The NA48/2 experiment - Measurement of Charged Kaon Decays



In 2003 and 2004, the new successor experiment NA48/2 recorded decays of charged kaons, aiming at the search for direct CP violation that would be reflected in an asymmetry between K+ and K- in the Dalitz plot of decays into three pions. For that, K+ and K- beams were produced simultaneously and directed symmetrically through the decay region. In total, about four billion K->3π decays were recorded and reconstructed.

Based on this data sample, limits of a few 10-4 could be put on the existence of direct CP violation in charged kaon decays. As the trigger system recorded not only decays into three pions, NA48/2 has the largest sample of rare K+/K- decays worldwide, so that many decays can be studied with high precision.

Another important goal of the NA48/2 experiment is the measurement of s-wave pipi scattering lengths, which are closely connected with the so-called quark condensate, a fundamental parameter of chiral perturbation theory (ChPT). The quark condensate cannot be predicted but must be measured experimentally; in ChPT, the expansion in powers of momenta is allowed only if the condensate is found to be large enough. PiPi scattering lenths can be measured precisely in Ke4 decays (K+ -> π+ π- e+ ν) and in decays of K+ -> π+ π0 π0. In the latter case this is possible via a so-called Wigner cusp at the m(π0 π0) = 2 * m(π+) threshold, which has been discovered surprisingly in a different analysis.

Apart from the decays given above, a number of other rare and very rare decays of charged kaons are being studied. For most analyses the data samples available are larger by approx. two orders of magnitude compared to previous measurements.

Finally, measurements of semileptonic decays are performed to determine the CKM parameter |Vus| in decays of charged kaons as well.

Analyses with contribution from Mainz

Many results obtained by NA48/2 have been published or presented as preliminary at conferences. The Mainz working group has been or is presently involved in the following data analyses:
  • Search for direct CP violation in the decays of K+- -> π+- π0 π0
  • Measurement of the form factors of the decay K+ -> π+ π- e+ ν and the s-wave π π scattering lengths a00 und a02
  • Measurement of the rate and the form factor of the decay K+ -> π0 π0 e+ ν
  • Measurement of the rate and the form factor of the decay K+ -> π+ π- μ+ ν
  • Measurement of the decay K+ -> π+ γ γ
  • First measurement of the decay K+ -> π+ γ e+ e-
  • Measurement of the rate and the form factor of the decay K+ -> π+ e+ e-
  • Measurement of the π0 double Dalitz decay π0 -> e+ e- e+ e- and determination of the parity of the π0.