I believe they use magnetic field containment.
Yes, magnetic containment to form the beams. But inertial containment for the actual collision reactions.
That is, the mass of the particles prevents them getting out of each other’s way for a femtosecond or so, then they all go blooey and the detectors pick up the fragments to see what happened.
The momenta of many of the charged fragments from this type of experiment make it impossible to contain them in the available magnetic field.