Refined Bounds on MeV-scale Thermal Dark Sectors from BBN and the CMB (1910.01649v2)

Abstract: New light states thermally coupled to the Standard Model plasma alter the expansion history of the Universe and impact the synthesis of the primordial light elements. In this work, we carry out an exhaustive and precise analysis of the implications of MeV-scale BSM particles in Big Bang Nucleosynthesis (BBN) and for Cosmic Microwave Background (CMB) observations. We find that, BBN observations set a lower bound on the thermal dark matter mass of $m_\chi > 0.4\,\text{MeV}$ at $2\sigma$. This bound is independent of the spin and number of internal degrees of freedom of the particle, of the annihilation being s-wave or p-wave, and of the annihilation final state. Furthermore, we show that current BBN plus CMB observations constrain purely electrophilic and neutrinophilic BSM species to have a mass, $m_\chi > 3.7\,\text{MeV}$ at $2\sigma$. We explore the reach of future BBN measurements and show that upcoming CMB missions should improve the bounds on light BSM thermal states to $m_\chi > (10-15)\,\text{MeV}$. Finally, we demonstrate that very light BSM species thermally coupled to the SM plasma are highly disfavoured by current cosmological observations.

Insights into MeV-scale Thermal Dark Sectors: Constraints from BBN and CMB

The cosmological implications of light, thermally coupled particles to the Standard Model (SM) plasma during the early Universe represent a critical area of research in understanding the expansion history and synthesis of primordial elements. This paper presents a detailed examination of MeV-scale thermal dark sectors and their effects as inferred from Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) observations. Highlighting precise methodologies, the authors derive stringent constraints on new physics beyond the Standard Model (BSM), addressing stable and unstable particles that were in thermal equilibrium in the early Universe.

Key Findings and Constraints

The paper leverages a modified version of the state-of-the-art PRIMAT code for nucleosynthesis predictions, incorporating up-to-date reaction rates and cosmological parameters. Through careful modeling, it examines how different particles with masses in the MeV range affect key observables like the primordial helium abundance ($Y_{\rm P}$), deuterium-to-hydrogen ratio (${\rm D/H}|_{\rm P}$), and $N_{\rm eff}$, the effective number of relativistic species during recombination.

The paper finds that:

1. BBN Implications: For particles in thermal contact with neutrinos or electromagnetically, the analysis reveals that BBN constraints are sensitive to the particle mass due to their effect on the expansion rate and entropy release. Specifically, the authors establish a lower bound of $m_\chi > 0.4$ MeV on thermal dark matter mass at 95.4% CL, independent of annihilation channels.
2. Neutrinophilic Constraints: Stronger bounds are placed on neutrinophilic BSM species—at $m_\chi > 3.7$ MeV—illustrating their significant role in modifying $N_{\rm eff}$ and thus the CMB. These particles impact the neutrino-to-photon temperature ratio, affecting the primordial soup evolution.
3. Electrophilic Constraints: Electrophilic particles exhibit slightly relaxed constraints at $m_\chi > 2.4$ MeV under current observations, highlighting distinct sensitivity differences arising from their coupling to different SM components.
4. Combined Observations: Integrating Planck CMB datasets fortifies these constraints further, linking tightly with $N_{\rm eff}$ measurements.

Implications for Theoretical Models

The constraints mapped out in this paper have far-reaching implications for a variety of theoretical models:

• Dark Matter Candidates: The analysis applies robust constraints on thermal relic scenarios, impacting models such as light annihilating dark matter with neutrino or electron interactions. The bounds are critical for frameworks focusing on neutrino portal dark matter or dark photon models where mediator masses also lie in the MeV range.
• BSM Particle Interactions: For unstable particles and scenarios involving light dark sectors, these bounds ensure that the implications of annihilation rates into SM particles are accounted for comprehensively.
• CMB Probes: Future CMB experiments, including those planned by the Simons Observatory and CMB-S4, are expected to refine these bounds significantly, offering finer resolution into potential sub-eV variations in $N_{\rm eff}$ that might indicate slight, yet pivotal deviations from the expected particle content during recombination.

Future Prospects

The research underscores the capability of cosmological tools to investigate light BSM scenarios, which are challenging to probe via terrestrial experiments. As observational precision improves, combining data from upcoming CMB missions with increasingly accurate primordial abundance measurements promises further constrainment on the parameter space of MeV-scale dark sectors. These advancements will be instrumental to discover or rule out regions of parameter space for light dark matter candidates, influencing both theoretical investigations and future experimental searches in astroparticle physics.

In conclusion, this paper advances our understanding of the early Universe's inventory, providing a rigorous framework to limit or discover new particle physics phenomena at low energy scales relevant during the epoch of nucleosynthesis. The constraints derived offer essential guidance for ongoing theoretical model developments and experimental validations in high energy physics and cosmology.