Dark Sectors and New, Light, Weakly-Coupled Particles (1311.0029v1)

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.

• The paper outlines dark sectors as comprising light, weakly-coupled particles—including axions, dark photons, and millicharged particles—that extend the standard model and address phenomena like dark matter and the strong CP problem.
• The paper presents a range of experimental methods, from ADMX and ALPS-II to beam dump experiments, to probe the parameter space and detect signals of these elusive particles.
• The paper emphasizes interdisciplinary approaches by integrating cosmological, astrophysical, and laboratory data to test theoretical predictions and resolve anomalies such as the muon g-2 discrepancy.

Analysis of "Dark Sectors and New, Light, Weakly-Coupled Particles"

The document summarizes the physics motivations and experimental opportunities for exploring dark sectors, hypothesized to consist of new, light, weakly-coupled particles that do not interact significantly with known standard model forces. This concept provides a framework to understand phenomena such as dark matter and the strong CP problem, fostering the potential for identifying new particles like axions, dark photons, and other light particles that may interact through additional forces or portals.

Highlights of the Paper

The research discusses various theoretical motivations underlying the existence of dark sectors:

1. Axions and Axion-Like Particles (ALPs): Axions, originally proposed to solve the strong CP problem, and ALPs, potential dark matter candidates, are explored within this paradigm. Existing experiments like ADMX and future ones such as ALPS-II and IAXO aim to probe their parameter space, especially targeting axion-induced cosmic microwave background isocurvature perturbations, which significantly constrain theoretical models.
2. Dark Photons: These hypothetical particles could provide the coupling mechanism between dark matter and regular matter through kinetic mixing with the photon. A notable aspect is their role in explaining anomalies such as the muon g-2 discrepancy and cosmic ray excesses. Experimental searches for dark photons include fixed-target electron beam experiments (APEX, HPS), e+e− colliders like KLOE and BaBar, and proton beam dumps such as at LSND and MiniBooNE.
3. Light Dark Matter (LDM) and Dark-Sector States: The exploration of sub-GeV dark matter has gained traction due to potential cosmological and astrophysical inconsistencies with cold dark matter (CDM) models on galactic scales. The implications of having LDM and associated particles accessible to current and future detectors are substantial, raising new questions about dark sector complexity.
4. Millicharged Particles (MCPs): These particles arise naturally from dark sectors with massless dark photons. Experiments like MilliQ and SLAC have set constraints on MCPs, but upcoming experiments, leveraging existing infrastructures like those at JLab or SuperKEK, could probe unexplored regions of parameter space.
5. Chameleons: As a dark energy candidate, the chameleon field offers testable predictions for self-interacting scalar fields that dynamically acquire mass in response to local matter density. Constraints on chameleons arise from various terrestrial experiments and are complemented by astrophysical observations.

Theoretical and Practical Implications

The implications of this paper are significant, both practically and theoretically:

• Unveiling New Physics: The existence of a dark sector would extend the standard model, possibly linking particle physics with cosmology in explaining dark matter, dark energy, and even neutrino masses.
• Broadened Parameter Space Exploration: The identification of dark photons or axion-like particles could redefine the accessible mass and coupling strength parameter spaces for new physics, encouraging diverse experimental approaches.
• Interdisciplinary Approaches: Combining data from cosmology, astrophysics, and particle physics experiments could validate or invalidate specific theoretical predictions, refining our understanding of fundamental forces and particles.

Future Developments

Future search avenues are discussed, suggesting enhancements to experimental setups and proposing new experiments to push the boundaries of current scientific knowledge. The convergence of multiple experimental techniques, from colliders to direct dark matter searches, illuminates potential pathways to uncovering what constitutes the majority of the universe's content.

The paper not only delineates a robust scientific exploration framework for these elusive particles but also highlights the synergy required among global scientific communities and experiments to make breakthroughs in understanding dark sectors and the fundamental structure of our universe.