neutral Kaon CP violation
What are neutral Kaons?

A Kaon is a very small particle. Its size is about one thousandth of a millionth of a millionth of a meter, which is about one hundred thousand times smaller than an atom. A Kaon lives for about five hundredth of a millionth of a second. Compared to a human being, which lives for about two thousand million seconds, a Kaon lives a very short time. But, many elementary particles only live some 10-23 seconds, five thousand million million times shorter than a Kaon. Hence, for particle physicists, a Kaon is a particle which lives very long.

It may be clear that, in order to observe a Kaon, one needs very special (and very large and expensive) equipment. There exist only a few places in the world where Kaon observation is performed.

The time is completely over, that a professor with one or two assistants set up his equipment in a laboratorium at his university. Now, laboratories for particle physics are very large, take huge accellerators (several kilometers in size) and hundreds of persons to run and administer it. And when the professor wants to do his experiment there, he has to write his proposal, apply for the necessary funds and form a credible team of collaborators, usually several hundreds of persons from different universities and research centers from all over the world.
Building the equipment, which next is transported and ensembled at the location of the accellerator center, takes usually several years. Before data taking, ten to fifteen years may easily have past since the professor wrote his first proposal. Testing the equipment and taking data may take another two, three years. And then he and his team can start analyzing the results; another five to ten years of work.
The results are sometimes spectacular. But it takes a lot of energy from hundreds of persons, which for many years just can't do anything else than only being very dedicated to their part of the job.
But, back to Kaons!

There exist four types of Kaons:
  • Kaons with negative electric charge, K- particles.
  • Kaons with positive electric charge, K+ particles.
  • neutral Kaons (2 different types), KL0 and KS0 particles.
How can one distinguish the two types of neutral Kaons?

Well, K-, K+ and KL0 particles have comparable life times. But, KS0 particles live some 500 times shorter. The L stands for long lived, the S for short lived. Moreover, KL0 particles decay into three pions, whereas, KS0 particles decay into two pions. Hence, the two types of neutral Kaons can be distinguished by studying their decay products.
They may also decay into other stuff, but let's not complicate the story yet.

What is CP violation?

The neutral Kaons which decay into two pions are said to have positive CP, the ones which decay into three pions negative CP. Consequently, KS0 is a CP=+ particle, whereas, KL0 is a CP=- particle.
So, when a beam of neutral Kaons is prepared, it happens that after a while all KS0 have disappeared, since they decayed rapidly into two pions. What remains in the beam are just the KL0 particles, which decay 500 times slower.
Now, in 1963 it was observed by a team of four (good old days!) physicists, that from the final beam of only KL0 particles, some 0.23 percent of those particles decayed into a pair of pions, instead of into three pions.
Now, 0.23 percent is only a small fraction. Nevertheless, it is way too large to be a remnant of KS0 particles, which managed to survive a bit longer. So, the KL0 particles do something which they not supposed to do: CP=- particles decay as if they are CP=+ particles.
Untill 1964 cientists believed that in Nature, CP cannot be changed. Hence, the above observation demonstrates a violation of that conviction.

Untill now this has puzzled many cientists while looking for a reasonable explanation.

What is CP?

The C stands for Charge-conjugation symmetry, which means the following. Consider a particle (for example a positively charged pion) which decays into certain other particles (for example into a positron and a neutrino) and consider also its antiparticle (a negatively charged pion which is the antiparticle of the positively charged pion) which decays into particles that are the antiparticles of the previous decay products (an electron and an antineutrino in this case).
In a decay process the decay products do not fly away in arbitrary directions, but follow certain patterns. Nature is said to be charge-conjugation symmetric when the two above discussed processes occur with exactly the same pattern.
It is however simple to understand that Nature is not completely charge-conjugation symmetric from the following example. Neutrinos are particles which move very fast and while moving, are spinning around an axis which is in the direction of motion. If you see it coming towards you it spins clockwise. Now, its charge-conjugate particle is the antineutrino. But, antineutrinos spin counterclockwise. Hence, Nature does not furnish the charge-conjugate particle of a neutrino, which, consequently, violates charge-conjugation symmetry.

The P stands for Parity or mirror-image symmetry, which is explained hereafter. Nature is said to be mirror-image symmetric, when decay processes follow the same patterns when seen through a mirror as when seen in reality. But, in 1956 it was discovered that Nature is also not completely mirror-image symmetric.

HOWEVER, when the pattern of the distribution of decay products from an antiparticle is seen through a mirror, it shows the same pattern as that from the corresponding particle when seen in reality. That combination of the two symmetries is called CP.
At first it appeared that CP was a real symmetry of Nature. But, since 1963 we know that also CP is not an exact symmetry of Nature.