||The quality factor of niobium
superconducting cavities shows a strong degradation (Q-slope) for
accelerating fields above 20 MV/m. These non quadratic losses are not
linked to a field emission because no electrons or X-rays are detected.
A simple in-situ bake-out around 120 C induces modifications of
the surface superconducting parameters and the Q-slope removal. The
Q-slope improvement seems more easy to achieve if the cavity is
electro-polished (EP) rather than etched by using a standard buffered
chemical polishing (BCP). On the basis of these experimental
observations, different models have been developed and many experiments
carried out to explain the Q-slope origin. We will try to make a
overall analysis of theories and experiments to make allowances and to
reduce the field of research.