Dark Sectors and New, Light, Weakly-Coupled Particles

TL;DR

Dark Sectors and New, Light, Weakly-Coupled Particles surveys the theoretical motivation and experimental prospects for light hidden-sector states, including axions/ALPs, dark photons, light dark matter, millicharged particles, and chameleons. It emphasizes the vector and axion portals as the most tractable low-energy connections between the SM and hidden sectors, detailing a diverse array of current and planned experiments across laser, fixed-target, beam-dump, collider, and haloscope platforms. The work highlights how intensity-frontier programs can probe DM and beyond-SM scenarios with orders of magnitude sensitivity improvements, often with existing infrastructure, and discusses future accelerator and detector technologies to extend reach. Overall, the paper argues for a broad, well-supported experimental program at the intensity frontier to pursue potential game-changing discoveries in particle physics and cosmology. The exploration has profound implications for understanding DM, the strong CP problem, dark energy, and possible new forces, and it calls for sustained US and global leadership in these efforts.

Abstract

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 low-cost 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.

Figures (11)

• Figure 1: Parameter space for axions (top) and axion-like particles (ALPs) (bottom). In the bottom plot, the QCD axion models lie within an order of magnitude from the explicitly shown "KSVZ" axion line (red band). Colored regions are: experimentally excluded regions (dark green), constraints from astronomical observations (gray) or from astrophysical or cosmological arguments (blue), and sensitivity of planned and suggested experiments (light green) (ADMX Asztalos:2011ei, ALPS-II Bahre:2013ywa, IAXO Irastorza:2011gsVogel:2013btaIrastorza:1567109, Dish antenna Horns:2012jf). Shown in red are boundaries where ALPs can account for all the dark matter produced either thermally in the big bang or non-thermally by the misalignment mechanism.
• Figure 2: (a) Simple photon regeneration to produce axions or axion-like particles. (b) Resonant photon regeneration, employing matched Fabry-Perot cavities. The overall envelope schematically shown by the thin dashed lines indicates the important condition that the axion wave, and thus the Fabry-Perot mode, in the photon regeneration cavity must follow that of the hypothetically unimpeded photon wave from the Fabry-Perot mode in the axion generation magnet. Between the laser and the cavity are optics (IO) that manage mode matching of the laser to the cavity, imposes RF sidebands for reflection locking of the laser to the cavity, and provides isolation for the laser. The detection system is also fed by matching and beam-steering optics. Not shown is the second laser for locking the regeneration cavity and for heterodyne readout.
• Figure 3: Exclusion plot of mass and photon coupling $(m_{a},g_{a\gamma\gamma})$ for the axion, and the 95% CL exclusion limit for the resonantly enhanced photon regeneration (REPR) experiment. The existing exclusion limits indicated on the plot include the best direct solar axion search (CAST collaboration) cast, the Horizontal Branch Star limit RaffeltBook, and previous laser experiments Chou08alps.
• Figure 4: Exclusion regions for axions and axion-like particles in the $m_{a}-g_{a\gamma\gamma}$ plane achieved by CAST in the vacuum CAST_PRL05cast_jcap07, $^{4}$He cast_jcap09, and $^{3}$He phase CAST_PRL11CAST_PRL13. We also show constraints from the Tokyo helioscope, horizontal branch (HB) stars Raffelt08, and the hot dark matter (HDM) bound Hannestad10. The yellow band labeled "Axion models" represents typical theoretical models with $\left|E/N-1.95\right| = 0.07-7$. The green solid line inside the band is for $E/N=0$ (KSVZ model).
• Figure 5: Expected sensitivity of IAXO compared with current bounds from CAST and ADMX. Also future prospects of ADMX are shown (dashed brown region).