Information Being A Form Of Matter: The backstory

Scientists want to use an experiment to confirm that elementary particles have measurable mass. It would involve a matter-antimatter annihilation process that would shoot a beam of positrons at electrons in a piece of metal. Positrons and electrons are both subatomic particles, with the same mass and magnitude of charge.

However, positrons are positively charged, and electrons are negatively charged. A sheet of metal has many free electrons, increasing the probability of collision with the incoming positrons.