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  • EPJ manuscript No.(will be inserted by the editor)

    A precise measurement of the direct CP violationparameter Re(/)

    NA48 Collaboration

    A. Lai, D. MarrasDipartimento di Fisica dellUniversita e Sezione dellINFN di Cagliari, I-09100 Cagliari, Italy.

    A. Bevana, R.S. Dosanjh, T.J. Gershonb, B. Hayc, G.E. Kalmus, C. Lazzeroni, D.J. Munday, M.D. Needhamd, E. Olaiya,M.A. Parker, T.O. White, S.A. Wotton

    Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, U.K.e.

    G. Barr, G. Bocquet, A. Ceccucci, T. Cuhadar, D. Cundy, G. DAgostini, N. Doble, V. Falaleev, W. Funk, L. Gatignon,A. Gonidec, B. Gorini, G. Govi, P. Grafstrom, W. Kubischta, A. Lacourt, M. Lentif , S. Luitzg, J.P. Matheys, I. Mikulech,

    A. Norton, S. Palestini, B. Panzer-Steindel, D. Schinzel, G. Tatishvilii, H. Taureg, M. Velasco, O. Vossnack, H. WahlCERN, CH-1211 Geneve 23, Switzerland.

    C. Cheshkov, A. Gaponenkoj, P. Hristov, V. Kekelidze, D. Madigojine, N. Molokanova, Yu. Potrebenikov, A. Tkatchev,A. Zinchenko

    Joint Institute for Nuclear Research, Dubna, Russian Federation.

    I. Knowles, V. Martin, H. Parsons, R. Sacco, A. WalkerDepartment of Physics and Astronomy, University of Edinburgh, Edinburgh, EH9 3JZ, U.K.e.

    M. Contalbrigo, P. Dalpiaz, J. Duclos, P.L. Frabettik, A. Gianoli, M. Martini, F. Petrucci, M. Savrie, M. ScarpaDipartimento di Fisica dellUniversita e Sezione dellINFN di Ferrara, I-44100 Ferrara, Italy.

    A. Bizzetil, M. Calvetti, G. Collazuol, G. Graziani, E. Iacopini, F. Martellim, M. Veltrim

    Dipartimento di Fisica dellUniversita e Sezione dellINFN di Firenze, I-50125 Firenze, Italy.

    H.G. Becker, H. Blumer, D. Coward, M. Eppard, H. Fox, A. Hirstius, K. Holtz, A. Kalter, K. Kleinknecht, U. Koch, L. Kopke,P. Lopes da Silva, P. Marouelli, I. Pellmann, A. Peters, S.A. Schmidt, V. Schonharting, Y. Schue, R. Wanke, A. Winhart,

    M. WittgenInstitut fur Physik, Universitat Mainz, D-55099 Mainz, Germanyn.

    J.C. Chollet, S. Crepe, L. Fayard, L. Iconomidou-Fayard, J. Ocariz, G. Unal, I. Wingerter-SeezLaboratoire de lAccelerateur Lineaire, IN2P3-CNRS,Universite de Paris-Sud, 91898 Orsay, Franceo.

    G. Anzivino, P. Cenci, E. Imbergamo, P. Lubrano, A. Mestvirishvili, A. Nappi, M. Pepe, M. PicciniDipartimento di Fisica dellUniversita e Sezione dellINFN di Perugia, I-06100 Perugia, Italy.

    L. Bertanza, P. Calafiura, R. Carosi, R. Casali, C. Cerri, M. Cirillip, F. Costantini, R. Fantechi, S. Giudici, I. Mannelli,V. Marzulli, G. Pierazzini, M. Sozzi

    Dipartimento di Fisica, Scuola Normale Superiore e SezioneINFN di Pisa, I-56100 Pisa, Italy.

    J.B. Cheze, J. Cogan, M. De Beer, P. Debu, F. Derue, A. Formica, R. Granier de Cassagnac, E. Mazzucato, B. Peyaud,R. Turlay, B. Vallage

    DSM/DAPNIA - CEA Saclay, F-91191 Gif-sur-Yvette, France.

    I. Augustin, M. Bender, M. Holder, A. Maier, M. ZiolkowskiFachbereich Physik, Universitat Siegen, D-57068 Siegen, Germanyq.

    R. Arcidiacono, C. Biino, N. Cartiglia, R. Guida, F. Marchetto, E. Menichetti, N. PastroneDipartimento di Fisica Sperimentale dellUniversita e Sezione dellINFN di Torino, I-10125 Torino, Italy.

    J. Nassalski, E. Rondio, M. Szleper, W. Wislicki, S. WronkaSoltan Institute for Nuclear Studies, Laboratory for High Energy Physics, PL-00-681 Warsaw, Polandr.

    H. Dibon, G. Fischer, M. Jeitler, M. Markytan, G. Neuhofer, M. Pernicka, A. Taurok, L. WidhalmOsterreichische Akademie der Wissenschaften, Institut fur Hochenergiephysik, A-1050 Wien, Austrias.

    Received: date / Revised version: date

    Abstract. The direct CP violation parameter Re(/) has been measured from the decay rates of neutralkaons into two pions using the NA48 detector at the CERN SPS. With 3.3 million KL

    00 eventscollected during the 1998 and 1999 running periods, a result of Re(/) = (15.0 2.7)104 has beenobtained. The result combined with the published 1997 sample is Re(/) = (15.3 2.6)104.

    PACS. PACS-key describing text of that key PACS-key describing text of that key

  • 2 NA48 Collaboration: A precise measurement of the direct CP violation parameter Re(/)

    1 Introduction

    The violation of CP symmetry was first reported in 1964by J.H. Christenson, J.W. Cronin, V. Fitch and R. Turlay,who detected a clean signal of KL + decays [1]. CPconservation implies that the KS and KL particles arepure CP eigenstates and that KL decays only into CP=1and KS into CP=+1 final states. The observed signal ofthe forbidden KL 2 decays (CP=+1) indicates thatCP is not a conserved symmetry.

    CP violation can occur via the mixing of CP eigen-states, called indirect CP violation, represented by theparameter . CP violation can also occur in the decayprocess itself, through the interference of final states withdifferent isospins. This is represented by the parameter

    and is called direct CP violation. L. Wolfenstein in 1964 [2]proposed a super-weak force responsible for S=2 transi-

    a Present address: Oliver Lodge Laboratory, University ofLiverpool, Liverpool L69 7ZE, U.K.

    b Present address: High Energy Accelerator Research Orga-nization (KEK), Tsukuba, Ibaraki, 305-0801, Japan.

    c Present address: EP Division, CERN, 1211 Geneve 23,Switzerland.

    d Present address: NIKHEF, PO Box 41882, 1009 DB Ams-terdam, The Netherlands.

    e Funded by the U.K. Particle Physics and Astronomy Re-search Council.

    f On leave from Sezione dellINFN di Firenze, I-50125Firenze, Italy.

    g Present address: SLAC, Stanford, CA., 94309, USA.h On leave from Osterreichische Akademie der Wis-

    senschaften, Institut fur Hochenergiephysik, A-1050 Wien,Austria.

    i On leave from Joint Institute for Nuclear Research, Dubna,141980, Russian Federation.

    j Present address: University of Alberta, Edmonton AlbertaT6G 2J1, Canada.

    k Dipartimento di Fisica e INFN Bologna, viale Berti-Pichat6/2, I-40127 Bologna, Italy.

    l Dipartimento di Fisica dellUniversita di Modena e ReggioEmilia, via G. Campi 213/A I-41100, Modena, Italy.m Instituto di Fisica Universita di Urbinon Funded by the German Federal Minister for Research and

    Technology (BMBF) under contract 7MZ18P(4)-TP2.o Funded by Institut National de Physique des Particules et

    de Physique Nucleaire (IN2P3), Francep Present address: Dipartimento di Fisica dellUniversita di

    Roma La Sapienza e Sezione INFN di Roma, I-00185 Roma,Italy.

    q Funded by the German Federal Minister for Research andTechnology (BMBF) under contract 056SI74.

    r Supported by the Committee for Scientific Re-search grants 5P03B10120, 2P03B11719 and SPUB-M/CERN/P03/DZ210/2000 and using computing resourcesof the Interdisciplinary Center for Mathematical and Compu-tational Modelling of the University of Warsaw.

    s Funded by the Austrian Ministry of Education, Science andCulture under contract GZ 616.360/2-IV GZ 616.363/2-VIII,and by the Fund for Promotion of Scientific Research in Austria(FWF) under contract P08929-PHY.

    tions, so that all observed CP violation phenomena comefrom mixing and =0. In 1973, Kobayashi and Maskawaproposed a matrix representation of the coupling betweenfermion families [3]. In the case of three fermion genera-tions, both direct and indirect CP violation are naturallyaccommodated in their model, via an irreducible phase.

    The parameters and are related to the amplituderatios

    + =A(KL +)A(KS +)

    = +


    00 =A(KL 00)A(KS 00)

    = 2

    which represent the strength of the CP violating amplitudewith respect to the CP conserving one, in each mode. Ex-perimentally, it is convenient to measure the double ratioR which is related to the ratio /:

    R =(KL 00)(KS 00)

    /(KL +)(KS +)

    1 6 Re(/)(1)

    By the mid-1970s, experiments had demonstrated thatCP violation in the neutral kaon system is dominated bymixing, with the limit Re(/) 102[4]. On the otherhand, theoretical work showed that direct CP violationin the Standard Model could be large enough to be mea-surable [5]. This stimulated experimental effort with so-phisticated detectors to measure Re(/). The first evi-dence for the existence of a direct component of CP viola-tion was published in 1988 [6]. In 1993, two experimentspublished their final results without a conclusive answeron the existence of this component. NA31 [7] measuredRe(/) = (23.0 6.5)104, indicating a 3.5 effect.The result of E731 [8], Re(/) = (7.4 5.9)104, wascompatible with no effect. Recently, a new more precisegeneration of experiments announced results from samplesof their total statistics. The KTeV collaboration measuredan effect of Re(/) = (28.0 4.1)104 [9] and NA48published a first result of Re(/)=(18.57.3)104 [10].These observations confirmed the existence of a direct CPviolation component. Current theoretical predictions arein the range up to 30104 [11].

    This paper reports a measurement of Re(/) withincreased precision, performed by the NA48 experiment,using data samples recorded in 1998 and 1999. The cor-responding statistics is seven times larger than that usedfor the published 1997 result [10].

    2 The method

    Measuring Re(/) to a precision of 104 requires se-veral million KL and KS decays. A sufficiently lar-ge flux of kaons is produced by the high intensity protonbeam from the SPS accelerator. Data are accumulatedusing a fast and efficient data acquisition system, including

  • NA48 Collaboration: A precise measurement of the direct CP violation parameter Re(/) 3

    a trigger with high rejection power and a large capacitydata storage system.

    The design of the experiment and the analysis methodfocus on making the inevitable systematic biases in theevent counting symmetric between at least two of the fourcomponents of the double ratio. In this way, most of theimportant systematic effects cancel to first order, and onlythe differences between t