NA31 experiment
NA31 is a CERN experiment which was proposed in 1982 as a "Measurement of |η00 /η+-|2 by the CERN-Edinburgh-Mainz-Pisa-Siegen collaboration. It took data between 1986 and 1989, using a proton beam from the SPS through the K4 neutral beam-line. Its aim was to experimentally prove direct CP-violation. CP violation While charge symmetry and parity symmetry are both violated for any transformation under the weak interaction, the CP violation is known only to appear in particular phenomena - kaon and B-meson decays - under the weak interaction. CP-violation was first theoretically developed for the Standard Model by Kobayashi and Maskawa in 1973 when they introduced a third generation of quark (bottom and top) and thus extended the Cabibbo matrix to the 3x3 CKM matrix, parameterizing the couplings between quark-mass eigenstates and the charge weak gauge bosons. CP violation then appears through the presence of complex parameters in this matrix. Determined from the relative decay rates of short- and long-lived neutral kaons into two neutral and charged pions, respectively, the so-called ε'/ε ratio which expresses the relative strength of direct CP-violation was known to be small but expected to be different from zero in the Standard Model. The measurement of this small