Precise dark matter relic abundance in decoupled sectors

Dark matter (DM) as a thermal relic of the primordial plasma is increasingly pressured by direct and indirect searches, while the same production mechanism in a decoupled sector is much less constrained. We extend the standard treatment of the freeze-out process to such scenarios and perform precision calculations of the annihilation cross section required to match the observed DM abundance. We demonstrate that the difference to the canonical value is generally sizeable, and can reach orders of magnitude. Our results directly impact the interpretation of DM searches in hidden sector scenarios.Comment: 8 pages, 4 figure

Do pulsar timing arrays observe merging primordial black holes?

In this letter we evaluate whether the gravitational wave background recently observed by a number of different pulsar timing arrays could be due to merging primordial supermassive black hole binaries. We find that for homogeneously distributed primordial black holes this possibility is inconsistent with strong cosmological and astrophysical constraints on their total abundance. If the distribution exhibits some clustering, however, the merger rate will in general be enhanced, opening the window for a consistent interpretation of the PTA data in terms of merging primordial black holes.Comment: 8 pages, 3 figure

Strong constraints on clustered primordial black holes as dark matter

The idea of dark matter in the form of primordial black holes has seen a recent revival triggered by the LIGO detection of gravitational waves from binary black hole mergers. In this context, it has been argued that a large initial clustering of primordial black holes can help alleviate the strong constraints on this scenario. In this work, we show that on the contrary, with large initial clustering the problem is exacerbated and constraints on primordial black hole dark matter become overwhelmingly strong.Comment: 9 pages, 6 figures. Discussion extended, in particular to include extended PBH mass functions. Version to appear in PR

Complete leading-order standard model corrections to quantum leptogenesis

Thermal leptogenesis, in the framework of the standard model with three additional heavy Majorana neutrinos, provides an attractive scenario to explain the observed baryon asymmetry in the universe. It is based on the out-of-equilibrium decay of Majorana neutrinos in a thermal bath of standard model particles, which in a fully quantum field theoretical formalism is obtained by solving Kadanoff-Baym equations. So far, the leading two-loop contributions from leptons and Higgs particles are included, but not yet gauge corrections. These enter at three-loop level but, in certain kinematical regimes, require a resummation to infinite loop order for a result to leading order in the gauge coupling. In this work, we apply such a resummation to the calculation of the lepton number density. The full result for the simplest "vanilla leptogenesis" scenario is by $\mathcal{O}(1)$ increased compared to that of quantum Boltzmann equations, and for the first time permits an estimate of all theoretical uncertainties. This step completes the quantum theory of leptogenesis and forms the basis for quantitative evaluations, as well as extensions to other scenarios.Comment: 30 pages, 13 figures, expanded Sec. 3.2 with additional appendix, small additions and corrections, matches published versio

A new life for sterile neutrino dark matter after the pandemic

We propose a novel mechanism to generate sterile neutrinos $\nu_s$ in the early Universe, by converting ordinary neutrinos $\nu_\alpha$ in scattering processes $\nu_s\nu_\alpha\to\nu_s\nu_s$. After initial production by oscillations, this leads to an exponential growth in the $\nu_s$ abundance. We show that such a production regime naturally occurs for self-interacting $\nu_s$, and that this opens up significant new parameter space where $\nu_s$ make up all of the observed dark matter. Our results provide strong motivation to further push the sensitivity of X-ray line searches, and to improve on constraints from structure formation.Comment: 12 pages revtex 4, 5 figures; matches published versio

Dark Sector Cosmologies: Evolution and Constraints

Many models of dark matter place their dark matter candidate inside a broader dark sector with other particles and fields for theoretical and phenomenological reasons. The cosmological evolution of these dark sectors can differ significantly from standard scenarios, in particular due to their interplay with the Standard Model of particle physics. In this thesis, we study various dark sector cosmologies, including their evolution and resulting constraints. In view of ever more stringent limits from direct and indirect searches on dark matter as a thermal relic from the primordial plasma encompassing the SM particles, scenarios of a dark sector decoupled from the Standard Model receive increasing interest. Interestingly, the corresponding dark matter production mechanism of thermal freeze-out can also occur entirely in a decoupled dark sector. Still, certain modifications to the standard treatment need to be taken into account. We study these changes and find significant deviations of the annihilation cross-section required to obtain the observed dark matter abundance, in particular if the dark matter particle and its annihilation product are similar in mass. After the first gravitational waves were observed by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), interest in primordial black holes as a candidate for dark matter has been renewed. As gravitational waves can be produced in primordial black hole binary mergers, this setup can be considered a dark sector with purely gravitational interactions. Large initial primordial black hole clustering has been suggested to circumvent the strong limits on this scenario. We show that instead, gravitational wave constraints are enhanced by large clustering such that highly clustered primordial black holes with the masses corresponding to the LIGO events cannot account for all of the dark matter in the Universe. A major part of this thesis studies Big Bang Nucleosynthesis (BBN) as a probe for MeV-scale dark sectors. Due to the remarkable agreement of predictions for the primordial light element abundances from BBN with observations, alterations are generally strongly constrained. Dark sectors can change the predictions for the primordial light element abundances by the influence of their cosmological evolution during BBN itself and subsequent disintegration processes, e.g. photodisintegration. The former proceed via alterations of the Hubble rate, neutrino decoupling, the time-temperature relation, and the best-fit value for the baryon-to-photon ratio. The latter are due to late-time high-energy electromagnetic injections into the Standard Model plasma, which induce an electromagnetic cascade producing an abundance of non-thermal photons that can disintegrate light nuclei. We derive these constraints for annihilations of MeV-scale dark matter, electromagnetic decays of MeV-scale dark sector particles, and axion-like particles coupled to photons

Do pulsar timing arrays observe merging primordial black holes?

In this letter we evaluate whether the gravitational wave background recently observed by a number of different pulsar timing arrays could be due to merging primordial supermassive black hole binaries. We find that for homogeneously distributed primordial black holes this possibility is inconsistent with the strong cosmological and astrophysical constraints on their total abundance. If the distribution exhibits some clustering, however, the merger rate will in general be enhanced, opening the window for a consistent interpretation of the PTA data in terms of merging primordial black holes

ACROPOLIS: A generiC fRamework fOr Photodisintegration Of LIght elementS

The remarkable agreement between observations of the primordial light element abundances and the corresponding theoretical predictions within the standard cosmological history provides a powerful method to constrain physics beyond the standard model of particle physics (BSM). For a given BSM model these primordial element abundances are generally determined by (i) Big Bang Nucleosynthesis and (ii) possible subsequent disintegration processes. The latter potentially change the abundances due to late-time high-energy injections which may be present in these scenarios. While there are a number of public codes for the first part, no such code is currently available for the second. Here we close this gap and present ACROPOLIS, A generiC fRamework fOr Photodisintegration Of LIght elementS. The widely discussed cases of decays as well as annihilations can be run without prior coding knowledge within example programs. Furthermore, due to its modular structure, can easily be extended also to other scenarios. For the most recent version of this manual, please visit the GitHub repository at https://github.com/skumblex/acropolis