Exploring the Role of Axions and Other WISPs in the Dark Universe (1210.5081v2)

Abstract: Axions and other very weakly interacting slim particles (WISPs) may be non-thermally produced in the early universe and survive as constituents of the dark universe. We describe their theoretical motivation and their phenomenology. A huge region in parameter space spanned by their couplings to photons and their masses can give rise to the observed cold dark matter abundance. A wide range of experiments - direct dark matter searches exploiting microwave cavities, searches for solar axions or WISPs, and light-shining-through-a-wall searches - can probe large parts of this parameter space in the foreseeable future.

• The paper introduces axions and WISPs to address the strong CP problem and extend dark matter paradigms.
• It evaluates diverse experimental approaches like haloscopes, helioscopes, and LSW techniques to probe these particles.
• The findings underscore the need for future experiments such as IAXO and ADMX to refine the search for dark matter.

Axions and WISPs in the Dark Universe: An Examination

The exploration of axions and other very weakly interacting slim particles (WISPs) offers a promising direction in understanding the constituents of the dark universe. This paper by Andreas Ringwald discusses the potential roles of axions and analogous particles, not traditionally covered under the weakly interacting massive particles (WIMPs) framework, as viable candidates for dark matter. The text provides an extensive theoretical background, experimental efforts, and implications of the possible existence of these particles, thereby creating a compelling case for further research and development in this area.

Theoretical Foundations

Axions and WISPs arise from the necessity to address certain limitations in the Standard Model, including the strong CP problem and the dark matter conundrum. The paper begins by revisiting the motivation behind axions, primarily introduced to resolve the strong CP problem. This problem is effectively addressed by making the CP-violating $\theta$ parameter dynamically equivalent to zero through the Peccei-Quinn mechanism, leading to the realization of axions as light pseudoscalar particles.

In addition to QCD axions, extensions such as axion-like particles (ALPs) and hidden sector photons (HPs) find their relevance in theoretical frameworks like string theory, where they emerge naturally in compactification scenarios. These extensions predict a complex spectrum of these particles, often referred to as the "axiverse," whose properties are yet to be explored experimentally.

WISPs as Dark Matter

The possibility of non-thermally produced axions and WISPs surviving as constituents of dark matter is noteworthy. Their weak interactions, cold nature at the time of structure formation, and stability over cosmological timescales make them suitable candidates. The paper outlines how sufficient dark matter abundance can be achieved across a wide parameter space by tuning their mass and couplings, a parameter space increasingly within reach of direct detection experiments.

Experimental Searches and Implications

Experimental endeavors to detect axions and WISPs have broadened, encompassing various strategies such as haloscopes, helioscopes, and light-shining-through-walls (LSW) experiments. Notably, constraint analyses from astrophysical observations like white dwarf cooling, gamma-ray transparency, and stellar evolution provide indirect evidence and limitations, pushing forward the sensitivity of experimental bounds. These methods are crucial for narrowing down the viable mass and coupling ranges for axions, paving the way for tailored experiments.

The continued lack of any direct WIMP detection signals further underscores the value of these experimental explorations for axions and WISPs. Developments like the International Axion Observatory (IAXO) and ADMX HF are set to expand the search capabilities significantly, highlighting the importance of these particles in potentially unraveling the mysteries of dark matter composition.

Implications and Future Prospects

Ringwald's paper implicitly argues the necessity to extend beyond traditional dark matter models, exploring the axion and WISP landscape more comprehensively. This shift not only expands the scope of possible dark matter candidates but also aligns with broader theoretical predictions from advanced field theories and string phenomenology. Should these particles be detected, the impact would extend across particle physics, cosmology, and astrophysics, with far-reaching implications for our understanding of fundamental forces and the universe's evolution.

In sum, the paper provides a thorough exposition on the potential role of axions and WISPs in the cosmic landscape. It acknowledges both the theoretical challenges and the promising prospects of ongoing experimental searches, ultimately advocating for a more diversified approach in the quest to decode the dark universe. This work resonates with a scientific paradigm shift towards more complex and rich models of fundamental and cosmological physics.