CP Violation Results from the NA48 experiments

Abstract

The main goal of the NA48 experiments at the CERN SPS has been the search for direct CP violation (CPV) in kaon decays. In this article two selected results will be presented. In the neutral kaon sector, NA48 determined η+−, related to the parameters of indirect and direct CPV (η+− = ε+ε′), from data collected during a dedicated run in 1999 with a pure KL beam. Using a sample of 47000 KL→π+π− and 5 million KL→π±e∓ν, we extract |η+−| = (2.223±0.012)×10−3. In the charged kaon decays, NA48/2 has measured the asymmetry Ag = (g+−g−)/(g++g−). of the linear slope parameter g in the Dalitz plot of K±→3π decays from data collected in 2003 and 2004. Any non‐zero value of Ag would be evidence for direct CPV. A new technique of asymmetry measurement, involving simultaneous K+ and K− beam and a large data sample allowed a result of unprecedented precision. The charge asymmetry parameter was measured to be Agc = (−1.5±2.1)×10−4 with 3.11×109 K±→π±π+π− decays, and Agn = (1.8±1.8)×10−4 with 9.13×107 K±→π±π0π0 decays. The results are compatible with the SM predictions, i.e. no evidence for direct CP violation at the order of 10−4 has been found.