[Paper Review] Flavor-specific Interaction Favours Strong Neutrino Self-coupling

Anirban Das, Subhajit Ghosh|arXiv (Cornell University)|Nov 26, 2020

Neutrino Physics Research6 citations

TL;DR

This paper proposes a flavor-specific neutrino self-interaction model in the three-active-neutrino framework, motivated by experimental constraints on secret interactions. It shows that allowing only one or two neutrino flavors to self-interact enhances the statistical significance of a strongly interacting neutrino mode—linked to late decoupling near matter-radiation equality—while preserving the coupling strength central value, offering a better fit to CMB data features.

ABSTRACT

Flavor universal neutrino self-interaction have been shown to ease the tension in the measurements of Hubble constant between the early and late Universe data. We introduce a self-interaction structure that is flavor-specific in the three active neutrino framework. This is motivated by stringent constraints on new secret interactions among electron and muon neutrinos from several laboratory experiments. Our study indicates the presence of a strongly interaction mode which implies a late-decoupling of the neutrinos just prior to matter radiation equality. Using the degeneracy of the coupling strength with other cosmological parameters, we explain the origin of this new mode as a result of better fit to certain features in the CMB data. We find that if only one or two of the three active neutrino flavors are interacting, then the statistical significance of the strongly-interacting neutrino mode increases substantially relative to the flavor universal case. However, the central value of the coupling strength for this interaction mode does not change by a appreciable amount in flavor-specific cases.

Motivation & Objective

• Address the Hubble tension between early and late Universe measurements using modified neutrino self-interactions.
• Overcome constraints from laboratory experiments on secret interactions between electron and muon neutrinos.
• Explore whether flavor-specific self-interaction structures can better fit cosmological data than flavor-universal models.
• Investigate the impact of partial neutrino flavor interaction on the statistical significance and coupling strength of a strongly interacting neutrino mode.

Proposed method

• Introduce a flavor-specific self-interaction Hamiltonian in the three-active-neutrino framework, breaking flavor universality.
• Apply constraints from laboratory experiments on electron and muon neutrino interactions to restrict viable interaction structures.
• Use cosmological parameter fitting to assess the statistical significance of the strongly interacting neutrino mode in the context of CMB data.
• Perform degeneracy analysis between the self-coupling strength and other cosmological parameters to isolate the origin of the new interaction mode.
• Compare the statistical significance of the interaction mode in flavor-universal versus flavor-specific scenarios using likelihood analysis.

Experimental results

Research questions

• RQ1 Can flavor-specific neutrino self-interactions improve the fit to CMB data compared to flavor-universal models?
• RQ2 What is the impact of restricting self-interaction to only one or two neutrino flavors on the statistical significance of a strongly interacting neutrino mode?
• RQ3 How does the coupling strength of the self-interaction mode change when flavor universality is broken?
• RQ4 What cosmological features in the CMB data drive the emergence of the strongly interacting neutrino mode in the flavor-specific model?
• RQ5 To what extent is the coupling strength degenerate with other cosmological parameters in the flavor-specific scenario?

Key findings

• Flavor-specific self-interaction models significantly increase the statistical significance of the strongly interacting neutrino mode compared to the flavor-universal case.
• The late-decoupling of neutrinos just before matter-radiation equality is strongly favored in the flavor-specific model, linked to the new interaction mode.
• The central value of the self-coupling strength remains largely unchanged in flavor-specific scenarios, indicating robustness of the coupling magnitude.
• The improved fit to CMB data features arises from the degeneracy of the coupling strength with other cosmological parameters, which is more effectively exploited in flavor-specific models.
• When only one or two neutrino flavors participate in self-interaction, the model achieves a better fit to observational data, particularly in resolving features near the matter-radiation equality epoch.