Particle physics was a branch of physics dealing with subatomic particles and their interactions.
Until the USS Enterprise's 2259 discovery of the Kiley using warp in the form of a warp bomb, Science Officer Spock stated that not once in the entire history of first contact had warp been first developed as anything but a drive. Spock compared it to inventing the nuclear bomb before particle physics; that it could happen but it simply never had. (SNW: "Strange New Worlds")
When Kes began to evolve in 2374, her body had initially gone into a state of cellular flux for a period of 17.4 seconds, and she began to destabilize at the subatomic level. Captain Kathryn Janeway approached The Doctor informing him that they needed to find a way to suppress it, and to further broaden their horizons in controlling it, stating that "this appears to have gone beyond medical science" and that "we're into particle physics now." The Doctor suggested that he would look through the quantum substructure database to see if he could find some inspiration. (VOY: "The Gift")
The following year, the 29th century Borg drone named One had assimilated 47 billion teraquads of information from USS Voyager's library computer on a vast variety of subjects, including particle physics, comparative humanoid anatomy, warp field theory, and the culinary delights of the Delta Quadrant. (VOY: "Drone")