The Collaborative International Dictionary of English v.0.48:

Particle accelerator \Par"ti*cle ac*cel"er*a*tor\, n. (Physics)
   A large and expensive scientific instrument used by
   physicists to accelerate elementary particles (such as
   protons or electrons) to speeds near that of light, for the
   purpose of investigating the fundamental properties of
   matter; sometimes also called an atom smasher, since the
   particles thus accelerated are often directed at targets of
   atoms which are fragmented by the impact into their more
   fundamental component particles.

   Note: The particles generated by impact of a beam in an
         accelerator on its target are detected by various types
         of detecting apparatus, and procedures are required to
         sort and identify the many particles created. The
         fundamental particles generated by impacts in a
         particle accelerator are often those not actually
         present inside atoms; and in certain types of particle
         accelerator, such as the colliding beam accelerator,
         the impact which generates energetic particles is with
         other fundamental particles, and not with atoms.
         [PJC]

WordNet (r) 3.0 (2006):

particle accelerator
    n 1: a scientific instrument that increases the kinetic energy
         of charged particles [syn: accelerator, particle
         accelerator, atom smasher]