1,279 research outputs found
Clockwork Dark Matter
I give a pedagogical discussion of thermal dark matter (DM) within the
clockwork mechanism. The clockwork mechanism, which is a natural way to
generate small numbers starting from order-one couplings, allows to have a
long-lived, but unstable, DM particle that nevertheless has O(1) couplings with
electroweak- or TeV-scale states. Remarkably, DM decays on time scales much
longer than the age of the Universe and has at the same time sizeable couplings
with light states, which therefore allow to produce it thermally within the
WIMP paradigm. These new particles with large couplings can be searched for at
current or future colliders. I also briefly comment on how this setup can
minimally emerge from the deconstruction of an extra dimension in flat
spacetime.Comment: 7 pages, 3 figures; Contribution to proceedings of the 52nd
Rencontres de Moriond on EW Interactions and Unified Theories, 18-25 March,
2017, La Thuile, Italy. Comments on extra-dimensional constructions adde
Cold keV dark matter from decays and scatterings
We explore ways of creating cold keV-scale dark matter by means of decays and
scatterings. The main observation is that certain thermal freeze-in processes
can lead to a cold dark matter distribution in regions with small available
phase space. In this way the free-streaming length of keV particles can be
suppressed without decoupling them too much from the Standard Model. In all
cases, dark matter needs to be produced together with a heavy particle that
carries away most of the initial momentum. For decays, this simply requires an
off-diagonal DM coupling to two heavy particles; for scatterings, the coupling
of soft DM to two heavy particles needs to be diagonal, in particular in spin
space. Decays can thus lead to cold light DM of any spin, while scatterings
only work for bosons with specific couplings. We explore a number of simple
models and also comment on the connection to the tentative 3.5 keV line.Comment: 19 pages, 6 figures. published versio
Symmetry Improved 2PI Effective Action and the Infrared Divergences of the Standard Model
Resummations of infinite sets of higher-order perturbative contributions are
often needed both in thermal field theory and at zero temperature. For
instance, the behaviour of the Standard Model (SM) effective potential
extrapolated to very high energies is known to be extremely sensitive to
higher-order effects. The 2PI effective action provides a systematic approach
to consistently perform such resummations. However, one of its major
limitations was that its loopwise expansion introduces residual violations of
possible global symmetries, thus giving rise to massive Goldstone bosons in the
spontaneously broken phase of the theory. We review the recently developed
symmetry-improved 2PI formalism for consistently encoding global symmetries in
the 2PI approach, and discuss its satisfactory field-theoretical properties. We
then apply the formalism to study the infrared divergences of the SM effective
potential due to Goldstone bosons, which may affect the stability analyses of
the SM. We present quantitative comparisons, for the scalar sector of the SM,
with the approximate partial resummation procedure recently developed to
address this problem, and show the quantitative discrepancy of the latter with
the more complete 2PI approach, thus motivating further studies in this
direction.Comment: 19 pages, 14 figures; Contribution to the Proceedings of DISCRETE
2014, London, United Kingdo
Pati-Salam explanations of the B-meson anomalies
We provide a combined explanation of the increasingly tantalizing -meson
anomalies, both in and , in the Pati-Salam model
with minimal matter content. This well-known model, based on the gauge group
, naturally contains a variety of
scalar leptoquarks with related and restricted couplings. In particular we show
that the seesaw-motivated scalar leptoquark within the representation
and its right-handed parity partner can solve both anomalies while making testable predictions for related
observables such as and . The solution of the
anomaly alone can be related to a type-II seesaw neutrino mass
structure. Explaining also requires the existence of a light
right-handed neutrino, which constrains the UV structure of the model.Comment: 26 pages, includes discussion of RGEs; to appear in JHE
Higgs doublet decay as the origin of the baryon asymmetry
We consider a question which curiously had not been properly considered so
far: in the standard seesaw model what is the minimum value the mass of a
right-handed (RH) neutrino must have for allowing successful leptogenesis via
CP-violating decays? To answer this question requires to take into account a
number of thermal effects. We show that, for low RH neutrino masses and thanks
to these effects, leptogenesis turns out to proceed efficiently from the decay
of the Standard Model (SM) scalar doublet components into a RH neutrino and a
lepton. Such decays produce the asymmetry at low temperatures, slightly before
sphaleron decoupling. If the RH neutrino has thermalized prior from producing
the asymmetry, this mechanism turns out to lead to the bound GeV. If,
instead, the RH neutrinos have not thermalized, leptogenesis from these decays
is enhanced further and can be easily successful, even at lower scales. This
Higgs-decay leptogenesis new mechanism works without requiring an interplay of
flavor effects and/or cancellations of large Yukawa couplings in the neutrino
mass matrix. Last but not least, such a scenario turns out to be testable, from
direct production of the RH neutrino(s).Comment: 6 pages, 5 figures. Accepted for PRL. References adde
Dark matter and observable Lepton Flavour Violation
Seesaw models with leptonic symmetries allow right-handed (RH) neutrino
masses at the electroweak scale, or even lower, at the same time having large
Yukawa couplings with the Standard Model leptons, thus yielding observable
effects at current or near-future lepton-flavour-violation (LFV) experiments.
These models have been previously considered also in connection to low-scale
leptogenesis, but the combination of observable LFV and successful leptogenesis
has appeared to be difficult to achieve unless the leptonic symmetry is
embedded into a larger one. In this paper, instead, we follow a different route
and consider a possible connection between large LFV rates and Dark Matter
(DM). We present a model in which the same leptonic symmetry responsible for
the large Yukawa couplings guarantees the stability of the DM candidate,
identified as the lightest of the RH neutrinos. The spontaneous breaking of
this symmetry, caused by a Majoron-like field, also provides a mechanism to
produce the observed relic density via the decays of the latter. The
phenomenological implications of the model are discussed, finding that large
LFV rates, observable in the near-future conversion experiments,
require the DM mass to be in the keV range. Moreover, the active-neutrino
coupling to the Majoron-like scalar field could be probed in future detections
of supernova neutrino bursts.Comment: 10 pages, 8 figures; extended discussion, version to appear in PR
Symmetry Improved CJT Effective Action
The formalism introduced by Cornwall, Jackiw and Tomboulis (CJT) provides a
systematic approach to consistently resumming non-perturbative effects in
Quantum Thermal Field Theory. One major limitation of the CJT effective action
is that its loopwise expansion introduces residual violations of possible
global symmetries, thus giving rise to massive Goldstone bosons in the
spontaneously broken phase of the theory. In this paper we develop a novel
symmetry-improved CJT formalism for consistently encoding global symmetries in
a loopwise expansion. In our formalism, the extremal solutions of the fields
and propagators to a loopwise truncated CJT effective action are subject to
additional constraints given by the Ward identities due to global symmetries.
By considering a simple O(2) scalar model, we show that, unlike other methods,
our approach satisfies a number of important field-theoretic properties. In
particular, we find that the Goldstone boson resulting from spontaneous
symmetry breaking of O(2) is massless and the phase transition is a second
order one, already in the Hartree-Fock approximation. After taking the sunset
diagrams into account, we show how our approach properly describes the
threshold properties of the massless Goldstone boson and the Higgs particle in
the loops. Finally, assuming minimal modifications to the Hartree-Fock
approximated CJT effective action, we calculate the corresponding
symmetry-improved CJT effective potential and discuss the conditions for its
uniqueness for scalar-field values away from its minimum.Comment: 31 pages, 8 figures. Comments on thermodynamic consistency added.
Version published in Nuclear Physics
Baryogenesis from L-violating Higgs-doublet decay in the density-matrix formalism
We compute in the density-matrix formalism the baryon asymmetry generated by
the decay of the Higgs doublet into a right-handed (RH) neutrino and a Standard
Model lepton. The emphasis is put on the baryon asymmetry produced by the total
lepton-number violating decay. From the derivation of the corresponding
evolution equations, and from their integration, we find that this contribution
is fully relevant for large parts of the parameter space. This confirms the
results found recently in the CP-violating decay formalism with thermal
corrections and shows in particular that the lepton-number violating processes
are important not only for high-scale leptogenesis but also when the
RH-neutrino masses are in the GeV range. For large values of the Yukawa
couplings, we also find that the strong washout is generically much milder for
this total lepton-number violating part than for the usual RH-neutrino
oscillation flavour part.Comment: 12 pages, 10 figure
Cosmological constant: relaxation vs multiverse
We consider a scalar field with a bottom-less potential, such as ,
finding that cosmologies unavoidably end up with a crunch, late enough to be
compatible with observations if . If
rebounces avoid singularities, the multiverse acquires new features; in
particular probabilities avoid some of the usual ambiguities. If rebounces
change the vacuum energy by a small enough amount, this dynamics selects a
small vacuum energy and becomes the most likely source of universes with
anthropically small cosmological constant. Its probability distribution could
avoid the gap by 2 orders of magnitude that seems left by standard anthropic
selection.Comment: 15 pages, 3 figures; published versio
Actuation performances of anisotropic gels
We investigated the actuation performances of anisotropic gels driven by
mechanical and chemical stimuli, in terms of both deformation processes and
stroke--curves, and distinguished between the fast response of gels before
diffusion starts and the asymptotic response attained at the steady state. We
also showed as the range of forces that an anisotropic hydrogel can exert when
constrained is especially wide;indeed, changing fiber orientation allows to
induce shear as well as transversely isotropic extensions.Comment: 11 pages, 11 figure
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