Particle

Harvey had a tattoo of Schrödinger equation for the wave function of the particle. (AUDIO: The Big Blue Book)

Types of particle
The Higgs particle was, according to Jack Harkness, the "fundamental particle of existence". It was the particle that protons and other subatomic matter were made out of. After years of research, in 2009, scientists at CERN discovered the particle as a side effect of colliding neutron eaters' gateway with protons and antiprotons. (AUDIO: Lost Souls)

Retro-genitor particles were a type of radiation that could stop Time Lords from regenerating. (AUDIO: X and the Daleks) They could prevent an individual from travelling in time without a time capsule. (AUDIO: The White Room) Close proximity to someone who were infused with retro-genitor particles would cause their eyes to turn dark periodically. (AUDIO: Eyes of the Master)

The death particle was a single particle created by the Cyberium that, when activated, was capable of destroying all organic life on at least one planet. (TV: The Timeless Children)