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Volume 218, number 3 PHYSICS LETTERS B 23 February 1989

PRECISE DETERMINATION OF THE LIFETIME OF THE CHARMED BARYON A~

A C C M O R Col labora t ion

A m s t e r d a m - B r i s t o l - C E R N - C r a c o w - M u n i c h - R u t h e r f o r d - V a l e n c i a Col labora t ion

S. B A R L A G a.l, H. B E C K E R a,2 T. B O H R I N G E R b.3, M. B O S M A N a, V. CASTILLO b.4, V. C H A B A U D b, C. D A M E R E L L c, C. D A U M d, H. D I E T L a, A. G I L L M A N c, R. G I L M O R E e, T. G O O C H e, L. G O R L I C H f, P. G R A S g, Z. H A J D U K f, E. H I G O N g, D.P. KELSEY b,5, R. K L A N N E R a'6, S. K W A N b, B. L U C K I N G a, G. L O T J E N S a, V. L U T H b,7 G. L U T Z ~, J. MALOS ",

W. MA.NNER ", E. N E U G E B A U E R ,,8, H. P A L K A ~, M. PEPE ~.9, j . R I C H A R D S O N ~.lo, K. R Y B I C K I f, H.J. S E E B R U N N E R b, U. S T I E R L I N ", H.G. T I E C K E d, G. W A L T E R M A N N a, S. W A T T S h, p. W E I L H A M M E R b, F. W I C K E N S c, L.W. W I G G E R S d, M. W I T E K f and

T. Z E L U D Z I E W I C Z f

Max Planck Institut fiir Physik, D-8000 Munich 40, Fed. Rep. Germany b CERN, CH-1211 Geneva 23, Switzerland ' RutherfordAppleton Laboratory, Chilton, Didcot O X l l OQX, UK

NIKHEF-H, NL-1009 DB Amsterdam, The Netherlands University' o f Bristol, Bristol BS8 1 TL, UK

f Institute o f Nuclear Physics, PL-30055 Cracow, Poland

Received l 9 December 1988

We present the final result on the measurement of the lifetime of the At-baryon from a sample of At-decays obtained in the NA32 experiment at the CERN SPS using silicon microstrip detectors and charge-coupled devices for vertex reconstruction. We observe a total of 160 Ac in five different decay modes. A sample of 101 unambiguous decays of A~.-~pK-~ + (and charge conju- gate) above a background of 7 events is used for the Ac mass and lifetime measurement. The mean A,. lifetime is determined to be 1.96_+°:~] × 10 t3s and the Ac mass to be 2285.8 _+ 0.6 _+ 1.2 MeV/c 2.

1. Introduction

Present address: LAL, F-91405 Orsay, France. 2 Present address: Gesamthochschule, D-6600 Saarbrticken, Fed.

Rep. Germany. Present address: University of Lausanne, CH- 1015 Lausanne, Switzerland.

4 Present address: University of Valencia, Valencia, Spain. Present address: Rutherford Appleton Laboratory, Chitton, Didcot OXI 1 0QX, UK.

~' Present address: DESY, D-2000 Hamburg 52, Fed. Rep. Germany.

7 Visitor from SLAC, Stanford, CA 94305, USA. 8 Present address: Universitfit-GH Siegen, D-5900 Siegen, Fed.

Rep. Germany. ,7 Present address: CERN, CH-121 I Geneva 23, Switzerland. "~ Present address: University of Geneva, CH-1211 Geneva 4,

Switzerland.

The s tudy of the decay proper t ies of the weakly decaying cha rm particles is a rich source of i n f o r m a t i o n abou t the in terplay of weak and s t rong in te rac t ions at short distances. On the one hand the weak decays of heavy quarks offer the possibi l i ty to test predic- t ions of the S U ( 2 ) × U ( 1 ) s t andard mode l of elec- t roweak in teract ions and on the other hand short- and long-dis tance Q C D correct ions to the weak decay h a m i l t o n i a n are of great impor t ance for a comple te u n d e r s t a n d i n g of the exper imenta l results.

A ma jo r b reak through for fixed target cha rm ex- pe r imen t s was the deve lopm en t of si l icon micros t r ip detectors. In 1981 the A C C M O R col labora t ion at

374 0370-2693 /89 /$ 03.50 © Elsevier Science Pub l i she r s B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g D i v i s i o n )


CERN was the first group to introduce silicon microstrip detectors in an experiment (NA11 ) [ 1 ] and to complement these in 1985 with charge-coupled devices (CCDs) in the NA32 experiment [ 2 ].

Many experiments contributed to a better under- standing of the decay properties of the three weakly decaying charmed mesons (¢15 ~°, D + , D + ), whereas still very little is known about the four weakly decaying charmed baryons (A,?, -c-+-, (e~o_ c, ( ~ o ) . In con- trast to the mesons the baryons offer a unique possi- bility to study the importance of W exchange graphs in the weak decays of charm particles, since in the case of baryons these amplitudes are neither colour nor helicity suppressed [ 3 ].

2. The experiment

During the year 1984 the NA32 experiment at the CERN SPS studied charm production in hadronic re- actions and measured the lifetimes of the charmed mesons [ 4 ], using an interaction trigger and a vertex detector consisting of a segmented silicon active target and silicon microstrip detectors. We report here on the second phase of NA32 which was designed to be a dedicated A c and Ds-experiment and had two data taking periods in 1985 and 1986. For this pur- pose a ~ p)K* and K + K ~ pair trigger was used, which was based on the FAMP (Fast Amsterdam Multi Processor) system [5]. This trigger had been used originally in the NA11 experiment in 1982 to collect 600 000 0's [ 6 ], but it had also proved to be a selec- tive charm trigger yielding a sample of D~-~0~ for mass determination and lifetime measurement [7]. In order to be more sensitive in the range of very short lifetimes the silicon microstrip vertex detector was upgraded with the addition of two CCDs [ 2 ]. These detectors have pixels o f 22 jam × 22 ~am and allow the measurement of space points with a precision of ~ 5 gm in two orthogonal coordinates. A two-track resolution of 40 gm was achieved. The two CCDs were located 10 and 20 mm downstream of the 2.5 mm long Cu target, allowing the observation of secondary vertices in vacuum immediately behind the target. They are followed by a setup of eight silicon microstrip detectors ranging from 65 to 180 m m downstream of the target. A beam telescope consisting of seven silicon microstrip detectors measured the tra-

jectory of the incoming beam particle with a precision of 3 gm vertically and 7 gm horizontally at the position of the target.

The FAMP trigger reduced the rate of accepted events by a factor 12 compared to an interaction trigger. The enrichment factor of the recorded data sample in A , . ~ p K - n + and Ac-,15K+n - decays is 7, taking into account the ratio of acceptances between this trigger and an interaction trigger. However, the gain in sensitivity (defined as the number of events per unit t ime) for Ac decays was only a factor 5, due to the increase of the overall deadtime of the experiment.

The experiment was located in the H6 beam of the North Area at the CERN SPS and used a negative beam with a momentum of 230 GeV/c. Hadronic charm decays into charged particles were fully reconstructed with the large acceptance forward spectrometer [6], which consisted of two magnets and 48 planes of drift chambers. Three multicellular thresh- old Cerenkov counters were used to identify n, K, p in the momentum range 4-80 GeV/c. More details about the experiment, in particular about the trigger and the vertex detector, can be found in ref. [ 8 ].

3. Data analysis

In order to ensure a high efficiency for finding charm decays two independent approaches to the data analysis have been developed. The comparison between the two analysis programs during development was found useful to achieve a high efficiency. In the following we describe only the analysis on which the results presented in this paper are based. Details about the other approach can be found in ref. [ 8 ].

The search for charm decay vertices is carried out in five steps.

First, tracks are reconstructed for all events in the drift chambers of the forward spectrometer.

In the second step, the beam track and all outgoing tracks are reconstructed in the beam and vertex tele- scopes independently of the drift chamber track information. Then, tracks found in the drift chamber and in the vertex telescope are matched. Unmatched drift chamber tracks are used as a loose guidance to recover tracks out of complex clusters of signals in the vertex telescope. Particles are identified using the information of the Cerenkov hodoscopes. The recon-

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struction of the primary vertex is performed using all tracks except for tracks with a large impact parameter (the distance of closest approach in space of a track to the primary vertex), i.e. tracks with a X 2 probability of less than 1% for originating from the primary vertex. Fig. 1 shows the calculated error distributions of fitted coordinates of the primary vertices (x refers to the horizontal, y to the vertical coordinate and z points along the beam axis). We checked that all Z 2 distributions are well normalized. Fig. 2 shows the measured impact parameter distribution of all tracks. The impact parameter of a track is measured with a precision ofae~ad,,~ [52+ ( 1 8 / p ) 2 ] 1/2 gm (dx, dy:

minimum projected distance of a track from the primary vertex) where the momentum dependent term

3 .10 1.2 a

- - I I I. I

E Ji-I < o-~> ~2.3/.zm

0.4 ] 1 < °y> °2"O/'zm

~o 0 . 8 - d

0.6 -

t~ I -- o',

0.2

° 0 2 4 6 ( ~ )

200 b

175

~ 150

125

100

7 5

w 50

25

0 0 80 160 240

a , ( f fm)

Fig. 1. Error distributions of the fitted primary vertices (z-axis is beam direction ).

.10

1.6

1.4 E :::l. 1.2

1.

o.8 to

2 0.6 b -

0.4

0.2

O. 0

<i.p.> =7.9,u.m

10 20 30 40 i.p. (,u,m)

Fig. 2. Impact parameter (i.p.) distribution of all tracks with respect to primary vertex.

(p is measured in GeV/c) is the contribution from multiple scattering.

The third step is a loose preselection on charm decays. All events are selected which have a primary vertex inside the Cu target and at least two tracks not originating from the primary vertex (as defined under step two) or one such track and a K ° or A ° reconstructed in the drift chambers. This reduced the orig- inal data sample by about a factor seven.

In the fourth step a secondary vertex search is performed. First the tracks which do not originate from the primary vertex are used to fit one or more secondary vertices. Then tracks which were originally used in the fit of the primary vertex are checked for compatibility with any of the secondary vertices. Fre- quently, this leads to multiple assignments of the tracks all of which are used in the final analysis. Fig. 3 shows the measurement error on the distance between fitted primary and secondary vertices. The selection of all events with at least one good secondary vertex outside the Cu target reduces the remaining data sample by another factor four.

In the last step, the secondary vertices are scanned for fully reconstructed charm decays by checking all effective mass combinations compatible with the particle identification. Also "wrong sign" combinations are formed for background studies. This approach based on the purely topological search for secondary vertices enables us to detect some rare decay modes which have not been observed before [9 ]. In

376


140

E120 < ~ > = 190/.~m

2 lOO

8O

6o

t~ 4o

20

0 0 200 400 600

~ (/~rn)

Fig. 3. Distribution of errors on decay lengths.

order to be kept as a charm candida te the total mo- men tum vector of the decay has to have a Z 2 probabil i ty to originate from the pr imary vertex greater than 1%.

4. Resul t s

The results presented are based on the analysis of the full data sample of 16.7 × 106 triggers. We observe clean Ac decays in five different decay channels: A c ~ p K n ( ~ 135 events) , Ac~I~°pn+n ( ~ 10 events) , A c ~ p K - n + r t - n + ( ~ 4 events) , A c ~ p ¢ ( ~ 3 events) and A c ~ Z + n + n - ( ~ 11 events) w i t h e + decaying to pn ° [ 9 ]. All event numbers given are background subtracted. For the de te rmina t ion of the Ac l ifet ime we only retain the p K - n + (and c.c.) com- binat ions where we observe a signal o f 135 events above a background of 31 events. The rather loose cuts of the general da ta analysis are further t ightened to obta in a bias-free measurement of the l ifet ime of the A~ for which the removal of background is essen- tial. A serious source of background is reflections from the decay channels D + ~ K + K - n ± and D ± - , K + K - rt +- which occur due to ambigui t ies in the iden- t i f icat ion of protons and kaons. These decays are re- moved from the sample. Reflect ions from D ±--,K;rt-+r~ +- decays are negligible because o f the small probabi l i ty of ambigui ty between pro ton and pion. To improve the s igna l /background rat io even further, the following addi t iona l cuts are imposed:

( i ) we require impact parameters of more than 3 a

with respect to the pr imary vertex (see fig. 2) for at least two of the decay tracks, and 1 a for the third decay track,

( i i ) we require a m i n i m u m separat ion of 3 aA_-between pr imary and secondary vertex (see fig. 3 ),

( i i i ) we l imit decay volume to the posi t ion of the second CCD, which is located 20 m m downst ream of the end of the target. The probabi l i ty for a Ac with a proper l ifetime of 2 X 10- ~3 s and a m o m e n t u m of 100 G e V / c to decay downst ream of this decay volume is below 10 -4 .

Final ly we end up with 108 events within 3 a o f t h e A~ mass (where a is the calculated error on the mass) and a background of 75 events in the mass interval from 2.1 to 2.5 GeV. The es t imated background below the signal is 7 events. Fig. 4 shows the invar iant mass dis t r ibut ion.

For the extract ion of the l ifetime from the data, we have to correct for the acceptance of the selection cri- ter ia [ 1 ]. For each event we de termine the m i n i m u m and m a x i m u m detectable lifetime, tmin and t . . . . respectively. This is done by moving the decay vertex along the straight line connecting the pr imary and secondary vertex backwards and forwards keeping all other parameters o f the event unchanged. The minimum and ma x imum distances from the pr imary vertex, / m i n and/max respectively, where the event fails the imposed cuts, de termine tmin and tmax. Here tmax is defined by the end of the decay volume, i.e. the posi t ion o f CCD2. As discussed in ref. [ 1 ], we fit the d is t r ibut ion of corrected lifetimes, tcor/r= (t . . . . - -

tmi°) /r , where z is the mean lifetime. We perform a

40

~u 35

30 >

25 t~ 2O

15 _+..,

E 10 Q )

>

t~ 5

0

m

m

m

m

m

L . . . , . . . . . . ~ . . . . . ~ . ~ . ~

2.1 2.2 2..3 2.4 2.5

M(pK-'n -+) (GeV/c ' )

Fig. 4. Invariant mass distribution of the pK ~+ and 15K+~ - system after all cuts for determination of the Ac lifetime.

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combined max imum l ikel ihood fit to the mass and corrected l ifetime dis t r ibut ions for signal and background, where the l ifet ime dis t r ibut ion o f the background is also taken to be exponential . For the mass spectrum we assume a gaussian signal above a l inear background. The width is obta ined from the calculated mass resolution of each event. Figs. 5 and 6 show the decay length and the corrected l ifetime distr ibu- tions for the A~ signal and the background respectively. The fit yields a mean A~ lifetime of

1 96 +0.23 "fA = . _ 0 . 2 0 X I 0 - 1 3 S

and a mass of

mA, =2285.8 + 0.6 M e V / c 2 .

f -

55 a ~- b

25

~" 20 ,~1o

~> 5 I o I ] R I rl I I 1

0 8 16 24 0 4 8 12 Decoy length (mm) Corrected Lifetime (10-'~S)

Fig. 5. (a) Decay length and (b) corrected lifetime distribution for the A~ signal region, for events with a mass between 2.26 and 2.30 G e V / c 2.

20.

17.5 1 E15.

E 12.5

7.5 > I11 5.

2.5

O. 0 8

I-I--h I 16 24

a

b ~1o

c © >

Ld 1

I

b

0 8 16 24 Decay length Bckg(mm) Corr. Lifetime Bckg(lO-'~S)

Fig. 6. (a) Decay length and (b) corrected lifetime distribution for the background, for events with a mass between 2.10 and 2.25 GeV or 2.31 and 2.50 GeV/c 2.

We have checked for possible sources of systematic errors on the Ac lifetime, in par t icular a non-constant geometrical acceptance of the appara tus between the es t imated lm~n and l . . . . . d is tor t ion in the reconstruction of secondary vertices and a varying efficiency for f inding vertices within the decay volume•

We checked that the ma x imum change of geometrical acceptance of the detector within the decay volume is small and es t imated with a Monte Carlo sim- ulation o f the A, product ion and decay and of the appara tus that the effect on the Ac lifetime is only 0 .02× 10-t3 s.

We checked for dis tor t ion of reconstructed decay vertices which may occur if the m i n i m u m distance between any two decay tracks is less than 50 tam in a CCD. We es t imated with the Monte Carlo program that the ma x imum effect on the A,, l ifetime is 0 . 0 4 X 10 -13 s.

The compar ison between the two independent analysis programs on a total of 700 charm decays (Ac- ,pK- r~ +, D ° - - , K - ~ + ~ ~ + , D + - ~ K ~+~+) has shown a relative efficiency for fully reconstructing charm decays of about 85% for each program. This corresponds to an absolute track reconstruction efficiency of about 95%. No dependence of the relative efficiency on the decay length is observed. We checked with the Monte Carlo program that the ma x imum var ia t ion of efficiency compat ib le with the results of the above compar ison would result in a systematic error o f 0.03 X 10-t3 s on the A~. lifetime. The second analysis program yields a A~ lifetime of r x = 1.84~I~:~X 10 -~3 s and a A~ mass of 2286 .0+0 .6 M e V / c 2 with a sample of 102 A~ events on top of a background of 6 events.

As a further check, in order to detect the presence of systematic effects, we have var ied the selection cri- teria and verif ied the stabil i ty of the result for the different event samples. Fig. 7 shows the mean At. life- t ime as a function of a cut on the m i n i m u m distance AZmm between the pr imary and secondary vertex. Our quoted mean l ifet ime is taken at AZmm = 0 mm. From A Z m i n = 3 m m onwards tm~n is exclusively de te rmined by the Az~m cut and not by the 3 era: requirement . The var ia t ion of r for various other cuts is shown in fig. 8, details are explained in table 1. Our quoted value corresponds to set A. All the variat ions of the fi t ted l ifetime are compat ib le with the f luctuations expected due to the exclusion of part of the sample.

378


T A EVENTS

[10 - ' ] s]

2.5

.°tt 1.5

" 100

-50

1.0 i o 4

--I I I

2 3

i

I I i

5 AZmin[mm ]

Fig. 7. The mean Ac lifetime (data points) and the number of Ac events in the corresponding sample (solid line) as function of the Azmin cut.

T

[10-" 4 ! 2.5

2.0

1.5

1.0

EVENTS

-100 ' - - I _ _ I

5O

i I I i i

A B C D E 4 CUT

Figs. 7 a n d 8 do no t s h o w e v i d e n c e for the p r e s e n c e

o f a s y s t e m a t i c e r ror .

In the m a x i m u m l ike l ihood fit we p a r a m e t r i z e d the

l i f e t ime d i s t r i b u t i o n o f t he b a c k g r o u n d by a n expo-

nen t i a l . T h e b a c k g r o u n d ha s a l i f e t ime o f 5.5+°:87

X 10- ~ 3 s (see fig. 6 ). We s t u d i e d v a r i o u s p a r a m e t r i -

z a t i o n s o f t he b a c k g r o u n d a n d e s t i m a t e d t h a t the

m a x i m u m s y s t e m a t i c e r r o r o n the Ac l i f e t ime due to

t he b a c k g r o u n d s u b t r a c t i o n is 0 .04 × 10-~3 s.

We also h a v e r e a n a l y s e d w i t h o u r i m p r o v e d ana l -

ysis p r o g r a m the pa r t i a l d a t a s a m p l e s u sed for o u r

ea r l i e r p u b l i c a t i o n s [ 8 ,10 ]. We r e p r o d u c e t he ea r l i e r

va lues a n d t he f l u c t u a t i o n s in t he resu l t s a re en t i r e ly

c o m p a t i b l e w i t h t h a t e x p e c t e d f r o m the size o f the

s a m p l e used.

We conc lude , t h a t all sources o f s y s t e m a t i c e r r o r

o n t he m e a n Ac l i f e t ime are negl ig ib le w i th r e spec t to

the s ta t i s t i ca l e r rors .

We e s t i m a t e d the s y s t e m a t i c e r ro r on the A~ m a s s

to be 1.2 M e V / c 2 where the m a i n c o n t r i b u t i o n c o m e s

f r o m the u n c e r t a i n t y o n the c a l i b r a t i o n o f t he mag-

ne t i c field. We c h e c k e d t h a t o u r D o a n d D + m a s s e s

are in good a g r e e m e n t w i th the wor ld ave raged values.

Fig. 8. The mean Ac lifetime (data points) and the number of Ac events in the corresponding sample (solid line) for the various cuts (A-H) defined in table 1.

5. Conclusions

In th i s e x p e r i m e n t , we o b t a i n e d a large Ac s a m p l e

Table 1 The A~. lifetime for various values of the parameters of the cuts.

Cut Impact Probability that parameter total momentum of decay tracks vector with respect to originates from primary vertex primary vertex greater than na (%) F/I, /7~, /7 3

Decay volume between target and

Remove decays r~, 1, of Ac ~ of with 2 tracks ( 10-~3 s ) events background within 50 gm within 3 ~r events in 2nd CCD of the A,. in the sidcbands

mass in the mass interval 2.1-2.5 GeV/c 2

A 3, 3, 1 > O. 1 CCD2 B 3, 3, 1 > 1.0 CCD2 C 3, 3, 2 > 1.0 CCD2 D 3, 3, 3 > 0.1 CCD2 E 5, 5, 5 > 0.1 CCD2 F 3, 3, 1 >0.1 CCD1 G 3, 3. 1 > 1.0 CCDI H 3, 3, 1 > 1.0 CCDI

no 1.96_+~L~i] 108 75 no 2.06+81~ 98 46 no 2.00 + ~J,:~ 90 57 no 1.89+~i~ 84 51 no 1.84+[Ii~ 72 38 no 2.04+~I:~ 101 59 no 2.03 + ~i~ 97 39 yes 2.04 +~:3~ 84 31

379


due to a select ive trigger. A very low background level

was ach ieved by in t roduc ing charge-coupled dev ices

in a d d i t i o n to the s i l icon mic ros t r i p de tec tors for ver-

tex recons t ruc t ion . We have m e a s u r e d the m e a n life- t i m e o f the Ac to be

1 O ~ + 0 - 2 3 r,x, . . . . . . 0.20 X 10-13 s

and the Ac mass to be

m A , = 2 2 8 5 . 8 + 0.6 + 1.2 M e V / c 2 .

O u r Ac l i f e t ime agrees well wi th m e a s u r e m e n t s o f

o ther exper iments (refs. [ 11 ] - [ 14] ). This shorter Ac

l i fe t ime c o m p a r e d to the l i fe t imes o f o the r weakly

decaying c h a r m par t ic les suppor t s theore t i ca l mode l s

[ 3 ] that p red ic t an inc reased h a d r o n i c decay rate for

the Ac ba ryon due to the W exchange con t r ibu t ion .

O u r Ac mass is in good a g r e e m e n t wi th the wor ld av- eraged va lues [ 15 ].

Acknowledgement

We wish to t hank R.L. Engl ish and A.L. L in t e rn o f

the R u t h e r f o r d A p p l e t o n Labo ra to ry for the i r inva-

luable work on the C C D de tec tors and r eadou t elec-

t ronics , A. B jo rkebo and H. Kars tens for process ing

very large a m o u n t s o f da ta and C. Pon t ing for the her skillful typ ing of the manusc r ip t .

References

[ 1 ] R. Bailey et al. Z. Phys. C 28 ( 1985 ) 357. [ 2 ] R. Bailey et al., Nucl. Instrum. Methods 213 ( 1983 ) 201. [ 3 ] B. Guberina et al., Z. Phys. C 33 ( 1986 ) 297. [4 ] S. Barlag et al., Z. Phys. C 37 (1987) 17; C 39 ( 1988 ) 451 ;

H. Becker et al., Phys. Lett. B 184 ( 1987 ) 277. [5] C. Daum et al., Nucl. Instrum. Methods 2t7 (1983) 361. [6] H. Dijkstra et al., Z. Phys. C 31 ( 1986 ) 375. [7] ACCMOR Collab., R. Bailey et al., Phys. Lett. B 139 (1984)

320. [ 8 ] ACCMOR Collab., S. Barlag et al,, Phys. Lett. B 184 ( 1987 )

283. [9] S. Barlag et al., Results on A~ +, D~, D o and D + decay

properties from the NA32 experiment, preprint CERN-EP/ 88-103.

[10] D.P. Kelsey, Proc. Intern. Europhys. Conf. (Uppsala, Sweden, June 25-July 1 1987) p. 351.

[ 11 ] LEBC-EHS Collab., M. Aguilar-Benitez et al., Phys. Lett. B 189 (1987) 254.

[ 12 ] S.R. Amendolia et al., Z. Phys. C 36 ( 1987 ) 513. [ 13 ] J.C. Anjos et al., Phys. Rev. Len. 60 (1988) 1379. [ 14] A. Filippas et al., preprint CERN-EP/87-211. [ 15 ] Particle Data Group, M. Aguilar-Benitez et al., Review of

particle properties, Phys. Lett. B 204 ( 1988 ) 243.

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