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.

Hidden sectors with light extra U(1) gauge bosons, so-called hidden photons, have recently attracted some attention because they are a common feature of physics beyond the Standard Model like string theory and supersymmetry and additionally are phenomenologically of great interest regarding recent astrophysical observations. The hidden photon is already constrained by various laboratory experiments and presently searched for in running as well as upcoming experiments. We summarize the current status of limits on hidden photons from past electron beam dump experiments including two new limits from such experiments at the High Energy Accelerator Research Organization in Japan (KEK) and the Laboratoire de l'accelérateur linéaire (LAL, Orsay) that have so far not been considered. All our limits take into account the experimental acceptances obtained from Monte Carlo simulations.

In the WIMP scenario, there is a one-to-one relation between the dark matter (DM) relic density and spin independent direct detection rate if both the annihilation of DM and its elastic scattering on nuclei go dominantly through Higgs exchange. In particular, for DM masses much smaller than the Higgs boson mass, the ratio of the relevant cross sections depends only on the DM mass. Assuming DM mass and direct detection rate within the ranges allowed by the recent DAMA collaboration results -taking account of the channelling effect on energy threshold and the null results of the other direct detection experiments-gives a definite range for the relic density. For scalar DM models, like the Higgs portal models or the inert doublet model, the relic density range turns out to be in agreement with WMAP. This scenario implies that the Higgs boson has a large branching ratio to pairs of DM particles, a prediction which might challenge its search at the LHC.The DAMA collaboration has recently provided evidence for an annual modulation of the rate of nuclear recoils in their detector [1], confirming at a firmer level their previous results [2]. Taking into account the null results of the other direct Dark Matter (DM) detection experiments [3] and the recently discovered channelling effect on the threshold energy in DAMA, points towards a nuclear recoil due to dark matternucleon elastic scatterings, with a spin independent (SI) cross section in the range (see [4])and dark matter mass in the range 3 GeV m DM 8 GeV.These results have been already the object of various studies in specific models [5,6,7,8]. In this short letter, working in the WIMP framework which assumes a DM relic density determined by the thermal freeze-out of DM annihilation, we emphasize the importance of Higgs exchange diagrams for DM mass in the range of (2). If the DM candidate is light, there is a limited number of possible (2-body) annihilation channels to SM particles, i.e. the Inert Doublet Model (IDM), a two Higgs model extension of the Standard Model with a scalar dark matter candidate [10,11,12,13,14,15,16], and a fortiori for a singlet scalar DM candidate [17,18,19,20,21,22,23,24], the channels of Fig.1 are the only possible non-negligible ones in the range of (2). In more sophisticated models, such as with the neutralino DM candidate of the MSSM, these channels coexist a priori with many other channels, in particular with other intermediate Higgs scalar particle or squarks channels [7,25].Both processes in Fig.1 are tightly related. The Higgs boson mass dependence is the same, mh , because in both cases the momentum of the Higgs is negligible compared with m h . Furthermore the dependence in the unknown DM-DM-h coupling is the same. This means that, up to uncertainties in the Yukawa couplings of the SM fermions and in the Higgs to nucleon coupling, the only left parameter is the mass of the dark matter. This dependence is however limited if we take the DM mass within the range (2). In other words, if Higgs mediated processes are dominant, the r...

If dark matter (DM) simply consists in a scalar particle interacting dominantly with the Higgs boson, the ratio of its annihilation cross section -which is relevant both for the relic abundance and indirect detection-and its spin-independent scattering cross section on nuclei depends only on the DM mass. It is an intriguing result that, fixing the mass and direct detection rate to fit the annual modulation observed by the DAMA experiment, one obtains a relic density in perfect agreement with its observed value. In this short letter we update this result and confront the model to the recent CoGeNT data, tentatively interpreting the excess of events in the recoil energy spectrum as being due to DM. CoGeNT, as DAMA, points toward a light DM candidate, with somewhat different (but not necessarily incompatible) masses and cross sections. For the CoGeNT region too, we find an intriguing agreement between the scalar DM relic density and direct detection constraints. We also give the one σ region favoured by the CDMS-II events, that suggest a light DM candidate too, and the limits from Xenon10 2009 data, which, depending on the assumed scintillation efficiency, may exclude both CoGeNT and DAMA. Assuming CoGeNT and/or DAMA to be due to DM leads to definite predictions regarding indirect detection and Higgs search at the LHC.Recently, there has been some effervescence regarding what may be first hints of direct detection of dark matter (DM) from the galactic halo. The most recent is related to the CoGeNT experiment, a low background germanium crystal based detector, with a rather modest exposure time and detector mass, but very low threshold energy, which has announced an anomaly in the form of an excess of events at low recoil energies [1]. Although the collaboration clearly leans toward natural radioactivity as the cause for the observed excess, they do put forward the possibility that the events may be explained by the elastic collisions of DM from the galactic halo, with a mass in the ∼ 7 − 10 GeV range, and a rather large spin independent (SI) cross section on nuclei, σ SI ∼ 7 × 10 −41 cm 2 . Surprizingly, these values for the mass and scattering cross section are not too different from those required to fit the DAMA/Libra and DAMA/NaI (DAMA in the sequel) events. DAMA has observed 11 successive cycles of annual modulation in the rate of nuclear recoils, with a statistical significance of 8.2σ [2]. These measurements are consistent with the signal that would arise from elastic scattering of a WIMP from the galactic halo with the nuclei in the detectors, the flux of DM particles being modulated by the periodic motion of the Earth around the Sun [3,4].There has been much work on the DM interpretation of the recent DAMA data (see e.g. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]). In [21] (see also [22]), it has been shown that the DAMA results may be explained as being caused by the elastic scattering of a singlet scalar DM candidate interacting through the Higgs portal. This particle may be a true singl...