When particle accelerators collide protons or lead ions, they do so within special sections of an instrument that can capture fine details. These huge detectors are like giant digital cameras that take photographs of the outcome of each collision. (Researchers keep only a subset of these photographs, but that is another story.) These pictures record information about each outgoing particle produced in the collision. Astronomy magazine subscribers can read the full answer for free. Just make sure you're registered with the website.