What is inside the detector in a particle accelerator?? We need E and B fields to steer and focus beams in a particle accelerator, but all the calculations we do in QFT assume (apart from the colliding particles) that we are in the vacuum state. Does this mean there are no fields at the point of interaction of two beams? i.e., are detector chambers designed to be vacuum sealed and do they shield out external E and B fields? How then do the colliding particles enter the chamber? Thanks.