# Schedule for the summer term 2018:

[table delimiter=”|”]Date|Speaker[attr style=”width:200px”]|Title (hover/click for abstract)[attr style=”width:400px”]

09.04.| * no seminar * |

16.04.| Ludwig Hoffmann (ITP) |

23.04.| Maik Wessling (ITP) |

30.04.| * no seminar * |

07.05.| Jisuke Kubo (Kanazawa) |

14.05.| * no seminar * |

21.05.| * public holiday * |

28.05.| Francesca Arici (MPI MIS) |

04.06.| Asger Ipsen (HU Berlin) |

11.06.| Marek Kozon (ITP) |

18.06.| Hans-Bert Rademacher (Leipzig) |

25.06.| Felix Kurpicz (ITP) |

02.07.| Angkana Rüland (MPI MIS)|

09.07. 16:15| Andrea Thamm (CERN) |

[/table]

**Abstract:**In most classical field theories the total mass of a system is defined using the stress-energy tensor. However, in general relativity this definition ceases to work. The difficulties arise because there is no stress-energy tensor for the gravitational field and thus the total mass has to be defined otherwise. By introducing a convenient coordinate system (Bondi coordinates) far away from the system it is possible to find a satisfying description of the total mass. Building on previous results in even dimensions we show how the Bondi mass is defined in odd dimensions.

**Abstract:**We propose a conjecture: For analytic FLWR spacetimes (i.e. with analytic expansion factor a(t)), the construction method for states of low energy (SLEs) produces states that satisfy the analytic microlocal spectrum condition, if an analytic smearing function f(t) is used in their construction. Because proving the conjecture was found to be not fitting the framework of a master thesis we rather do a consistency check. We proceed to construct SLEs for the Klein-Gordon field in deSitter spacetime while specifying the mass of the Klein-Gordon field and the form of the smearing function. It is then shown that the difference of two SLEs – expressed as two-point functions – behaves analytically.

**Abstract:**We assume that the electroweak scale is generated in a hidden sector which is described by a non-abelian gauge theory. The non-perturbative effect in the hidden sector generates dark matter as well. Since this dynamical scale genesis appears as a first-order phase transition at finite temperature, it can produce a gravitational wave background.

**Abstract:**Reflection positivity is a central ingredient in the Osterwalder-Schrader reconstruction theorem, which relates Eucliedean QFTs to their Lorentzian counterparts. In this talk I will present an explicit proof of violation of reflection positivity for a large class of propagators. Based on joint work with D. Becker, C. Ripken, F. Saueressig and W. van Suijlekom.

**Abstract:**Classically it is possible to introduce defects (or interfaces) in N = 4 while preserving half the super(-conformal) symmetry. I will consider the case where the defect has codimension 1 and separates spacetime into two regions with different gauge groups. An argument based on holography suggests that there should be a corresponding consistent QFT. Recently there has been progress in understanding this setup at the quantum level. I will present our computation of the one-loop corrections to certain one-point functions, and show that they match a ‘prediction’ from string theory. Joint work with I. Buhl-Mortensen, M. de Leeuw, C. Kristjansen, K.E. Vardinghus and M. Wilhelm.

**Abstract:**We present a semi-classical quantization scheme of closed Nambu-Goto string, generalizing the earlier work done for the cases of open and circular closed strings. Using methods of quantum field theory in curved space-times, we calculate the expectation value of energy for a string rotating in two spatial planes perpendicular to each other, in a space-time of general dimension and compare the resulting Regge intercept to previous work. We also provide a comparison of the state spectrum of such string with the one obtained by covariant quantization.

**Abstract:**Twistor spinors are conformally covariant spinor fields generalizing parallel spinors as well as Killing spinors. In General Relativity the associated conformal vector field defines a first integral for lightlike geodesics. We discuss twistor spinors with zeros in Riemannian geometry. This is joint work with Wolfgang Kuehnel, Florin Belgun and Nicolas Ginoux.

**Abstract:**In this talk we discuss the thermal behavior of quantum fields near the apparent horizon of a dynamical spherically symmetric spacetime. When considering a suitable scaling procedure of the two-point function of a scalar field in a Hadamard state, a thermal spectrum can be detected along integral lines of the Kodama vector field. This scaling limit can also be related to the tunneling probability of particles over the apparent horizon. Joint work with N. Pinamonti and R. Verch.

**Abstract:**In this talk I discuss a nonlocal inverse problem, the fractional Calderón problem. This is an inverse problem for a fractional Schrödinger equation in which one seeks to recover information on an unknown potential by exterior measurements. In the talk, I prove uniqueness and stability of the “infinite data problem” and then address the recovery question. This also yields (at first sight) surprising insights on the uniqueness properties of the inverse problem in that it turns out that a single measurement suffices to uniquely recover the potential. These properties are based on the very strong unique continuation and approximation properties of fractional Schrödinger operators, which are of independent interest and which I also discuss in the talk. This is based on joint work with T. Ghosh, M. Salo and G. Uhlmann.

**Abstract:**I will show that the study of rare Higgs decays in the high-luminosity run of the LHC can probe axions and axion-like particles (ALPs) in a wide range of parameter space, which is otherwise inaccessible to experimental searches. If the ALP decays predominantly into photons, our strategy covers the current “gap” in the mass range between 1 MeV and 60 GeV down to a photon-axion coupling as small as 10^-6/TeV. An ALP in this parameter range can explain the anomalous magnetic moment of the muon and is consistent with electroweak precisions tests. In our analysis we consider the most general effective Lagrangian for a spin-0 particle protected by a shift symmetry, motivated by many extensions of the Standard Model with a spontaneously broken global symmetry.