A t-channel singularity of a scattering process can occur when a stable mediator is kinematically allowed to be on-shell. Then, the cross section appears to be infinite which, obviously, cannot be a physical result. Since the mediator has no width, it is impossible to regularize the singularity using the Breit-Wigner approach.
In this talk, I will discuss the conditions that are necessary for the singularity to occur. I will briefly summarize attempts to regularize the singularity that are already present in literature, especially in the case of collider physics.
After showing that none of previously proposed ways to solve the problem is satisfactory in the cosmological case of a thermal medium of particles, I will present a natural solution developed within the Keldysch-Schwinger formalism. A non-zero imaginary part of the mediator's self-energy appears as a consequence of interactions between the mediator and the thermal medium. Consequently, the mediator acquires a non-zero effective decay width and the cross section becomes finite.
slides: https://mycloud.fuw.edu.pl/index.php/s/ZCarLbB3HiZHwj7
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Anna Socha (IFT UW)
According to the standard model of cosmology, the Universe at its very beginning underwent a phase of a rapid, exponential expansion, followed by a reheating period. During this epoch, the energy density, initially accumulated in oscillations of the inflaton field, was injected into the visible sector, eventually setting the initial conditions for the hot big bang. In this talk, I will discuss the production of the Standard Model (SM) radiation and dark matter adopting a generic post-inflationary scenario with a non-standard expansion history. In particular, I will explore the Higgs boson-induced reheating, assuming that it is achieved through a universal inflaton-Higgs coupling $\phi |\mathcal{H}|^2$. In presence of such interaction, the Higgs doublet acquires a $\phi$-dependent mass which generates a vacuum expectation value due to the inflaton oscillations and breaks the SM gauge symmetry. The non-zero mass of the Higgs field leads to a time-dependent inflaton decay rate during the reheating period in both standard and modified cosmologies. This, in turn, has non-trivial consequences for the dynamics of the reheating period, affecting the evolution of the radiation energy density and thus the UV freeze-in dark matter production.
slides: https://mycloud.fuw.edu.pl/index.php/s/RsX9Lozq6ZCTstR
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Andrzej Hryczuk (NCBJ)
In this talk I will discuss the freeze-in dark matter paradigm focusing on various recent new ideas and observations regarding this production mechanism. In particular, after broader pedagogical introduction, I will present a novel scenario based on the freeze-in through semi-production, i.e. the inverse semi-annihilation processes. Its peculiar feature is that the production rate is suppressed by a small initial abundance of dark matter and consequently creating the observed relic density requires much larger coupling values than for the usual freeze-in. Even up to a point of leading to prospects for indirect detection searches, a rather surprising property given how difficult it is to detect dark matter in the typical freeze-in paradigm.
slides: https://mycloud.fuw.edu.pl/index.php/s/rWqf2GsEfZc76do
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Apostolos Pilaftsis (Manchester University)
For more than half of a century, covariant and differential geometric methods have been playing a central role in the development of Quantum Field Theory (QFT). After a brief historic overview of the major scientific achievements using these methods, I will focus on the covariant and differential geometric formalism originally proposed by Vilkovisky and DeWitt (VDW). I will discuss recent developments made in addressing the uniqueness of the path-integral measure of the VDW effective action, and so address the problem of quantum frame dependence in cosmologically relevant scalar-tensor theories beyond the classical approximation. I will then turn my attention to a long-standing problem concerning the obstacles that the VDW formalism is facing from its original conception in describing generic QFTs that include fermions. I will show how in addition to bosons the VDW effective action can be extended to supermanifolds to include fermions. The so-extended formulation appears to be very promising for a complete geometrisation of realistic theories of micro-cosmos, such as the Standard Model and its gravitational sector.
slides: https://mycloud.fuw.edu.pl/index.php/s/GDgdxGDkcLn73dk