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Identifier 000451421
Title Dark matter bound state formation and thermal decoupling in the early universe
Alternative Title Δημιουργία δέσμιων καταστάσεων και θερμική παραγωγή σκοτεινής ύλης στο πρώιμο σύμπαν
Author Βασιλάκη, Χριστιάνα Σ.
Thesis advisor Παυλίδου, Βασιλική
Πετράκη, Καλλιόπη
Reviewer Κυρίτσης, Ηλίας
Νιάρχος, Βασίλειος
Abstract In this thesis we focus on the thermal production of dark matter (DM) that couples to force mediators significantly lighter than itself. In this case, the interactions between DM particles are long-range. Long-range interactions give rise to non-perturbative effects, including the existence of bound states. The formation and subsequent decay of metastable DM bound states opens up an additional DM annihilation channel. This affects the DM relic density, thereby altering the predicted DM mass and couplings that can account for the observed abundance. Here, we consider DM consisting of a pair of Dirac fermions coupled to a massless dark photon. Due to the long-range interaction, the dark particle-antiparticle pairs can form positronium-like bound states. The computation of the DM density requires solving a system of coupled Boltzmann equations that track the densities of the free particles and the bound species, and describe the interplay among DM annihilation, boundstate formation, bound-state decay and the inverse processes, in an expanding universe. However, metastable bound states remain in a quasi-steady state that can be employed to reduce the coupled system of equations to a single one with an effective DM depletion cross-section that encodes all these effects. The effective equation is solved numerically and the validity of the steady-state approximation is investigated. The coupling required to reproduce the observed DM density is determined as a function of the DM mass. It is found that for DM masses larger than a few TeV, the DM density is significantly affected by bound-state formation and decay. Lastly, the back-reaction of the resonant radiation produced during bound-state formation is examined. It is shown that rather than ionising the bound states, the resonant dark photons redshift due to the expansion of the universe and thermalise with the dark particles at much smaller timescales.
Language English
Subject Bound-state formation
Long-range intraction
Δέσμιες καταστάσεις
Σκοτεινό φωτόνιο
Issue date 2022-11-25
Collection   School/Department--School of Sciences and Engineering--Department of Physics--Post-graduate theses
  Type of Work--Post-graduate theses
Permanent Link https://elocus.lib.uoc.gr//dlib/0/7/0/metadata-dlib-1664881943-89784-7799.tkl Bookmark and Share
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