Relic Abundance of Dark Photon Dark Matter (1810.07188v1)

Abstract: We present a new mechanism for producing the correct relic abundance of dark photon dark matter over a wide range of its mass, extending down to $10^{-20}\,\mathrm{eV}$. The dark matter abundance is initially stored in an axion which is misaligned from its minimum. When the axion starts oscillating, it efficiently transfers its energy into dark photons via a tachyonic instability. If the dark photon mass is within a few orders of magnitude of the axion mass, $m_{\gamma'}/m_a = {\cal O}(10^{-3} - 1)$, then dark photons make up the dominant form of dark matter today. We present a numerical lattice simulation for a benchmark model that explicitly realizes our mechanism. This mechanism firms up the motivation for a number of experiments searching for dark photon dark matter.

• The paper establishes that a tachyonic instability efficiently transfers energy from axions to dark photons, driving their relic abundance over an extensive mass range.
• It employs large-scale numerical lattice simulations to quantify production efficiency and characterize peak momentum of the dark photons.
• The work outlines model-building constraints, highlighting the need for ultra-small gauge couplings and feasible mass generation mechanisms for dark photons.

Relic Abundance of Dark Photon Dark Matter: A Comprehensive Evaluation

The paper "Relic Abundance of Dark Photon Dark Matter" by Agrawal et al. explores a novel mechanism for generating the relic abundance of dark photon dark matter (DPDM) across an exceptionally wide mass range, from as low as $10^{-20}\text{ eV}$. This exploration extends the prevailing conceptions of dark matter, particularly regarding spin-1 bosons and their potential to act as the results of the fundamental constituents.

Overview of the Mechanism

The core mechanism introduced in the paper rests on the initial storage of dark matter (DM) abundance in a misaligned axion that is connected to the dark photon through a specific coupling. This coupling, alongside a notable range of mass conditions for the dark photon — particularly when $m_{\gamma'}/m_a = {\cal O}(10^{-3} - 1)$ — allows for an efficient energy transfer from the axion to the dark photon via a tachyonic instability as the axion begins to oscillate. The process ensures that the dark photon becomes the dominant form of DM today. By leveraging large numerical lattice simulations for benchmark models, the authors effectively substantiate their theoretical propositions.

Numerical and Phenomenological Implications

The methodology employs direct numerical simulations that reveal a striking production efficiency capable of inflating the dark photon number density significantly above that of the axion. The lattice simulations demonstrated that the efficient energy transfer, via the outlined instability, results in dark photons assuming a characteristic peak momentum around $10^{-2} \beta m_a$, translating into a significant cold dark matter contribution.

Agrawal et al. rigorously quantify the dark photon's relic abundance, providing analytical expressions that define the relationship among axion initial misalignment angles, decay constants, and the dark photon masses pertinent for achieving cold DM densities when accounting for specific astrophysical and cosmological constraints.

Theoretical Extensions and Model Building

Beyond simulation, the authors explore crucial theoretical aspects and model-building constraints necessary for the tachyonic production mechanism to function without contradiction. One notable challenge is ensuring the mass-giving mechanism for the dark photon—whether through the Higgs or Stueckelberg processes—properly aligns with the requirement of ultra-small gauge couplings to avoid detrimental self-interactions. Additionally, the paper provides viable pathways using frameworks such as the clockwork mechanism to address the large charge issue typically required to facilitate the demonstrated axion-dark photon coupling.

The paper also examines potential phenomenological scenarios for DPDM detected in current and upcoming experimental setups. Incorporating a broader examination of kinetic mixing and its implications allows the authors to present strategic parametric landscapes, highlighting accessible areas by next-generation instruments like DM radios and dish antenna experiments.

Conclusions and Future Directions

The authors contribute a substantial theoretical advancement by not only detailing a viable mechanism for DPDM abundance across an expansive mass range but also by synergizing simulation data with potential experimental probing. Furthermore, the work compels the field to consider further refinements in both theoretical and observational strategies for investigating very light bosonic DM candidates and raises important considerations for future astrophysical studies.

Researchers are encouraged to pursue further exploration of these mechanistic pathways, especially those associated with unconventional but potentially significant effects of dark condensed matter physics on galactic structures. Moreover, understanding the potential interplay with standard model fields could lead to even more comprehensive insights in the near future, as theoretical and experimental techniques continue to evolve and intersect in novel ways.