The seminar of the Theory Section takes place Mondays (alternating with the Journal Club) in D3-114 (Onsager) and on zoom, starting normally at 14.15. If you would like to suggest a seminar speaker or want to be added to the email list, please contact the organizer (Michael.Kachelriess@ntnu.no).
A list of seminars in the previous and the current years can be accessed in the sidebar; the (planned) seminars in 2021 are
- 22.01., Marco Stein Muzio (New York University): Closing in on the sources of UHECRs: A multimessenger approach
Abstract: Today ultrahigh energy cosmic rays (UHECRs), with energies in excess of an EeV (or 10^18 eV), provide us with a glimpse both into the most extreme astrophysical environments in the universe and into fundamental interactions beyond LHC energies. However, the source of these particles has long remained a mystery. In this talk, I will focus on progress made in characterizing the sources of UHECRs by combining a phenomenological model of these sources with multimessenger measurements. Recent elaborations of our model have allowed us to make more concrete statements about the types of environments required to explain UHECR data, and possibly astrophysical neutrino data. I will present these constraints and their implications for astrophysical sources.
slides
- 19.02., Alessandro Mirizzi (INFN, U Bari): Axions: Bounds and Discovery Opportunities
Abstract: Axions have been introduced in relation to the strong CP problem of the QCD and are predicted in many extensions of the Standard Model of particle physics. Depending on the actual value of their mass, axions can play an important role in cosmology, acting as cold or hot dark matter. The coupling with photons allows for axion-photon mixing in external electromagnetic fields. This effect is exploited for direct searches of axions in laboratory experiments. Axions can be searched also through astrophysical observations. Notably they can be produced in stellar cores leading to an excessive energy loss, that would alter the standard stellar evolution. Furthermore, the two-photon vertex would also induce the mixing with axions for photons emitted by distant astrophysical sources, and propagating in the large-scale cosmic magnetic fields, leading to peculiar observational signature. In this talk I will present an overview of current bounds on axions and discovery opportunities in the planned laboratory and astrophysical experiments.
slides - 19.03., Magdalena Eriksson (NTNU, UiS): Quantum corrections to inflation postponed
- 16.04., Martin Mohajed (NTNU): Chiral perturbation theory at finite isospin
Abstract: Chiral perturbation theory (ChiPT) is a low-energy effective field theory of QCD. I will discuss the QCD phase diagram at finite isospin chemical potential and the application of ChiPT to the problem of pion condensation at zero temperature. Since QCD at finite isospin is free of the fermion sign problem, one can use lattice simulations. We compare our two- and three-flavor results for various condensates obtained using ChiPT with recent Monte Carlo data.
slides
- 30.04., Tomas Brauner (UiS): Topology of spontaneous symmetry breaking, emergent higher-form and higher-group symmetries
Abstract: Spontaneous breaking of a continuous global symmetry implies as a rule the existence of new, emergent symmetries, associated with the topology of the vacuum manifold. These may take the form of ordinary symmetries associated to a conserved current and localized charged excitations, or higher-form symmetries, acting on extended topological defects. I will give a brief introduction to the above-mentioned concepts, and then show how higher-form symmetries can be combined into a more general mathematical structure still, referred to as higher-group symmetry. This is a concept that has recently attracted attention in the high-energy theory community. It however turns out to be also relevant for systems as mundane as superfluid mixtures.
slides - 07.05., Magnus Malmquist (NTNU): Polarization Sums and Unitarity in QCD
Abstract: One of the first techniques one encounters when learning quantum field theory is the method of replacing the sum over photon polarizations by minus the metric tensor. This is not an equality; instead the QED Ward identity ensures that the replacement gives the correct result. We ask the simple question of whether this replacement can be made in a non-abelian theory like QCD. The answer is "yes, we can". Using unitarity we explain how the method should be applied in QCD, which requires considering external Faddeev-Popov ghosts. Understanding the procedure of replacing the polarization sum by minus the metric tensor then leads us to Slavnov-Taylor identities which are a core stone of the mathematical consistency of QCD.
slides
- 21.05., Magdalena Eriksson (NTNU, UiS): Quantum corrections to inflation
Abstract: The theory of cosmic inflation solves the problems of Big Bang cosmology and provides an explanation to the origin of the cosmic microwave background (CMB) radiation. Inflation is often modelled with a scalar field slowly rolling down some potential in FRLW spacetime. In cosmological perturbation theory one perturbs this inflaton field as well as the FRLW metric, and these perturbations can be related to the temperature fluctuations observed in the CMB. In this talk I give an introduction to inflationary cosmology and cosmological correlation functions. I introduce the concept of corrections to these correlation functions and how they can affect the inflationary dynamics. In particular I differentiate between corrections to cosmological observables, i.e. n-point correlation functions, and corrections to the evolving inflaton field and background metric. Corrections to the evolution equations can be viewed as corrections to the slow-rolling parameters, which also enter into observables.
- Summer break...
- 06.09., Bjoern Eichmann (RUB, NTNU):
Germano Nardini (UiS), Alexander Stasik (UiO)