Large ν\nu-νˉ\bar{\nu} Oscillations from High-Dimensional Lepton Number Violating Operator

It is usually believed that the observation of the neutrino-antineutrino (${\nu}$-$\bar{\nu}$) oscillations is almost impossible since the oscillation probabilities are expected to be greatly suppressed by the square of tiny ratio of neutrino masses to energies. Such an argument is applicable to most models for neutrino mass generation based on the Weinberg operator, including the seesaw models. However, in the present paper, we shall give a counterexample to this argument, and show that large $\nu$-$\bar{\nu}$ oscillation probabilities can be obtained in a class of models in which both neutrino masses and neutrinoless double beta ($0\nu\beta\beta$) decays are induced by the high-dimensional lepton number violating operator ${\cal O}_7 = \bar{u}_R l^c_R \bar{L}_L H^*d_R + {\rm H.c.}$ with $u$ and $d$ representing the first two generations of quarks. In particular, we find that the predicted $0\nu\beta\beta$ decay rates have already placed interesting constraints on the $\nu_e \leftrightarrow \bar{\nu}_e$ oscillation. Moreover, we provide an UV-complete model to realize this scenario, in which a dark matter candidate naturally appears due to the new $U(1)_d$ symmetry.Comment: 21 pages, 6 figure

Strongly self-interacting vector dark matter via freeze-in

We study a vector dark matter (VDM) model in which the dark sector couples to the Standard Model sector via a Higgs portal. If the portal coupling is small enough the VDM can be produced via the freeze-in mechanism. It turns out that the electroweak phase transition have a substantial impact on the prediction of the VDM relic density. We further assume that the dark Higgs boson which gives the VDM mass is so light that it can induce strong VDM self-interactions and solve the small-scale structure problems of the Universe. As illustrated by the latest LUX data, the extreme smallness of the Higgs portal coupling required by the freeze-in mechanism implies that the dark matter direct detection bounds are easily satisfied. However, the model is well constrained by the indirect detections of VDM from BBN, CMB, AMS-02, and diffuse $\gamma$/X-rays. Consequently, only when the dark Higgs boson mass is at most of ${\cal O}({\rm keV})$ does there exist a parameter region which leads to a right amount of VDM relic abundance and an appropriate VDM self-scattering while satisfying all other constraints simultaneously.Comment: 29 pages, 8 figure

Controlling the second-harmonic in a phase matched negative-index metamaterial

Nonlinear metamaterials (NLMMs) have been predicted to support new and exciting domains in the manipulation of light, including novel phase matching schemes for wave mixing. Most notable is the so-called nonlinear-optical mirror, in which a nonlinear negative-index medium emits the generated frequency towards the source of the pump. For the first time, we experimentally demonstrate the nonlinear-optical mirror effect in a bulk negative-index NLMM, along with two other novel phase matching configurations, utilizing periodic poling to switch between the three phase matching domains.Comment: 5 pages, 7 figure