Electromagnetic strangeness production at CEBAF

NUCLEAR PHYSICS A ElY,EVIER

Nuclear PhysicsA585 (1995) 63c-74c

Electromagnetic Strangeness Production at CEBAF Reinhard A. Schumacher Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA This is a summary of the presently approved experiments at CEBAF that involve the production of strangeness. These experiments cover the study of hyperon electromagnetic decays, the elementary photo- and electro-production of hyperons on nucleons and light nuclei, and the electroproduction of light hypernuclear species. Experiments involving hidden strangeness are briefly discussed. Advances made possible by the CLAS spectrometer in Hall B, and the HMS and SOS spectrometers in Hall C are emphasized.

1. INTRODUCTION A cartoon of idealized electromagnetic quark-pair creation is shown in Figure 1. When the electron scatters, it produces a virtual photon with four-momentum q = (v,--~), leading to an interaction that results in an sg- pair, which hadronizes into a kaon and a hyperon. While it would be marvelous if we could compute such processes directly from QCD, at CEBAF energies one cannot ignore the many baryonic resonances, or non-perturbative degrees of freedom, which play a role in arriving at the final state. Thus the current state of the art is much more along the lines of Figure 2, which shows some of the tree-level Feynman diagrams that describe the elementary production process. As we shall see, the mechanism of strangeness production is not yet well in hand even at this level of description, and this motivates new experiments at CEBAF. In passing we note that total cross sections for real strangeness photoproduction are on the order of a few microbarns, quite small fractions of the total photoproduction cross section. The CEBAF accelerator is nearing completion; first extracted beam was achieved in July 1994. The approved experiments will be carded out in Halls B and C [1]. Hall B contains the Large Acceptance Spectrometer (CLAS), which is a toroidal magnet design optimized to track charged particles from 8 to 140 degrees for momenta from about 250 MeWc to 4 GeV/c. An electromagnetic calorimeter and gas Cherenkov detector provide electron, photon, and G° detection for angles below 45 °. For electroproduction, the maximum luminosity is expected to be 1034 cm2sec-1. Momentum resolution is expected to be about 1%. For strangeness production, K/~ separation will be done by time-of-flight over a 4 meter flight path, and is expected to work up to about 2 GeV/c. For real photon experiments a photon tagging system operates from 20% to 95% of the bremsstrahlung endpoint energy, with about 5 MeV energy resolution. Completion of CLAS and first experiments are scheduled for 1996. In Hall C there are two principal spectrometers. The Short Orbit Spectrometer (SOS) is suitable for detecting kaons up to 1.5 GeV/c with a momentum resolution of 0.1% FWHMand 7 msr acceptance. The High Momentum Spectrometer (HMS) will detect electrons up to 6 GeV/c, with similar momentum resolution and acceptance. These spectrometers are in the 0375-9474/95/$09.50 © 1995 - ElsevierScienceB.V. All rights reserved. SSDI 0375-9474(94)00545-1

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commissioning phases now. For high resolution hypernuclear physics, there are long-range plans for dedicated spectrometers for low momentum electrons and for kaons, and also a dipole to separate forward-scattered electrons and kaons from the beam (see Sec. 4). Table 1 is a list of all experiments at CEBAF which involve strangeness. In this talk I will outline these experiments, except those which probe the strange sea using the parity violation in electro-weak scattering of electrons [3]. A, Z° K+ U

S

Figure 1. Quark-line strangeness production picture. Note that the photon need not couple directly to the strange quark pair.

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Figure 2. Resonance diagrams for the process y + p ---> K+ + A. Born term (left side) and resonant terms (right side) contribute. After Ref. [4].

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R.A. Schumacher /Nuclear Physics A585 (1995) 63c-74c

Table 1 Approved experiments at C E B A F involvin

1994 [2]. Contact person

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Title of Experiment

Exp #

Hall

Radiative Decays of the Low-Lying Hyperons

89-024

B

G. Mutchler

Beam Days 65

Electromagnetic Production of Hyperons

89-004

B

R. Schumacher

65

Measurement of the Structure Functions for Kaon 93-030 Electroproduction

B

M. Mestayer

50

L/T Cross Section Separation in p(e,e' K+)A(E °) for 93-018 0.5 1.7 GeV, and train>0.1 (GeV/c)2.

O.K. Baker

15

Measurements of the Electroproduction of the A(gnd), 89-043 A (1520), and fo(975) via the K+K-p and K+It-p Final States

L. Dennis

48

Study of Kaon Photo-production on Deuterium

89-045

B

B. Mecking

23

Quasi-free Strangeness Production in Nuclei

91-014

B

C. Hyde-Wright

25

Electroproduction of Kaons and Light Hypernuclei

91-016

C

B. Zeidman

21

Investigation of the Spin Dependence of the AN Effective 89-009 Interaction in the p Shell

C

E. Hungerford

25

E. Smith

15

Measurement of the Polarizarion of the ~(1020) in 93-022 Electroproduction Photoproduction of 11 and 1i' Mesons

91-008

B

B. Ritchie

26

GO: Measurement of the Flavor Singlet Form Factors of the 91-017 Proton

C

D. Beck

46+X

Mcasurement of Strange Quark Effect~ Using Parity-Violating 91-010 Elastic Scattering from 4He at Q2=0.6 GeV2

A

P. Souder

42

Parity Violation on Elastic Scattering from the Proton & 4He 91-004

A

E. Bei~

85

2. E L E M E N T A R Y P R O D U C T I O N The field of photo- and electro-production of strange particles benefits from the substantial body of theory and experiment that exists for single pion production. Essentially all of the formalism for these studies is taken from the pion production area. The general formalism for pseudo-scalar meson electro and photo production can be found in m a n y places, for example Refs [4,5,6,7,8]. Note there are many minor variants among authors in defining notation, cross sections, coordinate systems, and amplitudes.

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R.A. Schumacher I Nuclear Physics A585 (1995) 63c-74c

The relevant kinematic quantities for one-photon exchange are the 4-momentum of the virtual photon q=(v,-~) and the direction (0re,O) of the meson relative to "~ in the electron scattering plane, as shown in Figure 3. The photon energy is v = E - E ' , and its 3-momentum is -~. The electron vertex is perfectly well known, which means that the polarization of the virtual photon is determined purely from the electron kinematics. The polarization has three components, since the photon is off shell, two transverse and one longitudinal. The polarization parameter c, defined as £ = 1 - 2 -q2 - tan 2 ranges over +1 and gives the degree of linear transverse polarization along x (c = +1) or y (~ = -1), and is direcdy proportional to the longitudinal polarization along z. The reaction factorizes into a virtual photon cross section and a virtual photon flux. The virtual photon flux into dE'dD e is given by tz E ' W 2 - m n 2 1 1 F(E"D'e) = ~ x 2 E mn ~ 1-e (2) where W is the invariant photon-nucleon energy and m n is the nucleon mass. Y

Figure 3. Kinematic variables for meson electroproduction. When kaons are produced with unpolarized electron beams and only the electron and meson are detected, then there are four kinematically separable pieces to the cross section which can take the form ' ~d ~ a dD.m = F(E''D*) ( ~dGT + c ~dG L + c cos(2¢) dOTr _~ _- - ~ + ~e(c+l)/2 cos¢ d6LT~ dOln ) (3) dE The components of the cross section are each related to six complex gauge and Lorentz invariant amplitudes derived from the hadronic currents, J. Various equivalent sets of amplitudes are used in the literature: helicity, transversity, and extended CGLN basis amplitudes are used, and all can be written as linear combinations of each other. In any representation, t~ T is the

R.A. Schumacher / Nuclear Physics A585 (1995) 63c-74c

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unpolarized transverse cross section, which in rectangular coordinates is proportional to ~df~m[] (Ref. [6]). The longitudinal cross section, ~L, goes as ~dg~m. In inclusive experiments, where the meson is not detected, these two terms are extracted from data at fixed q~and W, but for differing ~, and doing the famous Rosenbluth separation. The transverse-transverse cross section ~Tr arises from the interference of the two components of transverse current, and is proportional to ~ d ~ m [ < J x * J x - Jy*Jy>]. The photon vertex dictates that this term is proportional to cos ~, where ~--0 corresponds to the meson being detected on the "+x" side in the electron scattering plane. This term can also be accessed with linearly polarized real photons. The (~LTterm is due to the interference of the longitudinal and transverse parts of the current and is proportional to Jdf~m[]. Because the longitudinal photons transfer no angular momentum, ~L is sensitive to the exchange of 0 ~-exchanges in the t-channel, and hence is sensitive to pions, diquarks, or other spinzero objects in the nucleon. At low q2 and near minimum t, the longitudinal cross section is dominant in pion production [6]. With suitable extrapolations it has been possible to extract the pion form factor from pion electroproduction data [9]. The analogous work for kaons has not been done, but will be the subject of several CEBAF experiments (see Sec 2.2). It may not be possible to compute the relevant amplitudes directly from QCD and the quark model. In practice models have been developed using hadronic and mesonic degrees of freedom. Figure 2 shows the diagrams that are needed. The lowest order Born terms involve proton, lambda, and kaon exchange in the s, u, and t-channels, respectively. Other important terms involve the exchange of the Z, the spin-1 kaon resonances, the spin 1/2 and spin 3/2 nucleon resonances, and spin 1/2 hyperon resonances. The coupling constants at the vertices can be extracted from the data and compared either with values predicted by SU(6), or with values obtained from other strangeness-producing reactions. The most elaborate theoretical treatment of kaon electroproduction, which includes crossing symmetry and s and t channel duality, is that of Cotanch and coworkers [10]. In photoproduction where only four complex amplitudes are needed, recent work has been done by Adelseck and Saghai [ 11 ], and by Adelseck, Bennhold & Wright [4]. 2.1 O n the proton - photoproduction

The elementary photoproduction reactions on the proton are 7 + P --->K+ + A ; Y + P --->K+ + Z ° ; and Y + P --~ K° + Z+- Total cross sections exist for all cases, differential cross section data exist for the A and Z °, and sparse hyperon and target polarization data exist for the A. While being restricted to q2=0, real photon experiments using tagged or untagged bremsstrahlung beams have been more numerous than electroproduction experiments because of their relative simplicity. Figure 4 gives an example of the existing A differential cross section data and polarization data [12,4]. One measure of the state of the theory these reactions is that about a dozen models have been published over the past thirty years, all based on the diagrammatic techniques mentioned earlier. The relatively poor quality of the data has led to a wide range of "best" fits, such that even the principal Born couplings gKAN and gK~N (see Fig. 2) vary over a wide range, differing in sign in some cases [13]. Unlike pion photoproduction where one strong resonance, the A(1232), dominates the reaction, for hyperons the resonance structure is just not well known.

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R.A. Schumacher / Nuclear Physics A585 (1995) 63c-74c

The contribution of CEBAF to this field will be to provide data for all three elementary channels, and to emphasize the polarization variables which show large sensitivity to the model parameters. Experiment E89-004 will use CLAS to obtain these data, using the detector to examine the self-analyzing weak decays of the hyperons to an extent impossible in earlier measurements. Figure 4, for example, shows an old analysis of Renard [12] which illustrates the sensitivity of that model to A polarization data: the hatched region shows the range of predictions due to reasonable variations of the couplings gAKN and gXKN. The polarization shown is that of the hyperon perpendicular to the (y,K) plane. In another category of polarization measurements, CLAS will be able to operate with circularly polarized photon beams produced by bremsstrahlung from polarized electrons. Preliminary discussions are underway to extend E89004 to include such measurements. Real photoproduction of the E+ and zo and A are of equal importance, since all three reactions are described within the same framework. A useful feature o f Z ° production is the absence of simple t-channel exchange because the photon does not couple to the K °. This results, for example, in a backward peak in the predicted differential cross section, which would be easy to find experimentally [14]. By detecting K°---->~+ x - this reaction will be accessible in CLAS. Even the most extensive model proposed to date [10] still fails to reproduce the sparse data available in these channels [15]. The zo polarization information will be accessible in CLAS because this hyperon decays 100% to AT. The polarization of the A, turns out to be -1/3 of the Z° polarization, thus preserving in diluted form the polarization produced in the reaction.

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:-T&

o.~

o

qi~

~o ~

~ ..--.

r,

¢~

"........

.-"

¢2

~

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1'6

Figure 4. (a) Differential cross sections for T+ p --->K+ + A as a function of kaon c.m. angle, from Ref [12]. The shaded region corresponds to gAKN/4x~varying from 1.1 to 2.8. (b) A polarization data for p0',K+)A for kaon c.m. angles of 90o+ 5°. 2.2 O n the proton - eleetroproducdon

Several CEBAF experiments will examine the (e,e' K) reaction on the proton. Each plans to separate the four components of the cross section in Equation (4). Experiment 93-030 will use CLAS to obtain information over as wide a kinematic range as possible to maximize the ability to do the separations. A related measurement in Hall C, Experiment 93-018, is planned using the SOS and HMS spectrometers to do the strange analog of the single pion electroproduction done

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R.A. Schumacher / Nuclear Physics A585 (1995) 63c-74c

in the past. C[LTwill be obtained over a small range by moving the SOS spectrometer left and right in the lepton scattering plane. The resonance structure of (e,e' K) is unknown, as mentioned above, and the reaction is not dominated by a single strong resonance. As one example of the physics to be addressed, Figure 5 shows the Q2 dependence (Q2=-q2)of the electroproduction of A and T'.° for a fixed value of the total c.m. energy W. For A production the cross section first rises from the real photon point at Q2=0, while for ~o production the drop is monotonic with Q2. This is interpreted as an indication that longitudinal photon contributions are important for A production but not ~ production. A rough longitudinal-transverse separation was obtained [16] by measuring at different £ for the same W and Q2, supporting the conclusion that a large longitudinal component contributes to A production only. A suggested explanation [17] is that longitudinal photons contribute strongly to kaon exchange in the t-channel, but this process is expected to be larger for A production than for I'~production since gKAN> gKT.,N,and hence the difference in Q dependence. The Q2 dependence of Z ° production is much steeper than that for A production. In quarkparton models [18,19,20] this feature was interpreted as a consequence of the decrease of the ratio F17n/FI~P, the deep inelastic electron-nucleon structure functions of the neutron and proton, as Bjorken x = Q2/2Mv goes to 1. In this limit the production of forward-going kaons off u-quarks tends to leave behind an isospin 0 pair u-d quarks, from which the production of I=1 baryons (E) is suppressed in favor of I--0 baryons (A). The t dependence of the longitudinal cross section can also be interpreted in terms of the kaon form factor, similar to work done on the pion form factor. Experiment 93-018 will emphasis this aspect of the analysis. T00 . . . . 6O0

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R.A. Schumacher /Nuclear Physics A585 (1995) 63c-74c

2.3 O n the d e u t e r o n - p h o t o p r o d u c l i o n

After the elementary production mechanism on the proton is established, it only makes sense to extend our knowledge using a deuteron target to get the three reactions on the neutron: T + n ---> K + + Z- ; y + n --->K ° + A ; Y + n ---> K ° + Z °. A set of data including all isospin channels will naturally provide the most rigorous test of the models. This will be done in CEBAF experiment 89-045. No data of this kind exist to date. Besides extracting the elementary cross sections [22], it may be possible to explore YN interaction effects; since the photon and the K + both interact "weakly," the deviations from quasi-free behavior in the (y,K) reaction will be due to the interaction of the final state interaction hyperon-nucleon system [23]. This measurement will also scan the cross section as a function of missing mass in the region between the A and the Z °. Here the amplitudes for production of the two hyperons will interfere, producing a cusp that has been predicted to be visible [24]. This channel-coupling cusp has been seen in D(x+,K +) and D(K-,lz-) reactions [25] but electromagnetic studies of the shape of the cusp will reveal the relative phase of the production amplitudes. Experiment 91-016 will also look for this cusp in electroproduction in Hall C. Another goal of E89-045 is to look for narrow structures near the ZN threshold which might be interpreted as S = -1 dibaryons [26]. The spin-one D 1 should be 20-40 MeV above the cusp while the spin-zero D O should be 20-40 MeV below the cusp. The D l can be reached from the deuteron using reactions with small spin-flip probability: (~,K), (K,~), and (y,K). But the (y,K) reaction is special because only it can also populate the spin-flipped D O state. This state should be narrower than the D 1, and therefore might be worth a look despite failure to detect the D 1 in the mesonic reactions [27]. 2.4 O n h e a v i e r n u c l e i - i n c l u s i v e k a o n p h o t o p r o d u c t i o n

Once the elementary amplitudes for kaon photoproduction are known, it is interesting to know if the inclusive nuclear response can be described in a simple quasi-free picture, i.e. incoherent production on individual nucleons. From inclusive A(e,e') and (e,e'p) studies it is known that medium modifications such as two-nucleon currents can substantially alter the quasifree behavior. There may be analogous effects in strangeness production, though no ernest theoretical estimates have been made. Experiment 91-014 will study the relative importance of the one-nucleon (y,K) versus either two-nucleon (y,K) currents or KY final state interactions (FSI). The targets will be 3He, 4He, and 12C. Due to strangeness conservation, the total A(T,K) cross sections will be insensitive to FSI, since either a K ÷ or a K ° must emerge once either has been created. The A dependence of the production ratio for D:4He:C thus will be sensitive to the presence of medium modifications of the elementary amplitudes. The ratio of K+/K° production yields on isoscalar nuclei also will be measured and compared to deuterium. This ratio would probe either two-nucleon currents or charge-exchange final state interactions. Finally, the differential 3He data will be a detailed test of the quasi-free model, because good wavefunctions exist and calculations should be most reliable.

R.A. Schumacher / Nuclear Physics A585 (1995) 63c-74c

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3. RADIATIVE HYPERON DECAYS

Radiative decays in general are useful for testing quark models of baryons. Table 2 lists widths of radiative transitions for several hyperons. These widths are sensitive to the detailed quark wave functions used in the models, with predictions presently varying by a factor of sixty. In an elementary quark model certain radiative transitions are forbidden: e.g. the I=0 A(1405) cannot decay to the two I=1 E states because a single photon, described by one-body operators, cannot connect states that involve changing two quarks. There is some belief that the A(1405) consists at least partly of a ~NI molecular state, owing to the proximity of the F,N threshold at 1437 MeV. Because the A(1405) and the 7.(1385) are below this threshold they cannot be studied using stopped K- on liquid hydrogen. Table 2 Radiative decays for low-lying hyperons using several quark-model wavefunctions [28]. The anticipated CEBAF numbers show the goals of E89-024. Transition [ NRQM I MIT BAG CHIRAL Experiment Anticipated [Ref 30,31] [Ref 30] BAG [29] Stopped K -p CEBAF A(1520) --->A?

I

156

46

32

A(1405)--)A?

200

17

75

E(1385) o A y

273

152

A(1520) --~ Zy A(1405) --->X?

55 72

17 2.7

E(I385)-+Ey

22

15

51 1.9

150-2_30[32] 33+11 [33] 27-+8 [34,35]

150_+6 30-+3 200-+10 302-8 270-2_10

47+17 [33] 10-2_4 [34,35] 23+7 [34,35]

The CLAS spectrometer is suited to making these states using the reaction yp ---) K + Y*. In Experiment 89-024, the incident real photons are tagged, the kaon is detected and used to construct the Y* missing mass; in the cases of the A(1405) and the E(1385) there is an overlap because the states 50 and 36 MeV wide, while the A(1520) is well separated. The radiative decays to the ground state lambda will be detected by following the decay chain Y* --->TA followed by A -->rCp. Detection in the EGN calorimeter of the photon, in addition to the charged particles, greatly suppresses the background of n ° decays. Detection of decays with the E(1193) as an intermediate state will be more difficult due to the necessity of detecting both photons to reject background.

4. HYPERNUCLEAR PRODUCTION

No data exist on electromagnetic production of hypernuclear states. The reasons are clear: the large momentum transfers (comparable to the (n,K) reaction) and small predicted cross sections (on the order of 10nb/sr for even the lowest momentum transfers) make such reactions challenging to measure. The 5 MeV photon energy resolution of the CLAS photon tagging system make hypemuclear photoproduction experiments in Hall B unlikely. In Hall C, however, the A(e,e'K)AA reaction will be studied. Detecting the electrons close to zero degrees will

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R.A. Schumacher / Nuclear Physics A585 (1995) 63c-74c

minimize the momentum transfer, and the count rate for populating specific states will be on the order of several counts per day, similar to (n,K) studies. There are two main benefits in doing (e,e'K) hypernuclear spectroscopy. The first is that up to an order of magnitude improvement in resolution will be possible over present hadronic data. The second is that while the (n,K) reaction populates high-spin natural-parity states, the (e,e'K) reaction also populates the unnatural parity states starting from 0 + nuclei via the spin-flip interaction. CEBAF can therefore provide data complimentary to the hadronic production reactions. Experiment 89-009 will study the spectroscopy of the p-shell, with a goal of measuring the A-nucleus spin-orbit splitting, which is related to the AN spin-orbit interaction. The strength of this interaction is known to be small [36], with splittings of under 0.5 MeV, so while gamma-ray studies have seen transitions between widely spaced levels, the fine structure has not been seen. At CEBAF the plan is to use a 1.8 GeV electron beam, a dedicated Enge split-pole magnet to detect low-energy scattered electrons at zero degrees, and the SOS spectrometer with about 1.2 MeV resolution for the kaons. In addition a "splitting magnet" is needed at the target to peel off the electrons and kaons to large enough angles to enter the spectrometers. In this configuration a survey of p-shell nuclei will be done to map the overall response and to evaluate rates and backgrounds. As a future development this program will use a dedicated kaon spectrometer with resolution in the range of 120 keV. Experiment 91-016 will examine electroproduction of kaons on the proton, deuterium, 3He and 4He. The physics goals are examination of the A-Z cusp in deuterium (see Sec. 3.3) and examination of bound state angular distributions in the helium isotopes. Other goals include dibaryon hunting near the ZN cusp, and searching for narrow Z states. The measurements are essentially exploratory to obtain cross sections and count rates. This experiment will use the "standard" Hall C configuration, combining the SOS for kaons (same as experiment 89-009) with the HMS for the electrons. Both spectrometers will be at angles greater than 12 degrees, and the missing mass-resolution is expected to be about 3 MeV.

5. H I D D E N S T R A N G E N E S S

Hidden strangeness means that the strangeness quantum number is zero in the initial and final states, but strange quarks manifest their presence in some particular property of a system. We will not discuss the parity violation experiments here [3], but mention instead plans to measure ~(1020) electroproduction as a method of measuring the strange quark content of the nucleon. In a hard scattering process, an sg component of the proton may be knocked out to create a final state <~. In a model due to Henley an coworkers[37], such contributions may be of the same order as the diffractive processes described using vector meson dominance if the strange quark sea of the nucleon in the range of 10% to 20%. Experiment 93-022 plans to look for such t~ knock-out via electroproduction with the CLAS spectrometer. The key to separating the diffractive from the knockout interactions is to examine the self-analyzing decay of the ~'s. Henley et al predict that the knockout mechanism of interest goes via pseudo-scalar (0-) exchange, different from production that involves scalar (0 +) exchange. In the rest frame of the decaying ~'s the kaons are produced with an angular distribution W(~ rest frame) = ~ sin20 (1 + e cos 2~)

(5)

R.A. Schumacher / Nuclear Physics A585 (1995) 63c-74c

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where 0 is the kaon polar angle and x¢ is the azimuthal angle with respect to the lepton plane. Pseudo-scalar and scalar exchange are characterized by the + and the - sign, respectively. By measuring this decay distribution, the experiment plans to achieve a sensitivity of a 5% admixture of pseudo-scalar exchange, at which level the OZI-suppressed rc and 11 exchange diagrams are also expected to contribute. Any larger measured amount of 0- exchange would be a possible signature of strange quark components in the nucleon. Experiment 91-008 may serve as a similar avenue for seeking hidden strangeness in the nucleon by looking at eta production. The proposers speculate that since the two mesons have differing strange-quark admixtures, their relative cross sections off the proton perhaps could be related to the primordial amount of strangeness in the nucleon. The theoretical justification follows the arguments by Dover and Fishbane [38] for rl and rl' scattering. This experiment should provide differential cross section data on both mesons (the first for the rl'), by detecting recoil protons using CLAS. REFERENCES

1. CEBAF Conceptual Design Report, April, (1990). 2. Copies of proposals are available upon request from the CEBAF Users' Liaison Office, at 804-249-7536. 3. See the contribution of Paul Souder in these proceedings. 4. R.A.Adelseck, C. Bennhold, and L.E.Wright, Phys. Rev. C32 (1985) 1681. 5. N. Dombey Hadronic Interactions of Electrons and Photons, Ed. J. Cumming and H. Osborn, Acad. Press (1971). 6. J. M. Laget, Can. J. Phys. 62 (1984) 1046. 7. C. W. Akerlof et al Phys Rev 163 (1967) 1482. 8. R.C.E. Devenish and D. H. Leith, Phys. Rev. D5 (1972) 47. 9. P. Brauel et al. Z. Phys. C 3 (1979) 101. 10. Robert A. Williams, Cheung-Ryong Ji, and Stephen R. Cotanch, Phys,. Rev. C46 (1992) 1617; Stephen Cotanch and Shian Hsiao, Nucl. Phys. A450 (1986) 419c. 11. R.A.Adelseck and B. Saghai, Phys. Rev. C42 (1990) 108. 12. Y.Renard Nucl. Phys. B40 (1972) 499 ; F. M. Renard and Y. Renard, Nucl. Phys. B25 (1971) 491. 13. R.A. Schumacher, Particle Production Near Threshold, AIP Conf. Proc. 41, H. Nann, E. Stephenson, Eds. (1990) 378. 14. C. Bennhold, private communication. 15. T. Mart, C. Cennhold, C. Hyde-Wright, these proceedings. 16. C. J. Bebek et al., Phys Rev D15 (1977) 3082. 17. T. Azemoon et al., Nucl. Phys. B95 (1975) 77. 18. F.E. Close, Nucl. Phys. B73 (1974) 410. 19. O. Nachtmann, Nucl. Phys. B74 (1974) 422. 2(I. J. Cleymans and F.E.Close, Nucl. Phys. B85 (1975) 429. 21. C. J. Bebek et al., Phys Rev D15 (1977) 594. 22. Xiaodong Li, L.E. Wright and C. Bennhold, Phys. Rev. C45 (1992) 2011. 23. R.A.Adelseck and L.E.Wright, Phys. Rev. C38 1965 (1988); Ralf Anton Adelseck, PhD thesis, Ohio University (1988), 24. S. R. Cotanch, Strangeness Production Studies at CEBAF, Porc. of the Conf. on Medium and High-Energy Physics, Taipei, (1989) 666.

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