signature of global symmetry breaking that can serve as an order parameter. As another, cleaner example of this, we show that CP symmetry breaking coincides precisely with the emergence of the partially-deconfined phase in supersymmetry-broken n = 1 super-Yang-Mills with a theta-angle theta = pi, for both large finite N and the formal large-N limit. Finally, we discuss consequences of these findings for holography, the QCD crossover, and the internal phase structure of neutron stars.

Abstract: I will review the recent results in the study of microstate geometries and describe some of the future directions for this research.

Abstract: I introduce the notion of anabasis in the context of extremal black holes

Abstract: The Chiral Magnetic Effect (CME) has recently been under investigation in the data of the isobar run at RHIC. Understanding how the effect is realized in heavy ion collisions is affected by many difficulties. We will address one of these: the question of how long it takes for the CME current to build up in a non-equilibrium situation. This is a relevant question since the life-time of the magnetic field is limited. To this end we shall study a holographic model which allows to monitor the out-of-equilibrium evolution of relevant quantities such as pressure anisotropy and CME. We will first introduce the physics behind the CME and later focus on its holographic realisation and results.

Abstract: I will describe an interesting new class of supersymmetric AdS_3 solutions in supergravity.