- The paper derives a novel, MeV-scale lower bound on cold thermal dark matter mass using Neff measurements from Planck's CMB data.
- It employs energy density equations and BBN constraints to link dark matter interactions with thermal equilibrium, excluding masses below key thresholds.
- It applies its findings to supersymmetric models, ruling out neutralino masses below 3.5 MeV and refining the viable dark matter parameter space.
A Lower Bound on the Mass of Cold Thermal Dark Matter from Planck
The paper "A lower bound on the mass of cold thermal dark matter from Planck" by Celine B\oe hm, Matthew J. Dolan, and Christopher McCabe addresses the longstanding challenge of establishing a robust lower bound on the mass of thermal relic dark matter particles. This paper leverages the precise cosmic microwave background (CMB) measurements provided by the Planck satellite, particularly analyzing the effective number of neutrinos, denoted as Neff, which contributes to establishing constraints on dark matter properties.
Key Contributions
The core contribution of this paper is the derivation of a novel lower bound on the mass of cold thermal dark matter, based on Neff measurements. The authors demonstrate that if dark matter remains in thermal equilibrium through interactions with electrons, photons, or neutrinos, a lower mass limit can be effectively established. This is shown to be approximately on the MeV scale, with minimal dependence on the annihilation cross-section's nature (whether s-wave or p-wave).
Methodology and Analysis
Employing the energy density equation related to neutrino and photon temperatures, the paper derives expressions linking the dark matter particle mass to the effective number of neutrinos. The analysis incorporates Planck's precise measurement of the CMB angular power spectrum, considering the constraints placed on Neff together with primordial helium and deuterium abundance from Big Bang Nucleosynthesis (BBN). The derived mass bounds indicate that for a Dirac fermion coupled to neutrinos, a mass of less than 7.3 MeV can be excluded, with similar constraints for other particle statistics and couplings.
Application to Supersymmetric Models
The research further applies these general findings to a specific scenario within supersymmetry: the case of a light neutralino acting as dark matter, with a sneutrino mediator facilitating interactions. This example underscores the paper’s broader implications, demonstrating that a neutralino mass less than 3.5 MeV can be ruled out, thereby enriching the discussion surrounding feasible mass ranges for supersymmetric dark matter candidates.
Implications and Future Directions
The implications of this paper are significant for both theoretical and experimental physics. On the theoretical side, it refines the parameter space for dark matter models, offering pivotal constraints that guide model development. Practically, it situates itself as a benchmark for future CMB observations and particle physics experiments looking to further delineate dark matter properties.
Future avenues of research could explore incorporating other potential sources of Neff modifications or extending this approach to alternative cosmological datasets. Additionally, investigating non-standard thermal histories or expanded interaction frameworks within early universe cosmology may yield further insights into dark matter characteristics.
Conclusion
In conclusion, the paper establishes a substantive contribution to the field of cosmology and particle physics by linking cosmic microwave background observations to tangible constraints on dark matter mass. These findings provide a crucial step towards narrowing down the broad spectrum of dark matter candidate theories and emphasize the intricate interplay between cosmological data and particle physics.