occur between the two counter-rotating beams of the LHC. At each end of the detector magnets focus the beams into the interaction point. At collision each beam has a radius of 17 um and the crossing angle between the beams is 285 urad.

At full design luminosity each of the two LHC beams will contain 2,808 bunches of protons. The interval between crossings is 25 ns, although the number of collisions per second is only