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.

At the APS Centennial meeting in Atlanta last week, both groups discussed their work. The KTeV group at Fermilab reported a definite result: a ratio of epsilon prime to epsilon equal to (28 +/- 4) x 10^-4, larger than the theoretical expectation. As for the NA48 group at CERN, Lydia Iconomidou-Fayard (lyfayard@in2p3.fr) said that data analysis was still proceeding and no definite measurement could be reported at this time. The principal conclusion was stated by KTeV co-spokesman Bruce Winstein (bruce@uchep.uchicago.edu, 773-702-7594): Before the new experiments direct CP violation had not been established, owing to the large uncertainty in the early measurements of epsilon prime; the new experiment, by contrast, does succeed in establishing a nonzero value for epsilon prime, thus providing a new way to probe (a parameter that can be measured in the lab) this cosmologically-important and most mysterious feature of particle physics. (See figure at Physics News Graphics; background article: Physics Today, October 1988.)