45,737 research outputs found

### Large $\nu$-$\bar{\nu}$ 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

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