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.

Particles with electric charge q ≤ 10−3 e and masses in the range 1-100 MeV/c 2 are not excluded by present experiments. An experiment uniquely suited to the production and detection of such "millicharged" particles has been carried out at SLAC. This experiment is sensitive to the infrequent excitation and ionization of matter expected from the passage of such a particle. Analysis of the data rules out a region of mass and charge, establishing, for example, a 95%-confidence upper limit on electric charge of 4.1×10−5 e for millicharged particles of mass 1 MeV/c 2 and 5.8×10 −4 e for mass 100 MeV/c 2 .PACS numbers: 14.80.-j, 95.35.+dThe quantization of electric charge is an empirically well-supported idea. Of the numerous searches for fractional charge carried out thus far, none has provided conclusive evidence for charge non-quantization. The current bounds on the proton-electron charge difference [1] and the neutron charge [2], of order 10−21 e, lend strong support to the notion that charge quantization is a fundamental principle. However, the Standard Model with three generations of quarks and leptons does not impose charge quantization. In order to enforce quantization of charge, physics beyond the Standard Model is necessary [3]. This observation has stimulated inquiry into mechanisms whereby charge quantization (and perhaps even charge conservation) might be violated [4]. Particles with small fractional charge (q < ∼ 10 −2 e) appear as a natural consequence of many of these mechanisms. There has been interest in the possibility of a small, nonzero electric charge for the neutrino [5], and the possibility that particles with small fractional charge account for a portion of the dark matter in the universe [6]. Additionally, a noteworthy model has been proposed wherein certain particles could exhibit apparent fractional charge without violating charge quantization [7]. Several authors have investigated constraints, imposed by laboratory experiments and by astrophysical and cosmological arguments, on the existence of (free) fractionally charged particles [8]. They point out that there remains a large domain in mass and charge (10

We present a search at the Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling α' to electrons. Such a particle A' can be produced in electron-nucleus fixed-target scattering and then decay to an e + e- pair, producing a narrow resonance in the QED trident spectrum. Using APEX test run data, we searched in the mass range 175-250 MeV, found no evidence for an A'→ e+ e- reaction, and set an upper limit of α'/α ~/= 10(-6). Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.

We present a new high-statistics measurement of the cross section for the process e+e~e+e m. +m at a center-of-mass energy of 29 GeV for invariant pion-pair masses M(m+m ) between 350 MeV/c' and 1.6 GeV/c2. We observe the f~( 1270) and measure its radiative width to be 3. 15+0.0420. 39 keV. We also observe an enhancement in the~+~spectrum near 1 GeV. General agreement is found with unitarized models of the yy~m m. reaction that include finalstate interactions.