Search

The coherent oscillation of axionic fields naturally drives copious production of dark photon particles in the early Universe, due to resonance and tachyonic enhancement. During the process, energy is abruptly transferred from the former to the latter, sourcing gravitational-wave generation. The resulting gravitational waves are eventually observed today as stochastic background. We report analytical results of this production and connect them to the recent pulsar-timing results from the NANOGrav Collaboration. We show an available parameter space for our mechanism to account for the signal around the mass mϕ ~ 10-13 eV and the decay constant fϕ ~1016 GeV, with a dimensionless coupling of O(1). A mechanism to keep the axion from dominating the Universe is a necessary ingredient of this model, and we discuss a possibility to recover a symmetry and render the axion massless after the production. We also comment on potential implications of the required effective number of relativistic species for the determination of the present Hubble constant. [ABSTRACT FROM AUTHOR]

The extension of the Standard Model by assuming U(1)B-L gauge symmetry is very well motivated since it naturally explains the presence of heavy right-handed neutrinos required to account for the small active neutrino masses via the seesaw mechanism and thermal leptogenesis. Traditionally, we introduce three right-handed neutrinos to cancel the [U(1)B-L]³ anomaly. However, it suffices to introduce two heavy right-handed neutrinos for these purposes and therefore we can replace one right-handed neutrino by new chiral fermions to cancel the U(1)B-L gauge anomaly. Then, one of the chiral fermions can naturally play a role of a dark matter candidate. In this paper, we demonstrate how this framework produces a dark matter candidate which can address the so-called "core-cusp problem". As one of the small-scale problems that the Λ cold dark matter paradigm encounters, it may imply an important clue for the nature of dark matter. One of resolutions among many is hypothesizing that sub-keV fermion dark matter halos in dwarf spheroidal galaxies are in a (quasi) degenerate configuration. We show how the degenerate sub-keV fermion dark matter candidate can be nonthermally originated in our model and thus can be consistent with the Lyman-a forest observation. Thereby, the small neutrino mass, baryon asymmetry, and the sub-keV dark matter become consequences of the broken B-L gauge symmetry. [ABSTRACT FROM AUTHOR]

We present a dark sector model addressing both the Hubble tension and the core-cusp problem. The model is based on a hidden Abelian gauge symmetry group with some chiral fermions required by the anomaly cancellation conditions, producing a candidate for the decaying fermion dark matter as a solution to the Hubble tension. Moreover, the sub-keV mass regime and the thermal history of the dark sector help the dark matter candidate resolve the core-cusp problem occurring in the standard Λ cold dark matter cosmology. [ABSTRACT FROM AUTHOR]

The domain wall problem and the isocurvature problem restrict possible combinations of axion models and inflation models. In this paper, we consider a new mechanism which solves those problems by dynamics of multiple scalar fields during/after inflation. The mechanism makes axion models with a nontrivial domain wall number compatible with inflation models with a large Hubble parameter, HI≫107-8  GeV. The isocurvature problem is also avoided in the mechanism. [ABSTRACT FROM AUTHOR]

The perfect fit of the matter fields of the Standard Model into the SU(5) multiplets has strongly supported the idea of the grand unified theory (GUT) for decades. In this paper, we discuss a novel framework which explains why the Standard Model matter fields form the apparently complete SU(5) multiplets. In the new framework, the apparent matter unification inevitably results from chiral SU(5) gauge theory even if the quarks and leptons are not embedded into the common SU(5) multiplets. We call this class of models the "fake GUT." The novel phenomenological prediction of the fake GUT is more variety of the nucleon decay modes than the conventional GUT, which reflects the rich structure of the origin of the matter fields. [ABSTRACT FROM AUTHOR]

A model with Lμ - Lτ gauge symmetry is the least constrained model as a resolution to the disagreement of the muon anomalous magnetic moment between the theoretical predictions and the experimental results. In this paper, we discuss how well the Lμ - Lτ model can be constrained by looking for decays of the charged kaon associated with a Lμ - Lτ gauge boson. More concretely, we consider searches for single muon tracks from the decays of stopped charged kaon as in the E949 experiment. In our conservative estimation, we find that the favored parameter region for the muon anomalous magnetic moment can be tested by using a 10 times larger number of the stopped charged kaons and about a 100 times better photon rejection rate than the E949 experiment. [ABSTRACT FROM AUTHOR]