Neutrino decoupling beyond the Standard Model: CMB constraints on the Dark Matter mass with a fast and precise $N_{\rm eff}$ evaluation (1812.05605v4)

Abstract: The number of effective relativistic neutrino species represents a fundamental probe of the thermal history of the early Universe, and as such of the Standard Model of Particle Physics. Traditional approaches to the neutrino decoupling are either very technical and computationally expensive, or assume that neutrinos decouple instantaneously. In this work, we aim to fill the gap between these two approaches by modeling the neutrino decoupling in terms of two simple coupled differential equations for the electromagnetic and neutrino sector temperatures, in which all the relevant interactions are taken into account and which allows for a straightforward implementation of BSM species. Upon including finite temperature QED corrections we reach an accuracy on $N_{\rm eff}$ in the SM of $0.01$. We illustrate the usefulness of this approach to the neutrino decoupling by considering, in a model independent manner, the impact of MeV thermal dark matter on $N_{\rm eff}$. We show that Planck rules out electrophilic and neutrinophilic thermal dark matter particles of $m< 3.0\,\text{MeV}$ at 95\% CL regardless of their spin, and of their annihilation being $s$-wave or $p$-wave. We point out that thermal dark matter particles with non-negligible interactions with both electrons and neutrinos are more elusive to CMB observations than purely electrophilic or neutrinophilic ones. In addition, assisted by the accuracy of our approach, we show that CMB Stage-IV experiments will generically test thermal dark matter particles with $m \lesssim 15\,\text{MeV}$. We make publicly available the codes developed for this study at https://github.com/MiguelEA/nudec_BSM .

Neutrino Decoupling Beyond the Standard Model and CMB Constraints on Dark Matter Mass

This paper by Miguel Escudero addresses a pressing question in cosmology and particle physics: the effective number of relativistic neutrino species, $N_{\text{eff}}$. This parameter serves as a critical probe into the thermal history of the early universe and offers insights into both the Standard Model (SM) of particle physics and potential Beyond Standard Model (BSM) extensions.

Methodology

Escudero proposes an innovative approach to modeling neutrino decoupling, situated between traditional methods that are either too technical and compute-intensive or rely on the oversimplification of instantaneous decoupling of neutrinos. The proposed method uses two coupled differential equations to simulate the temperatures of the electromagnetic and neutrino sectors, accurately incorporating all pertinent interactions.

This approach achieves an impressive $N_{\text{eff}}$ accuracy of 0.01 within the Standard Model, aligning closely with state-of-the-art predictions but with significantly reduced computational demands. Importantly, this framework also allows for straightforward integration of BSM particles, which could alter neutrino decoupling dynamics, affecting $N_{\text{eff}}$.

Key Findings

1. Constraints on Dark Matter (DM): This work utilizes Cosmic Microwave Background (CMB) observations from the Planck satellite to explore the influence of MeV-scale thermal dark matter, characterizing them as either electrophilic or neutrinophilic. The paper places stringent mass bounds on these particles, with Planck data excluding such particles with masses below 3.0 MeV at a 95% confidence level, irrespective of their spin or annihilation characteristics (s-wave or p-wave).
2. Future Sensitivity: The paper anticipates that upcoming CMB Stage-IV experiments will test DM masses up to around 15 MeV, significantly extending the empirical reach and potentially providing invaluable empirical data on the presence and properties of dark matter.
3. Oscillation and Interaction Effects: The treatment of neutrino decoupling considers significant phenomena such as neutrino oscillations within the cosmological context, where active neutrino oscillations are expected at temperatures ranging from 3 to 5 MeV. This inclusion underscores the robust nature of the proposed model and its potential adaptability to varying cosmological conditions and BSM physics.
4. Comparison with Previous Methods: Traditional treatments, which assume instant neutrino decoupling and rely on entropy conservation, seem to deviate from the results obtained with this nuanced differential method by up to 0.2 in $N_{\text{eff}}$. While current analysis suggests this variance does not considerably affect present measurements, it harbors the potential for substantial implications with the enhanced precision of future CMB data.

Implications and Future Directions

The results of this paper impose constraints on existing DM models, particularly those involving interactions exclusive to or simultaneously affecting electrons and neutrinos. These findings delineate the boundary conditions for the formulation of DM particles in BSM theories.

The significance of this work extends to its methodological framework, offering a simplified yet high-fidelity model for calculating $N_{\text{eff}}$. Given the intricate demands of accurately simulating neutrino decoupling, this method could become an invaluable tool for theorists exploring the implications of BSM scenarios.

Moving forward, integration of this model with numerical tools like modern Big Bang Nucleosynthesis (BBN) codes offers the opportunity for further refining predictions translated into measurable cosmic phenomena. Additionally, extending these methodologies to scenarios involving mixed interaction channels could deliver enhanced constraints, facilitating the exploration of more complex BSM theories and the continued search for dark components of the cosmos. This embodies a meaningful leap toward unraveling the intertwined narratives of neutrinos, dark matter, and the universe's evolution.