The redesigned ATLAS Forward Proton (AFP) spectrometer sits on either side of the main ATLAS cavern, just over 200 metres downstream from the collision point. Its detectors are based on high-resolution tracking 3D pixel silicon technology and high-precision Time-of-Flight (ToF) quartz-Cherenkov detectors, which reach directly into the LHC beam pipe to only two millimetres from the proton beam itself.
In many LHC collisions, the colliding protons are not destroyed, but remain intact, still emitting some energy that will produce particles in the central ATLAS detector. The two protons will have lost only a few percent of their energy in the collision, and will be deflected by the LHC magnet more than the other non-interacting ones in the beam. They will be measured by the AFP spectrometer, giving the full information of the collision together with the measurements from the central detector.