First measurement of the lifetime of the charmed strange baryon Ξc0

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  • Volume 236, number 4 PHYSICS LETTERS B 1 March 1990

    FIRST MEASUREMENT OF THE LIFETIME OF THE CHARMED STRANGE BARYON ~o

    ACCMOR Collaboration

    Amsterdam-Br isto l -CERN-Cracow-Munich-Rutherford-Valencia

    S. BARLAG a,1 H. BECKER a,2 T. BOHRINGER b,3, M. BOSMAN a, V. CASTILLO b,4, V. CHABAUD b, C. DAMERELL c, C. DAUM d, H. DIETL a, A. G ILLMAN c, R. G ILMORE e, T, GOOCH , L. GORL ICH r, p. GRAS g, Z. HA JDUK f, E. H IGON g, D.P. KELSEY b,5, R. KLANNER a.6, S. KWAN b, B. LOCKING a, G. LUTJENS a G. LUTZ a, j. MALOS e, W. MANNER ~, E. NEUGEBAUER a,7, H. PALKA r, M. PEPl~ c,8, j. R ICHARDSON ,9, K. RYBICKI f, H.J. SEEBRUNNER b, U. STIERLIN ~, H.G. T IECKE d, G. WALTERMANN a, S. WATTS h, P. WEILHAMMER b, F. WICKENS ~, L.W. WIGGERS d, M. WITEK f and T. ZELUDZIEWICZ f, lo

    Max-Planck-Institut y~r Physik, D-8000 Munich, FRG b CERN, CH-1211 Geneva 23, Switzerland c RutherfordAppleton Laboratory, Chilton, Didcot OX l l OQX, UK d NIKHEF-H, NL 1009 DB Amsterdam, The Netherlands e University of Bristol, Bristol BS8 ITL, UK f Institute of Nuclear Physics, PL-30055 Cracow, Poland g IFIC, CSIC and University of Valencia, Valencia, Spain h Brunel University, Uxbridge UB8 3PH, UK

    Received 14 December 1989

    We have observed four unambiguous decays of the charmed strange baryon -o in the NA32 experiment at CERN. Charge- coupled devices and silicon microstrip detectors were used to reconstruct the decay mode E-- ,pK-I(*(892) seen in events produced by the interaction of 230 GeV/c negative pions and kaons on a copper target. We present the first measurement of the lifetime of the E , together with a determination of its mass and production cross section. The resonant components of the E decay are studied. We use our earlier measurement of the mass of the -.E~ + in the determination of the isospin mass splitting of the Ec states.

    1 Present address: KNMI, De Bilt, The Netherlands. 2 Present address: Gesamthochschule, D-6600 Saarbriicken,

    FRG. 3 Present address: University of Lausanne, CH-1015 Lausanne,

    Switzerland. Present address: University of Valencia, Valencia, Spain. Present address: Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK. Present address: DESY, D-2000 Hamburg, FRG. Present address: Universit~it-GH Siegen, D-5900 Siegen, FRG. Present address: CERN, CH- 1211 Geneva 23, Switzerland. Present address: University of Geneva, CH- 1211 Geneva 04, Switzerland.

    1o Present address: University of Melbourne, Melbourne, Vic- toria, Australia.

    1. Introduction

    For more than ten years now, many experiments have studied the properties of charmed mesons. The situation regarding charmed baryons is much less clear and it is only recently that experiments have been able to make progress in this difficult subject. Four charmed baryons are expected to decay weakly: Ac + (cud), E~ (csu), .E (csd) and ~o (css). Only the first two appear in the Stable Particle Table of the latest Review of Particle Properties [ 1 ]. The A~ + is well established; we have recently published a mea- surement of its mass and lifetime [2]. The E~ + was

    0370-2693/90/$ 03.50 Elsevier Science Publishers B.V. ( North-Holland ) 495

  • Volume 236, number 4 PHYSICS LETTERS B 1 March 1990

    first observed by the hyperon beam WA62 experi- ment at CERN [3] and subsequently confirmed by the E400 experiment at FNAL [4].

    More recently, we have reported the observation of two new decay modes of the E~ (E-rt+rt + and Z+K-r t + ) ~ and measured its mass and lifetime to be 2466.5 + 2.7 + 1.2 MeV/c 2 and/v n+ l.~ ) 10-~3s

    - - k .~.v - -0 .6

    respectively [ 5 ]. The CLEO Collaboration has pub- lished a year ago the first observation of E (decay- ing to E -~ +) [6] and also confirmed the E+~ E-~+~ decay mode [7]. They have measured the masses of the Ec + and -c= to be 2467+3+5_ MeV/c 2 and 2472_+ 3_+ 4 MeV/c 2 respectively and the iso- spin mass splitting M( E~ + ) - M( E ) to be - 5 _+ 4 + 1 MeV/c 2. In this letter we present data confirming the existence of the ~o -c , seen in a new decay mode pK- I~*(892) , and measure its l ifetime for the first time.

    The data was taken at the CERN SPS in 1985-1986 and was a continuation of the 1984 NA32 experi- ment [8]. The main aim of this new phase was to study the hadroproduction and decay of charmed particles into hadronic final states containing a pK- or K+K - pair, particularly A~ + and D + . We used the ACCMOR spectrometer [9] with an improved ver- tex detector containing charge-coupled devices, CCDs [10], for the first time. A modif ied version of the FAMP trigger [11] was used to record a total of 17 10 6 events containing a pair of opposite charge kaons and/or protons.

    2. Experimental setup

    The NA32 experiment was situated in the North Area of the CERN SPS and used an unseparated neg- ative beam with a momentum of 230 GeV/c. Two CEDAR Cerenkov counters tagged incident pions (96%) and kaons (4%). The vertex detector con- sisted of a beam telescope of seven silicon microstrip detectors (MSDs) and a vertex telescope of two CCDs and eight MSDs. The CCDs, situated 1 cm and 2 cm in a vacuum downstream of a 2.5 mm thick cop- per target, measured high precision space points ( ~ 5 ~tm) on tracks close to the production vertex. Straight

    ~l Unless explicitly stated, a particle symbol stands for particle and antiparticle.

    line tracks are reconstructed with a precision on their transverse position of approximately [52+ (18/ p)2] ~/2 ~tm, where the momentum dependent term (p is in GeV/c is due to multiple scattering. The high precision vertex detector and the lack of material for producing secondary interactions allow the clean re- construction of charm decays with very few back- ground events. The choice of a thin target (2.5 mm) was essential to enable the observation of charmed particles with very short lifetimes.

    The large acceptance ACCMOR spectrometer con- sisted of two magnets and 48 planes of drift cham- bers [9]. Three multicellular threshold Cerenkov counters served to identify ~, K, p in the momentum range 4-80 GeV/c. A two-level trigger was used to increase by a factor of 5 the sensitivity of the experi- ment to decays containing oppositely charged pairs of kaons and/or protons. The decay mode presented in this letter, -o - - ,o -~c ~pK K , more than satisfies our trigger conditions. More details on the trigger are given in ref. [ 9 ].

    3. Data analysis

    The method used to search for decays is very sim- ilar to that applied in our A + analysis [2]. Briefly the data analysis consists of the following steps (more details may be found in ref. [ 2 ] ).

    Firstly, all tracks are reconstructed separately in the beam and vertex telescopes and the drift chambers of the spectrometer and then the vertex telescope and drift chamber tracks are matched together. Particles are identified using the information from the Cerenkov detectors.

    The next step is to search for vertices. The primary vertex is reconstructed from the beam track and as many of the outgoing tracks as possible. For those events where the primary vertex is found to be inside the copper target and where there are at least two tracks which do not originate from the primary ver- tex, a search is made for secondary vertices. Events are selected containing at least one secondary vertex outside of the copper (to remove secondary interac- t ions). Strange particle decays (K , A ) are re- moved. Approximately 300 000 events remain at this stage.

    The secondary vertices are then scanned for fully

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  • Volume 236, number 4 PHYSICS LETTERS B 1 March 1990

    reconstructed charm decays with no missing neutral particles, by restricting to those decays where the mo- mentum vector sum of the decay products points back to the primary vertex (X 2 probability greater than 1%). Invariant mass distributions consistent with the particle identification are then plotted. In our A + analysis [2], for example, the mass distribution for the pK-n + channel contained a clear Ac + signal of 135 events above a background of 31. The back- ground was mainly due to reflections from other charm particle decays where there was an ambiguity either in the topology of the decay or in the particle identification. The precise measurement of space points by our vertex detector allows the clean recon- struction of charm decays in a purely topological way with no restriction to particular decay modes. Using this method we have observed rare decay modes which have not been previously seen [ 12 ].

    The invariant mass distribution for the channel pK-K -n + is shown in fig. la. We have removed those events with secondary vertices which are consistent with interactions in the CCDs and we have also de- manded that all tracks have hits in at least one CCD. There are 33 decays passing the above cuts in the mass range 2.0-3.0 GeV/c a. The bin with the largest con-

    . . . . , no ,n,,, ?In 2 2.2 2.4 2.6 2.8

    ~ b)

    . . . . , n n , ,n , , , , , , , nn, , n 0 2 2.2 2.4 2.6 2.8 3

    m c) c -

    > , , , I , , , , I , N , I . . . . [h , PIN, [1 LH 2 2.2 2.4 2.6 2.8 ,3

    2 2.2 2.4 2.6 2.8 .3

    m(pK-K--rc +) (GeV/c 2)

    Fig. 1. Invariant mass distribution of the pK-K -n + channel: (a) After standard cuts (/> 2 displaced tracks). (b) After removal of reflections. (c) After tightening particle identification. (d) After demanding I(*-* K -x +.

    tent is that centred at 2475 MeV/c 2, suggesting that this may be evidence for a -=c o signal, although at this stage there is a substantial background.

    In the case of our A + analysis, further cuts were applied to the data, improving the ratio of signal to background by almost a factor 4. One of these was to demand that all three tracks were displaced