New Measurement of Direct CP Violation Parameter Re( /) by the NA48 Experiment at CERN

The NA48 experiment at CERN detects concurrently the four decay modes of the long and short lived neutral kaons. During 1998 data taking, statistics in each mode was increased by at least a factor two, with respect to the 1997 published data. A new measurement of the double ratio of the four modes has been obtained from these data. The new preliminary result on the direct CP violation parameter Re(# # /#) is (12.24.9)10 -4 which, combined with the NA48 published result gives Re(# # /#) = (14.0 DIRECT CP VIOLATION. The CP-violating decay of the long-lived neutral kaon, K L , into two pions is interpreted to be mainly due to the CP-conserving decay of the K 1 (CP = +1) component (CP violation in the K mixing) and is parametrized by the # parameter (|#| = (2.28 0.02) 10 -3 ). The interference of the amplitudes for final states with di#erent isospin could also induce a direct CP violation in the decay of the K 2 (CP = -1) component; this contribution to CP violation is quantified by parameter # # . Previous mesurements can be found in [1--4]. In the Standard Model, a value of # # di#erent from zero arises naturally from the complex phase of the CKM matrix. Theoretical predictions for the ratio Re(# # /#)are typically in the range (0 20) 10 -4 . The di#culty of the theoretical calculations come from QCD penguin diagrams and interactions in the final state [5]. 1) Cagliari, Cambridge, CERN, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Orsay, Perugia, Pisa, Saclay, Siegen, Torino, Vienna, Warsaw NA48 METHOD AND DETECTOR