Degravitation of the Cosmological Constant in Bigravity

In this article the phenomenon of degravitation of the cosmological constant is studied in the framework of bigravity. It is demonstrated that despite a sizable value of the cosmological constant its gravitational effect can be only mild. The bigravity framework is chosen for this demonstration as it leads to a consistent, ghost-free theory of massive gravity. We show that degravitation takes place in the limit where the physical graviton is dominantly a gauge invariant metric combination. We present and discuss several phenomenological consequences expected in this regime.Comment: 17 pages; v2 contains extended discussion on more general solutions; content matches published versio

Running of Radiative Neutrino Masses: The Scotogenic Model - REVISITED

A few years ago, it had been shown that effects stemming from renormalisation group running can be quite large in the scotogenic model, where neutrinos obtain their mass only via a 1-loop diagram (or, more generally, in many models in which the light neutrino mass is generated via quantum corrections at loop-level). We present a new computation of the renormalisation group equations (RGEs) for the scotogenic model, thereby updating previous results. We discuss the matching in detail, in particular in what regards the different mass spectra possible for the new particles involved. We furthermore develop approximate analytical solutions to the RGEs for an extensive list of illustrative cases, covering all general tendencies that can appear in the model. Comparing them with fully numerical solutions, we give a comprehensive discussion of the running in the scotogenic model. Our approach is mainly top-down, but we also discuss an attempt to get information on the values of the fundamental parameters when inputting the low-energy measured quantities in a bottom-up manner. This work serves the basis for a full parameter scan of the model, thereby relating its low- and high-energy phenomenology, to fully exploit the available information.Comment: 44 pages, 13 figures; continuation of 1205.0008 [hep-ph]; content matches published version; v3 corrected a minor typo in Eq. (25

Low-Scale Leptogenesis in the Scotogenic Neutrino Mass Model

The scotogenic model proposed by Ernest Ma represents an attractive and minimal example for the generation of small Standard Model neutrino masses via radiative corrections in the dark matter sector. In this paper, we demonstrate that, in addition to neutrino masses and dark matter, the scotogenic model also allows to explain the baryon asymmetry of the Universe via low-scale leptogenesis. First, we consider the case of two right-handed neutrinos (RHNs) N_{1,2}, for which we provide an analytical argument why it is impossible to push the RHN mass scale below M_1^min ~ 10^10 GeV, which is identical to the value in standard thermal leptogenesis in the type-I seesaw scenario with the same washout strength. Then, we present a detailed study of the three-RHN case based on both an analytical and a numerical analysis. In the case of three RHNs, we obtain a lower bound on the N_1 mass of around 10 TeV. Remarkably enough, successful low-scale leptogenesis can be achieved without any degeneracy in the RHN mass spectrum. The only necessary condition is a suppression in the N_1 Yukawa couplings, which results in suppressed washout and a small active neutrino mass of around 10^-12 eV. This leads to the fascinating realization that low-scale leptogenesis in the scotogenic model can be tested in experiments that aim at measuring the absolute neutrino mass scale.Comment: 13 pages, 2 figures; v2: minor changes to the text, updated discussion on direct detection bounds; content matches published versio

Gravitational Wave Oscillations in Bigravity

We derive consistent equations for gravitational wave oscillations in bigravity. In this framework a second dynamical tensor field is introduced in addition to General Relativity and coupled such that one massless and one massive linear combination arise. Only one of the two tensors is the physical metric coupling to matter, and thus the basis in which gravitational waves propagate is different from the basis where the wave is produced and detected. Therefore, one should expect -- in analogy to neutrino oscillations -- to observe an oscillatory behavior. We show how this behavior arises explicitly, discuss phenomenological implications and present new limits on the graviton parameter space in bigravity.Comment: 6 pages, 3 figures, journal versio

Decoherence of Gravitational Wave Oscillations in Bigravity

Following up on our recent study, we consider the regime of graviton masses and gravitational wave propagation distances at which decoherence of the wave packets plays a major role for phenomenology. This regime is of particular interest, as it can lead to very striking phenomena of echo events in the gravitational waves coming from coalescence events. The power of the experimental search in this case lies in the fact that it becomes sensitive to a large range of graviton masses, while not relying on a specific production mechanism. We are thus able to place new relevant limits on the parameter space of the graviton mixing angle.Comment: 7 pages, 2 figures; v2: extended discussion on the importance of the Vainshtein mechanism, content matches published versio

Low-scale leptogenesis assisted by a real scalar singlet

Standard thermal leptogenesis in the type-I seesaw model requires very heavy right-handed neutrinos (RHNs). This makes it hard to probe this scenario experimentally and results in large radiative corrections to the Higgs boson mass. In this paper, we demonstrate that the situation is considerably different in models that extend the Higgs sector by a real scalar singlet. Based on effective-theory arguments, the extra scalar is always allowed to couple to the heavy neutrinos via singlet Yukawa terms. This opens up new RHN decay channels leading to larger $CP$ violation as well as to a stronger departure from thermal equilibrium during leptogenesis. As a consequence, the baryon asymmetry can be generated for a lightest RHN mass as low as 500 GeV and without the need for a highly degenerate RHN mass spectrum. In fact, the requirement of successful leptogenesis via the Higgs portal coupling singles out an interesting parameter region that can be probed in on-going and future experiments. We derive a semianalytical fit function for the final baryon asymmetry that allows for an efficient study of parameter space, thus enabling us to identify viable parameter regions. Our results are applicable to a wide range of models featuring an additional real scalar singlet.Comment: 14 pages, 6 figures. v2 matches the published versio

Long Range Effects in Gravity Theories with Vainshtein Screening

In this paper we study long range modifications of gravity in the consistent framework of bigravity, which introduces a second massive spin-2 field and allows to continuously interpolate between the regime of General Relativity (mediated by a massless spin-2 field) and massive gravity (mediated by a massive spin-2 field). In particular we derive for the first time the equations for light deflection in this framework and study the effect on the lensing potential of galaxy clusters. By comparison of kinematic and lensing mass reconstructions, stringent bounds can be set on the parameter space of the new spin-2 fields. Furthermore, we investigate galactic rotation curves and the effect on the observable dark matter abundance within this framework.Comment: 32 pages, 9 figures; v2: minor changes to the body of the text; improved motivation of the framework and discussion of bullet cluster section; updated Fig. 9; content matches published versio

Fermionic WIMPs and Vacuum Stability in the Scotogenic Model

We demonstrate that the condition of vacuum stability severely restricts scenarios with fermionic WIMP dark matter in the scotogenic model. The sizable Yukawa couplings that are required to satisfy the dark matter constraint via thermal freeze-out in these scenarios tend to destabilise the vacuum at scales below that of the heaviest singlet fermion, rendering the model inconsistent from a theoretical point of view. By means of a scan over the parameter space, we study the impact of these renormalisation group effects on the viable regions of this model. Our analysis shows that a fraction of more than 90% of the points compatible with all known experimental constraints - including neutrino masses, the dark matter density, and lepton flavour violation - is actually inconsistent.Comment: 8 pages, 6 figures; content matches published versio

Consistency of WIMP Dark Matter as radiative neutrino mass messenger

The scotogenic scenario provides an attractive approach to both Dark Matter and neutrino mass generation, in which the same symmetry that stabilises Dark Matter also ensures the radiative seesaw origin of neutrino mass. However the simplest scenario may suffer from inconsistencies arising from the spontaneous breaking of the underlying $\mathbb{Z}_2$ symmetry. Here we show that the singlet-triplet extension of the simplest model naturally avoids this problem due to the presence of scalar triplets neutral under the $\mathbb{Z}_2$ which affect the evolution of the couplings in the scalar sector. The scenario offers good prospects for direct WIMP Dark Matter detection through the nuclear recoil method.Comment: 16 pages, 7 figure