Dark sectors, consisting of new, light, weakly-coupled particles that do not interact with the known strong, weak, or electromagnetic forces, are a particularly compelling possibility for new physics. Nature may contain numerous dark sectors, each with their own beautiful structure, distinct particles, and forces. This review summarizes the physics motivation for dark sectors and the exciting opportunities for experimental exploration. It is the summary of the Intensity Frontier subgroup "New, Light, Weakly-coupled Particles" of the Community Summer Study 2013 (Snowmass). We discuss axions, which solve the strong CP problem and are an excellent dark matter candidate, and their generalization to axion-like particles. We also review dark photons and other dark-sector particles, including sub-GeV dark matter, which are theoretically natural, provide for dark matter candidates or new dark matter interactions, and could resolve outstanding puzzles in particle and astro-particle physics. In many cases, the exploration of dark sectors can proceed with existing facilities and comparatively modest experiments. A rich, diverse, and lowcost experimental program has been identified that has the potential for one or more game-changing discoveries. These physics opportunities should be vigorously pursued in the US and elsewhere.

A new type of electromagnetic radiation by a neutrino with non-zero magnetic (and/or electric) moment moving in background matter and electromagnetic field is considered. This radiation originates from the quantum spin flip transitions and we have named it as "spin light of neutrino"($SL\nu$). The neutrino initially unpolarized beam (equal mixture of $\nu_{L}$ and $\nu_{R}$) can be converted to the totally polarized beam composed of only $\nu_{R}$ by the neutrino spin light in matter and electromagnetic fields. The quasi-classical theory of this radiation is developed on the basis of the generalized Bargmann-Michel-Telegdi equation. The considered radiation is important for environments with high effective densities, $n$, because the total radiation power is proportional to $n^{4}$. The spin light of neutrino, in contrast to the Cherenkov or transition radiation of neutrino in matter, does not vanish in the case of the refractive index of matter is equal to unit. The specific features of this new radiation are: (i) the total power of the radiation is proportional to $\gamma ^{4}$, and (ii) the radiation is beamed within a small angle $\delta \theta \sim \gamma^{-1}$, where $\gamma$ is the neutrino Lorentz factor. Applications of this new type of neutrino radiation to astrophysics, in particular to gamma-ray bursts, and the early universe should be important.Comment: accepted for publication in Phys.Lett.

The quasi-classical theory of the spin light of neutrino (SLν) in background matter, accounting for the neutrino polarization, is developed. The neutrino transitions ν L → ν R and ν R → ν L rates in matter are calculated. It is shown that the SLν in matter leads to the neutrino conversion from active ν L to sterile ν R states (neutrino self-polarization effect in matter).Convincing evidence in favour of non-zero neutrino mass that has been obtained during the last few years in atmospheric and solar-neutrino experiments, are also confirming in the reactor KamLAND and long-baseline accelerator experiments (see [1] for a review on the present status of neutrino mixing and oscillations). Even within the standard model (minimally extended with SU(2)−singlet right-handed neutrino) a massive neutrino inevitably has non-zero magnetic moment µ generated by the one-loop diagramme [2]. A recent studies of a massive neutrino electromagnetic properties within one-loop level, including discussion on the neutrino magnetic moment, can be found in [3]. It should be also noted here that a rather detailed discussion on the neutrino charge radius is presented in the two recent papers [4,5].In a series of our papers [6][7][8][9][10][11] we have developed the Lorentz invariant approach to neutrino oscillations which enables us to study, in particular, the neutrino spin procession in the background matter with effects of the presence of electromagnetic and gravitational fields being also accounted for. A review on these our studies can be found in [12]. * E-mail: studenik@srd.sinp.msu.ru 1 In [10,11] we have predicted the new mechanism of electromagnetic radiation by neutrino moving in background matter and/or electromagnetic and gravitational fields. We have named this radiation as "spin light of neutrino" and introduced the abbreviation SLν which we shall use below in this paper. The SLν originates from the neutrino spin precession that can be induced whether by weak interactions of neutrino with the background matter or by the external electromagnetic or gravitational fields that could be present in the background environment. It should be noted that the discussed mechanism of electromagnetic radiation by a neutrino moving in a constant magnetic field was also studied previously in [13].As we have shown in [10], the total power of the SLν in matter does not washed out when the emitted photon refractive index is equal to unit and the SLν can not be considered as the neutrino Cerenkov radiation (see, for example, [14] and references therein). It was also emphasized [10] that the initially unpolarized neutrino beam (equal mixture of active left-handed and sterile right-handed neutrinos) can be converted to the totally polarized beam composed of only ν R due to the spin light in contrast to the Cherenkov radiation which can not produce the neutrino spin self-polarization effect.The discovered important properties of SLν (such as strong beaming of the radiation along the neutrino momentum, the rapid growth of the total radiation...

The Earth's density distribution can be approximately considered piecewise continuous at the scale of two-flavor oscillations of typical solar neutrinos, such as the beryllium-7 and boron-8 neutrinos. This quite general assumption appears to be enough to analytically calculate the day-night asymmetry factor for such neutrinos. Using the explicit time averaging procedure, we show that, within the leading-order approximation, this factor is determined by the electron density within about one oscillation length under the detector, namely, in the Earth's crust (and upper mantle for high-energy neutrinos). We also evaluate the effect of the inner Earth's structure on the observed asymmetry and show that it is suppressed and mainly comes from the neutrinos observed near the winter and summer solstices. As a result, we arrive at the strict interval constraint on the asymmetry, which is valid within quite a wide class of Earth models.

We obtain an evolution equation for neutrinos in dense matter and electromagnetic field, which describes both flavor oscillations and neutrino spin rotation. Using this equation we construct a quasi-classical theory of these phenomena. We obtain the probabilities of arbitrary spin-flavor transitions assuming the external conditions to be constant. We demonstrate that the resonance behavior of the transition probabilities is possible only when the flavor neutrino states cannot be described as superpositions of the mass eigenstates. We discover that a resonance, which is similar to the Mikheev-Smirnov resonance, takes place for neutrinos in magnetic field due to the transition magnetic moments. * Electronic address: av.chukhnova@physics.msu.ru † Electronic address: lobanov@phys.msu.ru