Z. Phys. C Particles and Fields 47, 533-537 (1990) Zo,=r,. Partic fiJr Physik C

and �9 Springer-Verlag 1990

Search for narrow baryonia with negative and positive strangeness BIS-2 Collaboration

A.N. Aleev, V.A. Arefiev, V.P. Balandin, V.K. Berdyshev, V.K. Birulev, A.S. Chvyrov, I.I. Evsikov, T.S. Grigalashvili, B.N. Gus'kov, I.M. Ivanchenko, M.N. Kapishin, N.N. Karpenko, D.A. Kirillov, I.G. Kosarev, N.A. Kuz'min, M.F. Likhachev, A.L. Lyubimov, A.N. Maksimov, P.V. Moisenz, A.N. Morozov, V.V. Pal'chik, A.V. Pose, T.B. Progulova, A. Prokes, V.V. Rybakov, L.A. Slepets, A.I. Zinchenko

Joint Institute for Nuclear Research, Dubna, USSR

A.F. Kamburyan, A.A. Loktionov, Yu.K. Potrebenikov, V.I. Skorobogatova Institute of High Energy Physics of the Kaz.SSR Academy of Science, Alma-Ata, USSR

I. Pazoni, I. Veress, P. Zalan Central Institute for Physics of the Hungarian Academy of Science, Budapest, Hungary

A.S. Belousov, E.G. Devitsin, A.M. Fomenko, V.A. Kozlov, E.I. Malinovsky, S.Yu. Potashev, S.V. Rusakov, P.A. Smirnov, Yu.V. Soloviev, A.R. Terkulov, L.N. Shtarkov, Ya.A. Vazdyk, M.V. Zavertyaev Lebedev Physical Institute of the USSR Academy of Science, Moscow, USSR

V.D. Cholakov Hilendarski University of Plovdiv, Plovdiv, Bulgaria

J. Hladky, M. Novak, M. Smizanska, M. Vetsko Institute of Physics of the Czechoslovak Academy of Science, Prague, Czechoslovakia

V.I. Zayachky Higher Chemical-Technological Institute, Sofia, Bulgaria

V.R. Krastev Central Laboratory of Automation and Scientific Instrumentation of the Bulgarian Academy of Science, Sofia, Bulgaria

D.T. Burilkov, P.K. Markov, P.T. Todorov, R.K. Trayanov Institute of Nuclear Research and Nuclear Energetics of the Bulgarian Academy of Science, Sofia, Bulgaria

L.N. Abesalashvili, N.S. Amaglobeli, M.S. Chargeishvili, N.O. Kadagidze, V.D. Kekelidze, R.A. Kvatadze, N.L. Lomidze, G.V. Melitauri, G.I. Nikobadze, T.G. Pitskhelauri, R.G. Shanidze, G.T. Tatishvili Institute of High Energy Physics, Tbilisi State University, Tbilisi, USSR

E.A. Chudakov Institute of Nuclear Physics, Moscow State University, Moscow, USSR Received 11 July 1989; in revised form 9 March 1990

Abstract. A search for baryonia with negative and posi- tive strangeness decaying respectively into A +/~ + pions and z/+ p + pions has been carried out in a neutron beam with a mean momentum of -~ 40 GeV/c in an experiment performed at the Serpukhov accelerator. There is a strong indication of the existence of these baryonia. The following four charge states are observed for negative and positive strangeness: neutral, negative, positive and doubly charged. Their mean mass is 3055_+25 MeV/c 2 and the width F_<36+15 MeV/c 2. The data show that the isotopic spin of the baryonia is > 3/2. The baryonia production cross sections in the acceptable kinematic region Xv>0.2 and Pr< 1 GeV/c times the branching ratios of the observed decays are of the order of 1 gb per nucleon.

The problem of the existence of multiquark states is of great significance for the development of strong interac- tion theory. However, an experimental search for such states has not yet led to convincing results.

A narrow exotic state, a baryonium with negative strangeness decaying into A, /5 and pions, has been observed in the WA-62 experiment carried out in a hy- peron beam at the CERN SPS [1]. Unlike the experi- ment in the hyperon beam, the BIS-2 experiment per- formed in a neutron beam allowed one to search for baryonia with negative and positive strangeness alike. The existence of a narrow baryonium with negative strangeness was confirmed and the indications of the existence of a new baryonium with positive strangeness and also of doubly charged states of the baryonia were obtained [2]. The latter suggests a multiquark structure

534

X . 1 0 3

I ~ = i E i i I

I C3~_1 ~ ( a I [ 650 15 I

if!L [-- I 20 6 0 0

1 I-- I IH �9 Z

550

~C H2 [~ C2 Ct, NO C

4 5 0

x x'Y' Yx II IYxx'Y' X'Y' 400 I I I I I I I I I I I . . . . z o

XY ] XY I ~6 350

Fig. l. Layout of the BIS-2 spectrometer. TI/T2, 3 targets, ~ 300 COM 1/2 - target surrounding counters, PC 1-15 two-coordinate 4 proportional chambers. C 1/2 multicell threshold gas Cherenkov 250

counters. C 3/4- glass Cherenkov counters, MO neutron monitor, M analyzing magnet

0 2 0 0 - 8 0

of these states denoted as Ms (Ms) where s is the sign of strangeness.

New results on a search for and a study of Ms (Ms) produced in neutron-nucleus and neutron-proton inter- actions are presented. They are based on larger statistics in comparison with [2]. The experiment was carried out in a neutron beam of the Serpukhov accelerator. The mean momentum of the beam consisting mainly of neu- trons was 40 GeV/c. A layout of the main elements of the BIS-2 spectrometer [-3] is presented in Fig. 1. The magnetic field in the analyzing magnet M directed along the OY axis caused a 0.64 GeV/c change of the transver- sal momentum of charged particles crossing the field re- gion. The system consisting of multicell threshold gas Cerenkov counters C1 and C2 was used to identify charged hadrons [4]. The counter C1 was filled with air and C2 with freon-12 under atmospheric pressure. The identification system allowed one to identify with some probability charged hadrons in multiparticle events. The trigger conditions required the passage of more than three charged particles.

The thicknesses of the liquid-hydrogen target and the nuclear ones were 2.1 g/cm 2 and 3.4.A ~/3 g/cm 2 re- spectively, where A is the atomic weight of target nuclei. The nuclear targets interchanged each 40-50.103 re- corded events. The results presented here are based on the analysis of 1.9.10 v neutron-proton and 2.4.107 neu- tron-nucleus interactions.

The narrow baryonium with negative strangeness, M~, was searched for by its decays into A, # and pions:

Aprc +, ( la)

A pn + n - , (1 b)

A # n + n + (lc)

and Af in - , ( ld)

~b)

8 -8 0 8

M(p'rf')-M(A) ,MeV/c, M(15"R')-M(A),MeV/c, Fig. 2a, b. Effective mass spectra of V ~ for the systems p~ (a) and #~+ (b) near the A mass

and the charge conjugated baryonium with positive strangeness, ~rs, by its decays into:

3p ~ =, (2 a)

• p n + n - , (2b)

Aprc rc (2c)

and Apn +. (2d)

5.105 events with A candidates and 8.10 4 events with ~3 candidates were selected. A and ~3 were identified by their decays respectively into p~ and #re + with the to- pology of neutral Vee. A pair of oppositely charged par- ticles having a minimal distance between their tracks not exceeding a four-fold experimental resolution in this parameter was considered as V ~ The vertex of V ~ was required to be behind the liquid-hydrogen target or more than 10 cm downstream the back edge of the nuclear targets. A and A were selected according to the invariant masses of V ~ in the (pro-) and (fig+) systems. Figure 2 shows the distributions of the difference between the ef- fective mass of the prc-(#rc +) system and the A table mass for the events with more than one extra charged particle. Clear signals near the A mass are seen in both spectra. Further on those V~ were used, for which the effective mass of the (p n - ) or (# n +) systems were differed from the A table mass no more than 4-fold of the experi- mental resolution. The background among the selected A and .d was 15% and 80%, respectively.

The strange baryonia Ms and Ms were searched for among the events containing A/21 and more than one charged particle h +, all produced at the common vertex of interaction. It was required that the tracks of all the

535

particles A/21 and h e might have a good "convergence" in the target region, i.e. their r.m.s, of distances should be less than a four-fold value of the experimental resolu- tion. This resolution varied from 0.2 to 0.5 cm in runs with different targets. 70613(9253) events containing A (.3) and more than one charged particle were selected in accordance with the above criteria. In the events with A, the negative particle with the largest momentum was assumed to be a/3 candidate and the others to be pions. If the Vee was identified as .3, then the accompanying positive particle with a maximum momentum was taken as p and the others as pions.

The information obtained from C1/2 was used to reduce the background due to the charged particle erron- eously identified in the considered combinations. For each of the charged particles relative probabilities W(i) of identification with an /-type particle (i=n +, K + or p//5) were calculated by comparison of the signals in dif- ferent C1 and C2 channels with calculated distribution of Cherenkov light and the corresponding number of photoelectrons from the detected charged particles over these channels. The relative probabilities W(i) were nor- malized in such a way that their sum for each of the detected particle would be W(p//5)+ W(K+-)+ W(n-+)=3. Thus, the value W(i) = 3 means a 100% reliability of the particle identification as a type/ . Values W(p//5) =W(K+)=W(n+-)=I mean a completely unidentified particle and so on. Some ambiguities of the charged par- ticle identification were caused by the Cherenkov light from one particle to fall in several channels. The calculat- ed identification efficiency shows that this depends not only on the momentum of the charged particle, but on the total number of these particles in the event as well. Therefore, the numerically equal restrictions on W(i) lead to the different efficiencies of an / - type particle selection in the events with different final states.

In paper [-2] the results of the analysis were pre- sented, where the restrictions on W(i) for one and the same type (i) of the charged particle were different in the various final states (1 a-d) and (2a-d). The optimiza- tion of each analysed final state was performed to obtain the best signal to background ratio. This could lead to an artificial increase of the signals. In the present paper for all the final states the same W(i) criteria were adopted to identify a i type hadrons.

The identification of/5 is necessary mainly in search- ing for baryonium with negative strangeness among the final states (1 a-d). About 80% of all negative particles, candidates in/5, have momenta below the threshold of the Cherenkov radiation for kaons. For these particles W(/5)= W(K-) and the maximum values of W(/5) do not exceed 1.5. It was required for the searched events that

w(/5) __> 1.1. (3)

Using the selection criterion (3) the completely unidenti- fied negative particles were excluded. There remained a sufficient part of K - ' s among the candidates in p's, but were excluded 90% of g - ' s and not more than 20% of/5's. For the selection of particles candidates in ~+,

there was used a condition W(r~ +-) > 0.4. As a result more than 90% of all the pions were remained. After this anal- ysis of the final states (1 ~ d ) the number of investigated combinations was reduced by nearly tenfold. Taking into account the data for/5 production at the Serpukhov ener- gies [-5] and the selection criteria used, it was estimated that ~ 8 % of the selected combinations should contain A +/5 + pions.

Because of different background conditions of the final states ( l a -d ) and (2~d) , the similar efficiency of their selection was achieved by different conditions of the/5 and p identifications. The used proton identification criterium,

W(p) > 0.6, (4)

was less stringent, than condition (3) for the/5 identifica- tion. Thus, for the investigation of the states with positive strangeness (2a-d), only those combinations were ex- cluded in which candidates in p's with hight probability might be identified as r~+'s. This allowed one to save more than 90% of all p's. Antibaryons .3 in the final states were identified only by geometrical and kinemati- cal features. The used selection criteria almost halve the number of candidates in the final states (2a-d). So, the reduction of the background combinations in the final states (1 a-d) and (2a-d) was different.

The effective mass spectra obtained for the selected combinations of the neutral, charged and doubly charged final states with negative strangeness (1) are pre- sented in Figs. 3 a, 4 a and 5 a, respectively. The effective mass spectra of the final states with positive strangeness (2) are presented in Figs. 3b, 4b and 5b. The numbers of combinations entered into these spectra are presented

5 > 0 o C~

o o c-

c-

O

c

E 0 U

E z

140

120

100

8O

60

40

20

%

I (Q)

I | I I 1 I I

2.6 3. 3.4 3.8

240 ' ' ' ' J ' ' " i

200 . t it ( b ) / 1+0 . ]

120

80

40 '~

I f I I I i ~ 1 " " ~11

2.6 3. 3.4 3.8 E f f e c t i v e moss , G e V / c 2

Fig. 3a, b. Effective mass spectra of A/5~ + (a) and A p ~ - (b)

536

o CM 0

0 E

O

(3 E

55 E o (3

"6

.13

E -1

z

70

60

50

40

30

20

10

% 2.6 3.

,,,] ( ~ )

% I I I "1

3.4 3.8

l I I I I I I

140

120 ( l~ l

100

/11% % 26 3. 3.4 3.8

Effect ive moss, GeV/c~

Fig. 4a, b. Sum of the effective mass spectra for the final states A#~ + ~• (a) and 3p~z- ~-v (b)

> I1) (.9 c~ o o

O

O E

55 E O (9

~6

.13

E z

100

80

60

40

20

5.2

I I [ I I I I

, / l co)

t i r ' i II

2.6 3. 3.4 3.8

160 , , , , , ,

140 t ( b ) 12o

,oo tt 80

60

4o I I 20

Q2.~ 2.6 3. 3.4 3.8 Ef fect ive moss, GeV/c~

Fig. 5a, b. Effective mass spectra A /~z - (a) a n d / l p ~ + (b)

in Tab le 1. A b in wid th of 20 M e V / c 2 was chosen to be close to a two-fo ld value of the expe r imen ta l mass resolut ion. The spec t ra were a p p r o x i m a t e d by s m o o t h functions. E n h a n c e m e n t s of different s ta t is t ical signifi- cance were seen in all the spectra. They were fi t ted by a n o r m a l d i s t r ibu t ion . The o b t a i n e d p a r a m e t e r s of the

> (D c~ c) d c

0

0

_0

g O O

.13

E z

zOO

350

300

250

200

150

100

50

i I 1 i I l i

-i t l n Jl ~,~ % , ( O ) i h . l h l 111 I ,I U I I

dJ I1{~ I | n i '11 I i I

fJ,I u

% , , , , , , , 2.6 3. 3.4 3.8

350

3 0 0

250

2 0 0

150

100

50

2 2.6 3. 3.4 3.8

Ef fec t ive moss, GeV/c2

Fig. 6a, b. Sum of the final states: (a) ApTt • and A/Src + 7~ • effective mass for soft (dashed line) and stringent (solid line) criteria of charged track identification; (b) 3p~z + and Apn- ~ effective mass

Table 1

Final Number states of comb.

Parameters of the enhancements

Mean mass, Width, Number of MeV/c 2 MeV/c z combinations

A/Sg + 4815 3 0 6 0 _ + 3 0 55+15 92+33 Apg 5 7 1 5 3 0 4 0 _ + 3 0 40+15 47+24 A/57r+ ~ + 1 5 6 2 3 0 6 0 _ + 2 5 3 0 _ + 1 0 43+17 ApT~ 7z T 4295 3045-t-25 30+15 71+36 Ap~ 2694 3070-t-30 70+25 65+25 .4pTr + 4362 3 0 4 0 _ + 3 0 3 5 - 1 - 1 5 47-+20

enhancemen t s are shown in Tab le 1. A c o m b i n a t o r i a l b a c k g r o u n d in the p resen ted spec t ra is insignificant. So, the ra t io of the n u m b e r of c o m b i n a t i o n s to tha t of events does not exceed 1.1 wi thin the mass region of these en- hancements .

The existence of the enhancemen t s in all the pre- sented spec t ra of the final s tates at close masses a l lows one to suppose tha t they are due to signals f rom different charge states of the two resonances : one with negat ive and a no the r with posi t ive s t rangeness . The to ta l spec- t rum of all the final states with negat ive s t rangeness (1 d) is p resen ted in Fig. 6a by a sol id line. I t is o b t a i n e d by summar i z ing the d i s t r ibu t ions p resen ted in Figs. 3 a, 4 a and 5a. A clear peak caused by 150 c o m b i n a t i o n s

537

above a background of 380 ones is seen in this spectrum near a mass of 3060 MeV/c 2. The sum of the final states with positive strangeness (2a-d) is shown in Fig. 6b. In the same mass region, as in effective mass spectrum of final states (1 a-d), the effective mass spectrum of the states with positive strangeness (2a-d) contains a peak, which is caused by 140 combinations over the back- ground 522 ones. Statistical significances of the observed peaks are 7.5 and 6 standard deviations above the back- ground, respectively. This enables one to consider them as physical signals.

Narrowness of the observed enhancements and their presence in the invariant mass spectra of different sys- tems excludes the possibility of their interpretation as kinematic reflections of any resonances. However, the hypothesis of kinematic reflection was checked directly as p//5 had not been identified unambiguously. The dashed histogram in Fig. 6a presents the invariant mass spectrum of the sum of the final states (1 ~ d ) , obtained for the combinations, selected with (W(/5)>0.9) condi- tions of t5 identification softer than condition (3). The effective mass spectrum of the sum of the final states ( l a -d ) obtained for such combinations in which com- pletely unidentified h - ' s were also accepted as/5 candi- dates is presented in Fig. 6a by a dashed line. Almost 90% of all p's and 30% of all r t - ' s remained among negative particles under this selection. Among all the combinations, the fraction of combinations containing A + / 5 + p i o n s should be -~4%. As would be expected, in comparison with the solid-line histogram in Fig. 6a the background increased appreciably with an insignifi- cant change in the number of combinations in the peak. As another check, the masses of rc + and K + were as- cribed to the p//5 candidates. No narrow enhancements were observed in the obtained effective mass spectra. Thus, the hypothesis of enhancements as kinematic re- flections of resonances in other systems is excluded.

Statistical significances of the enhancements in each of the considered spectra (Figs. 3-5) do not allow one to establish unambiguously the existence of signals in all the final states ( l a -d ) and (2~d) . However, the numbers of combinations responsible for the signals in the total spectra of Fig. 6a and b coincide, within the errors, with the sum of the numbers responsible for the enhancements in the three spectra of the final states with negative and positive strangeness (see the table). Differ- ences between the central masses of the enhancements for each of the states (1 a-d) and (2a-d) are insignificant. This allows one to suppose that all or the majority of the observed enhancements are physical signals, i.e. four charge states of Ms and A4 s are observed.

The geometry of the spectrometer, the Coulomb scat- tering of charged particles, the spatial resolution of the proport ional chambers and their efficiency, and also the efficiency of event reconstruction by the used programs

have been taken into account for the Monte-Carlo calcu- lation of the detection efficiencies for the studied pro- cesses. The systems under consideration were accepted in the kinematic region:

Xe >-_ 0.2 and Pr--< 1 GeV/c, (5)

where Xv is the Feynman variable and Pr the transverse momen tum of the systems. Because of set-up asymmetry in the XOZ plane, the geometric acceptance for the final states containing A was slightly larger than for those containing A. The product ion cross sections of M s and Ms in the kinematic region (5) times the branching ratios of the observed decay modes were estimated taking into account the calculated efficiencies. They coincide within the errors and are of the order of 1 gb per nucleon. The dependence of A =/3 on the atomic weight A was used to recalculate the cross sections per nucleon.

Conclusion

Narrow enhancements have beed observed at almost the same masses in the effective mass spectra of different final states with negative (A/src -+, Aprc + rc -+) and positive (Aprc T-, Aprc-rc ~7) strangeness. They are not kinematic reflections of any resonances in other systems and can be considered as a strong indication of the existence of baryonia with negative and positive strangeness.

These baryonia decay into A/A,/5/p and pions. Their mean mass is 3055 +25 MeV/c= and the width smaller than 36_+ 15 MeV/c 2.

The existence of the enhancements in the spectra of the doubly charged final states A/5rc- and Aprc § shows that the isotopic spin of the baryonia is > 3/2.

Acknowledgements. The authors are greatly indebted to A.M. Bal- din, S.S. Gershtein, A.A. Komar, E.I. Maltsev, I.A. Savin, A.N. Sissakian, A.N. Tavkhelidze, N.E. Tyurin and P.A. Cherenvov for their support and permanent interest in the study; to Yu. Klabuhn, E.M. Likhacheva, H. Nowak, H.-E. Rysek, L.V. Sil'vestrov, G.G. Takhtamyshev and K. Hiller for their participation in the experi- ment.

References

1. M. Bourquin et al.: Phys. Lett. B172 (1986) 113; S. Cooper: Pro- ceeding of the XXIII International Conference on High Energy Physics vol. 1, p. 67. Berkeley 1986; H.W. Siebert ibid. vol. 2, p. 1015

2. A.N. Aleev et al.: JINR Rapid Communications, # 19 86, Dub- na 1986, p. 16; JINR, D1-88-368, Dubna 1988

3. G. Eichner et al.: JINR, 1-80-644, Dubna 1980; A.N. Maksimov: JINR, 1-81-574, Dubna 1981

4. M.A. Voichishin et al.: JINR, 13-84-161, Dubna 1984; PTE, # 3 (1985) 49; B.N. Gus'kov et al.: JINR, 13-84-373, Dubna 1984; PTE, 41- 5 (1985) 71, JINR, P1-86-248, Dubna 1986

5. K. Acherloff et al.: Preprint IHEP, 77-86, Serpukhov, 1977