University of Crete HEP Seminars


Comments on 4-derivative scalar theory in 4 dimensions

Speaker: Arkady Tseytlin
Institution: Imperial
Time: Tuesday 14 February 2023, 13:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.

The mysterious nature of the Big Bang singularity

Superradiance, black resonator strings and helical black strings

Speaker: Grigorios Fournodavlos
Institution: University of Crete
Time: Tuesday 21 February 2023, 13:00
Venue: Second floor seminar room
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.
Speaker: Takaaki Ishii
Institution: Rikkyo University
Time: Tuesday 28 February 2023, 13:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.

Hamiltonian truncation methods for the numerical simulation of relativistic QFT

Singularity theorems for worldvolume energy inequalities

Speaker: Spyros Sotiriadis
Institution: University of Crete
Time: Tuesday 07 March 2023, 13:00
Venue: Second floor seminar room
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.
Speaker: Eleni Kontou
Institution: King's College London
Time: Thursday 09 March 2023, 13:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.

The Off-shell String Effective Action

Speaker: Aron Wall
Institution: Cambridge
Time: Tuesday 14 March 2023, 13:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.

On the chaotic aspects of highly excited string amplitudes

Speaker: Maurizio Firrotta
Institution: INFN Rome
Time: Wednesday 15 March 2023, 13:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.

ALICE as a laboratory to study strongly interacting systems via particle correlations

Speaker: Lukasz Graczykowski
Institution: CERN
Time: Friday 17 March 2023, 16:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.

Accelerating cosmology from gravitational effective field theory with negative cosmological constant

Speaker: Brian Swingle
Institution: Brandeis University
Time: Tuesday 21 March 2023, 15:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.

Magic zeroes in the black hole response problem and a Love symmetry resolution

Speaker: Panos Charalambous
Institution: New York University
Time: Thursday 23 March 2023, 13:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.

Swampland Program, Extra Dimensions and Supersymmetry Breaking

Speaker: Ignatios Antoniadis
Institution: LPTHE
Time: Tuesday 28 March 2023, 18:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.

The Space of Gravitational Effective Theories from Amplitudes

Speaker: Dimitris Kosmopoulos
Institution: University of Geneva
Time: Thursday 30 March 2023, 13:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.

On the frame dependence of conductivities in anomalous hydrodynamics

Speaker: Ioannis Mathaiakakis
Institution: University of Genoa
Time: Thursday 06 April 2023, 13:00
Venue: Webinar
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.
Recording: The recorded talk can be found here.