[Paper Review] Dark Sectors and New, Light, Weakly-Coupled Particles

Rouven Essig, J. A. Jaros+87 more|arXiv (Cornell University)|Oct 31, 2013

Dark Matter and Cosmic Phenomena5 references157 citations

TL;DR

This paper proposes a comprehensive experimental and theoretical framework for discovering light, weakly-coupled particles in dark sectors—such as axions, axion-like particles, dark photons, and sub-GeV dark matter—through intensity frontier techniques. It demonstrates that existing and near-future experiments, leveraging photon regeneration, beam dumps, and sensitive detection in microwave cavities, can probe a broad parameter space with modest investment, offering a path to resolving dark matter, the strong CP problem, and astrophysical anomalies.

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.

Motivation & Objective

• To motivate the existence of dark sectors as a natural extension of the Standard Model, addressing unresolved problems like the strong CP problem and dark matter.
• To identify light, weakly-coupled particles—such as axions, axion-like particles, and dark photons—as compelling candidates for dark matter and new physics.
• To advocate for a focused, low-cost experimental program at the intensity frontier to probe these particles using existing and upgraded facilities.
• To highlight the synergy between theory and experiment in dark sector phenomenology, especially through photon coupling and kinetic mixing.
• To position the US as a global leader in this emerging field by sustaining and expanding experimental efforts in dark sector searches.

Proposed method

• Utilizing microwave cavity experiments and light-shining-through-walls setups originally designed for axion searches to detect dark photons via photon regeneration.
• Adapting helioscopes, which detect solar axions, to search for dark photons through similar coupling mechanisms.
• Reinterpreting beam dump experiments—originally for axion detection—as probes of dark photon couplings and masses.
• Employing high-intensity electron and proton beam dumps, especially at facilities like Project X, to search for light dark matter from dark photon decays.
• Applying sensitive, low-noise detection techniques such as superconducting microwave resonators, high-rate pixelated silicon detectors, and resonant optical cavities.
• Leveraging kinetic mixing between the photon and dark sector gauge bosons to enable indirect detection of dark-sector particles through their effective coupling to electromagnetism.

Experimental results

Research questions

• RQ1 Can axions and axion-like particles, which solve the strong CP problem, also account for the observed dark matter density?
• RQ2 Do dark photons and other weakly-coupled particles provide viable dark matter candidates and explain anomalies in particle and astrophysical data?
• RQ3 To what extent can existing experimental facilities—such as beam dumps, cavities, and helioscopes—be repurposed to probe the full parameter space of light dark sector particles?
• RQ4 How can modest investments in superconducting magnets, sensitive microwave detection, and upgraded accelerators enhance discovery reach for light, weakly-coupled particles?
• RQ5 What is the potential for educational and collaborative advancement in experimental particle physics through hands-on, small-scale dark sector searches?

Key findings

• Existing experiments, including microwave cavity and light-shining-through-walls setups, can be effectively repurposed to search for dark photons and axion-like particles.
• Beam dump experiments originally designed for axion searches now provide important limits on dark photon couplings and masses across a broad range of parameter space.
• Theoretical models predict that dark photons and other dark-sector particles can couple to the Standard Model via kinetic mixing, enabling indirect detection through photon-like signals.
• Sub-GeV dark matter candidates are viable and can be produced in beam dumps via dark photon decays, making them accessible to upcoming high-intensity experiments.
• The combination of high-intensity beams and highly sensitive detection techniques enables needle-in-the-haystack searches for weakly-coupled particles with minimal cost and high discovery potential.
• A coordinated, low-cost experimental program leveraging existing infrastructure and modest upgrades offers a high-impact pathway to discovering new physics beyond the Standard Model.