This physics today article about particle accelerator operation contains this figure, showing the evolution of peak luminosity over time for major proton-proton and proton-antiproton colliders. Notably they all increase monotonically over time, except for RHIC which shows a pronounced dip. Is there an identifiable cause for this?
In addition to proton beams, RHIC collides lots of different ion configurations at different energies.
The proton luminosity didn’t really go down:
(From this BNL RHIC overview page)
When running ions, the peak luminosity depends on species (really, on charge to mass ratio) and energy.
(You’re comparing the slopes in that last image)
So the apparent drop was primarily due to using the machine with new beam types to extend its physics life.