The Collaborative International Dictionary of English v.0.48:

Particle physics \Par"ti*cle phys`ics\, n.
   That branch of physics which investigates the nature of
   matter, and in particular the properties and behavior of the
   elementary particles (fundamental particles), of which matter
   is composed. Included in this field is the more specialized
   branch of high-energy physics.
   [PJC]

WordNet (r) 3.0 (2006):

particle physics
    n 1: the branch of physics that studies subatomic particles and
         their interactions [syn: particle physics, high-energy
         physics, high energy physics]