(a) The neutral K meson and its antimatter counterpart can both be thought of as a combination of a short-lived particle K1 (green squiggle) which mostly decays into two pions (each indicated by the letter p) and a long-lived particle K2 (red squiggle) which decays mostly into three pions. (b) In some rare cases, however, the K2 (CP= -1) turns into a K1 (CP= 1), which then decays into two pions. This is evidence for indirect CP violation. (c) To illustrate how K mixing comes about, consider the analogy with polarized light. Ordinary light from the sun contains light of all different polarizations (the direction of the light wave's electric field). But if the light is passed through a Polaroid filter oriented vertically, some of the light will be blocked and only that portion with a vertical polarization will emerge. In this beam there can be no light with a horizontal orientation. Next pass the light through a filter oriented at 45 degrees to the vertical. The light that emerges (at even lesser intensity) will now be oriented at the same 45 degrees; this light can be said to have a component which has vertical polarization and a component with horizontal polarization. The proof that some of the beam is now horizontally polarized (whereas a moment before the light was exclusively vertical) is that some light does emerge from a third polarizer oriented horizontally. Something like this is at work in converting K1's and K2's into each other just as vertically polarized light is turned into horizontal. Instead of polarizers, however, the K's are made to pass through thin slabs of matter, in which beams of short-lived K's are "regenerated" from beams of pure long-lived K's. (d) The recently observed case in which K2's are seen to be decaying directly into two pions. This is evidence of direct CP violation.
To view this figure in a larger, higher-resolution format, click here.
This research is reported by Fermilab physicists at the APS Centennial Meeting in Atlanta, Georgia, March 21 - 26, 1999.