Measurement of the CP-violation parameter η 00 using tagged K0 and K

The CP-violation parameter 00 is determined through the eigentime-dependent asymmetry in the rates of initially tagged K and K decaying to 0 . From the analysis of the complete data set we obtain the values j 00j = [2:47 0:31stat. 0:24syst.] 10 3 and 00 = 42:0 0 5:60stat. 1:9 0 syst.: (Submitted to Physics Letters B) 1) University of Athens, Greece 2) University of Basle, Switzerland 3) Boston University, USA 4) CERN, Geneva, Switzerland 5) LIP and University of Coimbra, Portugal 6) Delft University of Technology, Netherlands 7) University of Fribourg, Switzerland 8) University of Ioannina, Greece 9) University of Liverpool, UK 10) J. Stefan Inst. and Phys. Dep., University of Ljubljana, Slovenia 11) CPPM, IN2P3-CNRS et Universite d’Aix-Marseille II, France 12) CSNSM, IN2P3-CNRS, Orsay, France 13) Paul Scherrer Institut (PSI), Switzerland 14) CEA, DSM/DAPNIA, CE-Saclay, France 15) Royal Institute of Technology, Stockholm, Sweden 16) University of Thessaloniki, Greece CP violation in the mixing of neutral kaons decaying to 0 0 has previously been observed using KL and KS beams [1]. We report here results of a different approach, where CP violation is observed by measuring the asymmetry between the rates of initially pure K or K states decaying into 0 . In a previous CPLEAR publication, the first observation of a particle–antiparticle asymmetry in the decay of neutral kaons to 0 0 [2] was reported with partial statistics, where the details of the experimental method and the analysis were discussed. In this letter the measurements of j 00j and 00 based on the full statistics are presented. The neutral kaons are produced in the annihilations pp! KK and pp! KK . Owing to strangeness conservation in the annihilation process the strangeness of the neutral kaon is determined on an event-by-event basis by identifying the simultaneously produced charged kaon. This technique allows the measurement of any difference between the CP conjugate rates R(K ! 0 )( ) and R(K ! 0 )( ) to be made through the eigentimedependent rate asymmetry A00( ) R(K ! 0 )( ) R(K ! 0 )( ) R(K ! 0 )( ) +R(K ! 0 )( ) ; (1) which provides a direct proof of CP violation independently of phenomenological descriptions. The CP violation parameter 00 is derived from the time dependence of this asymmetry: A00( ) = 2Re( ) 2j 00je 1 2 (1= L 1= S) cos( m 00) 1 + j 00je (1= L 1= S) ; (2) where denotes the decay eigentime of the neutral kaon, S and L are the mean lives of KS and KL respectively, m is the KL–KS mass difference and describes the CP violation in K–K oscillations. A detailed description of the CPLEAR experiment can be found elsewhere [3]. Antiprotons of 200 MeV/c are delivered by LEAR and are stopped inside a high-pressure gaseoushydrogen target at a rate of about 10 per second. The cylindrical detector is placed inside a solenoid of 1 m radius and 3.6 m length, which provides a magnetic field of 0.44 T. The tracking system consists of two proportional chambers, six drift chambers and two layers of streamer tubes. Fast kaon identification is provided by a threshold Cherenkov counter sandwiched between two scintillators, which also provide ionization and time-of-flight measurements. An electromagnetic calorimeter made of 18 layers of lead converters and high-gain tubes is used for photon detection. Fast and efficient online data selection is achieved with a multi-level trigger system based on custom-made hardwired processors. The decay K(K) ! 0 0 ! 4 is selected by requiring exactly two charged tracks that have been identified as a kaon and a pion, and exactly four electromagnetic showers in the calorimeter [4]. The decay time of the neutral kaon is determined from a constrained fit to the data using the momenta obtained from the charged tracks, the K(K) production vertex, the photon conversion points and the photon energies in the electromagnetic calorimeter. The decay-time resolution is equally determined by the precision of the neutral-kaon fourmomentum as by the precision of the photon conversion-point positions. Three criteria based on the invariant masses of all -pair combinations, on a detailed study of the shape of the 2 function of the decay-time distribution and on the measured shower directions of the four photons are further applied to the data in order to improve the experimental decay-time resolution. The selection criteria have been optimized by providing the best sensitivity to the measured parameters and not to the decay-time resolution or background rejection [6]. 17) ETH-IPP Zurich, Switzerland