Self-Interacting Dark Matter Solves the Final Parsec Problem of Supermassive Black Hole Mergers.

Evidence for a stochastic gravitational wave (GW) background, plausibly originating from the merger of supermassive black holes (SMBHs), is accumulating with observations from pulsar timing arrays. An outstanding question is how inspiraling SMBHs get past the "final parsec" of separation, where they have a tendency to stall before GW emission alone can make the binary coalesce. We argue that dynamical friction from the dark matter (DM) spike surrounding the black holes is sufficient to resolve this puzzle, if the DM has a self-interaction cross section of order cm^{2}/g. The same effect leads to a softening of the GW spectrum at low frequencies as suggested by the current data. For collisionless cold DM, the friction deposits so much energy that the spike is disrupted and cannot bridge the final parsec, while for self-interacting DM, the isothermal core of the halo can act as a reservoir for the energy liberated from the SMBH orbits. A realistic velocity dependence, such as generated by the exchange of a massive mediator like a dark photon, is favored to give a good fit to the GW spectrum while providing a large enough core. A similar velocity dependence has been advocated for solving the small-scale structure problems of cold DM.

The first limit on invisible decays of Bs mesons comes from LEP.

Motivated by the recent evidence for B+→K+ν¯ν decays at Belle II, we point out that fully invisible Bd and Bs meson decays are strongly constrained by LEP. A reinterpretation of an old inclusive ALEPH search for b-hadron decays with large missing energy allows us to place the limits Br(Bd→invisible)<1.4×10-4 and Br(Bs→invisible)<5.6×10-4, both at 90% CL. The Bd limit is only a factor of 6 looser than the world-leading one provided by the BaBar collaboration, while the Bs one is the first limit in the literature on this decay mode. These results are relevant in the context of new light states coupled to quarks and exemplify the power of a future Tera-Z factory at FCC-ee to look for B meson decays containing missing energy.

New physics searches at kaon and hyperon factories.

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.