太阳对含$ν_τ$混合的重中性轻子的约束
Solar Constraints on Heavy Neutral Leptons with $ν_τ$ Mixing
AI总结：
本研究利用太阳中微子和SOHO数据，推导了MeV质量范围重中性轻子（HNL）与τ中微子混合的约束，较现有地面极限提升超一个数量级。
AI中文摘要：
重中性轻子（HNLs）与τ中微子的混合，相比其与电子中微子、μ中微子的混合，在很大程度上仍未受到约束。本研究探讨了利用太阳中微子来改善对MeV质量范围内HNL-ν_τ混合的约束的潜力。由于中微子振荡，太阳是所有味中微子的丰富来源，电子中微子在穿过太阳内部的过程中就会部分转化为μ中微子和τ中微子。若存在非零的HNL-ν_τ混合，τ中微子可通过与太阳内质子的散射产生HNLs；HNLs会逃逸太阳，并随后通过相同的相互作用衰变。对于质量为O(10) MeV的HNLs，其主导的可见衰变产物为电子和正电子，可被天基太阳观测站探测到。本研究利用太阳和日球层观测台（SOHO）的数据，搜寻此类信号，并推导了对HNL-ν_τ混合矩阵元平方|U_{τ N}|²的约束，在HNL质量约为5 MeV时，该约束低于10⁻²水平，较该质量范围内现有的地面极限提升了一个数量级以上。
英文摘要：
The mixing of heavy neutral leptons (HNLs) with tau neutrinos remains largely unconstrained compared to their mixing with electron and muon neutrinos. In this work, we investigate the potential of solar neutrinos to improve constraints on HNL-$ν_τ$ mixing in the MeV mass range. Due to neutrino oscillations, the Sun is a copious source of neutrinos of all flavors, and the partial conversion of electron neutrinos into muon and tau neutrinos occurs already during their propagation through the solar interior. Tau neutrinos can produce HNLs through scattering with protons in the Sun, provided nonzero HNL-$ν_τ$ mixing is present. The HNLs escape the Sun and subsequently decay through the same interaction. For $\mathcal{O}(10)$ MeV HNL masses, the dominant visible decay yields electrons and positrons that can be detected by space-based solar observatories. Using data from the Solar and Heliospheric Observatory (SOHO), we search for such signals and derive constraints on the squared HNL-$ν_τ$ mixing matrix element, $|U_{τN}|^2$, reaching below the $10^{-2}$ level for HNL masses of $\sim 5$ MeV. These constraints improve upon existing terrestrial limits in this mass range by more than an order of magnitude.
