Pseudo-Goldstone dark matter: gravitational waves and direct-detection blind spots

Abstract: Pseudo-Goldstone dark matter is a thermal relic with momentum-suppressed direct-detection cross section. We study the most general model of pseudo-Goldstone dark matter arising from the complex-singlet extension of the Standard Model. The new U(1) symmetry of the model is explicitly broken down to a CP-like symmetry stabilising dark matter. We study the interplay of direct-detection constraints with the strength of cosmic phase transitions and possible gravitational-wave signals. While large U(1)-breaking interactions can generate a large direct-detection cross section, there are blind spots where the cross section is suppressed. We find that sizeable cubic couplings can give rise to a first-order phase transition in the early universe. We show that there exist regions of the parameter space where the resulting gravitational-wave signal can be detected in future by the proposed Big Bang Observer detector.