New light, weakly-coupled particles

New light particles that couple only weakly to ordinary matter are ubiquitous innew physics extensions of the Standard Model. Their existence is motivated byseveral theoretical and observational puzzles, many of which are central in ourquest to obtain a comprehensive understanding of the constituents of ouruniverse and their interactions. These include the nature of dark matter anddark energy, the strong CP problem, and a variety of astrophysical puzzles anddark matter-related anomalies. Our working group examines axions, axion-likeparticles, hidden-sector photons, milli-charged particles, chameleons, andrelated particles (see [1] for a recent review). Their masses can range anywherefrom sub-femto-eV to the weak scale (∼ 100 GeV), and they are characterized bytheir small coupling or mixing with the photon. This allows them to be producedwith intense beams of photons, electrons, or protons and detected with sensitiveequipment. This makes them, by definition, targets for the intensity frontier.We will sometimes refer to these weakly interacting sub-eV (or " slim ")particles as " WISPs ". Axions are pseudo-scalar particles that solve the strongCP problem. They have extremely small masses, because they arise as pseudo-Nambu-Goldstone bosons of an almost exact symmetry, the " Peccei-Quinn "symmetry, which is spontaneously broken at a very high energy scale. Thespontaneous breaking of other, non-Peccei-Quinn global symmetries is common innew physics models (including string theory) and can give rise to light axion-like scalar or pseudo-scalar particles, called ALPs. Axions and ALPs canconstitute the dark matter of our universe and can explain a variety ofastrophysical observations. The axion couples to gluons, photons, and StandardModel fermions, and it is the latter two that are the most easily detected. ALPsalso naturally couple to photons, although this coupling is not guaranteed.Hidden-sector photons, called A bosons, are massive vector bosons that can mixwith the ordinary photon via " kinetic-mixing. " This mixing can permit photon-Aoscillations (observable for sub-eV A bosons) and produces a small coupling ofthe A to electrically charged matter. A sub-eV mass A could be the dark matterparticle or contribute to the observed number of relativistic degrees of freedomin the early universe. A MeV-GeV mass A could explain the discrepancy betweenthe measured and calculated muon anomalous magnetic moment in the StandardModel.