The LHC proton beams pass through the full length of the detector, safely encased within a beryllium pipe. The only point where the beams collide, and particles containing b and anti-b quarks are produced, is inside the VELO sub-detector.
The VELO’s job is to pick out B mesons from the multitude of other particles produced – a tricky task as their short lives will be spent close to the beam. To find them, the VELO’s 42 silicon detector elements must be positioned perilously close to the point where protons collide, at a distance of just five millimetres.
To prevent damage from the LHC proton beams, which both possess the energy of a high speed train, the VELO employs a novel design feature. Its sensitive detector elements are held out of harm’s way while the beams are being injected and stabilized, but once safe, the silicon elements are moved mechanically in towards the beam to hunt for B particles.
The VELO measures the distance between the point where protons collide (and where B particles are created) and the point where the B particles decay, spraying out other particles that VELO can detect. The B particles are therefore never measured directly – their presence is inferred from the separation between these two positions. Nevertheless, the VELO can locate the position of B particles to within 10 microns (100th of a millimetre).