超出BBN限制的重中性轻子：探测宇宙的轻子不对称性
Heavy neutral leptons beyond the BBN bound: probing the lepton asymmetry of the Universe
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中文总结 AI 辅助
本研究提出大轻子味不对称场景，使重中性轻子规避BBN限制，开辟新参数空间，SHiP或可探测，结合BBN与CMB可确定原初味不对称的剩余方向。
中文摘要 AI 辅助
寿命τ≥0.02秒的强子衰变粒子，几乎与其丰度无关，均被大爆炸核合成（BBN）排除：它们衰变产生的介子会使质子转化为中子的速度快于逆过程，进而导致氦元素过量产生。对于重中性轻子（HNLs），这一限制覆盖了未来加速器实验可探测的耦合范围。我们提出一种存在大单轻子味不对称的场景，其中HNLs可规避上述限制：狄拉克型HNL衰变产生π⁺，其反粒子衰变产生π⁻，两类粒子的不对称会注入过量π⁺，使中子恢复至标准丰度；同时，不对称与HNL衰变的抵消作用使Nₑff的偏移保持在较小水平。这开辟了mₗ+m_π≲m_N≲1GeV、τ_N≲1秒的参数空间，其中相当一部分区域处于SHiP的探测范围内。在SHiP发现该粒子后，结合BBN与宇宙微波背景（CMB）观测，可确定原初味不对称的剩余一个方向； relic中微子背景及可能的一阶宇宙QCD相变（引力波源）或可探测该方向，进而揭示宇宙的轻子不对称性。
英文摘要
Hadronically decaying particles with lifetimes $τ\gtrsim0.02\,{\rm s}$ are excluded by Big Bang Nucleosynthesis almost independently of their abundance: the mesons from their decays convert protons into neutrons faster than the reverse, and helium is overproduced. For heavy neutral leptons (HNLs), this blankets the couplings the upcoming accelerator searches can reach. We propose a scenario with large lepton flavor asymmetries in which HNLs evade it. A Dirac HNL decays into $π^+$ and its antiparticle into $π^-$, so an asymmetry between the two populations injects an excess of $π^+$, which converts the neutrons back to their standard abundance, while a cancellation between the asymmetries and the HNL decays keeps the shift in $N_{\rm eff}$ small. This opens the parameter space with $m_l+m_π\lesssim m_N\lesssim1\,{\rm GeV}$ and $τ_N\lesssim1\,{\rm s}$, including a substantial region within the reach of SHiP. A discovery there, combined with the BBN and CMB observables, would fix the primordial flavor asymmetries up to one remaining direction. The relic neutrino background and a possibly first-order cosmic QCD transition, a source of gravitational waves, may probe that direction, and with it the lepton asymmetry of the Universe.
