One of the unsolved theoretical questions in physics is why the universe is made chiefly of matter, rather than consisting of equal parts of matter and antimatter. It can be demonstrated that to create an imbalance in matter and antimatter from an initial condition of balance, the Sakharov conditions must be satisfied, one of which is the existence of CP violation during the extreme conditions of the first seconds after the Big Bang. Explanations which do not involve CP violation are less plausible, since they rely on the assumption that the matter-antimatter imbalance was present at the beginning, or on other admittedly exotic assumptions.

The Big Bang should have produced equal amounts of matter and anti-matter if CP-symmetry was preserved; as such, there should have been total cancellation of both. In other words, protons should have cancelled with anti-protons, electrons with positrons, neutrons with anti-neutrons, and so on for all elementary particles. This would have resulted in a sea of photons in the universe with no matter. Since this is quite evidently not the case, after the Big Bang, physical laws must have acted differently for matter and antimatter, i.e. violating CP symmetry.

The Standard Model contains only two ways to break CP symmetry. The first of these, discussed above, is in the QCD Lagrangian, and has not been found experimentally; but one would expect this to lead to either no CP violation or a CP violation that is many, many orders of magnitude too large. The second of these, involving the weak force, has been experimentally verified, but can account for only a small portion of CP-violation. It is predicted to be sufficient for a net mass of normal matter equivalent to only a single galaxy in the known universe.

Since the Standard Model does not accurately predict this discrepancy, it would seem that the current Standard Model has gaps (other than the obvious one of gravity and related matters) or physics is otherwise in error. Moreover, experiments to probe these CP-related gaps may not require the practically impossible-to-obtain energies that may be necessary to probe the gravity-related gaps (see Planck mass).

DIRECT CP VIOLATION has been observed at Fermilab by the KTeV collaboration. An important way of apprehending the basic nature of time and space (in the finest tradition of Greek philosophy) is to ask "what if" questions. For example, will a collision between particles be altered if we view the whole thing in a mirror? Or what if we turn all the particles into antiparticles? These propositions, called respectively parity (P) and charge conjugation (C) conservation, are upheld by all the forces of nature except the weak nuclear force. And even the weak force usually conserves the compound proposition of CP. In only one small corner of physics---the decay of K mesons---has CP violation been observed, although physicists suspect that CP violation must somehow operate on a large scale since it undoubtedly helped bring about the present-day preponderance of matter over antimatter.

K mesons (kaons) are unstable and do not exist outside the interiors of neutron stars and particle accelerators, where they are artificially spawned in K-antiK pairs amidst high energy collisions. K's might be born courtesy of the strong nuclear force, but the rest of their short lives are under control of the weak force, which compels a sort of split personality: neither the K nor anti-K leads a life of its own. Instead each transforms repeatedly into the other. A more practical way of viewing the matter is to suppose that the K and anti-K are each a combination of two other particles, a short lived entity called K1 which usually decays to two pions (giving K1 a CP value of +1) and a longer-lived entity, K2, which decays into three pions (giving K2 a CP value of -1). This bit of bookkeeping enshrined the idea then current that CP is conserved.

All of this was overthrown when in 1964 the experiment of Jim Cronin and Val Fitch showed that a small fraction of the time (about one case in every 500, a fraction called epsilon) the K2 turns into a K1, which subsequently decays into two pions. This form of CP violation is said to be indirect since the violation occurs in the way that K's mix with each other and not in the way that K's decay. One theoretical response was to say that this lone CP indiscretion was not the work of the weak force but of some other novel "superweak" force. Most theorists came to believe, however, that the weak force was responsible and, moreover, that CP violation should manifest itself directly in the decay of K2 into two pions. The strength of this direct CP violation, characterized by the parameter epsilon prime, would be far weaker than the indirect version. For twenty years detecting a nonzero value of epsilon prime has been the object of large-scale experiments at Fermilab and for nearly as long at CERN. In each case, beams of K's are sent down long pipes in which the K-decay pions could be culled in sensitive detectors.