Title: Characterizing Heavy Neutral Leptons: Measuring Parameters, Discriminating Majorana versus Dirac, and Using FASER2 as a Trigger for ATLAS Show Chinese title

Title: 表征重中性轻子：测量参数，区分马约拉纳与狄拉克型，以及使用FASER2作为ATLAS的触发器

Abstract: This work explores the potential of the proposed FASER2 experiment at the LHC to determine the properties of a discovered heavy neutral lepton (HNL), including its mass, couplings, and whether it is a Majorana or Dirac fermion. We first consider a Majorana HNL with mass $m_N = 1.84\,\rm{GeV}$ that is primarily produced through decays $D \to \mu N$ at the ATLAS interaction point. Such HNLs may travel macroscopic distances in the far-forward direction and then decay, yielding approximately 8600 $N \to \mu \pi$ decays in FASER2 at the High-Luminosity LHC. With FASER2 measurements alone, the HNL's mass and couplings can be measured to fractional uncertainties of approximately 0.1% and 3% at 95% CL, respectively, and the Dirac fermion hypothesis can be rejected at 99.8% CL. We then consider a second, more difficult, case of a Majorana HNL with mass $m_N = 2.00\,\rm{GeV}$, yielding only 80 $N \to \mu \pi$ decays in FASER2. With FASER2 alone, measurements of HNL properties are still possible, but somewhat less precise. However, by using FASER2 as a trigger for ATLAS and measuring the charge of the muon produced in association with the HNL at ATLAS to search for lepton number violation, one can precisely measure the HNL's properties and reject the Dirac fermion hypothesis at 99.7% CL. These results show that FASER2, sometimes in coordination with ATLAS, can precisely determine HNL properties, with far-reaching implications for our understanding of neutrino masses, baryogenesis, and the fundamental symmetries of nature. Show Chinese abstract

Abstract: 这项工作探讨了在大型强子对撞机（LHC）上提出的FASER2实验确定已发现的重中性轻子（HNL）特性（包括其质量、耦合以及它是马约拉纳费米子还是狄拉克费米子）的潜力。 我们首先考虑一种质量为$m_N = 1.84\,\rm{GeV}$的马约拉纳HNL，它主要通过ATLAS相互作用点的衰变$D \to \mu N$产生。 这种HNL可能在远前向方向上行进宏观距离，然后发生衰变，在高亮度LHC下的FASER2中产生大约8600个$N \to \mu \pi$衰变。 仅使用FASER2的测量，可以在95%置信水平下将HNL的质量和耦合分别测量到约0.1%和3%的分数不确定度，并且可以在99.8%置信水平下拒绝狄拉克费米子假设。 然后我们考虑第二种情况，即质量为$m_N = 2.00\,\rm{GeV}$的马约拉纳HNL，在FASER2中仅产生80个$N \to \mu \pi$衰变。 仅使用FASER2，仍然可以测量HNL的特性，但精度稍低。 然而，通过使用FASER2作为ATLAS的触发器，并在ATLAS上测量与HNL同时产生的μ子的电荷以寻找轻子数破坏，可以精确测量HNL的特性，并在99.7%置信水平下拒绝狄拉克费米子假设。 这些结果表明，FASER2有时与ATLAS协作，可以精确确定HNL的特性，这对理解中微子质量、重子生成以及自然的基本对称性具有深远的影响。