A Fermilab experiment known as KTeV has observed a correlation among the particles in the decay of the neutral kaon that signals a large violation in a symmetry between the behavior of particles and anti-particles (CP) and in the symmetry between forward and backward time directions. CP violation is still one of the deepest mysteries in nature and is thought to be responsible for the lack of any appreciable amount of anti-matter in our universe.
The experimenters studied the decay mode of the long-lived neutral kaon (KL) to 4 charged particles: a pair of oppositely charged pions (pi+pi-) with an electron and a positron (e- and e+). The pi- is the anti particle of the pi+ and the positron the anti particle of the electron.
The first observation of this rare mode was reported by the KTeV collaboration earlier this year in Physical Review Letters. Now with about 1800 observed decays, the collaboration can study the correlations among the final state particles.
Particle/anti-particle asymmetries require the (quantum mechanical) interference of different processes. Such asymmetries have been seen before, exclusively with the neutral kaon. These asymmetries have been small, about 0.3%.
The pi+pi-e+e- state arises from a decay of a kaon to two charged pions and a gamma (high energy photon). The gamma can be polarized and this can signal a violation of the symmetries. The KL-->pi+pi-gamma decay mode is being studied by KTeV as well. The polarization is expected to be sizable but difficult to observe directly.
The KTeV experiment now has enough sensitivity to study the gamma polarization by making use of the internal conversion of the gamma to an electron-positron pair; it is the orientation of the electron-positron particles with respect to that of the pions that allows the symmetries to be studied.
The polarization produces the largest particle/anti-particle asymmetry seen to date: the KTeV preliminary result is 13.5% with an error of 2.5% (statistical) and 3% (systematic). This serves to establish the asymmetry with certainty (more than 5 standard deviations); the systematic error means that its value is now known with a precision of about 30%.
The polarization is also what is called a "T-odd effect." This means that when the directions of all the particles are reversed, the polarization changes sign. So, if there is a non-zero polarization, this means that the the process is not symmetric with respect to the direction of time. Most fundamental physical processes are symmetric in time. The motion of the planets in the gravity field of the sun is reversable- a film of the motion of a planet around its sun can be shown backwards without anyone being able to tell. Similarly to gravity, the strong nuclear and electromagnetic forces are also time-symmetric. Only the weak nuclear force appears to violate this symmetry, and this so far only in the behavior of the neutral kaon.
Because there is good experimental evidence that the combined symmetry of CPT is "good," a violation of T-symmetry is expected to go along with a violation of CP symmetry. And the level of this violation seen by KTeV is in good agreement with predictions. But the establishment of T-violation had rested on rather indirect arguments. The direct observation by the KTeV collaboration is the first clear observation of its kind. Further data to be collected by KTeV in 1999 and by other experiments will allow a more thorough study of this interesting effect.
____________________________________________________________________________ Background and some technical issues.
1. This mode is analyzed by Sehgal and Wanninger, and in papers by Mark Wise.