The NA48 Experiment - CP Violation with Neutral Kaons


Measurement of direct CP violation

The aim of the NA48 experiment was the precise measurement of direct CP violation in the system of neutral kaons. The violation of the CP symmetry was discovered in 1964 (Nobel Price 1980) and is basically the result of the mixing between the neutral kaon K0 with its antiparticle. This mixing is leading to the so-called indirect CP violation, an effect of the order 10-3, which is described by the parameter epsilon.

In the case of direct CP violation, the CP eigenstate K2 (CP = -1) decays directly into two pions (CP = +1), which occurs even much less frequently than indirect CP violation. The parameter epsilon', which describes the amplitude of this process, is of the order 10-6. Direct CP violation is clearly more interesting than the indirect case: in the CKM mechanism of the weak interaction, both effects have the same origin, while in other theories (e.g. the superweak interaction) direct CP violation does not exist. In addition, it is needed to explain the matter-antimatter asymmetry observed in the universe, because only direct CP violation leads to an asymmetry in decay rates of particles and antiparticles.

The ratio of direct over indirect CP violation can be determined by measuring the decays into two pions of long-lived and short-lived neutral kaons, KL and KS, respectively:

          Γ(KL -> π0 π0)       /  Γ(KL -> π+ π-)

  R =  -----------------------    /    -----------------------   \approx  1 - 6 * Re(ε'/ε)

          Γ(KS -> π0 π0) /       Γ(KS -> π+ π-)

By collecting all four decay channels simultaneously, most systematic effects cancel in the double ratio. However, very high statistics is needed (several million decays of each channel) to measure any deviation from 1 on a permille level.

Data were taken by NA48 in the years 1997-2001. Two beams of neutral kaons were produced at two different targets. The first target was located 120m before the beginning  of the accepted decay region, so that the short-lived component had decayed almost completely, and the remaining beam could be regarded as a pure KL beam. From the second beam, produced a few meters before the decay region, mainly the rapidly decaying short-lived kaons were registered, classifying the beam as a KS beam. Beam line and detector were set up in a special way, so that the two beams pointed almost parallelly to the detector, and both decay channels, pi+pi- and pi0pi0, were recorded simultaneously. In total, about six million decays of the statistically limiting channel KL->pi0pi0 were reconstructed offline for the analysis. The final result is

   Re(ε'/ε) = (14.7 +- 2.2) * 10-4

clearly confirming the existence of direct CP violation in neutral kaon decays.

Measurement of other K0 decays

In addition to the measurement of direct CP violation, a number of rare and very rare decays could be studied, leading to a better understanding of chiral perturbation theory and allowing to search for CP violation in other decay channels. Less rare decay modes were of interest as well, in particular the precision measurement of the CKM parameter |Vus| in semileptonic decays.

The Mainz working group was involved in the following analyses:

  • Measurement of the decay KL -> π0 γ γ

  • Measurement of the decay rates KS -> γ γ and KL -> γ γ

  • Measurement of the decays KL -> e+ e- γ and KL -> e+ e- γ γ

  • Measurement of the very rare decays KL -> e+ e- e+ e- and KL -> μ+ μ- e+ e-

  • Search for the CP violating decay KS -> π0 π0 π0

  • Measurement of the decay rate KL -> π e ν and the CKM matrixelement |Vus|

  • Measurement of the rate of the CP violating decay KL -> π+ π-

  • Measurement of the Dalitz plot asymmetry in the decay KL -> π0 π0 π0

  • Search for the rare decay KL -> π0 π0 γ