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Why do proton-proton colliders need separate beam lines for the two proton beams?

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Diagram of a linear proton resonance accelerator, or linac, showing protons entering from a source, passing through a series of labeled drift tubes powered by a radio-frequency source, and exiting as a beam through a cavity. — particle accelerator The accelerator is a large-diameter tube within which an electric field oscillates at a high radio frequency. Within the accelerator tube are smaller diameter metallic drift tubes, which are carefully sized and spaced to shield the protons from decelerating oscillations of the electric field. In the spaces between the drift tubes, the electric field is oriented properly to accelerate the protons in their direction of travel.

Proton-proton colliders require separate beam lines because the two beams consist of identical particles with the same positive electric charge.1

In a particle accelerator, magnetic fields are used to guide particles along their orbits. Because protons carry the same charge, they must be steered in opposite directions to collide; however, a single magnetic field cannot guide two beams of identical particles in opposite directions simultaneously. Consequently, these machines must utilize two separate rings of magnets—or, in the case of the Large Hadron Collider, two separate beam pipes within a single structure—to guide the beams clockwise and counterclockwise so they can intersect at designated interaction regions.12

This setup differs from colliders that use particles of opposite charge, such as protons and antiprotons. Because those particles have opposite charges, they can be guided in opposite directions while circulating within the same vacuum chamber and the same magnetic field.12