LHC portal

The protons are collected into bunches which travel around the machine together. They go around 11.245 times a second. If there is 1 bunch in each beam they would cross each other at 2 locations in the machine. If there are 4 in each beam the number of crossing points is 8. We arrange it so that 4 of these crossing points occur in the experiments. The other 4 occur in areas where the 2 beams are kept apart.

Each time the two bunches meet there is a chance that some protons from one beam will come close enough to protons from the other beam to collide and interact. The rate at which such collisions occurs is therefore dependent on the number of protons in each beam and the physical dimensions of the beam where they meet. Most times a given proton will not meet another in the opposite direction and will just carry on circulating.

At the state we are at the moment with very low intensities, few bunches and relatively large beam sizes at the crossing points the chances are pretty low. The rate of interactions will probably be less than one per second of which a small fraction will be useful ‘events’ (most are just close encounters). When we approach nominal performance the rate will go up from a Hz to over 500MHz.

The beams can keep circulating while producing collisions for several hours. Slowly protons are lost from the beam and the rate of interactions will fall. Eventually it is not worth keeping the beams any more and we will dump and refill. For these initial physics runs at 450GeV I would expect runs of around 4 hours (depending on the beam lifetime).