Nelson-Barr ultralight dark matter - Wolfram Ratzinger (Weizmann Inst.)

Abstract: Light scalar fields are well motivated dark matter candidates, as they commonly arise as pseudo-Goldstone bosons and their abundance is set by the misalignment mechanism. The best known example is perhaps the celebrated QCD axion, further motivated as a solution to the strong CP-problem. I will discuss how in the orthogonal Nelson-Bar solution to the strong CP-problem, a naturally light scalar can arise in a similar way. The phenomenology is however completely new, if this field constitutes dark matter, as the CKM elements vary periodically in time. The model can further be tested using quantum sensors, which leads to an interesting synergy between different frontiers of physics. The most promising among those sensors are nuclear clocks, which will allow us to probe enormous parameter ranges of this and similar models in the near future. As I will show, even before the first nuclear clock becomes operational, we will be able to cover a host of new physics scenarios from the precise measurement of the transition frequency required to build the clock.