Thomas Hambye

We perform a detailed study of the cosmological constraints on the decay of a relic particle ϕ into neutrinos, ϕ → νν̅ , in particular those arising from the observed light-element abundances in the early Universe. Several processes are relevant, including final-state radiation associated with the decay, as well as subsequent interactions of the injected neutrinos with the thermal background neutrinos or between themselves. All processes generically contribute to the production of electromagnetic and often also hadronic material and may therefore induce late-time photodisintegration and hadrodisintegration reactions, i.e. the destruction of light elements that have previously been formed during BBN. Here, we examine this scenario with a Monte-Carlo inspired probabilistic approach rather than Boltzmann techniques, taking into account all of these different reactions as well as their interplay. We find the resulting constraints to be very significant, covering a broad range of previously unexplored masses and lifetimes of the relic source particle.

Among the few ways that allow or could allow us to probe the early Universe from the observation of a flux of primordial particles, there is one possibility which has been little studied: the observation today of high energy neutrinos which could have been emitted shortly after the Big Bang, from the decay or annihilation of early universe relics. We perform a general study of such a possibility. To this end, we first emphasise that these neutrinos could display various kinds of sharp spectral features, resulting from the primary energy spectrum at emission, and from how this spectrum is smoothed by redshift and radiative correction effects. Next, we determine the ranges of mass (from a fraction of eV all the way to the Planck scale) and lifetime of the source particles along which we do not/we do expect that the sharp spectral feature will be altered by interactions of the neutrinos on their way to the detector, mainly with the cosmic neutrino background or between themselves. We also study the theoretical (i.e. mainly BBN and CMB) and observational constraints which hold on such a possibility. This allows us to delineate the regions of parameter space (mass, lifetime and abundance) that are already excluded, hopeless for future observation or, instead, which could lead to the observation of such neutrinos in the near future.

The Sun may capture asymmetric dark matter (DM), which can subsequently form bound-states through the radiative emission of a sub-GeV scalar. This process enables generation of scalars without requiring DM annihilation. In addition to DM capture on nucleons, the DM-scalar coupling responsible for bound-state formation also induces capture from self-scatterings of ambient DM particles with DM particles already captured, as well as with DM bound-states formed in-situ within the Sun. This scenario is studied in detail by solving Boltzmann equations numerically and analytically. In particular, we take into consideration that the DM self-capture rates require a treatment beyond the conventional Born approximation. We show that, thanks to DM scatterings on bound-states, the number of DM particles captured increases exponentially, leading to enhanced emission of relativistic scalars through bound-state formation, whose final decay products could be observable. We explore phenomenological signatures with the example that the scalar mediator decays to neutrinos. We find that the neutrino flux emitted can be comparable to atmospheric neutrino fluxes within the range of energies below one hundred MeV. Future facilities like Hyper-K, and direct DM detection experiments can further test such scenario.

In models with spontaneous symmetry breaking by scalar fields in large group representations, we observe that some of the scalar masses can be loop-suppressed with respect to the naive expectation from symmetry selection rules. We present minimal models — the SU(2) five-plet and SU(3) ten-plet — with such accidentally light scalars, featuring compact tree-level flat directions lifted by radiative corrections. We sketch some potential applications, from stable relics and slow roll in cosmology, to hierarchy and fine-tuning problems in particle physics.

A massive particle decaying into neutrinos in the early Universe is known to be less constrained than if it was decaying into other standard model particles. However, even if the decay proceeds into neutrinos, the latter still inevitably emit secondary particles undergoing electromagnetic interactions that can be probed. We analyse in details how sensitive various cosmological probes are to such secondary particles, namely CMB anisotropies, CMB spectral distortions, and Big Bang Nucleosynthesis. For relics whose lifetime is shorter than the age of the Universe, this leads to original and stringent bounds on the particle's lifetime as a function of its abundance and mass.

A massive particle decaying into neutrinos in the early Universe is known to be less constrained than if it was decaying into other standard model particles. However, even if the decay proceeds into neutrinos, the latter still inevitably emit secondary particles undergoing electromagnetic interactions that can be probed. We analyse in details how sensitive various cosmological probes are to such secondary particles, namely CMB anisotropies, CMB spectral distortions, and Big Bang Nucleosynthesis. For relics whose lifetime is shorter than the age of the Universe, this leads to original and stringent bounds on the particle's lifetime as a function of its abundance and mass.

We perform a systematic study of the electric and magnetic dipole moments of dark matter (DM) that are induced at the one-loop level when DM experiences four-fermion interactions with Standard Model (SM) charged fermions. Related to their loop nature these moments can largely depend on the UV completion at the origin of the four-fermion operators. We illustrate this property by considering explicitly two simple ways to generate these operators, from t- or s-channel tree-level exchange. Fixing the strength of these interactions from the DM relic density constraint, we obtain in particular a magnetic moment that, depending on the interaction considered, lies typically between 10⁻²⁰ to 10⁻²³ ecm or identically vanishes. These non-vanishing values induce, via photon exchange, DM-nucleus scattering cross sections that could be probed by current or near future direct detection experiments.