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<p>The primordial neutrino asymmetry leaves profound imprints on the evolution history of the universe, which can be constrained by cosmological observations, including Big Bang Nucleosynthesis (BBN), Cosmic Microwave Background (CMB), and Large-Scale Structure (LSS). We present comprehensive analysis on implications and constraints of the primordial neutrino asymmetry, based on a precise treatment of neutrino decoupling by solving the complete (anti)neutrino quantum kinetic equations in the Closed-Time-Path formalism. Assuming the same primordial asymmetry <i>ξ</i><sub><i>ν</i></sub> for all neutrino flavors, the effective number of neutrinos <i>N</i><sub>eff</sub> and (anti)neutrino spectral distortions are calculated, and we find that the non-instantaneous decoupling correction is given by <i>δN</i><sub>eff</sub> = 0.0440 + 0.0102<InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13130_2025_26529_Article_IEq1.gif" Format="GIF" Height="20" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="MATHML"><math display="inline"> <msubsup> <mi>ξ</mi> <mi>ν</mi> <mn>2</mn> </msubsup> </math></EquationSource> <EquationSource Format="TEX">\( {\xi}_{\nu}^2 \)</EquationSource> </InlineEquation>. Then we perform the state-of-the-art calculation for the abundance of light elements including (anti)neutrino spectral distortions, which indicate a positive asymmetry 0.032 ≤ <i>ξ</i><sub><i>ν</i></sub> ≤ 0.052 from EMPRESS data. The implications of the neutrino asymmetry for the CMB and LSS are studied in detail, and we find that the Baryon Acoustic Oscillations (BAO) are also significantly affected by <i>ξ</i><sub><i>ν</i></sub> in addition to the sum of neutrino masses. A combined analysis with EMPRESS BBN, Planck CMB and BOSS BAO data yields a tighter constraint <i>ξ</i><sub><i>ν</i></sub> = 0.024 ± 0.012, which provides constraints on UV models capable of producing large asymmetries.</p>

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Primordial lepton asymmetries: neutrino transport, spectral distortions and cosmological constraints

  • 期刊论文

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  • 发表日期：

    2025年6月20日

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  • Yuan-Zhen Li,
  • Jiang-Hao Yu

摘要

The primordial neutrino asymmetry leaves profound imprints on the evolution history of the universe, which can be constrained by cosmological observations, including Big Bang Nucleosynthesis (BBN), Cosmic Microwave Background (CMB), and Large-Scale Structure (LSS). We present comprehensive analysis on implications and constraints of the primordial neutrino asymmetry, based on a precise treatment of neutrino decoupling by solving the complete (anti)neutrino quantum kinetic equations in the Closed-Time-Path formalism. Assuming the same primordial asymmetry ξν for all neutrino flavors, the effective number of neutrinos Neff and (anti)neutrino spectral distortions are calculated, and we find that the non-instantaneous decoupling correction is given by δNeff = 0.0440 + 0.0102 ξ ν 2 \( {\xi}_{\nu}^2 \) . Then we perform the state-of-the-art calculation for the abundance of light elements including (anti)neutrino spectral distortions, which indicate a positive asymmetry 0.032 ≤ ξν ≤ 0.052 from EMPRESS data. The implications of the neutrino asymmetry for the CMB and LSS are studied in detail, and we find that the Baryon Acoustic Oscillations (BAO) are also significantly affected by ξν in addition to the sum of neutrino masses. A combined analysis with EMPRESS BBN, Planck CMB and BOSS BAO data yields a tighter constraint ξν = 0.024 ± 0.012, which provides constraints on UV models capable of producing large asymmetries.