Novel collective excitations in a hot scalar field theory

We study the spectrum of quasiparticles in a scalar quantum field theory at high temperature. Our results indicate the existence of novel quasiparticles with purely collective origin at low momenta for some choices of the masses and coupling. Scalar fields play a prominent role in many models of cosmology, and their collective excitations could be relevant for transport phenomena in the early universe.Comment: In v1 there was an error in the self-energy, which is now corrected. This lead to some changes in the plots, but the basic results and conclusions remain unaffecte

On finite density effects on cosmic reheating and moduli decay and implications for Dark Matter production

We study the damping of an oscillating scalar field in a Friedmann-Robertson-Walker spacetime by perturbative processes, taking into account the finite density effects that interactions with the plasma of decay products have on the damping rate. The scalar field may be identified with the inflaton, in which case this process leads to the reheating of the universe after inflation. It can also resemble a modulus that dominates the energy density of the universe at later times. We find that the finite density corrections to the damping rate can have a drastic effect on the thermal history and considerably increase both, the maximal temperature in the early universe and the reheating temperature at the onset of the radiation dominated era. As a result abundance of some Dark Matter candidates may be considerably larger than previously estimated. We give improved analytic estimates for the maximal and the reheating temperatures and confirm them numerically in a simple model.Comment: 20 pages, 4 figures, matches version to appear in JCA

Measuring the Inflaton Coupling in the CMB

We study the conditions under which simple relations between the inflaton couplings and CMB observables can be established. The crucial criterion is to avoid feedback effects during reheating, which tend to introduce a complicated dependence of the CMB observables on a large number of microphysical parameters that prohibits the derivation of meaningful constraints on any individual one of them. We find that the inflaton coupling can be "measured" with cosmological data when the effective potential during reheating can be approximated by a parabola, and when the coupling constants are smaller than an upper bound that it determined by the ratios between the inflaton mass and the Planck mass or the scale of inflation. The power at which these ratios appear is determined by the power at which the inflaton appears in a given interaction term, and the value of the upper bound is largely independent of the type of produced particle. Our results show that next generation CMB observatories may be able to constrain the inflaton couplings for various types of interactions, providing an important clue to understand how a given model of inflation may be embedded into a more fundamental microphysical theory of nature.Comment: The initially somewhat sketchy idea presented in this manuscript is backed up with a more quantitative discussion, and references are added. 13 page

Combining Experimental and Cosmological Constraints on Heavy Neutrinos

We study experimental and cosmological constraints on the extension of the Standard Model by three right handed neutrinos with masses between those of the pion and W boson. We combine for the first time direct, indirect and cosmological constraints in this mass range. This includes experimental constraints from neutrino oscillation data, neutrinoless double $\beta$ decay, electroweak precision data, lepton universality, searches for rare lepton decays, tests of CKM unitarity and past direct searches at colliders or fixed target experiments. On the cosmological side, big bang nucleosynthesis has the most pronounced impact. Our results can be used to evaluate the discovery potential of searches for heavy neutrinos at LHCb, BELLE II, SHiP, ATLAS, CMS or a future lepton collider.Comment: 64 pages, 22 figures. Matches published versio

Neutrinoless double β\beta decay and low scale leptogenesis

The extension of the Standard Model by right handed neutrinos with masses in the GeV range can simultaneously explain the observed neutrino masses via the seesaw mechanism and the baryon asymmetry of the universe via leptogenesis. It has previously been claimed that the requirement for successful baryogenesis implies that the rate of neutrinoless double $\beta$ decay in this scenario is always smaller than the standard prediction from light neutrino exchange alone. In contrast, we find that the rate for this process can also be enhanced due to a dominant contribution from heavy neutrino exchange. In a small part of the parameter space it even exceeds the current experimental limit, while the properties of the heavy neutrinos are consistent with all other experimental constraints and the observed baryon asymmetry is reproduced. This implies that neutrinoless double $\beta$ decay experiments have already started to rule out part of the leptogenesis parameter space that is not constrained by any other experiment, and the lepton number violation that is responsible for the origin of baryonic matter in the universe may be observed in the near future.Comment: Discussion extended, figures added; 16 pages, 5 figures; identical to published version up to minor text correction

Sterile neutrino Dark Matter production from scalar decay in a thermal bath

We calculate the production rate of singlet fermions from the decay of neutral or charged scalar fields in a hot plasma. We find that there are considerable thermal corrections when the temperature of the plasma exceeds the mass of the decaying scalar. We give analytic expressions for the temperature-corrected production rates in the regime where the decay products are relativistic. We also study the regime of non-relativistic decay products numerically. Our results can be used to determine the abundance and momentum distribution of Dark Matter particles produced in scalar decays. The inclusion of thermal corrections helps to improve predictions for the free streaming of the Dark Matter particles, which is crucial to test the compatibility of a given model with cosmic structure formation. With some modifications, our results may be generalised to the production of other Dark Matter candidates in scalar decays.Comment: This version matches the one published in JHEP. 44 pages, 10 figure

Probing leptogenesis with GeV-scale sterile neutrinos at LHCb and BELLE II

We show that existing laboratory experiments have the potential to unveil the origin of matter by probing leptogenesis in the type-I seesaw model with three right-handed neutrinos and Majorana masses in the GeV range. The baryon asymmetry is generated by CP-violating flavour oscillations during the production of the right-handed neutrinos. In contrast to the case with only two right-handed neutrinos, no degeneracy in the Majorana masses is required. The right-handed neutrinos can be found in meson decays at BELLE II and LHCb.Comment: Title changed, discussion extended and references added. 12 pages, one figur