# 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 (26C 402 and zoom)

Lavinia Heisenberg (ETH Zürich)

Gravitational frontiers and the road ahead

**Abstract: **

General Relativity and the Cosmological Principle are the fundamental pillars of Cosmology. After introducing them and their underlying properties I will discuss the successes and challenges of the Standard Model of Big Bang Cosmology. I will then discuss how we can test General Relativity using different cosmological observations, from its geometrical properties down to testing the involved propagating degrees of freedom. This analysis will also help us to classify the attempts of going beyond General Relativity together with their explicit implications.

Host: J. Lesgourgues

### Thu 16.12.2021, 16.30 h

Philip Bechtle (Universität Bonn)

Making Useful Use of Particle Physics Experience in Epidemiology

**Abstract:**

The SARS-CoV-2 pandemic is now entering its third year. It poses ongoing and partially unforeseen challenges to our society as a whole and to science in particular. Apart from the (preventable) immediate and dire effects on long-term illness or death of far too many, and apart from the challenges of science-informed politics, it also unearthed a lack of common data management and data analysis structure in the health sector, and a disparity in modern scientific data analysis techniques between different sectors of science. Motivated by the fact that a large fraction of the German scientists with relevant contributions to the epidemiology of SARS-CoV-2 are actually physicists, the PUNCH4NFDI consortium in the National Research Data Initiative NFDI, composed of physicists from the fields of astronomy, nuclear- and hadronic physics, and particle physics, started to look for opportunities to support the data management, calibration and uncertainty estimation, statistical tools and modelling in general and to learn from the leaders of the field. The talk will report preliminary results and methodologies from the collaboration with Viola Priesemanns group on several projects, and discuss general lessons from particular physics analysis ideas for the improvement of the understanding of the pandemic and especially for an improved and consistent access to the relevant data.

Host: M. Krämer

### Thu 13.01.2022, 16.30 h

Juan Garcia-Bellido (IFT-UAM/CSIC)

Covariant formulation of non-equilibrium thermodynamics in General Relativity: Cosmic Acceleration from First Principles

**Abstract:**

We construct a generally-covariant formulation of non-equilibrium thermodynamics in General Relativity. We find covariant entropic forces arising from gradients of the entropy density, and a corresponding non-conservation of the energy momentum tensor in terms of these forces. We also provide a Hamiltonian formulation of General Relativity in the context of non-equilibrium phenomena and write the Raychaudhuri equations for a congruence of geodesics. We find that a fluid satisfying the strong energy condition could avoid collapse for a positive and sufficiently large entropic-force contribution. We then study the forces arising from gradients of the bulk entropy of hydrodynamical matter, as well as the entropy of boundary terms in the action, like those of black hole horizons. We apply the covariant formulation of non-equilibrium thermodynamics to the expanding universe and obtain the modified Friedmann equations, with an extra term corresponding to an entropic force satisfying the second law of thermodynamics. General relativistic entropic acceleration theory may explain the present cosmic acceleration from first principles without the need of introducing a cosmological constant. Following the covariant formulation of non-equilibrium phenomena in the context of a homogeneous and isotropic Friedmann-Lemaitre-Robertson-Walker (FLRW) metric, we find that the growth of entropy associated with the causal horizon of our universe (inside a finite bubble in eternal inflation) induces an acceleration that is essentially indistinguishable from that of ΛCDM, except for a slightly larger present rate of expansion compared to what would be expected from the CMB in ΛCDM, possibly solving the so-called H0 tension. The matter content of the universe is unchanged and the coincidence problem is resolved since it is the growth of the causal horizon of matter that introduces this new relativistic entropic force. The cosmological constant is made unnecessary and the future hypersurface is Minkowsky rather than de Sitter. We compare our predictions with CMB, BAO, SNIa and H(z) data and find that General Relativistic Entropic Acceleration Theory does significantly better than ΛCDM for the same data set.

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

**Abstract:**

In this seminar I will consider NNLO-accurate event generation for top-quark pair production. I will review the main steps of the derivation of the MiNNLOPS method for heavy quark production, and show phenomenological results for ttbar production at the LHC, comparing the NNLOPS predictions to particle-level data from ATLAS and CMS in all (fully leptonic, semi leptonic, and fully hadronic) decay modes.

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