Kaon

K

K0

The four kaons or K- mesons are particles from the group of mesons.

They contain a light u or d quark and a mid-heavy Strange anti- quark or each case the corresponding antiparticles. The strange quark (or antiquark Strange ) makes the kaons of the lightest mesons with strangeness (German: strangeness ). According to this property, the kaons are organized into two isospin doublets:

Like all mesons have integer spin and kaons are therefore bosons. They are subject to the strong interaction and thus belong to the class of hadrons.

• 3.1 mixture of neutral kaons
• 3.2 CP violation

Properties

• The positively charged K has a mass of 493.677 ± 0.016 MeV and an average life of ( 1.2380 ± 0.0021 ) · 10-8 s
• Its antiparticle is the negatively charged K-. Mass and mean life of k- have because of CPT invariance with the corresponding values ​​of K match. In fact, one finds experimentally compatible with zero mass difference 0.032 ± 0.090 MeV. The relative difference between the mean lifetimes of (0.10 ± 0.09) %.
• The mass of the electrically neutral K0 is 497.614 ± 0.024 MeV.
• Its antiparticle is also electrically neutral K0. The mass difference between the two neutral kaons is smaller than 10-15 MeV, which also confirms the CPT invariance.

All figures are in natural units and are from the Particle Data Group.

Discovery

The kaons were discovered in 1947 n of the process π → K Λ in the cosmic radiation. Originally they were named strange particles ( engl. strange particles ) because their life was significantly longer than that of other then-known unstable particles. To describe this, the quantum number " strangeness " was introduced. This is true of the strong interaction, which is responsible for the production of kaons is obtained, but violated by the weak interaction, which takes place via their decay.

Today we know that the reason for the relatively long lifetime of kaons the strange quark is (s- quark short). Strange quarks arise about the strong interaction pairs with Strange antiquarks, from which, for example, two kaons or - form a kaon and a baryon with strangeness - as in the discovery process.

Since the kaons fly in different directions after the production, the two strange quarks can not annihilate again in the reverse process. The decay takes place through the transformation of the strange quarks held in the lighter up quark, which explains their low probability with the strikingly long lifetime of kaons. The conversion can be done only through the weak interaction, since they do not will be the only strangeness.

The τ - θ puzzle

First, two different positively charged mesons with strangeness were known, which were distinguished by their decay products:

The final states of these two reactions have different parity. Since it was assumed at the time that parity would be conserved in all reactions, the initial states have τ and θ have different parity and hence must be two different particles. However, precision measurements of mass and lifetime of each showed no difference between the two particles, they appeared to be identical.

The solution to the puzzle lay in the parity violation of the weak interaction, run after both types of decay: it has - contrary to the original assumption - not received the parity. The two decays could therefore originate from the same particle, which was then called K .

CP symmetry

Mixing of neutral kaons

The Kaon assumed particular importance in the context of CP symmetry. Although the P- symmetry is maximally violated, but get the combined symmetry of parity P and charge conjugation C in all reactions to a good approximation.

In terms of strong ( and electromagnetic ) interaction alone would K0 and K0 the physical Kaonzustände (exact: the experimentally observable mass eigenstates ). Since there is by the weak interaction coupling between these two states, the physical Kaonzustände mixtures arising under the assumption of CP symmetry as follows:

The following applies:

Hence the CP eigenstates arise

And

The relative mass difference of these two states is smaller than 10-14.

Assuming CP symmetry, these states can only CP- conserving decay, resulting in two different decay channels with very different phase spaces and accordingly very different lifetimes:

In fact, they found two versions of neutral kaons, which strongly differ in their lifetime. These were as K0S ( short-lived, average life ( 8.953 ± 0.005 ) · 10-11 s ) designated and K0L (long -lived, average life ( 5.116 ± 0.020 ) · 10-8 s ). The average lifetime of the long-life variant is thus by a factor of about 600 greater than that of short-lived.

Due to the assumed CP- symmetry, it was natural to identify the observed K0S with K01 and K02 with the observed K0L; accordingly, the K0L would always fall into three and never in two pions.

CP violation

James Cronin and Val Fitch, however, found out in 1964 that the K0L decays with a small probability ( about 10-3) into two pions. It follows that the physical states are not pure CP eigenstates, but contain a small proportion to the other CP eigenstate:

This phenomenon has been examined closely in the experiments, and will be referred to as CP-violation by mixing, because it is characterized by a mixture of CP- eigenstates of the physical condition. As can be inferred only indirectly to this CP violation by observing the decay, it is very common in the literature known as indirect CP violation. Cronin and Fitch were awarded the Nobel Prize for Physics for their discovery in 1980.

In addition, there is also a direct CP violation, ie an injury directly in the observed decay itself This is again a factor of about 1000 times smaller than the indirect CP violation and was therefore also experimentally confirmed until three decades later at CERN: 1988 by the NA31 collaboration (Speaker Heinrich choice) and then specifically in the 1990s in the following experiment NA48.

It is remarkable that the CP violation occurs ( directly and indirectly ) only to a limited extent, in contrast to the maximum parity violation of the weak interaction. The reason for this remains unknown.