workshop II Nucleon-Nucleon

Nuclear Physics AS27 (1991) 377c-392c North-Holland, Amsterdam

3llc

NUCLEON-NUCLEON AND NUCLEON-NUCLEUS INTERACTIONS

John MCCLELLAND Los Alamos National Laboratory, Medium Energy Physics Division, MS H841, Los Alamos NM 87545, USA

and

Madeleine SOYEUR Laboratoire National Saturne, CEN de Saclay, F-91191 Gif-sur-Yvette Cedex, France

1. OVERVIEW

The parallel session “Nucleon-Nucleon and Nucleon-Nucleus Interactions” has been devoted to three main topics, the study of spin-isospin modes in charge exchange reactions, recent results on the baryon-baryon (and baryon-antibaryon) interaction and spin effects in proton-nucleus scattering. A contribution on a very new topic, the production of q-mesons in proton-nucleus collisions, has also been presented.

Spin-isospin modes have been studied in charge exchange and inelastic scattering processes induced by nucleons ([p, n], [n, p] and [P, p'])'*',by light ions ([d, 2p] and [ 3He,t])3,4 and by heavy ions ([‘“C, “N), [ ‘*C, ‘*B], [ *‘Ne, *OF], [ *‘Ne, *‘N],...)‘. At relativistic energies (500 MeV 5 ELab/N 6 1 GeV), th e nuclear response can be characterized as being concentrated in two energy domains : at low excitation energy (w < 100 MeV) and in the A region. The strength comes from the excitation of particle-hole and A-hole states respectively.

The study of particle-hole and A-hole spin-isospin excitations is particularly interesting because the interaction responsible for them can be rather simply related to A- and p-meson exchanges and to short range effects described by the Landau-Migdal parameter.’ Due to their peripheral nature’, charge exchange reactions provide a way of studying pionic modes in the nuclear medium at baryon densities lower than the saturation density.*

The new data presented at PANIC XII brought information on two important aspects of these reactions, the spin structure of spin-isospin excitations in both the quasi-free and A-hole sectorsg13 and the exclusive measurement of the in-medium A-decay modes following the [ 3 He, t] reaction. The measurements of polarization observables for quasi free [p,n] reactions taken at LAMPF are reviewed by Taddeuccig in this volume and the spin structure of the A excitation studied at SATUHNE in (d: 2p] reactions is discussed in section 2 of this paper. The 4x measurements of the A decay modes following the [ 3He, t] reaction made at SATURNE with the DIOGENE detector are presented in Hennino’s contribution” to this volume.

The theoretical understanding of the spin-isospin response of nuclei in the A resonance region, in particular the origin of the downward shift in energy of the A peak, is discussed

03759474Pl/$o3.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

378~ J. McClelland, M. Soyeur / Nucleon-nucleus interactions

by Osterfeld et al” in this volume.

The recent results presented on the baryon-baryon interaction focussed on its short range part, as described by vector meson exchanges, and on spin observables measured in the pn + pp~- reaction.

The dynamics of baryon-baryon interactions at short distance is of great interest to evaluate the limit of validity of the meson exchange picture and the possible explicit role of QCD degrees of freedom in this regime. A study of the NA, NC and K+N potentials’* indicated that all these systems have a repulsive core given mostly by the w-exchange, with a coupling constant for the w-meson much larger than expected from SU(6) symmetry. A discussion of K+N and K-N scatterings, in which the w-exchange gives rise to repulsive and attractive interactions respectively, shows the effective character of the large w coupling and gives indications on the nature of the additional repulsive force. This work is discussed by Speth in this volume13.

Another contribution related to vector meson exchange in the nucleon-nucleon interaction is the IUCF measurement of charge symmetry breaking in np scattering.i4 The spin-dependent left-right asymmetries are measured over a broad angular range by scattering polarized neutrons from polarized protons. The charge symmetry breaking is given by the difference between the neutron and proton analyzing powers. This quantity is shown to be non-zero and the value obtained can be understood when the p - w isospin mixing is included in the description of the np interaction. The large pNN and wNN coupling constants characteristic of the Bonn potential15 are favored by the data. This contribution is discussed in section 3.

Polarization data for the inelastic np - ~-pp process have been taken at

SATURNE” and at TRIUMFr7. Very few data were available until now for this reaction which has the interesting property that the 1= 0 channel cannot involve an intermediate AN state. The data taken at SATURNE use the polarized neutron beam and a new detector (ARCOLE) designed to identify and reconstruct completely the kinematics of events with 3-body final states, such as Zp + pp~- and i;p + cl&~-. Preliminary data for S-differential cross sections and asymmetries have been obtained for the i;p - ppn- reaction at 810 MeV.” The data taken at TRIUMF measure the analyzing power for the pn 4 ~-pp (‘So) reaction induced by polarized protons at energies of 353, 403 and 440 MeV over a large angular range (50” < 8 < 130”). l7 Both contributions are presented in

section 4.

The Skyrme model, based on an effective Lagrangian describing the low energy limit of QCD in terms of meson degrees of freedom, has provided a successful picture of the baryon structure. Baryon-baryon scattering has been studied in this modelr8; the first results for baryon-antibaryon annihilation were presented at PANIC XII.” This last process is especially interesting because is shows the time evolution of the baryon and energy densities and gives a new insight into the annihilation dynamics. This work is summarized in section 5.

The data presented on proton-nucleus collisions focussed on spin effects. Presentations ranged from elastic and low-lying to continuum excitations of the nucleus where spin plays a central role in experiment or theory. By bringing the spin of the projectile and/or the target nucleus explicitly into the measurements, particular components of the full spin- and isospin- dependent proton-nucleus interaction can be isolated.

Experimental and theoretical advances are providing new tools for these investigations. We heard of new polarized-beam, polarized nuclear target experiments using the High

J. McClelkzruJ M. Soyew / Nucleon-nucleus intemctions 379c

Resolution Spectrometer (HRS) at LAMPF “, high resolution inelastic proton scattering using the newly commissioned K600 spectrometer and focal plane polarimeter system at IUCFzl, and very small angle measurements of spin strength using the excellent tracking and focal plane polarimetry capabilities of the Medium Resolution Spectrometer (MRS) at TRIUMF and HRS at LAMPF”.

Microscopic DWIA-RPA calculations were presented for small angle spin-flip probabilities at 318 and 580 MeV for 40Ca at excitation energies up to 50 MeV23. The calculations were compared to data taken at LAMPF as a method to extract AS = 1 and AS = 0 strength in the continuum. The calculations demonstrate the importance of distortion effects in the interpretation of these data.

One of the important observations which came directly as a result of measurements of spin effects in proton-nucleus scattering was a serious discrepancy between non-relativistic multiple scattering (KMT) theory and experiment for small-angle analyzing power and spin-transfer data at intermediate energies. The possibility that second order spin-orbit interactions might bring KMT calculations into better agreement with the data was discussed.24 These two theoretical papers and the three experimental contributions on spin effects in proton-nucleus collisions are reviewed in section 6.

The last contribution presented the first data on n production in proton-nucleus collisions2’ The measurements have been made at SATURNE with the PINOT detector at Ep = 800, 900 and 1000 MeV, below the 17 threshold in nucleon-nucleon collisions

(% thresh = 1.25 GeV). These data provide the dependence of the n production cross section on the target mass, on the incident proton energy for a given target and also the energy distribution of the produced 7’s. This work is described in section 7. We give a few concluding remarks in section 8.

2. SPIN STRUCTURE OF THE A EXCITATION IN NUCLEI MEASURED WITH THE [ii, 2p(%,,)] PROBE

The [d? 2p(‘Ss)] reaction using a tensor polarized deuteron beam offers the possibility of measuring polarization data analogous to those that could be obtained in [Z, fl reactions if the polarizations of both the projectile and the ejectile were messured.3

Data were taken at SATURNE for a deuteron bombarding energy of 2 GeV on hydrogen, deuterium and 12C targets3 The two outgoing protons were detected with the magnetic spectrometer SPES 4 using two sets of drift chambers. The cuts from the spectrometer combined with cuts from the collimator restrict the relative motion of the two protons to the ‘SO state.

The spectra obtained at 0” show a downward shift of the A peak of the order of 65 MeV for the 12C target. The tensor polarization response is shown in Fig.1. It is defined by3

where pzc = 0.6 is the beam polarization, T2y and T2y spherical components of the tensor analyzing powers and cp the angle between the normal to the reaction plane and the direction of the beam polarization. Close to O”, the polarization response is only a function of the ratio T/L of the spin-transverse to spin-longitudinal cross sections3 The x-exchange gives rise to a purely longitudinal response and the p-exchange to a purely

38Oc J. McClelland, M. Soyeur 1 Nucleon-nucleus interactions

t d@2d - CkG’p) D 1 40 -

20 -

8 + a 0 ;;__I' .

.. -""".'......;..._--_...... i ’ ! ’ 3*o ‘_ 1.7

-20

-40 i \_ ---. L - I I ! / I

0 200 400 600

w (>feV)

Fig. 1. Tensor polarization response measured for d and 12C targets. The

long-short dashed curve corresponds to a purely longitudinal response and the dot-dashed curve to a purely transverse response. The two other curves show best fits for d and i2C.

transverse response. The zero-range force described by the Landau-Migdal parameter has both a longitudinal and a transverse component. An important part of the shift in the A peak, obtained by comparing data on the proton and on nuclear targets, is expected to come from a medium enhancement and downward shift of the longitudinal response while the transverse response should be quenched and shifted upwards.8~” The comparison of the tensor polarization response for d and “C targets indicates the opposite trend.

3. A MEASUREMENT OF CHARGE SYMMETRY BREAKING IN np SCATTERING

Charge symmetry breaking in the strong nucleon-nucleon interaction has been tested by measuring polarization observables in np scattering at 183 MeV. The data were taken at the Indiana University Cyclotron Facility using a polarized neutron facility, a polarized proton target and a particle - coincidence detection system.r4 The difference AA(B) =

An(B) - Ap(0) between analyzing powers associated with the neutron beam spin (A,) and the proton target spin (Ap) were measured over the angular range 60” < Pm < 120”. Charge symmetry of the strong interaction requires the neutron and proton analyzing powers to be equal at all angles and hence AA to be zero. There is an electromagnetic contribution to AA arising from the spin-orbit interaction between the proton current and the neutron magnetic moment which is small compared to expected effects from strong interactions over the angular range of interest for the experiment. We refer to Ref.14 for a discussion of the way of circumventing the ambiguity in AA(B) arising from the fact that the beam and target polarizations are known only to a few percent.

The measured value for AA is shown in Fig.2a which also includes the expected charge symmetry breaking from the np mass difference effect on single K- and single p-exchanges in addition to the electromagnetic contribution, the effect of p - w isospin mixing and previous TRIUMF data26 at 477 MeV with less statistical precision and a smaller angular range. Charge symmetry breaking arising from single K and p-exchanges

J. McClelku@ hf. Soyeur / Nucleon-nucleus interactions 381~

and the electromagnetic contribution could explain the TBIUMF data but falls about two standard deviations short of the IUCF result. The p - w mixing appears necessary to understand the value of AA = (32.1 f 6.1 f 6) x lo-* obtained at 183 MeV; this contribution is extremely small at 477 MeV at the p - w mixing term at that energy has a zero-crossing near the same angle as the analyzing power. The angular distribution of AA including p - w mixing has been calculated with the Reid and Bonn potentials and compared to the data (Fig.2b). It seems to favor the strong PNN and wNN couplings characteristic of the Bonn potential.

0

IUCF TRIUMF “AA(B)” (Minimal Variance)

- Bonn (WI). x’ * 6.1 ( ?.I) - - -‘- Reid @WI. x’ = 11.6 (10.0)

Y y ,

<AA> AA 8 cm

Fig. 2a (left). Difference between neutron and proton analyzing powers. The meaning of the diierent theoretical contributions is given in the text. Fig. 2b (right). Angular distribution of the difference between proton and neutron analyzing powers. The solid and dash-dot curves are obtained with the Bonn and the Reid potentials respectively.

4. MEASUREMENTS OF SPIN OBSERVABLES IN THE pn + ppn- REACTION

The NN ---+ NNz process is of basic importance to understand pion production in nucleon, light and heavy ion induced reactions. The pp -+ pn*+ reaction has been extensively studied.26 It proceeds dominantly through an intermediate AN state as the initial pp pair has total isospin 1. In the pn + ppn- reaction, the initial pn pair can couple to total isospin 0 or 1; in the I = 0 state, the intermediate AN states are forbidden and in the I = 1 state, the (Ahop) channel is less coupled to (np) than the dominant (A++“) channel is coupled to (pp). Therefore, the ap + ppn- reaction should carry information on the nonresonant contributions to pion production and on the effect of higher energy N’ resonances.

4.1. Study of the iip -+ pp?r- process with a polarized free neutron beam Exclusive data on the iip + ppx- reaction with a polarized free neutron beam

at 810 MeV have been taken at SATURNE using the ARCOLE detection system.i’ To

382~ J. McClellar& M. Soyeur / Nucleon-nucleus intemctions

reconstruct completely the kinematics of events with S-body final states the six angles (6

and ‘p) of the three emitted particles are measured. There are nine unknown quantities (the 3-momenta of the 3 produced particles) related by 4 energy-momentum conservation

relations, i.e. 5 quantities to be determined. The measurement of six angles provides

therefore a relation characterizing the angular correlations, which identifies the kinematics

of the np + ppx- reaction. The detection system consists mainly of a vertex detector which is a triple cylindrical wire chamber and a triggering system of plastic scintillators

surrounding the position counters. These scintillators are also used to get dE/dx and time

of flight information.

A first run has established that the i;p --t ppvr- and Gp ---) da+r- reactions can be

separated. The analysis of the data provides 5 differential cross sections and asymmetries. More integrated quantities can be derived such as invariant mass spectra of the two protons

or of the *- and either of the two protons. Asymmetries are studied as a function of the two

proton invariant mass. Fig.3 shows a preliminary measurement of the angular distribution

of the analyzing power when the two protons are in a ‘SO state. Further analyses and

calculations are in progress.

30.

IO.

-10.

-30.

-50.

Fig. 3. Analyzing power in percent as a function of the center of mass angle in degrees for the “m + pp(‘&)r- reaction at 810 MeV.

4.2. Analyzing power measurements in p’n - pp(%~)lr- The measurement of analyzing powers and differential cross sections for the & +

pp(‘&)~~- reaction has been made at TRIUMF with the equivalent d(p’,n-pp)p process,

in a geometry which isolated the quasi-free pn 4 pp(‘So)x- reaction.” The data were taken at energies of 353, 403 and 440 MeV, over the angular range of 50” to 130” in the center of mass, with statistical errors of k 0.03 on the analyzing powers. They extend

earlier measurements27 at 400 MeV. The analyzing power at 403 MeV is shown in Fig.4. The data favor a partial wave analysis solution with a sizeable amplitude for the transition from the 3Sl(p, n) state.

J. McClelland; M. Soyew f Nucleon-nucleus intemctiom 383c

Fig. 4. Analyzing power for the j& + pp(‘So)a- reaction at 403 MeV.

5. BARYON-BARYON ANNIHILATION IN THE SKYRME MODEL

The study of collision processes provides an interesting way of investigating the full dynamics of the Skyrme model beyond the static properties of baryons. Baryon-baryon collisions have been described previously in the Skyrme model by integrating numerically the (3+1)-dimensions classical field equations. r* Inelasticities and scattering angles at various impact parameters have been calculated.

Using the same method, the dynamics of baryon-antibaryon annihilation has been studied in (3+1) dimensions. I9 The initial conditions are defined by applying Lorentz boosts to well-separated static spherical skyrmion and antiskyrmion solutions. The time evolution of the solitons is then given by numerical integration of the classical equations of motion for the pion and the sigma fields using the staggered leap-frog method. As an example, the time evolution of the baryon number density and of the energy density distribution is shown in Fig.5. The center of mass energy is 500 MeV and the impact parameter 1.3 fm. A general result is that the baryon number disappears rapidly while the energy density remains concentrated in the annihilation region; the annihilation time is found to be about one pion mass unit time as a dynamical consequence of the Skyrme Lagrangian. A Fourier analysis of the pion field emitted after annihilation of the Skyrmions has been performed. The result may be interpreted as meaning that 3 to 4 pions are emitted in the course of the annihilation at W” = 500 MeV and b = 1.3 fm.

6. SPIN EFFECTS IN PROTON-NUCLEUS INTERACTIONS

6.1 13$(zpp) Elastic Scattering at 49’7 MeV Elastic scattering from J” = O+ targets is sensitive mainly to the spin- and isospin-

independent parts of the NN interaction and the isoscalar one-body densities of the target.

384~ J. McClelland M. Soyeur / Nucleon-nucleus interactions

Fig. 5. Time evolution of the baryon number density (integrated in the

direction perpendicular to the collision plane).

Other types of reactions, such as proton elastic scattering from polarized odd-mass nuclear targets, can be more sensitive to the spin- and isospin-dependent parts of the interaction

and new nuclear structure information through observation of new spin observables.

0.5 4Q7.5 IJeV ’ ’ ’ ’ ’ 0.5

447.5 tdeV ’ ’ ’ ’ ’ 0.4 - 0.4

0.3 + 13c -

+ 13c _ P 0.3 _ P

- - g 0.2 0.1 - *OOON 1 I J J

a 0.0 h

8 0.2 0.1 - *OONN

a I

0.0 - -0.1 - -0.1 -

-0.2 - -0.2 - -0.3 I I I I I I I

-0.3 I , I I I I I 0 5 10 15 20 25 30 35 40 0 5 lo 15 20 25 30 35 40

%,,. (ded %m. @cd

Fig. 6. Experimental data and theoretical predictions for $+13 Ctarget

spin observables AOOO,, (left) and AOOnn (right) at 497 MeV. Relativistic

(solid) and non-relativistic (dashed) predictions are shown.

Results of the first proton elastic scattering experiment from a polarized nuclear target

at intermediate energies was reported. The experiment used a polarized 13C target and

the HRS at LAMPF. The results for target analyzing power A,,,, and the spin-correlation

J. McClel[anci, M. Soyeur / Nucleon-nuckw interactiorw 385c

parameter A,,,, at 497 MeV are shown in Fig.6, along with theoretical predictions based on relativistic (solid curve) and non-relativistic (dashed curves) impulse approximation. There is reasonable agreement between both sets of calculations and these new data, although both are lacking the full structure of the data. Calculations based on density- dependent effective interactions and more realistic nuclear structure input are in progress in order to more fully understand the nuclear-medium effects. Based on experience with this measurement, an expanded program including other polarized target nuclei is being considered for further study.

6.2 0+ + O- Excitations in I60 at 200 MeV O+ + O- transitions excited in proton inelastic scattering from spin-zero nuclei offer

important advantages for nuclear structure and reaction studies. They provide one of the most favorable cases for the investigation of the longitudinal spin transition density, which is directly related to the pion field within the nucleus. Since the longitudinal spin density does not contribute in leading order to either (e, e’) or (A, a’), this represents a new aspect of nuclear structure which can be investigated with inelastic proton scattering.

Just as for elastic scattering, O+ -+ O- transitions in spin-zero nuclei are completely determined by a measurement of cross section (o), analyzing power (A,) and spin-rotation parameter (Q) for proton scattering. These unique transitions are also interesting since the spin-orbit component of the NN interaction cannot contribute and the central component is weak. Hence, the excitation occurs primarily through the tensor part of the interaction. Experimentally, these transitions are difficult to measure since they are generally weak and in regions of high level density. Excellent energy resolution is therefore required.

0-, T = 0, Ex = 10.957 MeV

0.6

0.2

a? -0.2

-0.6

-1 .o

9 c.m. (deg.)

Fig. 7. Cross section (left) and analyzing power (right) for 200 MeV proton inelastic excitation of the O-, T=O state at 10.957 inr60. DWgl calculations are shown for pure [lpi’, 1s; > single particle transitions

(dashed) and with (lp,‘, Id+ > admixture. I

A presentation was made on measurements of the two lowest O- (T=O, 10.957 MeV;

386~ J. McCleNanci, M. Soyeur / Nucleon-nucleus interactions

T=l, 12.797 MeV) excitations in 160 at 200 MeV using the newly commissioned K600 high- resolution spectrometer and focal plane polarimeter system at IUCF. An overall resolution of 25-35 keV was achieved for this experiment. High quality u and A, data for the T=O, 10.957 MeV state were presented, along with cross section data for the T=l, 12.797 MeV state and preliminary spin-transfer data. In general the T=O cross section data is very sensitive to the amplitude of the Ilp,‘, ldq > component of the wavefunction and good

agreement with the data can be achie:ed with reasonable admixtures as seen in Fig. 7. The analyzing power, however is insensitive to this admixture and is in very poor agreement with both relativistic and non-relativistic impulse calculations. Further measurements of the spin rotation parameter Q should allow extraction of information on the longitudinal spin density and the scattering amplitudes.

6.3 Spin Excitations in Heavy Deformed Nuclei Below excitation energies of about 4 MeV, strong low-lying Ml transitions in heavy

deformed nuclei have been well established through numerous electron scattering and nuclear resonance experiments. These transitions are mediated by the orbital part of the magnetic dipole operator, and thus are dominated by convection currents. Some calculations predict the orbital and spin components of the transitions to occur in well separated energy regimes, with the spin contribution concentrated around 7 MeV. Experimental data for Ml transitions above 4 MeV are, however. scarce.

25

20

5

0 0246610 2 4 6 8 IO 12

EX [MeVl

Fig. 8. Proton spectra taken at 200 MeV for target nuclei 154Sm,158 Gd

and r6s1 55 at two different forward angles.

The first measurements of this spin strength in heavy deformed nuclei were presented, obtained using the Medium Resolution Spectrometer (MRS) at TRIUMF. Small-angle proton inelastic scattering cross section data were presented on 154Sm,158 Gd and 16*Er

for excitations up to 12 MeV at a bombarding energy of 200 MeV as shown in Fig. 8,

.I. McClellm@ M. Soyeur / Nucleon-nucleus interactions 387c

as well as spin-flip data. Ml spin strength distribution functions were extracted from the data in the region of 6 to 10 MeV of excitation which is dominated by the tail of the Coulomb excitation of the giant dipole resonance. Instrumental background was rejected through track reconstruction, an important tool at the very forward angles where these measurements were taken (3.4’ to 6.1“). Th e extracted strength is compatible with microscopic calculations and with expectations from other measurements in this mass range. The strength function exhibits a double structure which is predicted by QRPA calculations, but not completely understood at present. Another feature of these calculations is that the spin strength should be independent of the nucleus in this mass range, which is confirmed by the data.

Data were also presented on the spin-flip probability obtained from transverse polarization-transfer measurements using the focal plane polarimeter on the MRS. These types of data are very sensitive to spin flip strength as noted in section 6.2 and confirm the Ml assignment.

6.4 The Nuclear Spin Response of 40Ca to Intermediate Energy Protons

Measurements of spin-flip probability (Snn = (1 - Dnn)/2) have been used in nucleon- induced reactions as a means of filtering out spin-flip (AS = 0 or 1) strength both for specific transitions and in the continuum. Its powerful signature has been used, for instance, to identify isovector spin-flip (GT) transitions in the (p,n) reaction and to locate mostly isoscalar spin-flip strength in continuum (p,p’) studies. The interpretation of these results is often based on the simple structure of the small momentum transfer form of the scattering amplitude. As measurements are extended to larger angles and higher excitations, more elaborate theoretical models are required.

Microscopic DWIA-RPA calculation of spin-flip probabilities and double differential cross sections for inclusive inelastic proton scattering from 40Ca at 318 and 580 MeV at forward angles and excitation energies (w) up to 50 MeV were presented. These results were compared to measurements carried out at LAMPF.

At 318 MeV, the double differential cross sections are well reproduced with a resonance at about 17 MeV excitation, having L=O,l and 2 contributions. The S=l component of this resonance is clearly seen in the spin-flip cross section (as,,) to be the spin dipole resonance. As seen in Fig. 9a, the calculated S,,, distributions reproduce the general trends of the data. Both the data and the calculations show S,, to be much smaller than the free NN value at lower w, increasing above this value at higher excitations, with a minimum in the resonance region which is dominantly isoscalar. The later effect is ascribed to exhaustion of S=O sum rule strength in the giant resonance region.

New data from LAMPF were presented for the spin-transverse (u x q) and spin- longitudinal (u. q) spin-flip probabilities (ST and SL) at 580 MeV, defined in terms of complete sets of polarization-transfer observables D,,, D,, and Da over the same range of excitation energies as seen in Fig. 9b. The SL and ST data at 5” show a significant enhancement over the free NN values, while at 8.5” they are consistant with the free values. Preliminary calculations were presented which suggest that this enhancement is not related to pionic field effects in the nucleus, but rather are due to distortion effects. In addition, the effects of distortion seem to be quite different for Sr, and ST, with Sr. having the largest effect. Distortion effects calculated for the 318 MeV S,, data were generally smaller than for SL.

6.5 Second Order Spin-Orbit Interactions for Proton-Nucleus Scattering In most comparisons of the experimental proton-nucleus scattering at moderate or

388c J. McClelland, M. Soyeur / Nucleon-nucleus interactions

0 (MeV) - DWIA

---- PWIA

w (MeV)

. . . . . . . Free

Fig. 9a (left). Spin-flip probability S,, for 40Ca(p,p’) at TI,b=318 MeV. Fig. 9b (right). Spin-longitudinal (SL) and spin-tram vers (ST) spin-flip probabilities for 40Ca at 580 MeV.

high energies with non-relativistic multiple scattering (KMT) theory , the effect of terms in the nucleon-nucleon t-matrix which depend bilinearly on the spin of each nucleon is neglected. These terms do not contribute to the first order optical potential for a spin zero target nucleus. In view of the significance attached to the failure of the non-relativistic

theory to predict intermediate energy polarization data, it is important to study the contribution of these second order terms. While significant work has been done in this area, a possible second order spin-orbit interaction was presented whose magnitude is of a size which might bring the non-relativistic theory into better agreement with the data.

The first order KMT yields a smooth behavior of the polarization parameters with angle at forward angles. This occurs because there is a phase relation between the

numerator and the denominator (cross section) in the ratio defining these parameters. A rather modest change in that phase relation was shown to replace the aforementioned smooth behavior with an oscillating one, more indicative of the data.

7. SUBTHRESHOLD 9 PRODUCTION ON NUCLEI BY PROTONS

The production of 17 mesons in nuclei is very interesting because 71 mesons belong to the chiral SU(3)xSU(3) octet of pseudo-scalar Goldstone bosons, because their wave function has a large ss component and because the 7 - 7’ mixing suggests that the n-meson could be affected by the chiral UA(~) anomaly.

The first experimental study of inclusive n production from nuclei by protons has been made at SATURNE below the n threshold in nucleon-nucleon collisionsz5. Data have been taken at TP = 800, 900 and 1000 MeV on several nuclear targets, LDz, ‘jLi, C, Al, Cu and Au. The q mesons were detected with a two-arm high energy resolution spectrometer

J. McClellar@ M. Soyeur / Nucleon-nucleus interactions 389c

designed to detect neutral mesons decaying with a high branching ratio in the ry channel (PINOT).

The doubly differential cross section as a function of the target mass number is shown in Fig.10 for Tp = 900 and 1000 MeV. For targets heavier than %, the dependence on A goes roughly like A213.

Measurements above threshold and studies of the NN --) NNq process should make it possible to understand the underlying dynamics of 7,~ production in nuclei and the many- body effects associated with subthreshold production.

oTp= 900MeV

<;ti 012345

.k’n A

Fig. 10. Doubly differential cross section for q production from nuclei versus the target mass number for incident protons of 900 and 1000 MeV kinetic energy.

8. CONCLUDING REMARKS

The session “Nucleon-Nucleon and Nucleon-Nucleus Interactions” has stressed new developments related to the interactions of baryons (antibaryons) and mesons and emphasized the importance of spin observables.

More exclusive data and polarization measurements in charge exchange reactions offer the possibility of gaining a deeper understanding of nuclear spin-isospin mode excitations. Of particular interest are the in medium propagation of rr- and p- mesons and the excitation and decay of the A resonance at baryon densities smaller than the saturation density.

Theoretical and experimental progress on the baryon-baryon interaction deals mostly with the role of vector meson exchanges and the dynamics of the NN 4 NNx reaction beyond the A resonance contribution. Polarized data are again very helpful in the latter case. A first study of baryon-antibaryon annihilation using the Skyrme model gives a very interesting picture of the dynamics of this process.

Spin effects in proton-nucleus reactions bring additional information on spin dependent interactions. Recent polarized beam, polarized target experiments using high resolution spectrometers have been discussed. These data lead to interesting new developments in reaction theory.

39oc J. McClellan M. Soyeur / Nucleon-nucleus interactions

Finally, a new field is beginning to open with the production of heavier mesons in proton-nucleus interactions. Data on the production of q-mesons appear very promising.

ACKNOWLEDGEMENT

We thank the contributors to the parallel session “Nucleon-Nucleon and Nucleon- Nucleus Interactions” for helpful lectures on their work and for providing us with the necessary material to write this paper.

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