Impact of non-standard neutrino-electron interactions on Big Bang Nucleosynthesis

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: We're looking at the paper: "Impact of non-standard neutrino-electron interactions on Big Bang Nucleosynthesis".

Jocelyn: Here's the gist of the paper. The gist Neutrino non-standard interactions (NSI) with electrons are studied to update previous analyses of neutrino decoupling and to provide a full study of their effect on light element production during Big Bang Nucleosynthesis (BBN).

Vera: Neutrino Non-Standard Interactions with Electrons Non-standard neutrino interactions arise in extensions of the Standard Model that generate neutrino masses and mixing

2: .

Jocelyn: For the range of cosmological temperatures during neutrino decoupling, the only abundant charged leptons are electrons and positrons, leading to a restriction to neutral-current NSI of neutrinos with electrons

4: .

Vera: The effective Lagrangian density describing these interactions is given by L = LSM + LNC−NSIe (2.1)

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Paper summary: Vera: So we started by setting up the framework for neutrino non-standard interactions with electrons, which are predicted in many extended theoretical models of particle physics. For those cosmological temperatures during neutrino decoupling, the only abundant charged leptons are electrons and positrons, which restricts us to neutral-current NSI of neutrinos with electrons.

Jocelyn: They give us this effective Lagrangian density describing these interactions as L = LSM + LNC-NSIe, where the SM contribution is described by that term involving e gamma mu mu P L e and gamma mu nu e.

Subrahmanyan: The NSI contribution is then LNC-NSIe = -two sqrt two G F epsilon X alpha beta (alpha gamma mu P L nu beta) (gamma mu P R e), where those dimensionless parameters epsilon X alpha beta quantify the strength of the non-standard interactions between neutrinos of flavours alpha and beta and electrons.

Vera: Those parameters can be flavour-changing when epsilon alpha beta is not zero for alpha different from beta, which means lepton flavour non-conserving interactions, or non-universal when epsilon alpha alpha - epsilon beta beta is not zero, which breaks lepton flavour universality.

Jocelyn: The paper then looks at the impact of these interactions on neutrino decoupling by modifying the thermal contact between neutrinos and the electromagnetic plasma. Specifically, NSI with electrons changes their momentum spectra, leading to a different value for N eff.

Subrahmanyan: They mention that tracking the NSI-induced modifications of these neutrino distributions is necessary to accurately predict the helium-four abundance because it cannot be derived from a pure N eff-driven change alone <ref:2602.23915#pg2>.

Vera: And they do provide results for N eff using a complete numerical computation with the code FortEPiaNO2, getting an error below five times ten to the negative four in the standard scenario.

Jocelyn: They also show that one-parameter studies indicate that the minimum of N eff occurs at specific values, like epsilon Lee = - L-zero point seven two seven for the left-handed electron sector, where the effective left-handed coupling to electrons vanishes.

Subrahmanyan: The paper then shows that by varying only one NSI parameter at a time, the predictions for primordial helium mass fraction Y p and deuterium abundance two H/H follow the qualitative behavior seen in Fig <ref:2602.23915#pg1>. one which is analogous to how N eff changes <ref:2602.23915#pg1>.

Vera: And they show that both those abundances largely follow the behavior expected from the NSI-induced changes in N eff, and regions of parameter space leading to a larger radiation density generally produce larger values for Y p and two H/H <ref:2602.23915#pg1>.

Conclusion: Jocelyn: So looking at the full picture of this paper, the authors explore the cosmological implications of neutrino non-standard interactions with electrons during neutrino decoupling and Big Bang Nucleosynthesis. They confirm that both primordial helium mass fraction Y p and deuterium abundance two H/H largely track the NSI-induced changes in N eff <ref:2602.23915#pg1>.

Subrahmanyan: That tracking is primarily driven by the modification of the radiation density, which generally produces larger values for both Y p and two H/H, consistent with the effect of a faster expansion rate during Big Bang Nucleosynthesis <ref:2602.23915#pg1>.

Vera: So what this means for us is that this work provides complementary constraints to laboratory limits by probing neutrino properties at different energy and time scales, which we can't access in the lab.

Jocelyn: It opens up a way to test these models using primordial abundances, and future improvements in the precision of those measurements could further enhance how sensitive these cosmological probes are to non-standard neutrino interactions.