Theory Seminars WS 23/24

Thu 26.10.2023, 16.30 h

Samuel J. Witte (Universitat de Barcelona)

Sourcing axions from pulsar magnetospheres

In this talk I will discuss how light axions are naturally (and unavoidably) sourced from the dynamics of the plasma which fills the magnetospheres of pulsars. I will demonstrate that, for axion masses roughly between 1e-10 eV and 1e-4 eV, this production mechanism leads to the generic prediction that all pulsars should be surrounded by extremely dense clouds of axions. The high densities realized in these axion clouds, which can exceed the local dark matter density by more than 20 orders of magnitude, can easily compensate for the feeble nature of the axion interactions, allowing these particles to exert a strong influence on their environments and produce a variety of distinctive signatures. I will highlight a number of observables which may already be present in existing data, and illustrate that axion clouds have the potential to dramatically expand indirect astrophysical searches for these particles.

Host: K. Nippel


Thu 02.11.2023, 16.30 h

Sebastian Baum (RWTH)

Paleo-Detectors & Co: Mineral Detectors for Neutrinos and Dark Matter

Minerals have been used as nuclear track detectors for more than 50 years - nuclear recoils leave latent damage in the crystal structure. In the past years, there has been much interest in fundamental physics applications for such detectors, not least because of advances in microscopy techniques that have revolutionized our abilities to image defects at the nm scale. In this talk, I will discuss a range of proposed applications of mineral detectors, in particular "paleo-detector" searches for Dark Matter and astrophysical neutrinos: Leveraging the 100 Myr - 1 Gyr exposure times natural minerals on Earth provide, one could not only measure such sources of nuclear recoils with unprecedented exposure, but also learn about their properties, for example, the distribution of Dark Matter in our Galaxy, the evolution of our Sun, or the star formation history of the Milky Way. Research groups in America, Asia, and Europe are pursuing feasibility studies of mineral detectors for neutrinos and Dark Matter, and I will also briefly report on the status and plans of these studies.

Host: S. Baum


Thu 09.11.2023, 16.30 h

Peter Matak (Comenius University Bratislava)

Diagrammar for the Early Universe: Asymmetries and Thermal Corrections

According to Sakharov's conditions, CP-violating processes are necessary for explaining the observed baryon asymmetry of the universe. The CP asymmetries are not independent and fulfil the CPT and unitarity constraints known in the literature. In this seminar, we employ the S-matrix unitarity and holomorphic cutting rules to reformulate these constraints diagrammatically in any perturbative order. As a result, the number of terms to be evaluated is reduced and tracking the asymmetry cancellations is simple. We further discuss the inclusion of thermal corrections reproducing the finite-temperature results, surprisingly, using zero-temperature Feynman rules for the amplitudes while further modifying the cutting rules.

Host: J. Heisig


Thu 16.11.2023, 16.30 h

Felix Wilsch (RWTH Aachen)

Functional one-loop matching of effective field theories

The separation of scales in effective field theories is essential for studying the low-energy phenomenology of BSM theories at the TeV-scale and above. An effective theory, containing only light degrees of freedom, can be obtained from an underlying UV model by integrating out the heavy states using path integral techniques, ensuring that both theories describe the same low-energy dynamics. It is important to perform this matching beyond the leading order as a great number of observables, like FCNC, might only appear at the loop level.  In this talk, we discuss how the one-loop matching of EFTs can be performed using functional methods. Furthermore, we show how the resulting EFT Lagrangian, usually containing many redundant operators, can be reduced to a basis. In this context, we pay particular attention to a proper treatment of evanescent operators, whose physical effects can be absorbed by a finite renormalization that can be computed using path integral techniques. Finally, we also present “Matchete”, a Mathematica code for the automatic one-loop matching and basis reduction of effective theories, based on functional methods.

Host: M. Krämer


Thu 23.11.2023, 16.30 h

Thibault Vieu (MPI Heidelberg)

Clustered supernovae: the hidden cosmic accelerators

Although supernova remnants are believed to be the most plausible sources of galactic cosmic rays, it is yet unclear how they can accelerate particles beyond 1 PeV, especially when they evolve in the warm phase of the interstellar medium (ISM). On the other hand, extra-galactic sources are not expected to contribute below hundreds of PeV. This indicates a glaring gap in our understanding of the cosmic ray spectrum observed near the Earth. The standard model does not however take into account the fact that most supernovae don't occur in the warm ISM, but rather in hot cavities blown by powerful star clusters. Within the latter, the mixing of stellar winds generate a strong turbulence and large magnetic fields which create favorable conditions for particle acceleration. In this talk I will show how supernova remnants expanding in the vicinity of young massive star clusters can explain the origin of galactic cosmic rays up to several hundreds of PeV. I will eventually discuss how this scenario can be probed with gamma-ray observations.

Host: P. Mertsch


Thu 30.11.2023, 16.30 h

Santiago Casas (RWTH)

What will Large Scale Structure tell us about Dark Energy in the next decade?

With the upcoming generation of galaxy surveys such as Euclid, LSST (Vera Rubin Observatory) DESI and SKAO, among others, the cosmological community will have groundbreaking measurements of the Large Scale Structure of the Universe. These measurements will provide very precise information about the expansion rate, the growth of non-linear structures as a function of scale and the impact of baryonic feedback. These measurements have to be confronted against theoretical models that aim to explain the nature of Dark Matter, Dark Energy and possibly resolve the ongoing tensions we face in cosmology, such as the H0 and the S8 discrepancies. In this talk I will recall some of the competing and alternative models to the standard LCDM paradigm that have been proposed in the literature in the last decades, focusing on scalar-tensor theories with and without screening and generalized parametrizations of Modified Gravity. I will discuss how, with the combined observations from Euclid (especially with the complementarity of Weak Lensing and Galaxy Clustering) and the advent of Radio-cosmology, we expect to be able to constrain that vast theoretical parameter space. By doing so, we will also hopefully measure the neutrino mass and learn about the fundamental physics of baryons and non-linear structure formation. Towards the end I will discuss also the challenges we face in computational complexity and how this is being resolved with emulators and the emerging field of differentiable programming.

Host: J. Lesgourgues


Thu 07.12.2023, 16.30 h

James Alvey (U. Amsterdam)

Can simulation-based inference be the future of gravitational wave data analysis?

Over the last 8 years, we have learnt a huge amount about the gravitational wave sky from observatories such as LIGO. A key facilitator of this success is a robust analysis pipeline for triggering and analysing localised binary sources and characterising noise in the various detectors. Looking towards later observing runs at current facilities, and future detectors such as the Einstein Telescope and LISA, however, some clear challenges start to appear. How do we deal with the vast increase in detection rate? How do we disentangle and analyse coincident signals in our detectors? Can we simultaneously calibrate instrumental noise and trigger/localise sources? Under what conditions can we detect a stochastic gravitational wave background? In this talk, I will give an introduction to the use of modern simulation-based inference methods in gravitational wave analysis, and provide some concrete examples of how they might be used to tackle these complex issues.

Host: S. Baum


Thu 14.12.2023, 11.00h in 26C 401 (note the special time and location!)

Guillem Domènech (Hannover U.)

A universal property of cosmic gravitational wave background anisotropies?

Gravitational Waves (GWs) may come from as far as the first instants of the universe. In fact, our universe might as well be filled with a GW Background (GWB). On one hand, its GW spectrum might help us probe the origin of such cosmic GWs, which concerns very small scales physics as compared to those probed by the Cosmic Microwave Background (CMB). On the other hand, anisotropies of the cosmic GWB provide an opportunity to have a new look at the universe on the largest scales. But, as opposed to the CMB, initial conditions of GWB fluctuations are set in the very early universe. I will discuss the meaning of adiabatic initial conditions and show that GWB anisotropies may be insensitive to the equation of state of the universe at the time of generation. I will then discuss how one may instead generate GWB isocurvature fluctuations within the curvaton mechanism.

Host: J.Lesgourgues


Thu 11.01.2024, 16.30 h

Renato Fonseca (Granada U.)

Automating group theory calculations in high energy physics

Most theories considered by physicists have one or more symmetries, such as the one of flat space-time and the one associated with gauge invariance. Fields are therefore representations of groups and, even before considering Feynman rules, one needs a Lagrangian with the interactions between these fields. For this and other reasons group theory is of great importance. In this presentation I will discuss my efforts over the years to automate several group theory calculations which are relevant in particle physics, including automatically building a Lagrangian.

Host: R. Harlander


Thu 18.01.2024, 16.30 h

Subodh Patil (Leiden U.)

On PBH formation in single clock inflation

I could have also titled this talk "On the uses and abuses of effective potentials in cosmology". Beginning with a general discussion of what one is operationally doing in early universe cosmology with scalar fields, I will recap what physical conclusions one is allowed to draw from effective potentials in cosmology. I will then apply this to scenarios that consider primordial black hole formation in single clock inflation, transitioning from critique to constructivism, and elaborate on technical and tuning issues that make this enterprise more challenging than the literature cares to admit. I'll conclude by highlighting what I consider to be the more plausible formation channels, and why.

Host: L. Lesgourgues


Thu 01.02.2024, 16.30 h

Davide Racco (ETH Zurich)

Utilizing the causal spectrum of gravitational waves: implications for Pulsar Timing Arrays

The low-frequency part of a primordial gravitational wave spectrum generated by local physics in a wide class of phenomena (such as a phase transition), is largely fixed by causality, offering a clean window into the early Universe. The physics of low-frequcncy modes allows to probe in a model-independent way the Hubble rate at the time of GW production, the equation-of-state of the Universe at subsequent times and the presence of free-streaming particles at primordial epochs. I will highlight the importance of these considerations for the GW background recently measured at Pulsar Timing Arrays, where the Standard Model has an unavoidable and distinctive effect on a primordial GW signal.

Host: L. Lesgourgues