Theory Seminars SS 23

Thu 20.04.2023, 16.30 h

Maximilian Berbig (Uni Bonn)

Recent advances in Leptogenesis

Abstract:
Leptogenesis is a very attractive scenario that ties  the generation of the matter-antimatter asymmetry in our universe to the tiny observed neutrino masses  from the Seesaw mechanism. We review the basic picture and recent developments in the field with a focus on right handed neutrino (RHN) masses  far above the electroweak scale. Gravitational wave astronomy is a novel way to probe both the Seesaw and the Leptogenesis scale, which are completely inaccessible to laboratory experiments in these high scale scenarios. We discuss the damping of inflationary gravitational waves (GW) that reenter the horizon before or during an epoch, where the energy budget of the universe is dominated by an unstable right handed neutrino, whose out of equilibrium decay releases entropy. Starting from the minimal Standard Model extension  with nothing more than 3 RHN for the Seesaw mechanism, we discuss the conditions for high scale Leptogenesis assuming a thermal initial population of RHN. One of our main findings is that the frequency, above which the damping of the tensor modes is potentially observable, is completely determined by successful Leptogenesis and a Davidson-Ibarra type bound to be at around 0.1 Hz. To quantify the detection prospects of this GW background for various proposed interferometers such as AEDGE, BBO, DECIGO, Einstein Telescope or LISA we compute the signal-to-noise ratio (SNR).

Host: P. Mertsch

 

Thu 04.05.2023, 16.30 h (MBP1 026)

Lorenzo Valbusa Dall’Armi (U. Of Padova)

The Dipole of the Astrophysical Gravitational Wave Background

Abstract:
In this seminar, we will characterize the Astrophysical Gravitational Wave Background (AGWB) generated by binary black hole systems, illustrating the main features of the homogeneous and isotropic contribution to the signal (the monopole) and discussing the sources of anisotropy in the energy density of the background. In particular, we will focus on the dipole, which is induced by three different effects (kinetic, intrinsic and shot noise terms), which carry distinct interesting physical information and should therefore be constrained individually. We will show that it is possible to do a very precise component separation, because of the different frequency scaling of the three contributions to the anisotropies and because of the large number of frequencies accessible by future experiments. We will conclude the seminar by showing that the detection of the angular power spectrum of the AGWB could be a very important tool to shed light on several tensions in cosmology, like the discrepancy in the estimate of our peculiar motion obtained by CMB and LSS measurements.

Host: J. Lesgourgues

 

Thu 11.05.2023, 16.30 h (MBP2 117)

Lara Nava (INAF Brera)

Very-high energy emission from Gamma-Ray Bursts: a tool to probe axion-like particles

Abstract:
Gamma-ray bursts (GRB) are cosmological, transient sources producing radiation on a very wide range of frequencies, from the radio band to gamma-rays. The origin of the emitted radiation is ascribed to the presence of an ultra-relativistic jet (with typical Lorentz factor of  ~100-300) undergoing energy dissipation within the jet itself and in  interactions with the surrounding medium (internal and external dissipation, respectively). In this talk, I review the status of our understanding of the physical processes responsible for the radiation. In particular, I will focus on the recent detection of very-high energy emission (around TeV energies) and on its possible use to improve our understanding of GRBs and particle acceleration in these extreme environments. Moreover, I will present GRB 221009A, the most powerful GRB ever detected, and I will discuss the possibility to use its very-high energy emission to constrain the existence and nature of axion-like particles.

Host: P. Mertsch

 

Thu 15.06.2023, 16.30 h (MBP2 117)

Pierre Cristofari (LUTH Paris)

Particle acceleration at non-relativistic collisionless shock waves: the case of starbust galaxies

Abstract:
How do particles get efficiently accelerated to the highest energies? It is now clear that in various astrophysical sources hosting collisionless shock waves, the first-order Fermi mechanism called Diffusive shock acceleration (DSA) can efficiently energize particles. However, the details of particle acceleration (spectrum, maximum energy, efficiency, etc.) often remain unclear. In the Galaxy, multi—wavelength observations of shocks have helped us gain valuable understanding on DSA. Outside the Galaxy, the case of starburst galaxies is of special interest. Indeed, the intense supernova activity in the nuclei leads to efficient particle acceleration, and to the formation of large-scale wind structures, that can additionally contribute to DSA. Multi-wavelength study, especially with next-generation gamma-ray instruments such as the Cherenkov Telescope Array (CTA) or the Square Kilometer Array (SKA) can help substantially improve our understanding on particle acceleration, and close in on the long-standing problem of the origin of the highest energy cosmic rays.

Host: P. Mertsch

 

Thu 06.07.2023, 16.30 h

Gabriele Franciolini (U. of La Sapienza, Rome)

Primordial Black Holes from Inflation: recent challenges and opportunities in the GW era

Abstract:
Primordial black holes might form in the early universe and comprise a significant fraction of the dark matter. If Primordial Black Holes (PBHs) are generated due to an enhanced scalar-perturbations at small scales, their formation is inevitably accompanied by the emission of gravitational waves (GWs) that could be seen by current and future GW experiments. We will review some recent progress on the characterization of such a signal and discuss the potential constraints Pulsar Timing Array observations could set on this scenario, as well as on the inflationary dynamics underlying these formation models.

Host: J. Lesgourgues

 

Thu 13.07.2023, 16.30 h (MBP2 117)

Marika Asgari (Hull U.)

Seing the dark Universe with observational cosmology

Abstract:
Probes of the large scale structures can give us the much needed insight into the nature of the dark Universe. In this talk I will show the latest results from the Kilo Degree Survey (KiDS) and its combination with other surveys and discuss the modelling and analysis of data. KiDS is a purpose-built gravitational lensing survey with high quality images and a wide photometric coverage, resulting in very high fidelity data.  Combining weak lensing data with other probes of the large scale structures, such as galaxy clustering, enables us to break degeneracies in cosmological parameters and control the systematics in the data. At the end of the talk, I will discuss the prospects for the KiDS final data release.

Host: J. Lesgourgues