  Many modern and proposed future particle accelerators rely on superconducting
radio frequency cavities made of bulk niobium as primary particle accelerating
structures. Such cavities suffer from the anomalous field dependence of their
quality factors Q0. High field degradation - so-called high field Q-slope - is
yet unexplained even though an empirical cure is known. Here we propose a
mechanism based on the presence of proximity-coupled niobium hydrides, which
can explain this effect. Furthermore, the same mechanism can be present in any
surface-sensitive experiments or superconducting devices involving niobium.
