c2

NUCLEAR PHYSICS A

Nuclear physics A558 (1993) 19Jc-200~ North-HolX%ud,Amsterdam

A. Ferrer”, A. A. Grigoryanb, V. F. Perepeiitsa”, P. Sondereggerd “Departamento de Fisica At&mica Molecular y Nuclear, T&&r. of Valencis Centro Mixto Univ. of Vaiencia - CSIC, Avda. Dr. Mofiner 50, E-46100 VsJencia, Spain

xznd IFIC, Burjassot,

bTheoretical Department r YerPhi, Yerevan Physics Institute, Dr. Alikhanian st . 2, 375036 Yerevan, Armenia “ITEP, B, Cberemushkin~~~~a 25,117259 “CERN,

Moscow, Russia

1211 Geneva 23, Switzerland

Abstract We report evidence for the existence of a nmrow p_i state with a mass of 242 GeV/c’ and a width of about 0.02 GeV/c’, produced centraHy in x”p(w-p) interactions at 2Of12) GeV/c in sever2 final states, proceeding via baryon exchange mechanism, and that could be interpreted as a ~~T~~~~~ candidate, The data come from the am+& of WA56 experiment made at the CERN 0 spectrometer,

Two mwrow g+ states of masses 2.02 and 2.20 GeVJc2 have been observed by Benkheiri et al. in a--p interactions at 9 and 12 GeV/c [l]. These results were interpreted as backward production of the observed states in association with fast forward A( 1232) and N(1520) baryons, following a quasi-two-body process. Those results were not confirmed Later in several backward production experiments f2-61, among which the most sensitive was just the WA56 experiment, based on 4 constraint events f$]. In a new analysis of the WA56 data we have studied the events of baryon exchange reactions

037~-~74~~~06.~

0 199‘3 - Bsevier Science Publishers B.V, AJX.tights resented.

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A. Ferrer

n-p +

et al. I Narrow ppstate

of mass 2.02 GeVtc~

PfPiji~“($i),

(4

at 12 GeV/c, where pf is a fast proton produced with a momentum greater than half the beam momentum, and the brackets refer to slow pions that go undetected by the R apparatus but are however reconstructed imposing 1-C kinematics fits, as explained below. The very good momentum resolution available from the Q tracks measurements allows a clear separation and identification of one missing pion channels in our data. As an example of this, we refer to the recently reported results on the central production of p”, fi and p!j mesons in the baryon exchange reaction: a+p

-+ pf7r+“-(“f)

(5)

at 20 GeV/c, based on the WA56 data (73, wh ere an experimental approach similar to the one presented here was used . In the analysis of reaction (5) it was shown that, in baryon exchange induced reactions, the central region is suitable for meson spectroscopy studies, providing a good signal/background ratio. The work described in this paper results from the continuation of the investigations in the central region, with events of reactions (1) to (4), where we have found the evidence for the 2.02 GeV/c’ pp state. Also included in this paper are the upper limits on several mesonic (n+a-,2nf2nand K+K-) decay modes of the observed state which suggest that this state is a firm baryonium candidate since it is coupled strongly to baryons and decoupled from mesons [g-12].

2. EXPERIMENTAL

DETAILS.

SELECTION

OF EVENTS

The layout of the 0 spectrometer as used in the WA56 experiment is described in detail in [6,7]. W e recall here that the main trigger requirement was the detection of a fast forward proton with momentum greater than 10 (7) GeV/c in ~+p (ST-P) reactions at an incident momentum of 20 (12) GeV/c. The fast proton signature was determined with the aid of two downstream multicell Cherenkov counters Cl and C2; Cl having momentum thresholds for pions, kaons and protons at 2.6, 9.6 and 18.2 GeV/c, respectively, and C2 having thresholds approximately a factor of 2 higher. We note that due to the high threshold of Cl for kaons we expect, in the case of x-p exposure, a large background of Kf+,,,t triggered events with kaon momenta close or lower than 9.6 GeVjc. In addition to the Cherenkov counters two other types of particle identificators were implemented in the WA56 experiment; the time of flight measuring system (TOF) and the 24-element scintillation hodoscope (“barrel”) surrounding the hydrogen target. The TOF, with a measured time resolution u = 0.7 ns, allowed p/x+ (F/T-) separation from 0.4 to 2.0 GeV/c momenta in vr+p (x-p) reactions. The barrel, with an average resolution of 30% in dE/dx measurement, allowed separation of p and ii from rr up to 1.0 GeV/c, and K from rr up to 0.5 GeVJc momenta, with a reasonable efficiency. The use of the response of these particle ident~ficators has proved to be crucial for the confirmation of the validity and quality of the method used for the association of events to each channel. Events were selected according to the following criteria. The data samples were composed of 3-prong events of _t + - topology (with one charged track undetected by 0 spectrometer) and 4-prong charge-balanced events with a missing neutral(s). Then, fol-

A. Ferrer et al. I Narrow pp state of mass 2.02 GeVlc2

193c

lowing the same method as used in [7] for the study of reaction (5), a 1-C fit procedure was used to reconstruct the 4-momenta of missed pions from reactions (l-4). First, the was calculated with appropriate mass hypotheses for the missing mass squared (MM’), detected particles, where prominent signals of lost n:, 7rf and “9 were seen; the events were retained if they felt in the rni band, namely ]MM2 - rnz] < 0.16(0.12) (G~V/C’)~ in the x+p (r-p) exposure, finally a 1-C momentum-energy balance fit was performed. For the analysis presented here we retained all those events which passed this 1-C kinematical fit with a P(x”) > 5%. H owever, for events fiiting reaction (2) we retained only those having rapidity values yrt > ypF and for events fitting reaction (4) we retained only those having p,- < %. The experimental sensitivities corresponding to the WA56 analysed data are 150 (30) eventsfnb in n+p (x-p) exposure.

3. THE pp MASS

SPECTRA

The pp mass spectra from reactions (1,2) are shown in Figures 1 and 2, where peaks at a mass close to 2.02 GeV/c’ are clearly seen. Though the signal/background ratios seem to be small, the mass spectra fits reveal that these peaks are well above the background.

500

“0

240

400

> al x 300 0

160

7 z 00 ii2

80

& 100

0

40

t t

2

I

I,

I,

2.2

.,

,

1,

2.4

p f~ mass,

0

GeV/c’

Figure 1. Spectra of pp mass for events from reaction (1): a ) a11events; b) events satisfying the cuts Ap = p,, - p,f > Aprnlr, and cos BJ < 0.6.

A. Ferrer et al. I Narrow ppstate of mass 2.02 GeVtcz

194c

The fits were made with a parameterization

of the pjj mass spectra of the form

A(M-Mo)“exp{(Mo-M)P}(l+BW)

(6)

where Ms = 2mp, and BW means a Breit-Wigner term. The resulting parameters of the BW together with the x2 values obtained, the number of degrees of freedom and the statistical significances of the signals (in the standard deviation units) are displayed in Table 1.

Table 1 Parameters of the fits of the pj~ mass spectra from reactions (l-4).

Spectrum

Statistical significance of the pj! peak

X;it/ndf

Mass of pp state,

Width of pp state,

(MeV)

(MeV)

(sd) 20 GeV/c s+p data Figure la Figure lb Figure 2a Figure 2b Figure 3a Figure 3b 12 GeV/c a-p data Figure 4a Figure 4b Figure 5b

6.0 7.0 6.9 6.0 4.5 6.4

39155 38 155 51155 45155 63155 61/55

2009 2012 2021 2016 2012 2014

& f f f f f

6 6 6 7 7 7

3.1 3.8 3.0

41152 42152 71152

2007 f 6 2006 f 6 2010 f 5

30 34 35 39 33 38

f 12 f 21 f 24 f 20 f 17 f 18

19 f 14 18 f 8 12 f 12

Two kinematical cuts were employed to suppress the background coming from the forward production of a p/p sytem via one-pion exchange mechanism (“diffractive” background) and to reinforce the yields of the central regions of reactions (l-4): i) cos 6~ < 0.6, where 6’~ is the Jackson angle of the pin the pp rest system, with the z axis taken along the ($bcom - &,,) and ($benm- &, - &+ ) directions for reactions (1) and (2), respectively (th e d irections of the virtual exchanged baryon in these reactions). In case of pp resonance production the cos BJ distribution is expected to be symmetrical. The cut given above was good enough to suppress the diffractive background, which produces the strong forward/backward asymmetry observed in the data. ii) The cut Ap = j&l - I&,1 > Ap,,,i,,, with Ap,i, = 1 and 0.8 GeV/c for reaction (1) and (2), respectively, turned out to be rather effective to isolate the pp system from the slow particles in all reactions under investigation. In addition to the kinematical cuts, we have selected samples of events according to the particle identificators response; that is, events in which the p was identified by the barrel or TOF system and/or the p was identified by the barrel (Figure 3). The 2.02 GeV/c2 pp signals are clearly present after this selection. This is important as it contradicts the

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A. Ferrer et al. I Narrow pjstate of mass 2.02 GeVlc2

1200 350 1000

300

"0 \

2800 I

250

0 - 600 \ .z

200 150

6> 400 LLJ

100

200

0

50 I

2

..*,I.,,,,,

2.2

2.4

p p mass, GeV/c2 Figure 2. The same as in Figure 1 for events from reaction (2) with the aditional requirement that y(pp) > y(?r+). 160 r

b)

p jj

mass,

GeV/c2

Figure 3. pp mass for events from reactions (l),a), and (2), b), which satisfy the cuts as in lb), 2b), and have the p identified by TOF or barrel and/or p identified by barrel.

A. Ferrer et al. I Narrow pjistate of mass 2.02 GeVIB

196~

hypothesis that the origin of the observed peaks could be due to a kinematical reflection of known resonances, e.g. the p meson. Figures 4, 5 shows the pp mass spectra for events of reactions (3,4) selected with similar cuts as above. In addition, the events of reaction (3) (Figure 4) were selected with an extra cut: MM2(Kfp?r-) > 0.32 (GeV/c’)’ used to suppress the background due to KT. We have found that events of reactions r-p --) KT~T-K; contaminate strongly the 3-prong sample, whereas the 4-prong sample is contaminated by the reactions r-p -+ Kfp?r-x-K0 and T-p + Kffp~-K-T’. It can be seen (Figure 5) that only the use of particle identificators allows a satisfactory suppression of the Kf’ background that is otherwise too strong to permit an observation of the pji signal in the reaction (4).

60

“0

50

> g

25

40

0 > 30 -e, 5 > Id

20

10

0

p p

mass,

GeV/c’

Figure 4. Spectra of pp mass for events from reaction (3): a) events satisfying Ap = p, -p,; > lGeV/c and COSSJ < 0.6, selected with the condition (MM)‘(KT~R-) > 0.32 (GeV/c’)‘; b) events which satisfy the cuts as in 4a), and have p identified by TOF or barrel and/or x,; identified by barrel Note that in the case of T-P exposure the particle identificators were used to identify p (by TOF or barrel), and to reject K,; among the 3-prong events of reaction (3) and the result of these selections gave the mass spectra shown in Figures 4b, 5b.

197c

A. Ferrer et al. I Narrow pc state of mass 2.02 GeVlc2

200 175 “0 150

60

> ul25 I 0 - 100 -2

75

-E

? w

50 25 0

GeV/c’

p f~ mass,

Figure

5.

p,, - p,:

Spectra

p identified

this

pp state

the second

widths

section

The region

rapidity are

features

for

indicating

the observed The

spectra

shown only

reactions which

that

(4): satisfy

the mass

agreement

with

GeV/c’,

reported

at a mass of 2.20

are compatible

with

a) events the cuts

of the fastest

in Figure reaction

for events

Ap

=

and have

for the

of Benkheiri

2.02

pji

GeV/c’

et al.[l].

However,

in [l], is not seen in our data.

The

resolution.

AND

INTEGRATED

CROSS

Though

(1)

( see Figure

the

of the central

three 6a)

corresponding the

region

other in what

structures

reactions concerns

show

are very

clearly similar

the production

of

pp state.

momentum

transfer

distributions

IL;,, (reactions

1 and 3) or u;,~*

4), and uh,,, where u’ = u rrrrr.r- u, for events in the 2.02 MeV/c’ exponential behaviour, with slopes in the range of 2-3 (GeV/c)-‘), production

satisfying

as in 5a),

(p,) and of the slowest (x,~) particles of reactions selected to have the pp mass in the 2.02 GeV/c’

6.

a dominance

fit results

those

our experimental

MECHANISM

and of the p~j system

separated

we remark

1, are in good

4. PRODUCTION SECTIONS (l-4),

from

b) events

or barrel.

given in Table

obtained

for events

and cos BJ < 0.6;

by TOF

To conclude states,

of pp mass

> 0.8GeVJc

of the p”, fL and plj mesons

studied

in [7].

(reactions

pp mass band, similarly

2 and show an

to the central

A. Ferrer et al. I Narrow pjistate of mass 2.02 GeVlc2

198c

180

160 140 120 100

80 60 40

320 280 240 200 160 120 80 40

0

Figure 6 a-d). Rapidity spectra for fast protons, slow pions and pp system for events from reactions (l-4), respectively.

Taking into account these u’ dependences and assuming isotropic angular distributions of pjj state decays, we have calculated the total acceptances of the experiment for the reactions (l-4). Using our mass fits to subtract the background under the observed pp peaks we have obtained the numbers of 2.02 GeV/ c2 mesons produced in reactions (l-4). With these, and for each channel we have obtained the integral cross sections given in Table 2 together with the acceptances and event numbers. At 12 GeV/c we can compare our cross sections for 2.02 GeV/c’ pj? state and those reported in [l], which were calculated in terms of backward production of this state, with a quasi-two-body peripheral production mechanism. Our cross section exceeds those reported in [l] by factors 4 to 9. We have recalculated the cross section for the data of [I] involving our model of central production of the 2.02 GeV/c’ pji state, and taking into account the geometrical acceptance of the [l] app aratus. We have obtained the cross section (120 f 30) nb, which is in agreement with our result.

A. Ferrer et al. I Narrow ppstate of mass 2.02 GeVlc2

199c

Table 2 Number of events, acceptances and integral cross sections times branching ratio (Q;,~x BR) for th e centrally produced 2.02 GeV/c2 pp state.

Channel

20 GeV/c

Acceptance

Q;"~x BR

(%)

(nb)

256 f 42 458 f 66 <20 <40 <60

7.4 3.2 3.4 3.8 4.2

23 f 4 95 f 14 <4 <7
47 i 15 51 zt 17 <5 <40 <20

1.9 0.8 1.6 2.1 2.2

84 f 27 210 f 70

r+p data

n+P --) PfPP(T?), r+P * PfPPn+(a:), n+p --$ pf?T+n-(qb), *+p --) pf27r+2n-(rr;),

7r’p --f pfK+K-(7rf), 12 GeV/c

Number of events (background subtracted) or upper limit at 95 % CL.

r-p

data

=-P --f PfPP(x,), a+P --f PfPPx-(x:),

r-p + pf7r+T-(n,), T-p j pj27r+2a-(a,), a-p --f pfK+K-(n,),

5. BARYONIUM

SIGNATURE

OF THE OBSERVED


pp

STATE

Together with the pp final state of the 2.02 GeV/ c2 resonance we have investigated also the mesonic, A+T-, 2x+2x- and K+K- final states, produced centrally in the baryon exchange reactions:

G-p + pfK+“-(K$)

(7)

n*p + pf2n+2K-(r:)

(8)

a*p + pfK+K-(a:)

(9)

at 20 (12) GeV/c incident X+ (rr-) momenta. No signals of 2.02 GeV/c2 state were found in any of the reactions (7-9). In Table 2 we give the upper limits for the 2.02 GeV/c’ state production cross sections multiplied by the mesonic branching ratios. From this result and taking into account the small width of the observed pp state we believe that it is a firm baryonium candidate, probably an exotic four quack state, coupled strongly to baryons and decoupled from mesons, as has already been predicted earlier by various theoretical models [8-121. We finish by recalling that a charged narrow state of this mass, decaying into pii and Pn has also been reported 1131. Th e authors claim that the state is not peripherally produced, which effectively implies that central production mechanism is favoured.

2ooc

A. Ferrer et al. / Narrow ppstate of mass 2.02 GeVlc2

6. CONCLUSION The baryon exchange reactions rip -+ pfpjhr:, ~+p -+ pfn+pj%rz at 20 GeV/c and R-P -+ PfPPK,, x-p --f pfpp?r-a,0 at 12 GeV/c incident momenta were identified using the WA56 data obtained at the CERN Q spectrometer. The narrow pa state of mass 2.02 GeV/c’ and width of about 0.02 GeV/c’ is clearly seen. Since mesonic decay modes of this state are not seen, we conclude that this state is a baryonium candidate. Its mass and width are in a good agreement with those of the state found by Benkheiri et al.[l]. The large difference in the production cross sections obtained here and reported in [l] can be explained by the different production mechanisms assumed. The apparent dominance of central production of 2.02 GeV/c’ pp state could also explain why Z. Ajaltouni et a1.[6], as well as other backward production experiments [2-51, h ave failed to confirm this state. 1 P. Benkheiri et al., Phys. Lett., 68B (1977) 483. 2 R.M. Bionta et al., Phys. Rev. Lett., 44 (1980) 909. 3 AS. Carroll et al., Phys. Rev. Lett., 44 (1980) 1572. 4 S.U. Chung et al., Phys. Rev. Lett., 45 (1980) 1611 5 T.A. Armstrong et al., Phys. Lett., 1OOB (1981) 191. 6 Z. Ajaltouni et al., Nucl. Phys., B209 (1982) 301. 7 V.F. Perepelitsa et al., Z. Physik, C52 (1991) 407. 8 Chan Hong-MO and H. Hijgaasen, Phys. Lett., 72B (1977) 121; Nucl. Phys., B136 (1978) 401. 9 M. De Crombrugghe, H. HGgaasen and P. Sorba, Nucl. Phys., B156 (1979) 347. 10 M. Uehara, Progr. Theor. Phys., 61 (1979) 1137. 11 L. Montanet, G.C. Rossi and G. Veneziano, Phys. Rep. 63 (1980) 149. 12 Chan Hong-MO, Nucl. Phys., A335 (1980) 219. 13 F. Azooz et al., Phys. Lett., 122B (1983) 471.