# Theory Seminars WS 21/22

### Thu 14.10.2021, 16.30 h

Rebecca Diesing (U Chicago)

A Revised Theory of Cosmic Ray Acceleration: Solving the Problem of Steep Spectra

**Abstract:**

Galactic cosmic rays (CRs) are accelerated at the forward shocks of supernova remnants (SNRs) via diffusive shock acceleration (DSA), an efficient acceleration mechanism that predicts power-law energy distributions of CRs. However, observations of nonthermal SNR emission imply CR energy distributions that are generally steeper than E−2, the standard DSA prediction. Recent results from kinetic hybrid simulations suggest that such steep spectra may arise from the drift of magnetic structures with respect to the thermal plasma behind the shock. I will discuss how, using a semi-analytic model of non-linear DSA, we can apply this new theory to a wide range of SNRs and other astrophysical shocks. I will further show how, by accounting for the motion of magnetic structures behind the shock, we produce CR energy distributions that are substantially steeper than E−2 and consistent with observations.

Host: P. Mertsch

### Thu 21.10.2021, 16.30 h

Andrea Shindler (Michigan)

Unravelling matter-antimatter asymmetry in the universe

**Abstract: **

The observed baryon asymmetry in the universe cannot be reconciled with the current form of the Standard Model (SM) of particle physics. The amount of CP-violation stemming from the Cabibbo-Kobayashi-Maskawa matrix is not sufficient to explain the observed matter-antimatter asymmetry. Historically, one of the first systems to be studied in the search for CP-violation is the electric dipole moment (EDM) of the neutron. The contribution to the neutron EDM coming from the SM is several order of magnitudes smaller than the current experimental bound, thus providing a unique background-free window for potential discovery of physics Beyond the Standard Model (BSM). Beside all the CP-violating effective operators describing the contributions from BSM, the neutron EDM can potentially be induced by the strong CP-violating theta term.

After a brief introduction on baryon asymmetry and a summary of the current status for experimental searches of a neutron EDM, I describe the different CP-violating sources and the challenges that present a calculation of the corresponding hadronic matrix elements. I then proceed detailing recent results obtained, with my collaborators, on the neutron EDM with a specific focus on the the main theoretical and numerical tool used: the gradient flow. I conclude with near-term goals, challenges and an optimistic view into the future.

Host: R. Harlander

### Thu 28.10.2021, 16.30 h

Andrea Caputo (Weizmann)

Dark Photon, CMB and radio data in our inhomogeneous universe

**Abtsract:**

The dark photon is a well-motivated extension of the Standard Model which can mix with the regular photon. Whenever the dark photon mass matches the photon plasma frequency, a resonant conversion between photons and dark photons is possible. These conversions can produce observable cosmological signatures, including distortions to the cosmic radiation background. In this talk, I will discuss a new analytic formalism for these conversions that can account for the inhomogeneous distribution of matter in our universe, leading to new limits on the mixing parameter of light dark photons using COBE/FIRAS measurement of the cosmic microwave background spectrum. I will also briefly describe how a simple dark sector can explain the longstanding ARCADE radio background excess through resonant conversions of dark photons.

Host: F. Kahlhoefer

### Thu 04.11.2021, 16.30 h

Sebastian Hoof (Göttingen)

Deﬁnitions and Probes of the Axion Model Landscape

**Abstract:**

New experiments such as the International Axion Observatory (IAXO) or other astrophysical and cosmological probes will start gathering data in the coming years, allowing them to test many traditional axion benchmark models or observational hints for axion-like particles. However, our understanding of the axion model landscape and its viable regions in parameter space is still evolving as we continue to study and learn about axion physics. In this talk, I will outline the history of the QCD axion band, focusing on recently compiled catalogues of so-called hadronic axion models and how they can be used for alternative definitions of the axion band. I will then discuss the implications of this for the upcoming IAXO experiment in terms of opportunities and challenges. These involve the various sources of uncertainties in calculating the solar axion flux and the possibility of solving the long-standing solar metallicity problem if certain axion models are realised in Nature. After this intermezzo I will summarise the ongoing work to "box in" the axion band by complementary mass constraints from cosmology (dark matter, effective number of relativisitc species) and in particular the difficulty to estimate the dark matter density from axions when topological defects need to be taken into account. While a possible outcome is that QCD axions live in a difficult-to-probe region of parameter space, I will close by briefly commenting on how cosmology and exotic materials called topological insulators might come to the rescue of axion detection efforts.

Host: F. Kahlhoefer

### Thu 11.11.2021, 16.30 h

Cora Uhlemann (Newcastle U)

The sky from psi - Semiclassical paths to the cosmic web

**Abstract:**

On large scales, dark matter can be treated as a perfect fluid. On small scales, gravitationally bound structures form through a cascade of multiple fluid streams. Capturing the development of this intricate structure in 6-dimensional phase space is challenging. We suggest approximating the time evolution of the phase-space distribution using the quantum-classical correspondence. In this framework, one can solve a Schroedinger equation in 3d position space, where the small parameter hbar acts as a phase-space resolution scale. This method is simultaneously a tool to describe phase-space dynamics of cold dark matter with wave mechanics in position-space, and the fundamental description for ultralight scalar fields and axion-like particles acting as fuzzy dark matter. To complement the numerical treatment of the Schroedinger equation, we formulate a semiclassical equivalent of the Zeldovich approximation, where particles move along straight trajectories. Using analytical calculations and numerical examples we show that our solutions resemble Lagrangian Perturbation Theory, but protect key symmetries of the system. Our field-level approach can be used for forward modeling the matter distribution and setting accurate initial conditions for joint baryon and dark matter simulations that are key for precision cosmology.

Host: J. Lesgourgues

### Fri 12.11.2021 11:00 (MBP2 117 and via zoom) (special day and time!)

Maria Schuld (Xanadu and University of KwaZulu-Natal)

Prospects of machine learning with quantum computers

**Abstract:**

Machine learning is frequently listed among the most promising applications for quantum computing. This is in fact a rather curious choice: Today’s machine learning algorithms are notoriously powerful in practice, but remain theoretically very hard to study. Quantum computing, in contrast, does not offer practical benchmarks on any relevant scales, and theory is often the only tool we have to judge whether it could become relevant. In this talk I discuss this mismatch between commercial expectations and scientific challenges in more detail, and explain why it is so hard to say something about the practical power of quantum computers for machine learning at this stage. On the other hand, I show how quantum computing and machine learning do fit together surprisingly elegantly in a number of ways, such as the natural interpretation of quantum circuits as linear models in high-dimensional data spaces, or the ease with which quantum circuits can be trained by existing deep learning pipelines. Fundamental research that uncovers more of these links could turn out to be an important game changer for the design of future quantum machine learning applications.

Host: M. Krämer

### Thu 18.11.2021, 16.30 h

Valentin V. Khoze (Durham U.)

Searching for QCD Instantons at Hadron Colliders

**Abstract:**

QCD instantons are arguably the best motivated yet unobserved nonperturbative effects predicted by the Standard Model. A discovery and detailed study of instanton-generated processes at colliders would provide a new window into the phenomenological exploration of QCD and a vastly improved fundamental understanding of its non-perturbative dynamics. We present for the first time a full calculation of QCD instanton-induced processes in proton-proton collisions accounting for quantum corrections due to both initial and final state gluon interactions. Although QCD instanton processes are predicted to be produced with a large scattering cross-section at small centre-of-mass partonic energies, discovering them at hadron colliders is a challenging task that requires dedicated search strategies.

Host: M. Worek

### Thu 25.11.2021, 16.30 h

Oleg Lebedev (U Helsinki)

Neglected effects in dark matter studies: relativistic corrections and collective phenomena

**Abstract:**

I discuss certain effects in dark matter (DM) evolution and production that are often neglected, yet can make a crucial impact on the results. These include collective phenomena during DM production such as resonances, backreaction and rescattering,as well as effects associated with quantum statistics of dark matter states.

Host: F. Kahlhoefer

### Thu 02.12.2021, 9.00 Uhr !!!

Tom Melia (Tokyo U, IPMU)

Partition functions for Effective Field Theories

**Abstract:**

I will discuss developments in the application of novel partition function-esque probes—named Hilbert series—to Effective Field Theories. Hilbert series can be leveraged to enumerate operator bases of phenomenological Lagrangians e.g. the Standard Model EFT, and most recently have been applied to the QCD Chiral Lagrangian. Beyond this, they have been used to gain deeper analytical understanding of the S-matrix through a study of their asymptotics (e.g. Cardy formulae for generic free QFTs), and have found practical application in high-loop perturbative EFT calculations. I will highlight these developments too.

Host: R. Harlander

### Thu 09.12.2021, 16.30 h

Lavinia Heisenberg (ETH Zürich)

TBA

Host: J. Lesgourgues

### Thu 16.12.2021, 16.30 h

Philip Bechtle (Universität Bonn)

Making Useful Use of Particle Physics Experience in Epidemiology

Host: M. Krämer

### Thu 13.01.2022, 16.30 h

Juan Garcia-Bellido (Universidad Autónoma de Madrid)

TBA

Host: J. Lesgourgues

### Thu 20.01.2022, 16.30 h

Javier Mazzitelli (MPI Munich)

Next-to-next-to-leading order event generation for top-quark pair production

Host: M. Worek

### Thu 27.01.2022, 16.30 h

Alexandre Marcowith (U Montpellier)

TBA

Host: P. Mertsch

### Thu 03.02.2022, 16.30 h

Maximilian Stahlhofen (U. Freiburg)

TBA

Host: M. Czakon