Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A particle beam is an accelerated stream of charged particles or neutrons (often moving at very near the speed of light) which may be directed by magnets and focused by electrostatic lenses, although they may also be self-focusing (see Pinch). Subatomic particles such as electrons, positrons, and protons can be accelerated to high velocities and energies, usually expressed in terms of center-of-mass energy, by machines that impart energy to the particles in small stages or nudges, ultimately achieving very high energy particle beams, measured in terms of billions and even trillions of electron volts. Thus, in terms of their scale, particles can be made to perform as powerful missiles for bombarding other particles in a target substance or for colliding with each other as they assume intersecting orbits.