Measurement of the mass and lifetime of the charmed strange baryon Ξ+c

Volume 233, number 3,4 PHYSICS LETTERS B 28 December 1989

M E A S U R E M E N T O F T H E M A S S A N D L I F E T I M E O F T H E C H A R M E D S T R A N G E B A R Y O N Ec +

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, U 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 L I C K I N G a, G. L U T J E N S a, G. L U T Z a, j . M ALOS ~, W. M A N N E R a, E. N E U G E B A U E R a.7, H. P A L K A f, M. PEPI~ c,8, j . R I C H A R D S O N ~,9, 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 a, H.G. T I E C K E ~, 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 ~, L.W. W I G G E R S a, M. W I T E K land T. Z E L U D Z I E W I C Z f:o

a Max-Planck-lnstituteJ~r Physik, D-8000 Munich, FRG b CERN, CH-1211 Geneva 23, Switzerland c RutherfordAppleton Laboratory, Chilton, Didcot OXII OQX, UK d NIKHEF-H, NL 1009 DBAmsterdam, The Netherlands e University of Bristol, Bristol BS8 1TL, UK f Institute of Nuclear Physics, PL-30055 Cracow, Poland

IFIC, CSIC and University of Valencia, Valencia, Spain h Brunel University, Uxbridge UB8 3PH, UK

Received 25 October 1989

We have observed six unambiguous decays of the charmed strange baryon E~ + (or charge conjugate -~- ) in the collision of 230 GeV/c negative pions or kaons on a copper target at the CERN SPS using silicon microstrip detectors and charge-coupled devices for vertex reconstruction. Three of them have been reconstructed through the decay chain E~ + --,F.-n+n +, E---*A°n -, A°~pn - and the other three through the decay chain .E¢ + -~Z+K-n ÷, Z +--,pn °. We present our measurements of the mass, lifetime and production cross-section of the -~+, as well as of the branching ratio for the two decay modes.

l 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. 4 Present address: University of Valencia, Valencia, Spain. 5 Present address: Rutherford Appleton Laboratory, Chilton,

Didcot OX 11 0QX, UK. 6 Present address: DESY, D-2000 Hamburg, FRG. 7 Present address: Universit~it-GH Siegen, D-5900 Siegen, FRG. 8 Present address: CERN, CH-1211 Geneva 23, Switzerland. 9 Present address: University of Geneva, CH-1211 Geneva 04,

Switzerland. ~o Present address: University of Melbourne, Melbourne, Vic-

toria, Australia.

1. Introduction

F o u r s ing ly-charmed ba ryons are expected to decay weakly: Ac + ( cud ) , E~ + (csu) , -=c + (csd) and ~o (css) . Un t i l a year ago there has been little exper- imen ta l i n f o r m a t i o n on the last three states. The

+ Ec had been first observed by a hype ron b e a m experi- m e n t at C E R N in the react ion Z - ( 1 3 5 G e V / c ) + B e - - . A ° K - T t + r t + + X [1] and con f i rmed by an ex- p e r i m e n t at F N A L in the react ion n ( ~ 600 G e V / c ) + W / S i / B e ~ A ° ( o r Y ° ) K - T t + T t + + X [2] , with an average mass of 2 4 6 0 + 19 M e V / c 2 [3] . The E ° had not been seen and the ~o had been observed by the C E R N hype ron b e a m expe r imen t th rough three de-

522 0370-2693/89/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland)


cays in the decay mode E - K - n + n + whose masses cluster around 2740 MeV/c 2 [ 1 ]. In addition there appears to be a discrepancy between the two experiments for ---~+, as the second one sees two peaks in the A ° K - n + n + mass spectrum, while the first one only sees one peak.

Even for experiments using a precise vertex detector, charmed baryon decays involving a A ° suffer from combinatorial background for it is generally impos- sible to decide which vertex the A ° is coming from. This is not the case for decays involving a -=- or E • hyperon, or a proton. In addition the baryon charac- ter of such decays can in general be better ascertained through a clean reconstruction of the characteristic decay of a -=- or E ± than through the identification of a proton with (2erenkov counters. Exploiting these ideas we reported a year ago the observation of two unambiguous decays of -=¢+ into a new decay mode E - n + n + [4]. Then the CLEO Collaboration reported the first observation of E ° in the decay mode E - n + at a mass of 2471 +3_+4 MeV/c 2 [5] and we reported the observation of another new decay mode o f - =+ - , Y + K - n + [6]. Recently the CLEO Collabo- ration confirmed the decay -=+ - , -= -n+n + and, after reanalysing t h e i r - ° - - + - ' c - , = n data, measured the iso- spin mass splitting of the -=c system to be m z g -

m-o = - 5 _+ 4 _+ 1 MeV/c 2 [ 7 ]. In this letter we present our final results on the decay of-= + into the decay modes -=-n+n + and Y + K - n +. Results on the observation of-=° in this experiment will be reported in a forthcoming letter.

Our experiment was performed in 1985-1986 at the CERN SPS. Its main aim was the study of production and decay properties of charmed particles produced in hadronic collisions and decaying had- ronically into final states containing a p - K - or K + - K - pair. It was a continuation of the NA32 experiment [8] using the ACCMOR spectrometer, an im- proved vertex detector and a dedicated trigger. For the first time charge-coupled devices, CCDs [ 9 ], were used as vertex detectors. They provide high precision ( ~ 5 ~tm) space points on tracks close to the target. A modified version of the FAMP trigger [ 10] was used to trigger on events containing a pair of opposite charge kaons and /o r protons. A total of 17 million triggers have been recorded.

2. The experiment

A negative beam with a momentum of 230 GeV/c was used. Two CEDAR Cerenkov counters served for tagging incident pions and kaons (96% and 4%, respectively). Hadronic charm decays into charged particles were fully reconstructed with the vertex detector and the large acceptance spectrometer [11] shown in fig. 1. The vertex detector consisted of two parts: a beam telescope with seven silicon microstrip detectors (MSDs) and a vertex telescope with two CCDs at 10 and 20 mm behind the target and eight MSDs positioned from 65 to 185 mm behind the target. A short 2.5 mm Cu target was used so that decay vertices could be observed in vacuum close to the primary vertex. The spectrometer consisted of two magnets M 1 and M2 and 48 planes of drift chambers ar- ranged in four groups, DCI to DC4. Three multicellular threshold (~erenkov counters, (~-t~3, were used to identify n, K, p in the momentum range 4-80 GeV/c.

The trigger had two levels, a hardware trigger and the FAMP trigger. They are described in ref. [ 11 ]. Compared t o a simple interaction trigger, this two- level trigger increased by a factor 5 the sensitivity of the experiment for decays with a pair of opposite charge K / p in the final state. While decays like X--, Y + K - n + ( E + - - , p n °) satisfy the trigger conditions by themselves, decays like X - , E - n + n - ( -=- - ,A°n - , A° - ,pn - ) do not ~. However kaons or protons from the primary vertex, or the kaon from the decay of the associated charmed partner, or accompanying pions, simulating kaons or protons, permit such reactions to fulfil the trigger conditions with a sizeable, though re- duced, efficiency. Through this mechanism we have obtained substantial samples of D + - , K - n + n + and D ° - , K - n + or K - n + n + n - in this experiment [ 12].

3. Reconstruction of ~ - hyperons

We look for the cascade E - - , A ° n - , A° - , pn - and, at the same time, ~ - - , A ° K - , A° - , pn - . In order to achieve a strong rejection of background, one ought to reconstruct the E / ~ track and the three charged

~ Unless explicitly stated, particle symbols will be used through- out to denote particles and antiparticles.

523


B1 B 2 . . . .

I I I ~ I ! I

0 ° +14 ° ± 1 4 ° 9

I , , , , 1 , , , o 50

CCD~

[ ~ M S D 1 . . . . . . . M R D R

I ! I

z (ram)

3Ct,

BEAI

' / / Ml,M2 : Magnets z-DC 1 • DC1...DC4 : Sets of Dr i f t Chambers j' /-MI PI,P2,P3: Proportional Chambers for Trigger £ - V e r t e x Detec to r HA,MB : Sc in t i l la t ion Hodoscopes

C~1,1~2,1~3 : Gas ~erenkov Counters S : Spherical Mirror

Fig. 1. Schematic view of the ACCMOR spectrometer with the details of the vertex detector in the inset. BI-B7: silicon microstrip detectors for beam track reconstruction; T: 2.5 mm Cu target; CCDs: charge-coupled devices; I: interaction counter; MSDI-MSD8: silicon microstrip detectors used together with CCDs for reconstruction of tracks and vertices (underneath are shown strip orientations with respect to the horizontal plane).

decay tracks, measure their momen ta and identify them. On the other hand one would like to keep the acceptance as large as possible. We restr icted the search for E / D to decays in front of the onset of the magnetic field in M 1 (70 cm downst ream of the target) to avoid compl ica t ions with the bending o f the E/f~ track in M 1 (fig. 1 ). We also restricted the search to decays beyond CCD2 so that the F./D t rack can always be reconstructed in the vertex detector. The asymmetr ic decay of the E and o f the A ° frequently prevents ei ther decay pion to reach DC> A reasona- ble compromise between background rejection and larger acceptance was to restrict the search to A ° decays in front of DCl (285 cm downst ream of the target) and to require the r t /K from -=/f~ decay and the pion from A ° decay to be detected in DCI . As a con- sequence ( i ) the m o m e n t u m of the r t /K can always

be measured, ( i i ) the m o m e n t u m of the pion from A ° decay can be measured for most A ° decays. The forward-going proton from A ° decay is required to be detected in DCI and DC2 so that its momen tum can be determined from its deflection in M2. Under these condi t ions the background was found to be small and it was decided not to require part icle ident i f icat ion in order not to decrease unnecessarily the acceptance.

The search for - / D was made in two phases: ( i ) search for .E/f2 decaying between CCD2 and plane MSD7 of the vertex telescope (decay range = 16 cm) , ( i i ) search for .E/D decaying between MSD8 and the onset o f the magnetic field in M1 (decay r ange= 52 cm) . In the first phase the ~ / K from .E/f~ decay can be detected in at least tow MSD planes and the E/f~ decay vertex can be precisely reconstructed, making the reconstruct ion of the A ° originating from this

524


3 .10

2.

1.75 % .~1.5 >

~- 1.25

0.75

,,>,0.5

0.25

O.

m

m

1.28 1.3 1.32

M (A°Tr - ) 1.34 1.36

(ceV/c 2) Fig. 2. Invariant mass distribution of the A°~ - system for the

E-/~, + sample.

point compara t ive ly straightforward. In the second phase we had to use an i terat ive procedure, looking for that point on a E / ~ t rack which minimizes the sum of the two Z2s for the n / K and for the A ° to orig- inate from it. In both phases we require ( i ) the invar iant mass of ei ther the p n - or the n+!b interpreta- t ion of a reconstructed V ° to be within + 20 M e V / c 2 of the A ° mass, ( i i ) the angle between the - / f ~ track and the sum of the m o m e n t u m vectors of the rt/K

and of the A ° to be less than about 2 mr. As we do not use particle identif icat ion, each cas-

cade can be interpreted ei ther as a E - - - ,A°n - candi- date or as a f ~ - ~ A ° K - one. The small fraction of

ambigui t ies is removed by requiring ImAoK--- ran_ I> 15 M e V / c 2 for the E - candidates . Fig. 2 shows the dis t r ibut ion of the A°~ - invar iant mass as obta ined from all triggers. The E mass peak contains approximate ly 7500 E - and 5700 -Z +. The tails of the A°n - mass dis t r ibut ion reflect some dis tor t ion induced by the reconstruct ion procedure for the second range of the E - decay, in par t icular when the A ° decays closely behind the E - .

n + tracks. Then we require ( i ) the effective mass of the A°n - system to be within + 25 M e V / c 2 o f the E - mass, ( i i ) all three tracks in a decay vertex to be incompat ib le with originating from the pr imary vertex, with a z 2 probabi l i ty less than 1%, in order to exclude topological ambiguit ies, ( i i i ) the decay vertex to be outside the target, in order to exclude secondary interactions, and to be upstream of CCD2, so that all three tracks can be reconstructed in at least one CCD, ( iv) the total momen tum vector of the decay vertex to be compat ib le with passing through the pr imary vertex with a Z 2 probabi l i ty bet ter than 1%.

Under these condi t ions three events were selected out of the total sample of 17 mil l ion triggers. The second requirement , stronger than the one used in ref. [ 12 ], turned out to be very powerful in suppressing the background. Table 1 gives the characterist ics of the three events. Their ,E~rt effective masses cluster a round 2465 M e V / c 2. Our A~ + studies already showed that the precision of the vertex detector al- lows the selection of clean decay vertices in vacuum and we found a background of only 7% under the mass peak in the decay mode A + -~pK-r t . Here in the Ertn decay mode the presence of a .E- in the final state gives an addi t ional signature which reduces the background still further. As a cross-check, using the same selection criteria, we also looked for ErtTt decay vertices where the two pions have opposi te charges and where no charmed particles are expected. No event was selected.

We therefore interpret the three events as decays of the E~ + baryon. All three events have been produced by incident pions. One decay is a E~ + , the other two are ZZc. Taking the E - mass to be 1321.32 M e V / c 2 [3] we find a weighted mean mass ms~+ = 2 4 6 5 . 4 + 4.0 M e V / c 2 in good agreement with the previous de- te rminat ions [3,7 ]. As for our measurement of the A + mass [ 13 ], we est imate a systematic uncertainty of + 1.2 M e V / c 2 on the Ec + mass, after checking that the effect of the small dis tor t ion ment ioned in sec- t ion 3 is negligible.

4. The decay mode X[ - ~ - n + n +

The analysis is s imilar to the one used for our A + , D + , D + and D o studies [ 12 ]. We first look for decay vertices, having a Z 2 probabi l i ty of bet ter than 1% between the track of a reconstructed =,E- and two other

5. Reconstruction of E ÷ hyperons

The Z + hyperon decays almost equally into plt ° and into nrt +. The Z - hyperon decays exclusively into mr - . In order to achieve a strong rejection of the

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Volume 233, number 3,4

Table 1 Summary of properties of the three E-n+rt + decays.

PHYSICS LETTERS B 28 December 1989

Event

1 2 3

mass of (pn) from A ° (MeV/c 2) mass of (A°n) from E- (MeV/c 2) mass of the (E~n) system (MeV/c 2) total charge and momentum (GeV/c) distance of total momentum vector

to primary vertex and its error (Ixm) distance of decay vertex

from target edge (standard deviations)

decay length l (mm) lmin (mm) corrected lifetime ( 10-13 s)

1132 1302 2461.0± 8.8 -34.2

9.1(4.0)

10 1.61 1.49 0.27

1115 1320 2462.5 ± 7.0 -58.0

5.0(7.5)

6 3.14 2.50 0.91

1108 1323 2468.5 ± 5.2 +83.5

9.6(4.8)

52 13.5 4.12 9.2

background we required a baryon to be ident i f ied in the decay products . Since our set-up was not detect- ing neutrons, we restr icted the search to decays o f Z + into prr °. Due to their compara t ive ly short l ifetime, most Z +'s decay between the last plane of the vertex detector and the first plane of the drift chamber group DC~. In addi t ion the proton from Z + decay is emit- ted forward, with an angle with respect to the Z + di- rection which has to be less than the kinematical l imit 0max= 189/p mr, wherep is the proton m o m e n t u m in GeV/c . Its m o m e n t u m can, therefore, be de te rmined from its deflect ion in M2 and it can be ident if ied in the Cerenkov counters.

In these ci rcumstances a Z + decay can be recog- nized as a kink between a t rack reconstructed in the vertex detector ( " Z " t rack) and a track reconstructed in DC~ and DC2-DC4 and identif ied as being that o f a proton ( " p r o t o n " t rack) . A kink is accepted if a " Z " track and a "p ro ton" t rack pass each other within about 2.56 m m in the interval MSDs-DCI and if they have an angle larger than about 1 mr and less than about (0ma x + 1 ) mr. Once a E + decay is recog- nized, the Z + m o m e n t u m can be de te rmined from the proton m o m e n t u m and from the kink angle with the well-known twofold ambiguity.

The strength of this procedure together with the cleanliness of the reconstruct ion of decay vertices in the vertex detector has been demons t ra ted in this exper iment by the observat ion of 11 decays A + Z + n + n + with a small background [ 12].

6. T h e decay mode -~+ - , E + K - n ÷

The analysis is very s imilar to the one descr ibed in section 4. We first look for decay vertices, having a Z 2 probabi l i ty of bet ter than 1%, between the track of a reconstructed Z + and two other tracks, ident i f ied as kaon and pion respectively. Then we impose the condi t ions ( i i ) , ( i i i ) and ( iv) specified in section 4.

In total twelve events have been selected, seven with the charge state E + K - n + (E + ), three with the charge state E + K + n - (A + , Cabibbo d is favoured) and two with the "wrong" charge state Z + K - r t - . To interpret these events we have to take into account the Z momentum ambigui ty as well as the rather frequent K / n ident i f icat ion ambiguity. In the charge state Z + K - n +, four decays have an unambiguous kaon, three of them having one of the two ZKn mass solu- t ions clustering around mzg, the fourth one having both mass solutions below 2060 M e V / c 2. The re- maining three decay shave ambiguous kaons but all may be interpreted as A + - ~ E + n - n + ( m = 2 2 9 0 , 2284, 2277 M e V / c 2, respect ively) . Similarly, in the charge state Z + K + n - , all three decays may be interpreted as Ac +, two A + - - , Z + K + n - ( m = 2 2 8 4 and 2278 M e V / c 2) and one A + - ~ Z + n - n + ( m = 2 2 8 7 MeV/c2) .

Consider ing the low level of unexpla ined background (one event with charge state Z + K - n +, two with "wrong" charge state Z + K - n - ) we interpret the first three E + K - n + decays as evidence for a new,

526

Volume 233, number 3,4

Table 2 Sum mary of properties of the three E + K - n + decays.

PHYSICS LETTERS B 28 December 1989

Event

1 2 3

angle (Z, p) (mr) 7.7 momentum of the proton (GeV/c) 16.7 mass of the (ZKn) system 2467.2 + 3.8 2 solutions (MeV/c 2) 2452.7 + 5.1 total charge and momentum

for the first solution (GeV/c) + 47.3 distance of total momentum vector

to primary vertex and its error (~tm) 5.4 ( 3.2 ) distance of decay vertex

from target edge (standard deviations) 2.6

decay length l (mm) 2.78 /rain ( m m ) 2.66 corrected lifetime ( 10 -13 s) 0.22

7.1 6.3 24.2 29.1

2474.6+ 18 2446.5+20 2393.2+ 7.3 2408.0+26

-46 .3 +60.4

5.0(5.4) 7.5(4.2)

27 9 1.98 1.63 1.29 1.52 1.23 0.16

Cabibbo favoured, decay mode of-=~+. Table 2 gives their characteristics. The first two events have been produced by incident pions, one decay is a -E~ + , the other one a -~- . The th i rd event has been produced by an incident kaon and the corresponding decay is a -c=+, containing an s-quarks like the K - . In addi t ion the second event appears as a remarkble instance of hadronic product ion of a charmed b a r y o n - c h a r m e d ant ibaryon pair, whose decays are fully reconstructed: n - + C u ~ X - + ~ ' ~ - +A~ + +neut ra l s , -~c ~ - K + ~ - , Ac + -~ pK-Tt +.

The three selected YKn masses of table 2 have widely varying associated errors, rapidly increasing when the kink angle approaches the l imit 0 . . . . This effect is the strongest for the th i rd event which, being the only one where the E + decays inside the magnetic field of M 1, also suffers from the uncer ta inty on the posi t ion of the Z + decay vertex. The error calculations being strongly non-l inear for kink angles close to 0max, we prefer to ignore this event. Taking the Z + mass to be 1189.37 M e V / c 2 [3] , we find from the first two events rnzg =2467.5 + 3.7 MeV/c 2, in good agreement with our de te rmina t ion of section 4. As before, we est imate a systematic uncer ta inty of _+ 1.2 M e V / c 2 on this result.

7. Results

Combining our mass measurements from the two decay modes we find m-_g =2466.5 + 2.7 + 1.2 M e V / c 2 in very good agreement with the CLEO measurement of 2 4 6 7 + 3 + 4 M e V / c 2 [7] .

To measure the E~ + lifetime, we have to correct the observed decay lengths for the acceptance of the selection cri teria [ 14]. For each event with a decay length l and Ec momen tum p ~ , we de termine the m i n i m u m detectable decay length/rain and calculate the corrected proper t ime A t = (l--/rain )D'l=Jcp=¢. The correct ion for the loss of decays beyond CCD2 is negligible. Since s imilar selection cri teria were used for the two decay modes, we combine the six events and

/ 2 n+ l - l~ 10-13 f ind a lifetime r(E~ + ) = ~ .~,-o.6J × s some- what smaller than the previous measurement of (4.3+1"71.2) X 10 -13 S [3] , but very close to our measurement of r ( A + ) +0.23 = (1.96_o.2o) × 10-~3 s [13].

There are two calculations of the l ifetimes of the four weakly-decaying charmed baryons based on a spectator diagram, W-exchange and quark interference with QCD effects [ 15,16 ]. While the first pro- cess leads to equal lifetimes, the W-exchange between c and d quarks shortens the l ifet ime o f A + and =o The constructive interference between the s quark w c .

from the c--.sua decay and the s quark in the init ial state shortens the lifetimes of E+, -¢=° and even more

527


that of f~o, while the destruct ive interference of the u quarks lengthens the l ifetimes of Ac + and =+

~ C •

Guber ina et al. [15] predict r ( f ~ ° ) ~ r ( E ° ) < r(A~ + ) < r ( E + ), whereas Voloshin and Shifman [16] predict r ( f ~ c ° ) < r ( E ° ) < r ( A c + ) ~ r ( E c + ) < r ( D °) ~ r (Ds + ) < r (D + ). Both authors also give es- t imates of the lifetimes but they stress that, while these est imates are subject to large uncertainties, the pre- dicted l ifetime hierarchies are much more reliable. In par t icular Voloshin and Shifman indicate that each inequal i ty in their predic ted hierarchy represents a factor 1.5-2.0. Our measurement o f r ( E c + ), together with the more precise measurements of the l ifet imes o f A + , D °, D~ + and D + [ 13,3 ] supports their predic- tions, while the previous measurement of r(E~ + ) =

4 "1+1.7"~ 0--13 [3] agrees bet ter with the hier- - - ' - l . 2 J X 1 S

archy o f G u b e r i n a et al. More statistics are needed to decide between these interest ing predict ions.

We have looked for possible resonant components in the E { decays. For the six E ~+ combina t ions in E~ + ~ E - r t + ~ + decays, we find the following effective masses: (1963, 1887), (1965, 1912) and (1928, 1939) M e V / c 2. These values are incompat ib le with E (1530) °. For the mass of the K-Tt + system in the three E + ~Y~+K-n + decays, we f ind 936, 908 and 828 M e V / c 2, respectively, which may be interpreted as I<*(892). Using a quark model calculation, K6r- net et al. [ 17 ] have given predic t ions for the decay rates of charmed baryons into various two-body and quasi- two-body final states, in par t icular the part ia l width f o r e + ~ E ( 1530 )0n + to vanish and the one for E~ + ~ Z + l ~ *° to be large. Our results are compat ib le with these predict ions.

In order to de te rmine the total p rodac t ion cross section of E~ + by pions we have assumed a production mechanism similar to what we have observed for A~ + [ 18 ]. In a Monte Carlo s imula t ion the E + is produced with the usual d is t r ibut ion d2a/dxvdp2xw_ ( 1 - x F ) " e x p ( - b p ~ ) (xv, P x = F e y n m a n variable and t ransverse m o m e n t u m of E + ) in conjunct ion with a mixture of 67% 13 o and 33% D - . We take n = 3, b = 1 GeV -2, r ( E + ) = 2 . 0 × 10 -~3 s. Decay tracks from Ec + and D decays are merged with addi t ional tracks from real events. The Monte Carlo program includes the geometrical acceptance of our set-up, the simulation of the trigger and the simulat ion of the off- line selection. The total acceptance is found to be 0.25% for the decay chain E + --, ---Tt+~ + , E - --, A°n - ,

A ° - , p n - and 2.3% for the decay chain E ,+~ Y + K - n +, Y + ~ pn °. Assuming a l inear A dependence we measure from the three E-n+n + events the E + product ion cross section t imes branching fraction of ~+ ~ E - n + r t + to be 0. l +007 3 + 0.08--0105 ~tb per nucleon ~ c

for Xv> O. The first error is statistical, the second one is systematic and reflects mainly the uncertaint ies on the E~ + lifetime, on the value of n and on the trigger s imulat ion (for n we assume an uncer ta inty A n = + 2 ). F rom the above total acceptances and the five p ion- induced Ec + events we measure the branching ratio of the two decay modes to be F ( Z + K - n + ) / F ( E - n +n +) n/qo+o.13 +0.03 with the same remark = ~"L':-O.06 --0.02, as before on the two errors.

8. Conclus ions

We have observed six unambiguous decays of-c=+ in the two decay modes Ec + - , E - r t + r t + and -Z~ +--, Z+K-Tt +. We measure the mass of E~ + to be 2 4 6 6 . 5 + 2 . 7 + 1 . 2 MeV/c 2 and its l ifetime to be

2 0+1-1) X 1 0 - 1 3 - -0.6 s. Assuming a product ion mechanism of Ec + by pions s imilar to the one for Ac +, we de te rmine the E~ + product ion cross section t imes branching fraction of E + - , E - n + n + to be 0 .13+ 0.,,,,_o.05n~+°°7 ~tb per nucleon for XF>0. We also deter- mine the branching rat io of the two decay modes to be F ( Y + K - r t + ) / F ( - = - r t + n + ) = n ~0+0.13 +o.o3

' J - ~ : - - 0 . 0 6 - - 0 . 0 2 •

A c k n o w l e d g e m e n t

We would like to thank A. Bjorkebo for her pain- staking work in processing about 1600 DSTs for the hyperon search, R.L. English and A.L. Lintern of the Rutherford Apple ton Laboratory for their invaluable work on the CCD detectors and readout electronics, H. Karstens for helping in data processing and C. Ponting for her skilful typing o f the manuscript .

References

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Measurement of the mass and lifetime of the charmed strange baryon Ξ+c