First glimpse of matter-antimatter asymmetry in KLOE
21 April 1999
The dying moments of one of nature's most bizarre unstable particles are captured in this week's image. The particle is the K0 (pronounced "K-zero") or electrically neutral kaon. One consequence of its unusual behaviour is that it appears in two forms, called the K-long (or KL) and the K-short (or KS). The K-long lives a 100 times longer than the K-short, although its lifetime is only 0.05 microseconds. The K-long and K-short are particularly interesting because they are different "quantum mixtures" of matter and antimatter.
If matter and antimatter always behaved in the same way - as they do most of the time - then the the different mixtures, K-long and K-short, would always decay in different ways. However, in 1964, physicists James Cronin and Val Fitch discovered that sometimes the two kaon mixtures do decay the same way (and in 1980 they received the Nobel prize for this discovery). This is possible only if there is a small but subtle difference between matter and antimatter, which is known technically as "CP-violation".
The KLOE detector has been built to study this effect in detail at Dafne, the new collider at the Laboratori Nazionali di Frascati in Italy. The electron and positron (antielectron) beams that collide in Dafne are tuned to make a particle called the "phi" which itself decays very quickly, often making a K-long and a K-short. In this image, KLOE reveals an interaction that could be due to CP-violation. There are two pairs of particle tracks which have been identified as due to pairs of positive and negative particles known as pions. One pair issues from close to the centre of the detector, where the beams collide to make the phi particles, and appears to be due to the decay of a K-short. The other pair, which materialises some distance (about 1 m) from the collision point, is probably from a K-long.
Credit: KLOE experiment
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