### University of Crete HEP Seminars

**
Super-Planckian scalars?
**

Speaker: | Lukas Witkowski |

Institution: | University of Crete |

Time: | Tuesday 26 September 2017, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Scalar fields with super-Planckian field ranges play an important role in models of large-field inflation and are also employed in cosmological solutions to the electroweak hierarchy problem. One difficulty in constructing such models is that for super-Planckian field values quantum-gravitational effects cannot be ignored. In particular, ultraviolet physics is expected to constrain the field range over which scalars can exhibit a monotonic potential, which in turn has consequences for models of inflation. To study theories of super-Planckian scalars some knowledge of ultraviolet physics is thus necessary. String Theory is a candidate theory of Quantum Gravity and a suitable framework for addressing this question. Another approach is based on the Weak Gravity Conjecture – an as yet unproven theorem regarding consistent theories of Quantum Gravity. In this talk I want to report on recent progress in the study of super-Planckian scalars. In particular, I will explain how the existence of super-Planckian scalars is increasingly called into question by both String Theory and the Weak Gravity Conjecture. |

**
Holographic RG flows on curved manifolds
**

Speaker: | Lukas Witkowski |

Institution: | University of Crete |

Time: | Thursday 28 September 2017, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Holographic RG is based on the idea that the radial coordinate of a spacetime with asymptotically AdS geometry can be identified with the RG flow parameter of the boundary field theory. Holographic RG flows have been studied intensively for the case of field theories defined on a flat background manifold. In this talk I will consider the case where the background spacetime for the field theory is curved. In particular, I will consider the cases of field theories defined on a sphere as well as dS and AdS spacetimes. In this setting I will explore solutions that have exotic properties from a RG point of view. There is also tantalising evidence for the existence of first-order phase transitions triggered by a change in curvature. |

**
Quasinormal modes of a strogly coupled non-conformal plasma and approach
to criticality
**

Speaker: | Panagiotis Betzios |

Institution: | University of Crete |

Time: | Tuesday 3 October 2017, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | We study fluctuations around equilibrium in a class of strongly interacting non-conformal plasmas using holographic techniques. In particular we calculate the quasi-normal mode spectrum of black hole backgrounds that approach to Chamblin-Reall plasmas in the IR. In a specific limit, related to the exact linear-dilaton background in string theory, we observe that the plasma approaches criticality and we obtain the quasi-normal spectrum analytically. We regulate the critical limit by gluing the IR geometry that corresponds to the non-conformal plasma to a part of AdS space-time in the UV. Near criticality, we find two sets of quasi-normal modes, related to the IR and UV parts of the geometry. In the critical limit, the quasi-normal modes accumulate to form a branch cut in the correlators of the energy-momentum tensor on the real axis of the complex frequency plane. |

**
Primordial Inflation and Power Spectra
**

Speaker: | Nikolaos Tsamis |

Institution: | University of Crete |

Time: | Thursday 5 October 2017, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | After an introduction to primordial inflation and de Sitter physics we present a formalism which allows to compute primordial power spectra for non-constant geometries analytically and we obtain interesting quantitative results. |

**
Holographic Phonons
**

Speaker: | Matteo Baggioli |

Institution: | University of Crete |

Time: | Tuesday 10 October 2017, 13:45 |

Venue: | 2nd floor seminar room |

Abstract: | We discuss the presence and the physical features of phonons in the context of the holographic framework with particular emphasis on the elastic and transport properties of the dual strongly coupled QFT. |

**
Scale invariant Lifshitz hydrodynamics and Torsional Newton-Cartan geometry
**

Speaker: | Elias Kiritsis |

Institution: | University of Crete |

Time: | Thursday 12 October 2017, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Hydrodynamics is the generic low energy theory that describes fluids and generalizes thermodynamics. There are however quantum field theories, whose hydrodynamics is not known. A large class are non-relativistic theories with Lifshitz scaling (appearing in condensed matter systems). The gauge/gravity correspondence has provided a novel and precise correspondence between fluid dynamics and the gravitational physics of horizons. We use this to find the hydrodynamic equations that describe non-relativistic quantum field theories with Lifshitz scaling and hyperscaling violation. We also find the (torsional) Newton-Cartan gravity plays an important role. |

**
Simplifying one-loop amplitudes in superstring theory
**

Speaker: | Dario Consoli |

Institution: | Universita' di Roma "Tor Vergata" |

Time: | Tuesday 14 November 2017, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | We show that 4-point vector boson one-loop amplitudes in the RNS formalism, around vacuum configurations with open unoriented strings, preserving at least N=1 SUSY in D=4, satisfy the correct supersymmetry Ward identities, in that they vanish for non MHV configurations (++++) and (-+++). In the MHV case (--++) we drastically simplify their expressions. We then study factorization and the limiting IR and UV behaviour and find some unexpected results. In particular no massless poles are exposed at generic values of the modular parameter. |

**
Moduli of AdS4 vacua: A fine-print study
**

Speaker: | Massimo Bianchi |

Institution: | Universita' di Roma "Tor Vergata" |

Time: | Thursday 16 November 2017, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | We analyze the moduli space near N = 4 supersymmetric AdS4 solutions of string theory by combining and comparing various approaches: - exact backgrounds of type IIB string theory, - gauged supergravity - 3d quiver theories - representations of the superconformal algebra. We discuss both supersymmetry-preserving discrete moduli, and continuous moduli breaking N = 4 to N = 2 supersymmetry. Our analysis lends new support to the underlying holographic duality. |