Accelerators used to collide electrons (sometimes including positrons) because the resulting collisions involved no quark collisions; electrons are not baryons like protons, and accordingly contain no quarks. This simplifies interpretation of the results, because the only particles that appear in the collisions are those that condensed out of the momentum energy of the colliding particles.

Nowadays such colliders have more or less yielded most of what people think they could learn from them, so new machines are colliding hadrons. They give more complex results that demand huge computational loads, but permit different kinds of investigations.