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arXiv · 2510.02438

A Limit on the Total Lepton Number in the Universe from BBN and the CMB

Valerie Domcke·Miguel Escudero·Mario Fernandez Navarro·Stefan Sandner
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Abstract

At temperatures below the QCD phase transition, any substantial lepton number in the Universe can only be present within the neutrino sector. In this work, we systematically explore the impact of a non-vanishing lepton number on Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB). Relying on our recently developed framework based on momentum averaged quantum kinetic equations for the neutrino density matrix, we solve the full BBN reaction network to obtain the abundances of primordial elements. We find that the maximal primordial total lepton number $L$ allowed by BBN and the CMB is $-0.12 \,(-0.10) \leq L \leq 0.13\,(0.12) $ for NH (IH), while specific flavor directions can be even more constrained. This bound is complementary to the limits obtained from avoiding baryon overproduction through sphaleron processes at the electroweak phase transition since, although numerically weaker, it applies at lower temperatures and is obtained completely independently. We publicly release the C++ code COFLASY-C on GitHub which solves for the evolution of the neutrino quantum kinetic equations numerically.

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Valerie Domcke, Miguel Escudero, Mario Fernandez Navarro, Stefan Sandner. 2025-12-05. A Limit on the Total Lepton Number in the Universe from BBN and the CMB. https://arxiv.org/abs/2510.02438

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