Nuclear Instruments and Methods in Physics Research A 371 ( 1996) 192-194 NUCLEAR

INSTRUMENTS & METHODS IN PHVSICS

Search for charmed-strange baryons in experiment WAS9*

Uhich Miller

Div. PPE, CERN, Geneva, Switzerland

For the WA89 collaboration ’

Abstract

Experiment WA89 studies charmed-strange baryons produced by a 340 GeVlc hyperon beam. The Omega RICH detector is used for identification of daughter particles, especially of charged kaons, over a wide momentum range. Results on several

decay modes of the charged baryons z:, and flc are presented.

1. Introduction

Experiment WA89 [I] studies the production and spec-

troscopy of strangeness and charm, using the 340 GeVlc charged hyperon beam at the CERN SPS. The data analysis focuses mainly on the investigation of charmed

baryons, exotic states, and hyperons. In the charm physics sector, the main goal of WA89 is to

detect all single-charmed baryons of the SU(4) multiplet with Jp =f’ (Fig. 1). Since Cabibbo-allowed charm

-A -4-t

c

decays produce an extra unit of strangeness, correct identification of the total strangeness among their daughter particles provides a powerful means of rejecting com- binatorial background. In particular, identification of kaons

against the by far higher pion background plays an essential role in the search for charm decays.

The experiment has completed three physics beam times

in 1991, 1993, and 1994. About 120 million, 180 million, and 320 million events were recorded on tape, respective-

ly. Results presented here are based on the data from 1991

and 1993.

2. Apparatus

The apparatus is designed as a forward spectrometer and consists of a target area, a decay zone and the Omega magnetic spectrometer, followed by a RICH detector and

calorimeters. Fig. 2 shows the setup as used for the 1993 run. A

hyperon beam of 340 GeV/c with an intensity of 6 X 10‘ particles per spill is used, composed of nTT- and Z- with a ratio of about 2.3: 1. Beam particles are detected in a set of nine silicon microstrip detectors. A transition radiation

detector used in the trigger rejects about 95% of all beam nr-, keeping 85% of the z-.

Fig. 1. The SU(4) multiplet of Jp = i + baryons.

* Work supported by the Bundesministerium fur Forschung und

Technologie under contract numbers 05 5HD151, 06 HD5241, and

06 MZ5265.

’ Bristol Univ., CERN, Genoa Univ./INFN, Grenoble ISN,

Heidelberg MPIfK, Heidelberg Univ., Maim Univ., MOSCOW

Lebedev Inst.

The longitudinally segmented target is composed of one copper slab (4 mm thick) and three sintered diamond plates (2 mm thick) spaced by 20 mm. Secondary particles are detected by 29 planes of silicon detectors (pitch 25 pm and 50 p,m) close to the target. Positioning the target about 14 m upstream of the Omega spectrometer provided a 10 m long decay zone for hyperons and Kt, filled with drift chambers (40 planes) interleaved with 1 mm pitch MWPCs (20 planes) for the central region. The particle momenta

0168-9002/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved

SSDI 0168-9002(95)01155-2

U. Miller I Nucl. Instr. and Meth. in Phys. Res. A 371 (1996) 192-194 193

TRD

Target area CGMWPC YWPC DC Hod 1 Hod2

Decay area Omega spectrometer RICH Calorimeters

Beam SI TRO seam 2.1 Targets vertex 2.1

I

sOmlcron 50mlcron I: SCi”U

Scln SCh 12 x 25mlcron 5.5r6.5cm 12 I 2Smlcron S.Ox5.Ocm

Target area 5 x 50mlcron 5.0x5&m

Angles: O.OO.-45.+45

Fig. 2. Setup of WA89 in the 1993 beam time.

are measured by the Omega spectrometer [2] consisting of a superconducting magnet with 7.4 Tm field integral,

equipped with MWPCs (45 planes) inside the field and drift chambers (8 planes) at its exit. The momentum resolution is up/p2 = lo-’ (GeV/c)-‘. Charged particles are identified by the Omega RICH detector [3]. It has a threshold y = 41 and provides r/K separation up to about 100 GeV/c. An electromagnetic calorimeter and a hadron

calorimeter are positioned downstream of the RICH. The trigger is relatively open. It requires a signal from

the interaction counters as well as a predefined correlation of hits in scintillator hodoscopes and MWPCs downstream

of the magnet to select events with at least two high momentum particles.

Between the 1991 and 1993 runs, the apparatus was upgraded. In the 1991 run the microvertex detector con-

sisted of 12 silicon planes. The decay area was equipped with the drift chambers only.

3. Reconstruction of charmed baryon decays

The selection of candidates for charmed baryon decays is based on particle identification [4], reconstruction of the interaction point and isolation of the secondary vertex.

As possible daughter particles of charmed-strange baryons, A + pr are identified through their decay geometry and kinematics. R- -+ AK- and E -+ AT- are reconstructed from negative particles forming a vertex with a A. Only combinations are kept in which the mother particle can be assigned a track in the vertex detector which has no matching track in downstream detectors. For the R sample, background is reduced by excluding

combinations ambiguous with a E- -+ 12~ and by iden-

tifying Km candidates in the RICH if their momenta are

greater than 25 GeV/c. For lower momenta of the K _

candidates, clearly RICH-identified IT are rejected. The reconstruction of the charm decay vertex follows a

“candidate-driven” approach. First, track combinations for which the identified particles or at least their charges

correspond to charm decays are selected, and a secondary

vertex is reconstructed. From the remaining tracks the

position of the main vertex is determined. As selection criteria for the vertices, cuts are applied on

vertex quality (x2), separation of the two vertices, and on the impact parameter of the tracks from the charm decay with respect to the main vertex. For the K candidates, cuts on the likelihood ratio rK = &/max(& _Yp, 4)).

with the likelihoods defined as in Ref. [4], were applied.

4. Observation of 8, decays

Fig. 3 (upper row) shows signals for EL: decaying into AK-~F+~+ and zmnTT+n+ from the 1991 data set. The lower row of Fig. 3 presents signals for a: decaying into AK-r+ and s r’. This is the first observation of the

decay a: -+ AK-r+. For the decay channels involving a K-, the kaon candidate was identified by the RICH with a cut rK > 10 (for the case of the a: ) or rK > I (for the a:). The masses measured are 24542327 and 24515427 MeV/c’ for a:, and 24652327 and 24552427 MeVlc’ for Ez, which is about 2a lower than the PDG values [5]. These mass shifts are not seen in the 1993 data [6]. Systematic errors in the mass determination are still under study.

V. PHYSICS RESULTS

194 U. Miilier I Nucl. Insir. and Meth. in Phys. Res. A 371 (1996) 192-194

3 40

u 35 50 E OM

G 40

5”

25

20 30

15 m

10

5 10

0 2.2 2.4 2.6

0

GCVkZ 2.2 2.4 2.6

G&k’

Fig. 3. Decay modes of ET and a: observed in 1991

The z:,’ lifetime was measured using the AK-rr’rr’ signal shown in Fig. 3. A preliminary value rzi = 318’~~%45 fs was obtained, in good agreement \;ith

existing measurements [5].

5. Observation of Cl: decays

Signals from 0: decays from the 1993 data set are shown in Fig. 4. Mass peaks of different statistical

significance for various decay modes containing R-, s-, and A are observed. K- candidates among the daughter

particles were identified by the RICH, using various

momentum-dependent cuts on the rKn and rKp likelihood ratios. In addition, Km from fiL- decays were identified, as explained in Section 3. The mean value of the observed masses is about 2707 MeVlc’, with an uncertainty of a few

MeVlc2. At the time of writing, a lifetime measurement for the

Szz has been completed. The lifetime was determined to be

ro; = 55”;:::; fs [7]. This makes the fi: the shortest

living weakly decaying hadron observed so far.

WA89 Q, preliminary

0 2.6 2.7 2.6 2.9

0 0 2.7 2.0 2.9 ’ 2.6 2.6

NCK-nrr’ N(‘K-YT'*'

Fig. 4. Decay modes of fI: observed in 1993. The mass scales are

given in GeVlc’.

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