In a way, antimatter is the flip side of matter. Each subatomic particle has an antiparticle with the same mass but the opposite charge. For instance, the electron's antiparticle is the positron, which is just like an electron except that it has a positive charge. When a particle collides with its antiparticle, they annihilate one other, zapping out of existence and leaving only a burst of energy in their place.
If the big bang created equal amounts of matter and antimatter, they should have annihilated each other, leaving neither behind. Yet the observable universe has about ten billion galaxies full of matter and zero antimatter. Something must have happened very soon after the big bang to give matter an edge.
The BaBar experiment at SLAC seeks to explain this puzzling imbalance by probing whether the laws of nature are the same for matter and antimatter. Physicists use the term charge parity, or CP, to talk about matter-antimatter symmetry. If nature treated matter and antimatter alike, then, in physics-speak, nature would be CP-symmetric. If not, CP is violated.
Experiments have shown that nature's weak force—which is responsible for the decay of particles—does in fact violate CP. Yet the weak force by itself can explain only a small part of CP violation—so little that it would not leave enough matter for even a single galaxy. Some other hidden force, not accounted for in our Standard Model of particles and forces, must have caused the extra CP violation that led to the universe we observe.
Current and future particle accelerator experiments are designed to search for sources of CP violation large enough to account for all of the matter in the universe.
