Transcript
Page 1: First measurement of the lifetime of the charmed strange baryon Ξc0

Volume 236, number 4 PHYSICS LETTERS B 1 March 1990

FIRST M E A S U R E M E N T OF THE LIFETIME OF THE C H A R M E D STRANGE BARYON ~o

A C C M O R Collaboration

Ams te rdam-Br i s to l -CERN-Cracow-Mun ich -Ru the r fo rd -Va lenc i a

S. BARLAG a,1 H. BECKER a,2 T. B O H R I N G E R b,3, M. BOSMAN a, V. CASTILLO b,4, V. CHABAUD b, C. D A M E R E L L c, C. DAUM 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 ¢, L. G O R L I C H r, p. GRAS 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. KWAN b, B. L O C K I N G a, G. LUTJENS a G. L U T Z a, j. MALOS e, W. M A N N E R ~, E. N E U G E B A U E R a,7, H. PALKA r, M. PEPl~ c,8, j. R I C H A R D S O N ¢,9, K. RYBICKI f, H.J. SEEBRUNNER b, U. STIERLIN ~, H.G. T IECKE d, G. W A L T E R M A N N a, S. WATTS h, P. WEILHAMMER b, F. WICKENS ~, L.W. W I G G E R S d, M. WITEK f and T. Z E L U D Z I E W I C Z f, lo

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

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first observed by the hyperon beam WA62 experi- ment at CERN [3] and subsequently conf i rmed by the E400 exper iment at F N A L [4] .

More recently, we have repor ted the observat ion of two new decay modes of the E~ (E-r t+r t + and Z + K - r t + ) ~ and measured its mass and l ifetime to be 2466.5 + 2.7 + 1.2 M e V / c 2 a n d / v n+ l.~ ) × 10-~3s

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

respectively [ 5 ]. The CLEO Col laborat ion has pub- lished a year ago the first observat ion of E ° (decay- ing to E - ~ +) [6] and also conf i rmed the E + ~ E - ~ + ~ ÷ decay mode [7]. They have measured the masses of the Ec + and -c=° to be 2467+3+5_ M e V / c 2 and 2472_+ 3_+ 4 M e V / c 2 respectively and the iso- spin mass split t ing M ( E~ + ) - M ( E °) to be - 5 _+ 4 + 1 M e V / c 2. In this letter we present data confirming the existence of the ~o - c , seen in a new decay mode p K - I ~ * ( 8 9 2 ) °, and measure its l i fet ime for the first t ime.

The data was taken at the CERN SPS in 1985-1986 and was a cont inuat ion of the 1984 NA32 experi- ment [8] . The main a im of this new phase was to study the hadroproduct ion and decay of charmed particles into hadronic final states containing a p K - or K + K - pair, part icularly A~ + and D + . We used the A C C M O R spectrometer [9] with an improved ver- tex detector containing charge-coupled devices, CCDs [10], for the first t ime. A modif ied version of the FAMP trigger [11] was used to record a total of 17 × 10 6 events containing a pair of opposi te charge kaons a n d / o r protons.

2. Experimental setup

The NA32 exper iment was si tuated in the Nor th Area of the CERN SPS and used an unsepara ted neg- ative beam with a momen tum 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 microst r ip 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 m m 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 t ransverse posi t ion of approximate ly [52+ (18 / p)2] ~/2 ~tm, where the momen tum dependent term (p is in G e V / c is due to mult iple scattering. The high precision vertex detector and the lack of mater ia l for producing secondary interact ions allow the clean re- construct ion of charm decays with very few back- ground events. The choice of a thin target (2.5 m m ) was essential to enable the observat ion of charmed particles with very short lifetimes.

The large acceptance A C C M O R spectrometer con- sisted of two magnets and 48 planes of drift cham- bers [9] . Three mult icel lular threshold Cerenkov counters served to identify ~, K, p in the m o m e n t u m range 4-80 GeV/c . A two-level trigger was used to increase by a factor of 5 the sensit ivity of the experi- ment to decays containing opposi tely charged pairs of kaons a n d / o r protons. The decay mode presented in this letter, - o - - , o -~c ~ p K K , more than satisfies our trigger condit ions. 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 appl ied 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. Part icles are identif ied using the informat ion from the Cerenkov detectors.

The next step is to search for vertices. The pr imary vertex is reconstructed from the beam track and as many of the outgoing tracks as possible. For those events where the pr imary vertex is found to be inside the copper target and where there are at least two tracks which do not originate from the pr imary 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. Approximate ly 300 000 events remain at this stage.

The secondary vertices are then scanned for fully

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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 p K - 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 p K - 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 p K - 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 M e V / 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 o f our A + analysis, further cuts were applied to the data, improving the ratio o f signal to background by almost a factor 4. One of these was to demand that all three tracks were displaced from the primary vertex (3a, 3a and l a). As it is possible that the -c=° has a very short lifetime, we were unable to use this approach here as this would have reduced our acceptance too much. However, other cuts could be used to purify the p K - K - n + sample, as follows:

(i) Reflections from other fully reconstructed charm events are removed (see fig. l b). The seven decays which are cut are mainly due to A~ + - - .pK-n + or D ° - * K + K - n + n - where there is an additional pointing track superimposed in the first case and where the particle identification is ambiguous in both cases .

(ii) By studying the A~ + ~ p K - n + decays we de- fine a tighter particle identification algorithm for se- lecting "non-pions" (i.e. kaons and protons). De- manding two such non-pions per decay in the A~ + sample only reduces the number of accepted decays by 15%, but at the same time decreases the back- ground by a factor of two. Many of the events in fig. lb contain particles which could be either kaons or pions according to our looser identification cuts. We therefore demand three non-pions in the p K - K - n + events thereby reducing the sample to twelve events (see fig. lc) .

(iii) In our E~ + analysis [ 5 ] we found that the three decays to Z + K - n + were all consistent with the K - n + originating from a I(*(892)°. Following this, we de- mand that one of the two K - n + combinations have an effective mass within the range 796-996 M e V / c 2 (i.e. + 2F). Fig. 1 d shows the resulting distribution: four events cluster about a mass o f approximately 2470 MeV/c 2 and there are only another three events, all above a mass of 2900 MeV/c 2.

Given the very clean signal with virtually no back- ground, these four events are clear evidence for the decay mode -=cO ~ p K - I ( * (892)o. Three of the decays are -cO while the fourth one is a -o -o .~c. Two of the ~ and the -=cO are produced by incoming pions while the re- maining -=cO is produced by an incoming K - . In the latter case the -=cO contains an s-quark, like the beam

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Table 1 Details of the four E °--,pK-K-~ + decays.

PHYSICS LETTERS B 1March 1990

Event

1 2 3 4

mass of ( pKKn ) system (MeV/c 2 ) 2470.5 +_ 3.7 2476.6 +_ 4.0 2478.1 + 4.0 2470.0 + 3.3 total momentum (GeV/c) 123.1 56.6 129.2 65.8 distance of total momentum vector

to primary vertex and its error (p.m) 6.5 (4.4) 3.6 (3.8) 3.8 (4.5) 1.9 (3.5) distance of decay vertex to target edge

(standard deviations ) 5.2 3.0 19 1.1 mass of (K-n + ) system

2 solutions (MeV/c 2) 868, 780 830, 932 865,791 848, 849 mass of associated (pK-) systems

(MeV/c 2) 1511, 1491 1561, 1435 1525, 1674 1529, 1545 decay length l (mm) 2.47 2.14 4.42 1.62 lmin (mm) 1.43 1.68 1.83 1.41 corrected lifetime ( 10-13S) 0.70 0.67 1.65 0.26 reaction ~- ~E ° it- _~.~o ~- --*E ° K- -,E °

particle. More details of the decays are presented in table 1.

4. R e s u l t s

The results presented here are based on the full data sample of 17 × 106 triggers. The weighted mean mass

of the four E ° ~ p K - I ~ *° decays is 2473.3 _+ 1.9 MeV/ c 2 in good agreement with the CLEO measurement of 2472 +_ 3 _+ 4 MeV/c 2 [ 5,6 ]. As in the case of our

A + analysis [2], we estimate the systematic uncer- tainty on the E ° mass to be + 1.2 MeV/c 2. Combin- ing this with the E~ + mass of 2466.5 + 2.7 MeV/c 2 from our earlier letter [5], we measure an isospin mass splitting of M ( E + ) - M ( E ° ) = - 6 . 8 + 3 . 3 MeV/c 2. The systematic error in this case is much smaller because of several effects which cancel out; we estimate it to be approximately +_ 0.5 MeV/c 2. This result, taken together with that of CLEO, indi- cates that the E ° is heavier than the Ec +, which puts a restriction on several theoretical predictions [see ref. [ 13 ] for an extensive discussion ].

The mean uncorrected flight t ime of the four E ° decays is 2 .4× 10-~3s. To measure the E ° lifetime, we must correct the observed decay lengths for the acceptance of the cuts of our analysis. For each event, we determine the m i n i m u m detectable decay length

/rain and calculate the corrected proper time t=

(l-lm~n)M(Ec)lcP(Ec), where l and P(Ec) are the decay length and m o m e n t u m of the E ° decay. The resulting mean E ° lifetime is + o 59 - (0.82_o:~o) × 10 13s.We have checked for possible sources of systematic error on the E ° lifetime. These include a non-constant geo- metrical acceptance as a function of decay position, distortion in the reconstruction of secondary vertices and a varying efficiency for f inding vertices within the decay volume. In a similar way to our A~ + analy- sis [ 2 ], we find that the systematic error is negligible compared to the quoted statistical errors. This is the first measurement of the -o lifetime and is the short- est charmed particle lifetime ever measured.

Two theoretical calculations of charmed baryon lifetimes have been published [ 14,15]. These are based on a spectator diagram, W-exchange and quark interference with QCD effects. Guber ina et al. [ 14] predict r ( ~ ° ) ~ r(Ec °) < z(A~ + ) < Z(Ec + ), whereas Voloshin and Shifman [15] predict z ( f~° )< z(-Z °) < z(Ac + ) ~ "c(E~ + ), where each inequality rep- resents a factor between 1.5 and 2.0. Our measure- ments of z(E°), z(E~ + ) = kid . . . . (~+ 1.1o.6 ).× 10-t3s [ 5 ] and z(A~ +) (1 9 ~+°'23 13s . . . . o.zo) × 10- [2] support the latter prediction, although more statistics is required to make a firm conclusion.

We have searched for resonant components in the E ° decays. There are two K - n + combinat ions per de- cay. The presence of at least one I~*(892) ° was de- manded in the extraction of the signal, but as may be

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seen in table 1, events 2 and 4 have two K - n + masses within _+ 2 F of the I~* mass. There are also two p K - mass combina t ions per decay. For events 1 and 3, us- ing the kaon which is not part of the I~* combinat ion , the p K - masses are found to be 1511 and 1525 M e V / c 2 respectively. For event 4, the p K - masses corre- sponding to the IZ* combina t ions are 1529 and 1545 M e V / c 2. These p K - masses are compat ib le with the decay of the narrow A (1520) resonance, suggesting that the E ° might decay to A(1520) I~* (892 ) °. We cannot interpret these data as being conclusive evi- dence for A (1520) because the avai lable phase space tends to force the p K - mass into the region of the A ( 1 5 2 0 ) .

Before we can measure the total product ion cross section o f E ° by pions, we must de te rmine the accep- tance of the geometry of our exper imenta l setup, the trigger and the cuts o f the analysis. This is calculated in a Monte Carlo s imulat ion of E ° product ion using the usual d is t r ibut ion d2o'/dXF dp 2 ~ ( 1 --XF) n × exp ( -- bp 2, ) where XF and Pt are the Feynman vari- able and transverse m o m e n t u m of the E °. We assume a s imilar product ion mechanism to that we have ob- served for A + [16] , i.e. n = 3 and b-- 1 GeV -2, and use the Z ° l i fet ime as presented in this letter. The E ° are s imulated together with D mesons (67% 17) ° and 33% D - ) and merged with addit ional tracks from real events. The acceptance for the channel ,Ec ° - , p K - K *° is found to be 1.1%, including allowance for the non- observed all neutral decay mode o f the K*(892 )o.

Assuming a l inear A-dependence, we measure from the three pion induced -c~° events a E ° product ion cross section t imes branching fraction of .-o__, p K - I ~ * ( 8 9 2 ) ° to be 0.019 _+ 0.011 +o.066 -0.o09 /ab per nu- cleon for xv> O. The first error is statistical, the sec- ond one systematic and reflects mainly the uncer- taint ies on the =o lifetime, on the value of n and on ~ c

the trigger s imulat ion (for n we assume an uncer- ta inty o f _+ 2 ).

We have looked for the decay mode --c-° ~ . - - n + as observed by CLEO [6] . E - part icles are recon- s tructed via their decay into A ° n - as in our -.E~ + anal- ysis [ 5 ]. Selecting two-prong secondary vertices con- taining a ---- and n + with both tracks displaced from the p r imary vertex results in eight decays all o f which have effective masses less than 2110 M e V / c 2. The decay of E ° to E - x + would be highly asymmetr ic in the labora tory frame making the .E- travel forwards;

relaxing the requirement that the E - be displaced from the pr imary vertex increases the number o f se- lected events to 160 but all masses are less than 2170 M e V / c 2. We therefore do not observe E ° ~ - - n +.

We have repor ted here on the possible observat ion o f the decay mode E ° ~ A (1520)I~ *°. In add i t ion to decaying to N K the A ( 1 5 2 0 ) also decays to Zn. We have therefore searched for the decay mode E ° - ~ Z + n - K - n +. Using very s imilar techniques to our analysis o f E + ~ Z + K - n + [5] we found no events in this channel. Given the fact that we are only able to reconstruct Y +, and not Z ° or Z - , and then only via its decay to pn ° means that the three possible -c-° decays to A (1520 )I~* lead us to expect to see ap- proximate ly one decay via Z + n - K - n +, fully consis- tent with our null result in this channel.

5. Conclusion

We have observed four unambiguous decays of F , ° -~pK-I~*(892) °. We measure the mass of E ° to be 2473.3_+ 1.9_+ 1.2 M e V / c 2 and the isospin mass split t ing M ( E c + ) - M ( E °) to be -6.8_+3.3_+0.5 M e V / c z. We have measured the E ° l ifetime for the first t ime and find it to be tn Q2+o.59 I . . . . 0.30) X 1 0 - 1 3 S . As- suming a s imilar product ion mechanism of E ° by pions as for A { , we de termine the -c~° product ion cross section t imes branching fraction of E°-~ p K - K * (892) ° to be 0.019 _+ 0.011+°°66-o.oo9 ~b for XF> 0. Three of the decays are consistent with the p K - being decay products of the A (1520) resonance.

Acknowledgement

We would like to thank R.L. English and A.L. Lintern for their work on the CCD detectors and readout electronics, A. Bjorkebo and H. Karstens for processing large amounts of data and C. Ponting for her skilful typing of the manuscript .

References

[ 1 ] Particle Data Group, G.P. Yost et al., Review of particle properties, Phys. Lett. B 204 (1988) 1.

[ 2 ] ACCMOR Collab., S. Barlag et al., Phys. Lett. B 218 ( 1989 ) 374.

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[3] S.F. Biagi et al., Phys. Lett. B 122 (1983) 455; Z. Phys. C 28 (1985) 175.

[4 ] p. Coteus et al., Phys. Rev. Lett. 59 (1987) 1530. [5 ] P. Singer and G. Eilam, eds., Proc. XII Intern. Workshop

on Weak interactions and neutrinos (Ginosar, Israel, April 1989), Nucl. Phys. B (Proc. Suppl.) 13 (1990) 1; ACCMOR CoUab., S. Barlag et al., Phys. Lett. B 233 (1989) 522.

[6] CLEO Collab., P. Avery et al., Phys. Rev. Lett. 62 (1989) 863.

[7] M.S. Alam et al., Phys. Lett. B 226 (1989) 401. [ 8 ] ACCMOR Collab., H. Becker et al., Phys. Lett. B 184 ( 1987 )

277; ACCMOR Collab., S. Barlag et al., Z. Phys. C 39 (1988) 451.

[ 9 ] ACCMOR Collab., S. Barlag et al., Phys. Lett. B 184 ( 1987 ) 283.

[ 10 ] R. Bailey et al., Nucl. Instrum. Methods 213 ( 1983 ) 201. [ 11 ] C. Daum et al., Nucl. Instrum. Methods 217 (1983) 361. [ 12 ] ACCMOR Collab., S. Barlag et al., CERN preprint CERN-

EP/88-103; Phys. Lett. B 232 (1989) 561. [13] S.R. Klein, Boston University preprint BU-HEP-89-16

( August 1989 ), and references therein. [ 14] B. Guberina et al., Z. Phys. C 33 (1986) 297. [ 15 ] M.B. Voloshin and M.A. Shifman, Zh. Eksp. Teor. Fiz. 91

(1986) 1180. [ 16] P. Gras, Ph.D. Thesis, University of Valencia (1988);

S. Barlag et al., Production of the charmed baryon A¢ + in n - - C u interactions at 230 GeV/c, to be submitted to Phys. Left. B.

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