CERN’s recent tweets have been cramming as much excitement as you can squeeze into 140 characters about the increasing number of bunches in the LHC beams, culminating with the record intensity for a hadron collider that was set with 480 bunches per beam last night. Time, then, to explain what that’s all about.
A beam in the LHC is not a continuous string of particles, but is divided into chunks a few centimetres long squeezed down to the size of a human hair at the collision point. Elsewhere in the ring the beam size varies but is normally less than a millimetre.
These chunks are what we call bunches. Each bunch contains about a hundred billion protons, and it’s a measure of just how small protons are that if you were to scale each one up to the size of a marble, the bunch length would be as far as the distance from Earth to Uranus and the width of the bunch would be something like the distance between the Earth and the Moon. Neighbouring marble sized protons would be as far apart as Geneva and Hamburg. So it’s not surprising that when bunches collide in the LHC, only a handful of proton-proton collisions happen.
Discovery in particle physics is a statistical process, so increasing the number of bunches is important. It increases the number of collisions, or the statistics as physicists put it. The LHC is designed to run with 2808 bunches per beam, separated by a gap of just 25 nanoseconds. Since this is still early days in LHC running, we’re still at relatively low numbers and the bunch spacing is 50 nanoseconds. Nevertheless, building the number of bunches steadily this year towards last night’s record-breaking 480 per beam and beyond means that LHC experiments have already collected far more data so far this year that they collected in all of 2010.
Increasing the number of bunches in the beam is a stepwise process, since although each proton only has the energy of a mosquito in flight, by the time you multiply that by hundreds of billions, you have a large amount of energy stored in the beams. The operators need to be sure that the systems designed to protect the machine from damage are all ready before increasing the number of bunches. When the LHC reaches its full design potential, the beams will carry the energy of a 20,000 tonne aircraft carrier travelling at 12 knots. With 480 bunches per beam at half the LHC’s design energy, the energy stored in the beams equates roughly to the same aircraft carrier travelling at the rather more sedate pace of a little under 3 knots. Not quite so impressive, but a significant amount of energy nevertheless.
James Gillies and Mike Lamont