Theorie Palaver

Axion-like-particle (ALP) is a well-motivated candidate for dark matter, and it has been subject to extensive theoretical and experimental research in recent years. The most popular ALP production mechanism studied in the literature is the misalignment mechanism, where the ALP field initially has negligible kinetic energy and starts oscillating when its mass becomes comparable to the Hubble scale. Recently, a new mechanism called Kinetic Misalignment has been proposed where the ALP field receives large kinetic energy at early times due to the explicit breaking of the Peccei-Quinn symmetry. This causes a delay in the onset of oscillations so that the ALP dark matter parameter space can be expanded to lower values of the axion decay constant. At the same time, the ALP fluctuations grow exponentially via parametric resonance in this setup, and most of the energy in the homogeneous mode is converted to ALP particles. This process is known as fragmentation. In this talk, I will discuss the observational consequences of fragmentation for the axion mini-clusters and show that a sizable region of the ALP parameter space can be tested by future experiments that probe the small-scale structure.

A non-minimal dark sector could explain why WIMP dark matter has evaded detection so far. Based on the extensively studied example of a simplified t-channel dark matter model involving a colored mediator, we demonstrate that the Sommerfeld effect and bound state formation must be considered for an accurate prediction of the relic density and thus also when inferring the experimental constraints on the model. We find that parameter space thought to be excluded by LHC searches and direct detection experiments still remains viable. Moreover, we point out that the search for bound state resonances at the LHC offers a unique opportunity to constrain a wide range of dark matter couplings inaccessible to prompt and long-lived particle searches.

Axion--Like--Particles are among the most economical and well motivated extensions of the Standard Model. In this talk ALP production from hadronic and leptonic meson decays are studied. The hadronization part of these decay amplitudes has been obtained using Brodsky--Lepage method or LQCD, at needs. In particular, the general expressions for ALP emission in mesonic s-- and t--channel tree--level processes are thoroughly discussed, for pseudoscalar and vector mesons. Accordingly, exact results as well as some useful approximation for meson-to-meson and meson leptonic decay amplitudes are presented. I will the discuss the phenomenology of various decays and highlight the most robust in terms of experimental searches and theoretical predictions. Finally, bounds on the (low--energy effective Lagrangian) ALP--fermion couplings are derived, from present and future flavour experiments. If I have time left I'll also cover some of the new form factors calculations in B mesogenesis.

Perturbative Quantum Gravity (pQG), the effective quantum field theory of gravitational fluctuations around a given background, is currently the only experimentally accessible theory of quantum gravity. Its tree-level predictions, in the form of temperature fluctuations of the Cosmic Microwave Background, have been experimentally confirmed, and it is possible that loop corrections are accessible to future experiments. However, while the tree-level results are well understood also from a theoretical point of view, the diffeomorphism symmetry of gravity makes the construction of invariant observables very difficult beyond this. Only recently, this issue has been overcome, and a class of causal invariant observables has been constructed. I will discuss this construction and how it can be related to observations, and present some predictions of pQG for graviton loop corrections to the Newtonian gravitational potential and the Hubble rate, the local expansion rate of the universe. Lastly, I show that pQG also predicts that spacetime becomes non-commutative at the Planck scale, but in a different way from previous approaches. The talk is based (in particular) on the recent papers arXiv:1806.11124, 2108.11960, 2109.09753 and 2207.03345.

Within the AdS/CFT correspondence, the entanglement properties of the CFT are related to wormholes in the dual gravity theory. This gives rise to questions about the factorisation properties of the Hilbert spaces on both sides of the correspondence. We show how the Berry phase, a geometrical phase encoding information about topology, may be used to reveal the Hilbert space structure. Wormholes are characterized by a non-exact symplectic form that gives rise to the Berry phase. For a wormholes connecting two spacelike regions in AdS3 spacetimes, we find that the non-exactness gives rise to one phase space variable appearing in each of the two boundary CFTs located at each end of the wormhole. The two CFTs are thus coupled, reflecting non-factorization. Mathematical concepts such as coadjoint orbits and geometric actions play an important role in this analysis. In addition to its relevance for quantum gravity, the approach presented also suggests how to experimentally realize the Berry phase and its relation to entanglement in table-top experiments involving photons or electrons. This provides a new example for relations between very different branches of physics that follow from the AdS/CFT correspondence and its generalizations. Based on 2202.11717 and 2109.06190.

While the matching of specific new physics scenarios onto the SMEFT framework is a well-understood procedure, the inverse problem, going from the SMEFT to UV scenarios, is more involved and requires the development of new methods to perform a systematic exploration of models. In this talk, I will discuss a diagrammatic approach to construct in an automated way a complete set of possible BSM models, given a certain set of well specified assumptions, that can reproduce specific patterns of SMEFT operators, and illustrate its use by generating models with no tree-level contributions to four-fermion operators. These class of models, which on the SMEFT only contribute to four-fermion operators at one-loop order, can contain relatively light particles that could be discovered at the LHC in direct searches, and even accommodate a dark matter candidate. In these scenarios, there is an interesting interplay between indirect SMEFT and direct searches, combining low-energy observables with the SMEFT Higgs-fermion analyses and searches for resonances at the LHC.

We address the notorious metastability of the standard model (SM) Higgs potential and promote it to a model building task: What are the new ingredients required to stabilize the SM up to the Planck scale without encountering subplanckian Landau poles? Using the SM extended by vector-like fermions, we chart out the corresponding landscape of Higgs vacuum stability. We find that the gauge portal mechanism, triggered by new SM charge carriers, opens up sizeable room for stability in a minimally invasive manner. We also find models with Yukawa portals into Higgs stability opening up at stronger coupling. Several models allow for vector-like fermions in the TeV-range, which can be searched for at the LHC. For nontrivial flavor structure of Yukawa couplings severe FCNC constraints arise which complement those from stability, and push lower fermion masses up to a few hundred TeV.

The characteristics of the cosmic microwave background provide circumstantial evidence that the hot radiation-dominated epoch in the early universe was preceded by a period of inflationary expansion. Here, it will be shown how a measurement of the stochastic gravitational wave background can reveal the cosmic history and the physical conditions during inflation, subsequent pre- and reheating, and the beginning of the hot big bang era. This will be exemplified with a particularly well-motivated and predictive minimal extension of the Standard Model which is known to provide a complete model for particle physics -- up to the Planck scale, and for cosmology -- back to inflation.

Dark matter (DM) from freeze-in or superWIMP production is well known to imprint non-cold DM signatures on cosmological observables. It will be discussed how to derive constraints from Lyman-α forest observations for both cases, based on a reinterpretation of the existing Lyman-α limits on thermal warm DM. Special emphasis is placed on the mixed scenario, where contributions from both freeze-in and superWIMP are similarly important. In this case, the imprint on cosmological observables can deviate significantly from thermal warm DM. The above will be illustrated by studying a coloured t-channel mediator DM model, in which case contributions from both freeze-in through scatterings and decays, as well as superWIMP production can be important. The entire cosmologically viable parameter space, cornered by bounds from Lyman-α observations, the LHC, and Big Bang Nucleosynthesis, will be mapped.

Given our detailed knowledge of the dark matter energy density in the present universe, it is of great interest to study its evolution at early times in order to understand the mechanism of dark matter production. A particularly intriguing scenario, known as freeze-in, is that dark matter particles have tiny couplings and never enter into equilibrium with the thermal bath of Standard Model particles. In this talk, I will discuss various technical challenges that arise in this scenario as a result of the high temperatures and densities in the early universe. Specifically, I will show how to consistently treat the spin statistics of relativistic quantum gases and how to accurately calculate dark matter production via the Higgs resonance. Finally, I will discuss the case of freeze-in with low reheating temperature, which may be testable through cosmological, astrophysical and laboratory observations.

Nuclear saturation is a key property of low-energy nuclear physics that depends on the fine details of the nuclear interaction. We develop a unified statistical framework that uses realistic nuclear forces to link the theoretical modeling of finite nuclei and infinite nuclear matter. We also construct fast and accurate emulators for nuclear-matter observables and employ an iterative history-matching approach to explore and reduce the enormous parameter domain of Delta-full chiral interactions.