The decay rate of the long-lived neutral K meson into the e+e−e+e− final state has been measured with the NA48 detector at the CERN SPS. Using data collected in 1998 and 1999, a total of 200 events has been observed with negligible background. This observation corresponds to a branching ratio of Br(KL→e+e−e+e−)=(3.30±0.24stat±0.23syst±0.10norm)×10−8.

A measurement of the K_S lifetime is presented using data recorded by the NA48 experiment at the CERN-SPS during 1998 and 1999. The K_S lifetime is derived from the ratio of decay time distributions in simultaneous, collinear K_S and K_L beams, giving a result which is approximately independent of the detector acceptance and with reduced systematic errors. The result obtained is tau_S=(0.89598 +- 0.00048 +- 0.00051)x10^(-10) s, where the first error is statistical and the second systematic.

The decay rate of K-L --> pi(o)gammagamma has been measured with the NA48 detector at the CERN SPS. A total of 2558 K-L --> pi(o)gammagamma candidates have been observed with a residual background of 3.2%. The branching ratio is determined to be (1.36 +/- 0.03((stat)) +/- 0.03((syst)) +/- 0.03((norm))) x 10(-6) and the vector coupling constant a(v) = -0.46 +/- 0.03((stat)) +/- 0.04((syst)). This result suggests that the CP-violation effects are dominating in the K-L --> pi(o)e(+)e(-) decay. An upper limit for the K-L --> pi(o)gammagamma decay rate in the two photon mass region m(gammagamma) < m(&pi;)o is also given. (C) 2002 Published by Elsevier Science B.V.

A measurement of the K S lifetime is presented using data recorded by the NA48 experiment at the CERN-SPS during 1998 and 1999. The KS lifetime is derived from the ratio of decay time distributions in simultaneous, collinear K S and KL beams, giving a result which is approximately independent of the detector acceptance and with reduced systematic errors. The result obtained isτS = (0.89598±0.00048±0.00051)×10−10 s, where the first error is statistical and the second systematic.  2002 Published by Elsevier Science B.V.

We present the first observation of the decay KS→π+π−e+e− based upon the data collected in 1998 by the NA48 experiment at CERN. We have identified a clean sample of 56 events with negligible background contamination. Using KL→π+π−π0D decays as normalization sample, the branching ratio is measured to be BR(KS→π+π−e+e−)=[4.5±0.7(stat)±0.4(syst)]×10−5. This result is in good agreement with the theoretical expectations from the mechanism of inner bremsstrahlung.

New measurements from the NA48 experiment on the Dalitz and double Dalitz decays of the K-L are presented. In addition a new measurement of the cascade mass is presented.

The decay rate of the neutral K meson K TMeeg has been measured with the NA48 detector at the CERN SPS. A L Ž total of 6864 events has been observed with an estimated background of 10 events. The branching ratio is G K TM L q y . Ž . Ž . y5 ) e e g rG K TMall s 1.06"0.02 "0.02 "0.04 =10 . The parameter a describing the relative strength L stat sys calc K of the two contributing amplitudes to this decay through intermediate pseudoscalar or vector mesons was measured to be a ) sy0.36"0.06 "0.02 . q 1999 Published by Elsevier Science B.V. All rights reserved. K stat sys 1 Present address: CERN, CH-1211 Geneva 23, Switzerland. 2 Present address: Physics-Astronomy Building, Michigan State University, East Lansing, MI 48824, USA. 3 Present address: Physics-Astronomy Building, Michigan State University, East Lansing, MI 48824, USA. 4 Permanent address: Dipartimento di Fisica dell’Universita e Sezione dell’INFN di Torino, I-10125 Turin, Italy. ` 5 Also Dipartimento di Fisica dell’Universita di Modena, I-41100 Modena, Italy. ` 6 Present address: CERN, CH-1211 Geneva 23, Switzerland. 7 Present address: Institute fur Physik, Universitat Dortmund, D-44221 Dortmund, Germany. ̈ ̈ 8 Present address: SLAC, Stanford, CA 94309, USA. 9 Permanent address: Departamento de Fısica, Universidad de los Andes, Merida 5101-A, Venezuela. ́ ́ 10 Present address: CERN, CH-1211 Geneva 23, Switzerland. 11 Permanent address: Joint Institute for Nuclear Research, Dubna, Russian Federation. 12 Funded by the UK Particle Physics and Astronomy Research Council. 13 Funded by the UK Particle Physics and Astronomy Research Council. 14 Ž . Ž . Funded by the German Federal Minister for Research and Technology BMBF under contract 7MZ18P 4 -TP2. 15 Ž . Funded by Institut National de Physique des Particules et de Physique Nucleaire IN2P3 , France. ́ 16 Ž . Funded by the German Federal Minister for Research and Technology BMBF under contract 056SI74. 17 Ž . Funded by the Committee for Scientific Research KBN , grant 2 P03B07615. 18 Funded by the Bundesministerium f. Wissenschaft und Verkehr under the contract GZ 616.360r3-IIIr2ar98. ( ) V. Fanti et al.rPhysics Letters B 458 1999 553–563 555

The decay rate of the neutral K meson KL→e+e−γ has been measured with the NA48 detector at the CERN SPS. A total of 6864 events has been observed with an estimated background of 10 events. The branching ratio is Γ(KL→e+e−γ)/Γ(KL→all) = (1.06±0.02stat±0.02sys±0.04calc)×10−5. The parameter αK∗ describing the relative strength of the two contributing amplitudes to this decay through intermediate pseudoscalar or vector mesons was measured to be αK∗=−0.36±0.06stat±0.02sys.

The NA48 experiment at CERN has performed a new measurement of direct CP violation, based on data taken in 1997 by simultaneously collecting KL and KS decays into 0 0 and + . The result for the CP violating parameter Re ("=") is (18:5 4:5(stat) 5:8(syst)) 10 . (Submitted to Physics Letters B) V. Fanti, A. Lai, D. Marras, L. Musa Dipartimento di Fisica dell'Universit a e Sezione dell'INFN di Cagliari, I-09100 Cagliari, Italy A.J. Bevan, T.J. Gershon, B. Hay, R.W. Moore, K.N. Moore, D.J. Munday, M.D. Needham, M.A. Parker, S.F. Takach, T.O. White, S.A. Wotton Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK G. Barr, H. Bl umer, G. Bocquet, J. Bremer, A. Ceccucci, J. Cogan, D. Cundy, N. Doble, W. Funk, L. Gatignon, A. Gianoli, A. Gonidec, G. Govi, P. Grafstrom, G. Kesseler, W. Kubischta, A. Lacourt, S. Luitz, J.P. Matheys, A. Norton, S. Palestini, B. Panzer-Steindel, B. Peyaud, D. Schinzel, H. Taureg, M. Velasco, O. Vossnack, H. Wahl,