CZECHOSLOVAK Volume 41 �9 1991

JOURNAL OF PHYSICS Number 4 �9 pp. 297-396

SEARCH FOR THE CHARMED STRANGE BARYON E~ + IN NEUTRON-PROTON INTERACTIONS AT ~/S ~ 10 GeV

BIS-2 Collaboration

M. Vecko, IV[. Novfik, J. Hladk~, J. KrAsovA, M. Smi~anskA

Institute o f Physics, Czechosl. Acad. Sci., Na Slovance 2, 180 40 Praha 8, Czechoslovakia

A. N. Aleev, V. A. Arefiev, V. P. Balandin, V. K. Berdyshev, V. K. Birulev, A. S. Chvyrov, B. N. Gus'kov, M. N. Kapishin, D. A. Kirillov, I. G. Kosarev, N.A. Kuz'min, M. F. Likhachev, A. L. Lyubimov, A. N. Maksimov, A. N. Morozov,

A. V. Pose, A. Prokeg, L. A. Slepets

Joint Institute for Nuclear Research, Dubna, USSR

A. S. Belousov, E. G. Devitsin, A. M. Fomenko, E. I. Malinovski, S. Yu. Potashov, S. V. Rusakov, Yu. V. Soloviev, L. N. Shtarkov, Ya. A. Vazdik, M. V. Zavertyaev

Lebedev Physical Institute, Acad. Sci. o f the USSR, Moscow, USSR

E. A. Chudakov

Institute o f Nuclear Physics, Moscow State University, USSR

V. J. Zayachky

Higher Chemical-Technological Institute, Sofia, Bulgaria

V. R. Krastev, P. K. Markov, P. T. Todorov

Institute o f Nuclear Research and Nuclear Energetics, Bulg. Acad. Sci., Sofia, Bulgaria

N. S. Amaglobeli, V. D. Kekelidze, R. A. Kvatadze, N. L. Lomidze, G. T. Tatishvili, R. G. Shanidze

Institute o f High Energy Physics, Tbilisi State University, USSR

A. F. Kamburian, A. A. Loktionov, Yu. K. Potrebenikov, V. I. Skorobogatova

Institute o f High Energy Physics, Acad. Sci. o f the Kazakh SSR, Alma-Ata, USSR

Received 31 July 1990; in revised form 18 October 1990

Czech. J, Phys. 41 (1991) 297

M . Vecko et al'.

In this paper we present the results of a search for the charmed strange baryon E+ in the final states A~ - and E-~+n +. The experiment was performed using the magnetic spectro- meter BIS-2 with a hydrogen target located in the neutron beam of the Serpukhov accelerator. A narrow peak in the A~ - state is observed at a mass of ~2440 and possibly also of ~2310 MeV/c 2, corresponding to signals for the E + ~A~ and E + --* Z~ (1~o ~ AOT) decays respectively. The statistics obtained for the -~-rc+n + state is too low to make any conclusion.

1. Introduction

Many experiments contributed to a better understanding of the decay properties of the 3 weakly decaying charmed mesons (D ~ D +, D +) and the A~ + baryon, whereas still noch much is known about other weakl~ decaying charmed baryons E~ +, ~c=~ and n o"

The BIS-2 experiment was the first case where a statistically significant direct observation of hadroproduction of charmed baryon A~ + at energies different from ISR was made. The first observation in neutron-carbon interactions [1] was later on supported by the observed production of A + on a hydrogen target [2] and o f mesons 13 o and 13- on a carbon target [3]. Using the hydrogen data sample we report here on the searches for a charmed strange baryon E~ +.

The -~+ has been observed up to now in 3 experiments. The first was the C E R N Hyperon group WA-62 using a 135 GeV ~ - beam and a Be target [4], who obtained a 6 standard deviation signal in A~ - system with a mass value of (2460 + 15) MeV/c 2. This value was confirmed by the value obtained by the E-400 experiment at Fermilab of (2459 + 5 + 30) MeV/c 2 using a neutron beam with a mean energy of about 600 GeV and W, Si and Be targets [5]. In the invariant mass A~ however, two peaks were observed, one at the E~ + mass, the other (76 + 8) MeV/c 2 lower, and the authors of ref. [5] attribute the second peak to the E~ + ~ Z~ - (S ~ ~ A~ decay chain with the Y unseen. The third observation is that of the: A C C M O R (NA-32) collaboration using 230 GeV/c ~ - ' s incident on a Cu target. As a by-product they found in their 1985/86 data 6 events interpretable as E + --* E-~z+~ +, "~- ~ A~ - and E~ + --* Z+K-rc +, 2 + --+ pn ~ (or charge conjugated decays) with a E~+ mass value equal to (2466-5 + 2.7 __ 1.2) MeV/c z [6].

2. Experimental apparatus and data analysis

Our experiment was performed with the use of the magnetic spectrometer BIS-2 [7] located in the neutron beam of the Serpukhov accelerator. Briefly the spectro- meter consisted of 26 multiwire proportional chamber planes located in front of and behind the analysing magnet, which changed the transverse momentum of charged particles by 0.625 GeV/c. Two multichannel threshold gas Cerenkov counters [8] were used for identification of charged particles in the momentum range of ( 3 - 4 0 )

298 Czech. J. Phys. 41 (1991~

Search f o r E + in np interactions. . .

GeV/c. A schematic view of the set-up is presented in Fig. 1. The multiplicity trigger required four or more charged particles passing through the selected proportional chambers and through the scintillation hodoscope (H 2). The neutron beam had a broad band momentum spectrum with a mean value near 50 GeV/c.

X

~ r / t l i ',. i I ,el i t--tlU i' u

u ~o~o:Z, TM

X'Y' YXI

/i H2 q C2 r

iUn I -"lfn lull I

;I; -~ Z

NO

;;; :- Z

Fig. 1. A schematic view of the BIS-2 set-up: A-scintillation anticoincidence counter, T-target, SOM-cylindrical scintillation hodoscope, PC-proportional chambers, M-analyzing magnet, C1, C2--Cerenkov threshold counters, H2--scintillation hodoseope, C3, C4- - l ead glass walls, M O - neutron beam monitor. (No information from SOM and C3, C4 has been used

in the present data analysis.)

A search was made for E + via the two Cabibbo favoured decay modes containing a neutral strange particle accompanied by charged particles in the final state:

1) E + --* A~ (i)

2) -~+ ~ E-7~+n + a - ~ A~ - (ii)

with a subsequent decay A ~ ~ pn- . A total of 9.9 million events were processed; the experimental material being the same as the one where A + production in the A~ - decay channel was observed [2].

The minimum spatial separation of a pair of oppositely charged tracks ("Vee") originating from a potential A ~ decay should be less 1 cm, and the reconstructed decay vertex should lie within the decay volume beginning 5 cm downstream from

Czech, J. Phys. 41 (1991) 2 9 9

M. Vecko et al.

the e n d o f the target up to the first proportional chamber. Figure 2 illustrates the (pn- ) effective mass spectra of these "Vee's" satisfying a further relatively weak condition, derived from a Monte Car lo calculation, restricting the ratio o f the momenta of t he positive and negative particles forming the "Vee" to lie between 3 and 15. If the effective mass of this "Vee" is within _+7 MeV/c z of the A ~ mass,

1000

~O

V" \

Z 500

, , ~ ~ j ~ ~ ~ ~ [ J a a I 1 ' ~ J E-

- d O - 5 O 5

MCp~-) -- MCA~

'I0

E M~V/c ~ ]

Fig. 2. The (pn-) effective mass spectrum.

it is identified as a A ~ (the additional requirement of the correct Cerenkov identifica- tion of the A ~ ~ p n - decay products influences this sample only very little). After fitting this distribution with a linear background and a Gaussian, the obtained mean value is shifted by (0.86 ___ 0-05)MeV/c 2 from the A ~ mass of the Particle Data Group, which is probably caused by systematic errors in determining the coordinates of the proportional chambers. The ratio o f the A ~ signal to background is 2.0. Altogether 4537 events with a A ~ each accompanied by at least two positive and one negative particle, were selecled. The mean multiplicily of charged tracks in these events is 5.34, and the maximum number of tracks in one event is 11. In our momentum region only ,-~1~ of the selected A ~ ~ pn - decays can be interpreted also as K ~

n + n " decays [2].

a) D e c a y (i)

Looking for the E + --* A~ - decay, combinations of A ~ and three additional charged hadrons (h+h+h -) were constructed. They should have a common vertex

3 0 0 Czech. J. Phys. 41 (1991)

Search f o r E + in np in t e rac t ions . . .

_+5 cm within the target region with a root-mean-square distance from all the trajectories smaller than 0.9 cm. The decay of A ~ should lie more than 15 cm down- stream of this common vertex, and the maximum allowed momentum of each combination is 65 GeV/c. The experimental procedure was the same as in the case of the A~ + ~ A~ -, but with the new requirement of the correct Cerenkov identification of charged hadrons in decay (i), namely that of K - . Therefore a complementary criterium accepting negatively charged particles (h- ) only with momenta greater than 3 GeV/c (threshold for the pion identification) is used. This cut quite significantly decreases the combinatorial background caused by the low energy non-identifiable particles. The (A~ -) effective mass of these combinations without the use of any information from Cerenkov threshold ccunters is shown in Fig. 3. The resulting distribution using as accompanying particles only

30

% \ > @ Z tO 60

0 "1.8

\

f

2 . 3 Z . 8 3 . 3

90

Fig. 3. The ( A ~ ; ) effective mass spectrum without the use of Cerenkov identification for rc + and K - .

those identified as ~+ and K - is shown in Fig. 4. Furthermore, when two or more combinations in one event have effective masses closer than 25 GeV/c 2, then the combinations with the largest RMS distance of the vertex from all the trajectories are disregarded. The combination-to-event ratio in this distribution is 1"08.

Two narrow peaks are seen jn Fig. 4, one at ~2440 MeV/c 2, the second at the top of the distribution at ~2310 MeV/c 2. We have checked that they are not reflections of the A + ~ A~ ~ peak observ'ed in ref. [2].

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M. Vecko et al.

\ 4O g

~ 30 \

Z

20

10

0 1 .8 2 .3 2 . 8

~ 0

1.3

1,4 C A~ ~*K- 3 t : G e V / c = : l

Fig. 4. T h e (A~ - ' ) effective m a s s spec t rum.

50

t~

o 40

5-

f,l 3(? \

7"

LI

20

.rl 0 , , . . . . . . , , n 1.8 2 .,3 2 . 8 ~ .3

M C Ao~*~:-K - ) I: G e V / c = 3 '

Fig. 5. T he ( A ~ ~) effective m a s s spec t rum.

302, Czech. J. Phys. 41 (1991)

Search for rze+ in np interactions...

No similar signal is observed in the same mass regions (marked with arrows) in the opposite charge state A~ - (Fig. 5). Similarly no signal is observed in the A~ - decay channel (Fig. 6) which is suppressed by Kobayashi-Mas-

70

r, 0 601 \ > r

~E 50

tN \ 40

7

30

20

10

0 I ;1.8

, i , i I ' ~ l i

2 .3 2.8 3 . 3

MC A~ - ) E G e V / J ' I

Fig. 6. The ( A ~ --') effective mass spectrum.

kawa matrix elements including a factor of sin 2 0c/cos 2 0c, 0c being the Cabibbo angle, compared to the decay (i). Each of these two effective mass distributions is obtained using the same cuts as for decay (i).

b) D e c a y (ii)

Looking for this decay channel, combinations of A ~ and one negatively charged hadron (h-) were constructed. Candidates for the E- -~ A~ decay should have a vertex with minimal distance between its two tracks smaller than 1"1 cm; the vertex should lie more than 5 cm downstream of the end of the target and have a z-separa- tion from the A ~ decay vertex greater than 10 cm. The maximum allowed momentum for a = - candidate was 55 GeV/c. Using Cerenkov identification for n - , the resulting effective mass spectrum of the (A~ -) system in events with total multiplicity of positively and negatively charged particles greater than 3 and 2 respectively, is shown in Fig. 7. A signal for E - is seen in this figure, and therefore 63 events from this distribution with effective mass within 4-15 MeV/c = of the E - mass value of the Particle Data Group are taken for further investigation.

Czech. J. Phys. 41 0991) 303

M . Vecko et al.

A pr imary vertex reconstructed f rom a A~ - system and two charged hadrons identified as rc+'s should lie within +__5 cm o f the 'target region and simultaneously

more than 10 cm in f ront of the subsequent potential "~- --* A~ - decay vertex.

.30

~o \ > iI) 7- 20 If) \

Z

1.25 1 .35 1 .45 .55

I"I C A ~ - ) [ G ' e V / c 2 ]

Fig. 7. The (A~ -) effective mass spectrum.

1~ ) ,

~0 \ > g IE

tO 5 N \ Z

13 1.3

1

1 | v I

1.8 2 . 3 2.6

I'4 ( =___,-~*~,~" 3 E G e V / c = 3

Fig. 8. The (E-~+rc +) effective mass spectrum.

The effective m a s s distribution o f these combinat ions with momenta n o t greater

than 65 GeV/c is shown in Fig. 8. Using the same binning as in Fig. 4, the arrow points to the region where one could expect the E~ + signal.

304 Czech. J. Phys. 41 (1991)

Search for E+r in np interactions:..

3. Results

We have observed two peaks in the A~ - mass distribution (Fig. 4). No signal is found in the charge conjugated [Fig. 5) and Cabibbo supressed (Fig. 6)" decay modes, and therefore we are lead to interprete the observed structure as two decay modes of the charmed strange baryon ~c'~-'. ~c'~- "* A~ - l upper signal) and E + ~ 2;~ (lower signal) with a subsequent decay E ~ --* A~ where 7 is not detected.

The absolute mass value of the upper E i signal, ~2440 MeV/c 2 is about 20 MeV/c a lower than the value obtained by other experiments [ 4 - 6]. However the same shift of ,-,20 MeV c 2 to lower masses was observed also for the A~

A~ - signal with the same experimental material E2] and is most probably caused by the systematic error in proportional chamber coordinates mentioned above.

Concerning the A%c+~+K - mass difference between the observed E~+ signals, the authors of ref. [5] claim that it should be 75 MeV/c 2 which corresponds to the energy of the missing 3' in the t2 o rest frame. But taking into account the non- zero momentmn of the 2; o in the E~ + rest frame, the predicted values cf this separation lie in the interval (42-136) MeV/c 2 [9]. Consequently, one expects also a broadening of the lower E~ + signal.

The separation of the peaks in Fig. 4 is in the predicted range, but the position of the lower one at the top of the experimental distribution makes a reliable determina- tion of its exact position and width more difficult.

The statistics of the E + --~ E - ~ + ~ + decay is too low to make any conclusion. Monte Carlo simulation has not been done, but as a first approximation, one could

expect for the decay (i) similar detector acceptance and efficiency as for A + --* A~ - decay [ 2 ] - the beam momentum dependent efficiency of E~+ de- tection rising sharply for beam momenta greater than ~45 GeY/c and effective mass resolution near ( 1 0 - 1 5 ) M e V / c 2. In this case the production cross section of the E + times the branching ratio of its decay (i) will be of the same order of magnitude as for the production of the A + followed by its decay A + ~ A~ - .

4. Conclusion

Considering the results obtained as a by-product of our data acquisition aimed at a search for A + and motivated by the results of previous experiments, we conclude that we observe structures in the (A~ - ) effective mass distribution caused by E + ~ A~ - and probably also by Z + --+ Y,~176 --. 1~ decays with a mass of the E + around (2440 • 20) MeV/c z.

We are grateful to K. Hiller, F. Mandl, M. Markytan and J.MacNaughton for useful discus- sions and valuable remarks.

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M. Vecko et al.: Search for ~+ in np interactions...

References

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[2] Veeko M. et al.: Czech. J. Phys. B 39 (1989) 297. Veeko M.: Thesis, Prague, 1987.

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Guskov B. N. et al.: Pdb. Tekh. Eksp. 5, (1985) 49. Guskov B. N. et al.: Preprint R1-86-248. JINR, Dubna, 1986.

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