ETD

• big bang nuclosynthesis
• cosmic microwave background
• cosmology
• dark matter phenomenology
• dark matter theory
• particle physics

Dark Matter (DM) constitutes approximately 80% of the total matter density of
the Universe. Despite extensive experimental and theoretical efforts in recent years,
its microscopic nature remains unknown. The coming decade will see a wide range
of experiments aiming to detect DM and probe its properties across multiple energy scales. In this broad experimental landscape, significant progress is expected;
a comprehensive theoretical program is therefore essential to understand the complementarity among different probes and to catalyze further experimental developments in a coordinated way.
An important objective is to move toward identifying the nature of DM by developing a robust and inclusive theoretical framework for the rich phenomenology
of viable thermal candidates. In principle, the landscape of DM models in thermal contact with the Standard Model (SM) can be explored along two timely and
complementary directions at opposite ends of the allowed mass range: i) heavy
candidates at the TeV scale and above, and ii) light, secluded dark sectors particles neutral under SM gauge interactions with masses well below the electroweak
scale. The latter is particularly attractive: feeble portal couplings to the SM permit
rich dynamics while evading stringent constraints. Several portals have been proposed, including dark photons and the Higgs portal, the latter typically implying
DM masses in the GeV range.
Among the best-motivated portals to a light, fermionic dark sector are axionlike particles (ALPs), which couple weakly to SM fields through symmetry-protected
operators. In this framework, the lightness of the dark fermion is protected by chiral
symmetry, while the scalar (the ALP) is naturally light as a pseudo–Goldstone boson
of an approximate Peccei–Quinn (PQ) symmetry. The DM–ALP interaction can
be sizable, enabling efficient thermal freeze-out via DM annihilation into ALPs.
Meanwhile, small ALP–SM couplings keep the dark sector secluded, rendering this
class of models theoretically appealing and phenomenologically viable.
In this thesis we focus on the low-mass edge of the thermal window, where
both DM and ALPs lie in the MeV range. This corner of parameter space is well
motivated: it can naturally account for the long-standing 511 keV γ-ray line from
the Galactic Centre and may shed light on other astrophysical anomalies (e.g.,
anomalous ionization in the central molecular zone). It also opens a novel arena for
cosmological constraints, since a thermal dark sector can inject energy into the SM
plasma well after freeze-out via a delayed cascade of ALP decays. This late-time
entropy injection has non-trivial implications for Big Bang Nucleosynthesis (BBN)
and the Cosmic Microwave Background (CMB).
We consider two main scenarios: i) Heavy DM (decoupled freeze-out, DFO):
the fermion χ is the heaviest particle in the dark sector. After the dark and SM
sectors decouple, the present-day DM abundance is set by a freeze-out mechanism
within the dark sector via χχ → aa. The residual ALP abundance is then diluted
by decays into light SM states; ii) Light DM (sequential freeze-in, SFI): the ALP
a is the heaviest dark-sector particle and decays into on-shell DM. The dark-sector
abundance is determined by sequential freeze-in: first a is produced out of equilibrium from the SM, and later its decay a → χχ transfers this abundance to χ,
thereby setting the relic density.
In both cases, the secluded dark sector is populated through lepton flavour violating (LFV) decays: right–handed lepton fields, charged under the PQ symmetry,
generate effective couplings to the dark sector after rotation to the mass basis. This
mechanism naturally links SM leptons to the hidden sector and induces a coupling
hierarchy between flavour–diagonal and off–diagonal ALP–SM interactions, governed by a mixing angle. Consequently, the dark sector can thermalise with the SM
at early times while remaining sufficiently feeble during BBN. These LFV decays
therefore fix the initial conditions for the subsequent cosmological evolution.
Main findings. In both the heavy–DM and light–DM regimes we identify viable
parameter regions that reproduce the observed relic abundance of χ while simultaneously satisfying cosmological (BBN, CMB) constraints and collider LFV bounds.
For the heavy–DM case, the dark–sector abundance is reproduced via LFV
muon and tau decays, (i.e. µ → e a, τ → e a respectively). Stringent LFV searches
(e.g. present TWIST, Belle/Belle II, BaBar, and future MEG II, Mu3e) severely
constrain the muon channel, preventing thermalisation with the SM progenitor;
in contrast, production via tau decays can achieve thermal equilibrium with the
SM. From this first step we extract the PQ charges of the SM leptons. After
the ALPs are produced, we then determine the PQ charges of χ and the required
mass–mixing angles to reproduce the relic density of χ and deplete the residual ALP
population, in agreement with cosmological bounds. We find that the PQ charges in
the dark sector and in the SM must be hierarchically different, suggesting significant
fine–tuning in plausible UV completions.
For the light–DM case, sequential freeze–in followed by a → χχ efficiently transfers the ALP abundance to χ. The required LFV couplings depend primarily on
ma, mχ, and Br(a → χχ), and naturally allow PQ charges of comparable magnitude with essentially unconstrained mixing angles. This scenario is unchallenged
by BBN and LFV bounds and provides a clean benchmark in which promoting the
ALP to the QCD axion is feasible, further strengthening the theoretical motivation
of the model.