### University of Crete HEP Seminars

**
Higher form symmetries and superfluids
**

Speaker: | Diego Hofman |

Institution: | University of Amsterdam |

Time: | Monday 23 September 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | I will describe superfluid hydrodynamics as the hydrodynamic theory of a system with an emergent anomalous higher-form symmetry. The higher-form charge counts the winding planes of the superfluid -- its constitutive relation replaces the Josephson relation of conventional superfluid hydrodynamics. This formulation puts all hydrodynamic equations on equal footing. The anomalous Ward identity can be used as an alternative starting point to prove the existence of a Goldstone boson, without reference to spontaneous symmetry breaking. This provides an alternative characterization of Landau phase transitions in terms of higher-form symmetries and their anomalies instead of how the symmetries are realized. This treatment is more general and, in particular, includes the case of BKT transitions. |

**
Dark matter search and gravitational wave detection using atom interferometry
**

Speaker: | Wolf von Klitzing |

Institution: | IESL-FORTH Crete |

Time: | Thursday 26 September 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | In a recent White Paper (arXiv:1908.00802) we proposed a concept for a space experiment using cold atoms to search for ultra-light dark matter, and to detect gravitational waves in the frequency range between the most sensitive ranges of LISA and the terrestrial LIGO/Virgo/KAGRA/INDIGO experiments. This interdisciplinary experiment, called Atomic Experiment for Dark Matter and Gravity Exploration (AEDGE), will also complement other planned searches for dark matter, and exploit synergies with other gravitational wave detectors. We give examples of the extended range of sensitivity to ultra- light dark matter offered by AEDGE, and how its gravitational-wave measurements could explore the assembly of super-massive black holes, first-order phase transitions in the early universe and cosmic strings. AEDGE will be based upon technologies now being developed for terrestrial experiments using cold atoms (also on Crete), and will benefit from the space experience obtained with, e.g., LISA and cold atom experiments in microgravity. |

**
The Geometric Trinity of Gravity: the TEGR case
**

Speaker: | Konstantinos Dialektopoulos |

Institution: | Center for Gravitation and Cosmology, Yangzhou University |

Time: | Friday 27 September 2019, 16:15 |

Venue: | 2nd floor seminar room |

Abstract: | It is well established by now that General Relativity and the LCDM model are very successful in describing the gravitational interactions at all scales. However, the theory is plagued with some shortcomings, enabling scientists to pursue an alternative formulation of gravity. In this talk, I am going to review the three different formulations of gravity; one based on the curvature of spacetime (General Relativity), one based on its torsion (Teleparallel Equivalent of General Relativity, TEGR) and one based on its non-metricity (Symmetric Teleparallel of General Relativity, STEGR). I will try to focus on the TEGR, explaining some of its key features. |

**
Fine Tuning Problems from Cosmological Coleman-Weinberg Potentials
**

Speaker: | Richard Woodard and S.P. Miao |

Institution: | UF (USA) and NCKU (Taiwan) |

Time: | Monday 21 October 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Cosmological Coleman-Weinberg potentials are quantum corrections to the effective potential of the inflaton which result from coupling it to ordinary matter in order make re-heating efficient. These corrections are problematic for inflation because they are not Planck-suppressed and tend to make the potential too steep. Therefore they must be subtracted off by additions to the classical action. Unfortunately, they cannot be completely subtracted off. Although they go over to the Coleman-Weinberg form in the flat space limit, their actual form on de Sitter background is the 4th power of the Hubble parameter times a complicated function of the coupling constant times the ratio of the inflaton to the Hubble parameter. On a general inflationary background they are not even local functions of the geometry, whereas the allowable subtractions consist of algebraic functions of the inflaton and the Ricci scalar. We discuss the disturbing results from the best possible local subtraction schemes. This talk is based on arXiv:1506.07306, 1806.02533, 1908.03814 and 1908.05558. |

**
Comments on the black hole S-matrix
**

Speaker: | Panagiotis Betzios |

Institution: | UOC |

Time: | Tuesday 5 November 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | I will review the shockwave construction of 't Hooft by which gravitational backreaction can lead to a unitary S-matrix for particles scattering on an eternal black hole background. In order to obtain a background with a single exterior, an antipodal (CPT) identification was recently proposed that could in principle give a resolution to the information paradox for single sided asymptotically flat black holes. An emphasis will be given in the various assumptions and caveats of this construction. I will then describe an explicit dynamical quantum mechanical system that results in the same S-matrix. If time permits I will conclude with some efforts towards understanding the experience of an infalling observer in this setup. |

**
The Skyrmion Paradox
**

Speaker: | Christos Panagopoulos |

Institution: | NTU Singapore |

Time: | Tuesday 12 November 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Magnetic skyrmions (or vortices) are spatially inhomogeneous spin textures localized in nanoscale cylindrical regions. Topological protection and small size make skyrmions especially attractive for the study of spin topology and technologies wherein information is carried by the electron spin further to, or instead of the electron charge. Despite achievements in the synthesis of materials where axisymmetric magnetic skyrmions can be stabilized and characterized, there is disproportionate progress in elucidating the basic properties. The talk will attempt to bridge this gap and deliver an intelligible guide on the physical principles governing these magnetic whirls. |

**
Bootstrapping three dimensional cubic theories
**

Speaker: | Stefanos Kousvos |

Institution: | UOC |

Time: | Thursday 14 November 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | After a concise introduction to the conformal bootstrap in general dimensions, I will discuss recent results regarding theories with cubic global symmetry in three spatial dimensions. Under certain assumptions, we find an isolated region in parameter space, which given prior intuition with the numerical conformal bootstrap, hints towards the existence of a CFT in this region of parameter space. We study the consistency of this CFT using multiple correlator systems. We find critical exponents for the conjectured CFT which are in discrepancy with the epsilon expansion (but in agreement with experiments for structural phase transitions). The disagreement of critical exponents for structural phase transitions calculated in the epsilon expansion with those measured in experiments is something that was noticed since the 70s. Given time I will briefly mention a resolution to this apparent discrepancy proposed at that time. |

**
Violation of the horizon effect after a quantum quench in the sine-Gordon model
**

Speaker: | Spyros Sotiriadis |

Institution: | University of Ljubljana |

Time: | Tuesday 4 February 2020, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | The study of non-equilibrium dynamics in quantum field theories is of central importance for a wide range of physics areas, from high-energy and cosmology to condensed matter and atomic physics. One of the most characteristic features of quantum dynamics in relativistic field theories (and more generally systems characterised by a maximum speed of information propagation) is the "light-cone spreading" of correlations, also known as "horizon effect": starting from an initially short-range correlated state, measurements of two observers at distant points are expected to remain independent until their past light-cones start overlapping. Surprisingly, we find that in the presence of topological excitations, correlations can develop outside of the horizon, even between infinitely distant points. We demonstrate this effect in the sine-Gordon model, showing that it can be attributed to the non-local nature of its topological excitations and interpret it as dynamical emergence of entanglement at large distances. Our findings are derived in two independent ways: (i) a novel numerical technique for the simulation of quantum many-body dynamics in continuous models, and (ii) an exact analytical method based on the duality between sine-Gordon and massive Thirring model. References: Phys. Rev. Lett. 121, 110402 (2018) arXiv:1802.08696 arXiv:1906.02750 |

**
The Origin of the Six-Particle Amplitude at Finite Coupling
**

Speaker: | Georgios Papathanasiou |

Institution: | DESY |

Time: | Thursday 6 February 2020, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Scattering amplitudes form a bridge connecting theoretical particle physics with the real world of collider experiments. Yet their computation by means of Feynman diagrams quickly becomes prohibitive, and is only valid at weak coupling. Focusing on the simplest interacting gauge theory, known as large-color maximally supersymmetric Yang - Mills, and exploiting its integrability, in this talk I present a finite-coupling expression for its six-particle amplitude in a particular kinematic limit: The origin in the space of its natural kinematic variables, where two-particle Mandelstam invariants become much smaller than three-particle ones. I also demonstrate that our expression is in perfect agreement with existing field- and string-theoretic predictions for its weak and strong coupling expansion, respectively. |