25 research outputs found
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 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 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 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