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.

We have searched for solar axions or similar particles that couple to two photons by using the CAST setup with improved conditions in all detectors. From the absence of excess X-rays when the magnet was pointing to the Sun, we set an upper limit on the axion-photon coupling of g a?? < 8.8×10-11 GeV-1 at 95% CL for m a < 0.02 eV [2] (see exclusion plot given below). This result is the best laboratory limit over a broad range of axion masses and for m a < 0.02 eV also supersedes the previous limit derived from energy-loss arguments on globularcluster stars. The scanned range up to an axion rest mass of ~0.4 eV/c 2 covered during 2005-2006 with 4 He (up to 14 mbar) as refractive gas is also shown, while the analysis is in progress. With 3 He as buffer gas (up to 60 mbar), CAST has the potential to reach ~0.8 eV/c 2 , with ~1.2 eV/c 2 being the envisaged upper limit in axion rest mass.

The cryogenic storage ring CSR Review of Scientific Instruments 87, 063115 (2016); https://doi

We report on a direct search for sub-GeV dark photons (A^{'}), which might be produced in the reaction e^{-}Z→e^{-}ZA^{'} via kinetic mixing with photons by 100 GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The dark photons would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75×10^{9} electrons on target. We set new limits on the γ-A^{'} mixing strength and exclude the invisible A^{'} with a mass ≲100  MeV as an explanation of the muon g_{μ}-2 anomaly.

DOI: will be assignedThe International Axion Observatory (IAXO) is a new generation axion helioscope aiming at a sensitivity to the axion-photon coupling of gaγ few × 10 −12 GeV −1 , i.e. 1-1.5 orders of magnitude beyond the one currently achieved by CAST. The project relies on improvements in magnetic field volume together with extensive use of x-ray focusing optics and low background detectors, innovations already successfully tested in CAST. Additional physics cases of IAXO could include the detection of electron-coupled axions invoked to explain the white dwarf cooling, relic axions, and a large variety of more generic axionlike particles (ALPs) and other novel excitations at the low-energy frontier of elementary particle physics. This contribution is a summary of our recent paper [1].