Proton

A proton was a particle. It was one of the components of an atom, along with electrons and neutrons. (PROSE: The Infinity Doctors)

According to Jack Harkness, the Higgs particle was the "fundamental particle of existence", of which protons and other subatomic matter were made. The Large Hadron Collider accidentally opened a gateway to the neutron eaters reality, which was only able to be closed by protons and antiprotons being fired at each other. As a side-effect, Higgs particles were created. (AUDIO: Lost Souls)