Kalliopi Petraki

Dark-matter sterile neutrinos in models with a gauge singlet in the Higgs sector

Sterile neutrino with mass of several keV can be the cosmological dark matter, can explain the observed velocities of pulsars, and can play an important role in the formation of the first stars. We describe the production of sterile neutrinos in a model with an extended Higgs sector, in which the Majorana mass term is generated by the vacuum expectation value of a gauge-singlet Higgs boson. In this model the relic abundance of sterile neutrinos does not necessarily depend on their mixing angles, the free-streaming length can be much smaller than in the case of warm dark matter produced by neutrino oscillations, and, therefore, some of the previously quoted bounds do not apply. The presence of the gauge singlet in the Higgs sector has important implications for the electroweak phase transition, baryogenesis, and the upcoming experiments at the Large Hadron Collider and a Linear Collider.

Bound-state formation for thermal relic dark matter and unitarity

We show that the relic abundance of thermal dark matter annihilating via a long-range interaction, is significantly affected by the formation and decay of dark matter bound states in the early universe, if the dark matter mass is above a few TeV. We determine the coupling required to obtain the observed dark matter density, taking into account both the direct 2-to-2 annihilations and the formation of bound states, and provide an analytical fit. We argue that the unitarity limit on the inelastic cross-section is realized only if dark matter annihilates via a long-range interaction, and we determine the upper bound on the mass of thermal-relic dark matter to be about 197 (139) TeV for (non)-self-conjugate dark matter.

Small-scale structure formation properties of chilled sterile neutrinos as dark matter

We calculate the free-streaming length and the phase space density of dark-matter sterile neutrinos produced from decays, at the electroweak scale, of a gauge singlet in the Higgs sector. These quantities, which depend on the dark-matter production mechanism, are relevant to the study of small-scale structure formation and may be used to constrain or rule out dark-matter candidates.

Dark-matter bound states from Feynman diagrams

A bstractIf dark matter couples directly to a light force mediator, then it may form bound states in the early universe and in the non-relativistic environment of haloes today. In this work, we establish a field-theoretic framework for the computation of bound-state formation cross-sections, de-excitation and decay rates, in theories with long-range interactions. Using this formalism, we carry out specific computations for scalar particles interacting either via a light scalar or vector mediator. At low relative velocities of the interacting particles, the formation of bound states is enhanced by the Sommerfeld effect. For particle-antiparticle pairs, we show that bound-state formation can be faster than annihilation into radiation in the regime where the Sommerfeld effect is important. The field-theoretic formalism outlined here can be generalised to compute bound-state formation cross-sections in a variety of theories, including theories featuring non-Abelian (albeit non-confining) interactions, such as the electroweak interactions.

Heavy sterile neutrinos and supernova explosions

Abstract We consider sterile neutrinos with rest masses ∼ 0.2 GeV and with vacuum flavor mixing angles θ 2 > 10 −8 for mixing with τ-neutrinos, or 10 −8 θ 2 10 −7 for mixing with muon neutrinos. Such sterile neutrinos could augment core collapse supernova shock energies by enhancing energy transport from the core to the vicinity of the shock front. The decay of these neutrinos could produce a flux of very energetic active neutrinos, detectable by future neutrino observations from galactic supernova. The relevant range of sterile neutrino masses and mixing angles can be probed in future laboratory experiments.

Realistic neutron star constraints on bosonic asymmetric dark matter

It has been argued that the existence of old neutron stars excludes the possibility of non-annihilating light bosonic dark matter, such as that arising in asymmetric dark matter scenarios. If non-annihilating dark matter is captured by neutron stars, the density will eventually become sufficient for black hole formation. However, the dynamics of collapse is highly sensitive to dark-matter self-interactions. Repulsive self-interactions, even if extremely weak, can prevent black hole formation. We argue that self-interactions will necessarily be present, and estimate their strength in representative models. We also consider co-annihilation of dark matter with nucleons, which arises naturally in many asymmetric dark matter models, and which again acts to prevent black hole formation. We demonstrate how the excluded region of the dark-matter parameter space shrinks as the strength of such interactions is increased, and conclude that neutron star observations do not exclude most realistic bosonic asymmetric dark matter models.

Two radiative inverse seesaw models, dark matter, and baryogenesis

The inverse seesaw mechanism allows the neutrino masses to be generated by new physics at an experimentally accessible scale, even with (1) Yukawa couplings. In the inverse seesaw scenario, the smallness of neutrino masses is linked to the smallness of a lepton number violating parameter. This parameter may arise radiatively. In this paper, we study the cosmological implications of two contrasting radiative inverse seesaw models, one due to Ma and the other to Law and McDonald. The former features spontaneous, the latter explicit lepton number violation. First, we examine the effect of the lepton-number violating interactions introduced in these models on the baryon asymmetry of the universe. We investigate under what conditions a pre-existing baryon asymmetry does not get washed out. While both models allow a baryon asymmetry to survive only once the temperature has dropped below the mass of their heaviest fields, the Ma model can create the baryon asymmetry through resonant leptogenesis. Then we investigate the viability of the dark matter candidates arising within these models, and explore the prospects for direct detection. We find that the Law/McDonald model allows a simple dark matter scenario similar to the Higgs portal, while in the Ma model the simplest cold dark matter scenario would tend to overclose the universe.

Enhanced neutrino signals from dark matter annihilation in the Sun via metastable mediators

We calculate the neutrino signal resulting from annihilation of secluded dark matter in the Sun. In this class of models, dark matter annihilates first into metastable mediators, which subsequently decay into Standard Model particles. If the mediators are long lived, they will propagate out from the dense solar core before decaying. High energy neutrinos undergo absorption in the Sun. In the standard scenario in which neutrinos are produced directly in the centre of the Sun, absorption is relevant for E > 100 GeV, resulting in a significant suppression of the neutrino spectrum beyond E ~ 1 TeV. In the secluded dark matter scenario, the neutrino signal is greatly enhanced because neutrinos are injected away from the core, at lower density. Since the solar density falls exponentially with radius, metastable mediators have a significant effect on the neutrino flux, even for decay lengths which are small compared to the solar radius. Moreover, since neutrino detection cross sections grow with energy, this enhancement of the high energy region of the neutrino spectrum would have a large effect on overall event rates.

Collider signatures of sterile neutrinos in models with a gauge-singlet Higgs

Sterile neutrinos have been invoked to explain the observed neutrino masses, but they can also have significant implications for cosmology and accelerator experiments. We explore the collider signatures of a simple extension of the Standard Model, where sterile neutrinos acquire their mass after electroweak symmetry breaking, via their coupling to a real singlet Higgs. In this model, heavy sterile neutrinos can be produced in accelerators from decays of the Higgs bosons. Their own decay can yield distinct signals, suggesting both the presence of an extended Higgs sector and the coupling of the singlet fermions to the latter. In the same scenario, a relic matter abundance arises from the decay of the singlet Higgs into weakly coupled keV sterile neutrinos. The coupling of the Higgs doublet to particles outside the Standard Model relaxes the current experimental bounds on its mass.

Signals from dark atom formation in halos

We consider indirect detection signals of atomic dark matter, with a massive dark photon which mixes kinetically with hypercharge. In signicant regions of parameter space, dark matter remains at least partially ionized today, and dark atom formation can occur eciently in dense regions, such as the centers of galactic halos. The formation of dark atoms is accompanied by emission of a dark photon, which can subsequently decay into Standard Model particles. We discuss the expected signal strength and compare it to that of annihilating dark matter. As a case study, we explore the possibility that dark atom formation can account for the observed 511 keV line and outline the relevant parameter space.