Detection of ultra-peripheral collisions between relavistic Pb ions at the Large Hadron Collider

In the Large Hadron Collider collisions between protons and between lead nuclei will occur in which the impact parameter will be greater than the sum of the radii of the colliding partners, ultra-peripheral collisions (UPC's). A novel detector (the ATLAS Zero Degree Calorimeter, ZDC) has been constructed to selectively trigger on such collisions. It detects extremely forward neutrons that are emitted from the projectile/target by photons via a dipole excitation. Cerenkov light, generated by showers in tungsten plates, is funneled to photomultiplier tubes via quartz fibers. The detector has excellent timing and energy resolution.