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): Can radio galaxies solve the UHECR puzzle?

Abstract: The origin of the ultra-high-energy cosmic rays (UHECRs) is one of the great enigmas of modern astroparticle physics. Radio galaxies (RGs) - which were divided by Fanaroff and Riley into two classes (FR-I and FR-II) - show great potential to be the birthplace of these nuclei with energies above 1 EeV. In this talk some recent findings on the contribution of two individual, promising radio galaxies, Centaurus A and Cygnus A, will be discussed as well as the CR contribution from the bulk of these galaxies. Using the radio luminosity as a robust estimator for the CR luminosity, it is shown that the FR-I source Centaurus A is able to provide the dominant UHECR contribution at the highest energies. But there is an other contributor between 5 EeV and 30 EeV needed. The exceptionally bright radio source Cygnus A is a prominent source candidate, but it is shown in this talk, that the impact by the extragalactic magnetic field on the CR propagation causes some serious issues: Either the arrival directions of the CRs provide a high degree of anisotropy or the delay exceeds the source age. Alternatively, the low-energetic UHECRs can originate in the bulk of FR-I or FR-II sources. For such a scenario, the necessary jet dynamics of FR sources are discussed, showing that FR-I RGs can in principle provide the observed amount of UHECR energy as well as a proper spectral behavior. In contrast, the bulk of FR-II RGs most likely contribute less than 25%.

slides

- 25.10.,
**Jonas Tjemsland**(NTNU): The Almighty Axion

The Standard Model of particle physics has had immense success over the years. Yet, it has several shortcomings that illustrate its incompleteness, including for example the strong CP problem. However, already back in 1977 Peccei and Quinn postulated the existence of the so-called axion—a hypothetical, light pseudo-scalar boson—as a solution to the strong CP problem. Not only that, the axion and axion-like particles turn out to be potential miracle cures for many of the biggest problems humanity has ever faced: identifying dark matter, explaining dark energy, explaining inflation and ending world hunger. First, though, one must address the elephant in the room: the axion has not yet been detected. In this talk, I will give a theoretical introduction to the axion and its potential importance to modern physics, and I will discuss how the characteristic two-photon coupling enables the detection of axion-like particles. In particular, I will explain how axion-like particles can be detected by measuring photons from distant gamma-ray sources by exploiting a phenomenon known as ``photon-axion oscillation'' that occurs as photons propagate through astrophysical environments.

slides

- 28.10.,
**Alexander Rothkopf**(UiS): Open Quantum Systems: Thermometry at the Extremes

The study of quantum systems coupled to an environment plays a vital role in how we measure temperatures of the coldest and hottest matter in the universe. The strategy relies on introducing impurities into the system of interest and on observing how these probe particles evolve towards or in equilibrium with their surroundings, from which we may in turn deduce the thermal properties of that environment. Originally studied in the context of condensed matter physics, open quantum systems nowadays provide a common language to research spanning multiple orders of magnitude in temperature, ranging from Bose Einstein condensates made of ultracold atoms to the Quark-Gluon plasma created in ultra-relativistic collisions of heavy ions. This talk builds a bridge from polaron impurities in the former to quarkonium particles in the latter as two manifestations of quantum Brownian motion, a phenomenon ideally described by open quantum systems.

slides

- 15.11.,
**David Garofalo**(Kennesaw State University): Radio galaxies and their supermassive black holes

Despite the universe being only a few hundred million years old, a merger between two gas-rich galaxies produces a rapidly spinning massive black hole weighing 4 billion solar masses. But the bright disk of gas that settles around this black hole does so in an unusual way, with the direction of gas flow opposite to black hole rotation. A powerful jet emerges from the hole that enhances star formation in the galaxy for 400 million years. But as the black hole spin decreases, the jet turns off and remains so for about a billion years, after which a new jet emerges, but with a fundamentally different feature, its orientation tilted with respect to the previous jet. This allows the jet energy to couple directly to the ISM, heating it and suppressing star formation. Despite 10 billion years of accretion, the black hole fails to spin rapidly again, reaching only about 50% of its maximum possible when it is observed with the EHT. This is the black hole at the center of M87. I will describe the theoretical framework that accounts for this story.