The CPLEAR experiment at CERN studies the CP, T and CPT symmetries in the neutral kaon system using initially tagged K0 and K0• Results from the 1990 to 1993 data on 71+• 71+-o , A m and x are reported, as well as the first direct measurement of T violation and cs. Our results are consistent with CPT invariance and lead to an improved limit on the K0 K0 mass difference. ' University of Athens, ' University of Basie, 3Boston University, 4CERN, 5LIP and University of Coirnbra, 6Delft University of Technology, 7University of Fribourg, 8University of loannina, 9 University of Liverpool, 10J. Stefan Inst. and Phys. Dep., University of Ljubljana, 11CPPM, IN2P3-CNRS et Universite d'Aix-Marseille II, 12CSNSM, IN2P3-CNRS, Orsay, 13Paul-Scherrer-Institut(PSI), 14CEA, DSM/DAPNIA, CE-Saclay, 15KTH Stockholm, 16University of Thessaloniki, 17ETH-IPP Ziirich

We present a new measurement of the KL-K~ mass difference (Am) using semileptonic decays of neutral kaons. The measurement yields Am = (0.5274 * 0.0029,Va1. f O.OOOS,,,,.) x lO”ti/s.

R. Adler a, T. Alhalel b A. Angelopoulos c, A. Apostolakis c, E. Aslanides d,b G. Backenstoss a, C.P. Bee d,e, j. Bennet e, J.K. Bienlein f, P. Bloch d, Ch. Bula g, G. Burgun h, p. Carlson i, J. Carvalho J, E. Cawley e, S. Charalambous k, M. Chardalas k, G. Chardin h, H. Cobbaert J, S. Dedoussis k, M. Dejardin h, j. Derre h, M. Dodgson e, J.C. Dousse ~, J. Duclos h, A. Ealet b, B. Eckart a, L. Faravel ~, P. Fassnacht b, J.L. Faure h, C. Felder a, R. Ferreira-Marques J, W. Fetscher f, M. Fidecaro d, A. Filip6i6 m, D. Francis h, J.R. Fry ~, C. Fuglesang i, E. Gabathuler e, R. Garnet d.~, D. Garreta h, T. Geralis b, H.-J. Gerber f, A. Go °, P. Gumplinger f.l, C. Guyot h, P.F. Harrison e,2, p.j. Hayman ~, W.G. Heyes d.3, R.W. Hollander o, K. Jansson i, H.U. Johner ~, K. Jon-And i, A. Kerek i, j. Kern ~, P.-R. Kettle g, C. Kochowski h, p. Kokkas P, R. Kreuger o, T. Lawry n, R. Le Gac q, E. Machado j,d, p. Maley ¢, N. Manthos P, G. Marel d.h, M. Miku~ m, j.p. Miller n, F. Montanet b, T. Nakada g, A. Onofre J, B. Pagels ", P. Pavlopoulos d.4, F. Pelucchi b, j. Pinto da Cunha J, A. Policarpo J, G. Polivka a, H. Postma o, R. Rickenbach a, B.L. Roberts n, E. Rozaki c, T. Ruf f, L. Sacks e, L. Sakeliou c, P. Sanders e, C. Santoni a, K. Sarigiannis c~4, M. Sch~ifer f, L. Schaller ~, A. Schopper d, P. Schune h, U. Seljak m, S. Szilagyi i, L. Tauscher a, C. Thibault q, F. Touchard q, C. Touramanis k, F. Triantis P, D.A. Troester a.5, E. Van Beveren J, M. Van den Putte o, C.W.E. Van Eijk o, G. Varner ", S. Vlachos ~, P. Weber f, C. Witzig f.6, C. Yeche h, D. Zavrtanik m and D. Zimmerman "

The CPLEAR experiment at CERN uses time-dependent decay-rate asymmetries between K-0 and (K) over bar(0) to measure eta(+-). The analysis and the systematic errors are discussed. The preliminary results from all our data are phi(+-) = 43.5 degrees +/- 0.5(stat)degrees +/- 0.5(syst)+/-0.4(Delta m) and \eta(+-)\ = (2.316 +/- 0.025(stat) +/- 0.028(syst) +/- 0.009(tau S)) x 10(-3). The dominant part in the systematic error of phi(+-) is due to the regeneration correction. A dedicated experiment was performed to reduce this error considerably.

A large-acceptance lead/gas sampling electromagnetic calorimeter (ECAL) was constructed for the CPLEAR experiment to detect photons from decays of π0s with momentum pπ0 ≤ 800 MeV/c. The main purpose of the ECAL is to determine the decay vertex of neutral-kaon decays K0 → π0π0 → 4γ and K0 → π0π0π0 → 6γ. This requires a position-sensitive photon detector with high spatial granularity in r−, ϕ−, and z−coordinates. The ECAL - a barrel without end-caps located inside a magnetic field of 0.44 T - consists of 18 identical concentric layers. Each layer of 13 radiation length (X0) contains a converter plate followed by small cross-section high-gain tubes of 2640 mm active length which are sandwiched by passive pick-up strip plates. The ECAL, with a total of 6X0 has an energy resolution of α(E)E ≈ 13%(E(GeV)and a position resolution of 4.5 mm for the shower foot. The shower topology allows separation of electrons from pions. The design, construction, read-out electronics, and performance of the detector are described.

The CPLEAR experiment uses tagged K-0 and (K) over bar K-0 produced in p (p) over bar annihilation at rest to measure CP-, T- and CPT-violation parameters in the neutral kaon system. The results of these measurements and some implications are reported.

The ratio R between the branching fractions of pp → KSKS and pp → KSKL for antiprotons annihilating at rest in gaseous hydrogen at 27 bar pressure was measured with the CPLEAR detector to be R(27 bar) = 0.037 ± 0.002. The fraction of P-wave annihilation at rest at this target density was deduced to be 0.45 ± 0.06. A value of R was also measured at 15 bar, with less statistics, yielding R(15 bar) = 0.041 ± 0.009.

The CPLEAR experiment measured time-dependent decay-rate asymmetries of K0 and K0 decaying to π+π−π0 in order to study the interference between the decay amplitudes of KS0 — either CP-violating or CP-conserving — and the CP-conserving KL0 decay amplitude. From the analysis of the complete data set we find for the CP-violating parameter η+−0, Re(η+−0) = (−2 ± 7 stat. −1+4 syst.) × 10−3, Im(η+−0) = (−2 ± 9 stat. −1+2 syst.) × 10−3 and for the CP-conserving parameter λ, Re(λ) = (+28 ± 7 stat. ± 3 syst.) × 10−3, Im(λ) = (−10 ± 8 stat. ± 2 syst.) × 10−3. From the latter, the branching ratio of the CP-conserving K0S → π+π−π0 decay is deduced to be B = (2.5−1.0+1.3 stat. −0.6+0.5 syst.) × 10−7.

The time-dependent rate asymmetry of initial K-0 and (K) over bar(0) decaying into pi(+)pi(-)pi(0) CP-violating amplitude of the K-S --> pi(+)pi(-)pi(0) decay. For the real and the imaginary parts of eta(+-0), we find Re(eta(+-0)) = (6 +/- 13(stat.) +/- 1(syst.)) x 10(-3) and Im(eta(+-0)) = (-2 +/- 18(stat.) +/- 3(syst.)) x 10(-3) which correspond to \eta(+-0)\ < 0.037 with 90% CL.

The best estimation of ϕ+− (the phase of the CP violation parameter η+−) and of Δm (the KL - KS mass difference) is obtained by averaging the results of different experiments, taking into account the different correlation, existing for most of the experiments, between the measurement of ϕ+− and Δm. Including the recent measurements, we obtain the average values 〈Δm〉 = (530.7 ± 1.3) × 107 ħ/s and 〈ϕ+−〉 = 43.82° ± 0.63°. This value of ϕ+− is in good agreement with the superweak phase ϕSW = 43.49° ± 0.08°.

The CPLEAR collaboration has constructed a detector at CERN for an extensive programme of CP-, T- and CPT-symmetry studies using K0 and K0 produced by the annihilation of p's in a hydrogen gas target. The K0 and K0 are identified by their companion products of the annihilation K±π∓ which are tracked with multiwire proportional chambers, drift chambers and streamer tubes. Particle identification is carried out with a liquid Cherenkov detector for fast separation of pions and kaons and with scintillators which allow the measurement of time of flight and energy loss. Photons are measured with a lead/gas sampling electromagnetic calorimeter. The required antiproton annihilation modes are selected by fast online processors using the tracking chamber and particle identification information. All the detectors are mounted in a 0.44 T uniform field of an axial solenoid of diameter 2 m and length 3.6 m to form a magnetic spectrometer capable of full on-line reconstruction and selection of events. The design, operating parameters and performance of the subdetectors are described.

The CPLEAR experiment at CERN studies theCP, T andCPT symmetries in the neutral-kaon system by measuring the rate asymmetries between the decays of initially pure K0 and\(\overline {K^0 } \) states. Using data taken between 1990 and mid 1994, precise measurements of |η+-|,φ +- and Δm are reported. The comparison of φ+- the superweak phase gives one of the most sensitive tests ofCPT invariance and leads to a new limit on the\(K^0 - \overline {K^0 } \) mass difference.

The CPLEAR experiment at CERN studies the CP, T and CPT symmetries in the neutral-kaon system by measuring the rate asymmetries between the decays of initially pure K-0 and (K-0) over bar states. Using data taken between 1900 and mid 1994, precise measurements of \eta(+ -)\, phi(+ -) and Delta m are reported. The comparison of phi(+ -) with the superweak phase gives one of the most sensitive tests of CPT invariance and leads to a new limit on the K-0-(K-0) over bar mass difference.

The Bose-Einstein correlations (BEG) observed in hadronic experiments are reviewed. Emphasis is given to the factors that can modify the BEC parameters. The experimental results are studied and the behavior of the parameters is examined. The available data and the currently used parametrizations seem to indicate that the radius of particle sources does not depend on the energy of the system. However, new types of parametrization have been introduced and applied in certain experiments, which show a different behavior. In these cases, the data are not conclusive about the type of parametrization and the parameter values.

We review a phenomenological parametrization of an open quantum-mechanical formalism for CPT violation in the neutral kaon system, and constrain the parameters using fits to recent CPLEAR data.

We present the first direct observation of T violation in the neutral kaon system, showing a positive signal with a significance of more than two standard deviations. The result does not rely on the validity of the CPT theorem.

Antiproton annihilation at rest in gaseous hydrogen (16 bar pressure) to the final states ρ(770)X andf2(1270)X, whereX stands for one or more neutral mesons, has been studied at LEAR using the CPLEAR detector. Relative branching ratios are given forX =π0,2π0, 3π0 ω in the ρ case, and forX =π0, η in thef2 case. The annihilation channel\(\bar pp \to \rho 3\pi ^0 \) has not been measured before. The fraction of P-wave annihilation is deduced from the ratiof2π0/ρπ0 to be 0.38±0.07, in good agreement with the result previously obtained by CPLEAR from the ratioK+K-/π+π-.