Volume 64B, number 1 PHYSICS LETTERS 30 August 1976

S E A R C H F O R E X O T I C M E S O N S P R O D U C E D BY B A R Y O N E X C H A N G E

IN T H E R E A C T I O N n+p --* A + x ++ A T 12 G e V / c

H. BRUNDIERS, R. HARTUNG, K. RUNGE, O. SCHAILE, C. WEBER, Universitiit Freiburg, Germany

F. BRUN, G. CARLES, M. CRIBIER, J.R. HUBBARD, G. LAURENS, L. MOSCOSO, A. MULLER, S. ZYLBERAJCH,

DPhPE CEN, Saclay, France

L. FLURI Laboratorium f'ur Hochenergie ETH, Ziirtch, Switzerland

Received 14 June 1976

A search for exotic mesons has been carried out at the CERN Omega spectrometer. The baryon-exchange reaction ,r+p ~ A + x +÷ was studied at 12 GeV/c incident beam momentum. No evidence was found for the existence of doubly-charged strange mesons. The upper limit (95% confidence level) for this reaction is 60 nb for exotic mesons with squared masses from 1 to 7 (GeV/c 2)2 and 150 nb for 7 to 13 (GeV/c 2 )2. These fimits are of the same order of magnitude as the exotic cross-sections expected from two-component duality arguments.

Extensive efforts have been made in the last few years to look for the so-called exotic resonances. All known particle states can be described in a simple quark model in terms of the quark-antiquark coupling for mesons or three-quark coupling for baryons. The exotic states cannot be described in this way. Evi- dence for their existence has been sought in several mechanisms: 1) Exotic meson production through meson ex-

change 2) Exotic resonance formation in the s-channel 3) Exotic particle exchange in the t-channel. No convincing evidence has been found up to now [1].

The motivation of the present experiment was to look for exotic meson production in a baryon ex- change reaction. Rosner [2], Jacob and Wyers [3], and Lipkin [4] have shown that the baryon-anti- baryon system requires exotic states in order to satis- fy two-component duality. Thus, exotic states might be expected to couple preferentially to a baryon anti- baryon vertex. Furthermore, this production mecha- nism can be described by a simple allowed quark dia- gram in which neither the direct channel nor the ex- change is exotic. Two-component duality suggests that for baryon-exchange, the cross-sections for the pro-

duction of exotic mesons should be comparable to those for non-exotic mesons.

The purpose of the present experiment is to study the baryon exchange reaction:

lr + + p ~ A + x ++ (1)

where x ++ is a doubly-charged strange meson. The in- cident pion momentum was 12 GeV[c. The diagram for reaction (1) requires a baryon-antibaryon coupling as shown in fig. la. The corresponding quark diagram is shown in fig. lb. The experiment would be sensitive to an exotic production cross-section equal to the non-exotic baryon-exchange cross-section for the re- action:

n - + p ~ A + K *°. (2)

The experiment was performed with the Omega spectrometer at the CERN IS. The experimental set- up is shown in fig. 2. A general description of the Omega spectrometer has been given in ref. [5]. A large aperture magnet with super-conducting coils yields a field of 1.8 T over a volume of 11 m 3. A liq- uid hydrogen target 30 cm long is placed inside the magnet. Optical spark chambers occupy a volume of

107

Volume 64B, number 1 PHYSICS LETTERS 30 August 1976

IT+

P

-__ _- A

- X ÷4"

a)

'11

l n P P

b) Fig. 1. Diagram for reaction 1.

3.2 X 1.5 X 1.2 m 3 in the central region of the magnet. These chambers are viewed by television cameras of the plumbicon type through a large hole in the top pole of the magnet. The spark measurement accuracy after all corrections is ± 0.5 mm.

The incident beam is defined by a scintillator tele- scope. The parameters of each beam particle are meas- ured by a scintillator hodoscope and 5 proportional

lOre S m t I

chambers. The measurement accuracy was ± 0.2% for the momenta and ± 0.15 mrad for the direction. The particle nature was flagged by threshold Cerenkov counters. The beam intensity, limited by the spark chamber memory time, was 1.5 X 105 particles per burst. The dead time of the data acquisition system was 18ms.

The trigger was designed to select a forward A pro- duced with any number of charged tracks at the pri- mary vertex. Charged multiplicities were counted in two proportional chambers (PC1, PC2). PC1 was just downstream of the hydrogen target and PC2 was 1.4 m further downstream. Neutral vees were selected by requiring a higher multiplicity in PC2 than in PC1. The proton from the A decay was identified by a multi-ceU Cerenkov counter f'dled with isobutane at atmospheric pressure. Scintillator hodoscopes (HI, H2) were placed before and after the Cerenkov counter. Each hodoscope had 33 vertical strips - 6 cm wide by 150 cm high for HI, and 9 cm by 220 cm for H2. An H1 H2 correlation with the Cerenkov counter in anti- coincidence selected events having a positively charged particle with momentum greater than 3 GeV/c and heavier than a pion.

An average of 8 events were recorded per burst. About 300000 events were recorded in three days of production run.

The pattern recognition, geometrical reconstruc-

0 -4m 4 I i

Fig. 2. Experimental apparatus.

108

Volume 64B, number 1 PHYSICS LETTERS 30 August 1976

tion and vertex f'mding were performed by the off-line program ROMEO [6]. The criteria for defining a V ° topology were loose in order to increase the recon- struction efficiency. As a consequence, some V°'s were simulated by other phenomena such as plate interactions. The pattern recognition efficiency for finding fast V°'s was 93.5%. The efficiency for finding secondary tracks and associating them to a vertex in the target was substantially smaller, especially for low momentum tracks at large angles. The errors on the reconstructed parameters for the A are Ap/p + 1% and A0 ~ ±0.1 mrad. No kinematic fits were performed. There is no A-K ° ambiguity, because the Cerenkov inefficiency was negligible. Fig. 3 shows the mass distribution for the V°'s when a proton mass was assumed for the positive particle and a pion mass for the negative. The peak is well centered on the A mass, and the background is negligible.

The following analysis is based on 8051 events with 1.22 < M 2 < 1.27 (GeV/c2) 2. The acceptance of the apparatus depends on the A parameters; it is maximum ("~ 70%) for fast A's with small production angles, for which the main losses are due to decays

1OOO

SOO

250

i 1 1

M2IA)

0 1.15

g q

I-- Iii IM

12455 EVENTS

1 I 1~0 1.25 1.30 1.3S

M 2 (p n'-) (GeV/¢ ' ) 2

Fig. 3. A mass s q u a r e d d i s t r i b u t i o n .

before PC1. All accepted A's go.forward in the center of mass system. We have checked that the A decay angles and proper time distributions agree with those calculated by Monte Carlo techniques. A more com- plete experimental description will be given in a forth- coming publication [7].

In fig. 4a we present the distribution o f MM 2, MM is the missing mass associated to the A.

No significant structure is visible. Another relativistic useful invariation is u' = u - u o,

where u is the usual four-momentum transfer from the incident n + to the A, and u o is the kinematic limit of u for the recoil mass under consideration.

In order to enhance the baryon-exchange mecha- nism, we selected events with u ' > - 0 . 5 (GeV/c) 2

Il l All topologie's " ' ~ '

2OO • 3535 Ewmts # ~

tO0

! ° 4 b) 9;2 PRONGS

O 5003 Events 200 m 2229 Events

I,- Z ul

c) >2 PRONGS

O 3048 Events 200 B 1306 Events

100

_ _ ~ 1 ~ . - ' - - , ~ ~ ~ , . 0 2 4 6 8 10 12 14

MM2A (GeV/c 2) 2 Fig. 4. Mass squared recoiling from the A: a) for all topologies, b) for chargod multiplicities less than or equal to two, c) for charged multipficities greater than two. The shaded events have u' > -0.5 (GeV/c2) z.

109

Volume 64B, number 1 PHYSICS LETTERS 30 August 1976

(cross-hatched in fig. 4). Still no effect was detected. Another way to enhance the signal for exotic

mesons with masses above 2 GeV/c 2 would be to look for decays into a baryon and an antibaryon. Thus, the x ÷+ could decay into p + Z - . If this were the case, the multiplicity of the decay would be small. We have investigated the missing mass distribution, with and without the u' cut, for charged multiplicities less than or equal to two (fig. 4b), or greater than two (fig. 4c). Again, no effect was seen.

Cross-section upper limits have been calculated. The experimental resolution for the missing mass ranges from 0.2 (GeV/c2) 2 for low MM, to 0.1 (GeV/c2) 2 for high MM. Taking into account the nor- malisation and the experimental corrections, and as- suming that the resonance width is smaller than the experimental resolution, we find an upper limit for exotic production which ranges from 60 nb for the low masses to 150 nb for the high masses. These are 95% confidence level upper limits.

The cross-section for backward K*(890) production in ~r-p interactions at 8 GeV/c has been measured to be 550 -+ 70 nb, using the same apparatus [8]. Assum- ing a p - 3 energy dependence, we find 160 nb for backward K* production at 12 GeV/c, which is com- parable to the exotic upper limits found above. Thus, the absence of exotics .in this experiment can be inter- preted as weakening the two component duality pre- dictions.

This result can be compared to other similar exper- iments. Baltay et al. [9] found an upper limit of 1.7 to 2.6 tab for the same reaction at 15 GeV/c. Alam et

al. [10] found a limit of 1 to 2tab for non-strange exotic mesons in another baryon-exchange reaction:

+ X++ n + p -+ n + at 8.4 GeV/c.

We are grateful to the [2 staff and all persons who contributed to the success of this experiment. We also want to acknowledge the participation of P. Bareyre, P. Miihlemann, and G. Villet in the early stages of this experiment.

References

I 1 ] New directions in hadron spectroscopy, Proc. of the Summer Symposium held at Argonne National Labora- tory, July, 1975, ANL-HEP-CP-75-58.

[2] J.L. Rosner, Phys. Rev. Lett. 21 (1968) 950. [3] M. Jacob and J. Wyers, Nuovo Cim. 69 (1970) 521. [4] H.J. Lipkin, Phys. Rev. D7 (1973) 2262. [5J O. Gildemeiser, Intern. Conf. on Instrumentation for

high energy physics, Frascati 1973. [6] F. Bourgeois, H. Grote and J.C. Lassalle, Pattern recog-

nition method for OMEGA and SFM spark chamber ex- periments, DD/DH/70-13; H. Grote, M. Hansroul, J.C. Lassalle and P. Zanella, Iden- tification of digitized particle trajectories, Intern. Com- puting Symposium 1973 (North-Holland, 1974).

[7] Inclusive A production in n+p, n-p and K-p interactions, Nucl. Phys., to be published.

[8] Backward AK ° and AK~90 final states produced by n-p interactions at 8 GeV/c, Nucl. Phys., to be published.

[9] C. Baltay et al., Phys. Lett. 57B 0975) 293. [10] M.S. Alam, Phys. Lett. 53B (1974) 207.

110