报告题目：Pseudo-Nambu-Goldstone Dark Matter, Two Higgs Doublets, and Gravitational Waves
报告摘要：We study a dark matter model with one singlet complex scalar and two Higgs doublets. The scalar potential respects a softly broken global symmetry, which makes the imaginary part of the singlet become a pseudo-Nambu-Goldstone boson acting as a dark matter candidate. The pseudo-Nambu-Goldstone nature of the boson leads to the vanishing of its tree-level scattering amplitude off nucleons at zero momentum transfer. Therefore, although the interaction strength could be sufficiently large to yield a viable relic abundance via thermal mechanism, direct detection is incapable of probing this candidate. We further investigate the constraints from Higgs measurements, relic abundance observation, indirect detection, and B-meson decays of flavor-changing neutral currents. Moreover, three scalar fields in the model obtain vacuum expectation values, related to phase transitions at the early Universe. We find that some parameter points can cause strong first-order phase transitions, resulting in stochastic gravitational waves that could be detected in future space-based interferometer experiments like LISA, Taiji, and TianQin.