THE INFLUENCE OF SHORT RANGE CORRELATIONS ON PHOTONUCLEAR REACTIONS

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THE INFLUENCE OF SHORT RANGE CORRELATIONS ON PHOTONUCLEAR

REACTIONS

Max Huber

To cite this version:

Max Huber. THE INFLUENCE OF SHORT RANGE CORRELATIONS ON PHOTONU- CLEAR REACTIONS. Journal de Physique Colloques, 1971, 32 (C5), pp.C5b-243-C5b-245.

�10.1051/jphyscol:19715136�. �jpa-00214713�

THE INFLUENCE OF SIIORT RANGE CORRELATIONS ON PHOTONUCLEAR REACTIONS ^{+ )} Max G. Huber

Institut für Theoretische Physik, Universitat Erlangen-Nürnberg

Abstract: The effects of the short ranged nucleon nucleon interaction on

~hotonuciear processes are discussed; some of the available data on (y,pn)-, (y ,p) -, (y ,n)- and (e,eT)-rcactions in the intermediate energy rcgion are analyzed in terms of a consistent treatment of the nucleus in- cluding short range correlations. The nuclear structure information contained in those experiments is compared with that deduced from pion

lnduced 2eactions. The prospects of a unified picure of the nucleus is discusse

.

The nucleus currently is understood as an ensemble of inert and structureless nuc- leons which interact via two body forces.

From the niany body character of the problem it is obvious that the nucleus can be des- cribed only in terms of a model; such a model is tailored to reflcct the most im- portant nuclear properties. Any model, how-

ting both energies and wave functions especially of the low lying excitations of the nucleus. In principle this method of configuration mixing is exact and straight- forward, since the IPM provides a complete set of states. In practice, however, this procedure is restricted to the admixture of only a limited number of states. For ever, is only approximate in character, at obvious reasons only configurations in the best it is a carricature to the true nature vicinity of the nuclear ground state are of the object. Therefore, sooner or later usually taken into account explicitely. Due experimental facts will be found that to those restrictions of the configuration necessarily lead to a modification of space, only the long range character of the established models. Frequently such modi- residual nucleon-nucleon interaction can be fications are called correlations. treated by this method. The modifications Our present understanding of the nucleus introduced in this way are therefore calleck is based essentially on the Independent "long range cor relation^'^.

Particle Mode1 (IPM): it is assumed that At small internucleonic distances ( r < 1 fm) the nucleons move independently in a self- the nucleon-nucleon interaction behaves consistent potential (Hartree-Fock-field). rather strangely: it is strongly repulsive Considering the nature of the nucleon-nuc- and attractive for distances smaller and leon interaction, especially at small larger than 0.5 fm, respectively. The two internucleonic distances (r < 1 fm), the effects almost cancel, thus leading to an success of this model, both for bound and effectively weak interaction. This seems continuum States, is rather surprising. to be the reason for the apparent success Actually from the observation of low of the IPM concerning the properties of the lying collective excitations it turned out ground and the low lying excited nuclear to be necessary to modify this picture by States. At high excitation energies, how- the introduction of an effective residual ever, for example when interacting with interaction; this leads to a mixture of photons of sufficiently small wavelengths various IPM configurations, thereby affec- discrepancies between the predictions of

') Supported in part by the German Bundcsministerium für Bildung u. Wissenschaft.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19715136

M. G. HUBER

the IPM and the experiments are to be ex- pected, due to the short range character of the NN interaction.

Incidentally with the advent of new experi- mental facilities (especially with good quality beams of pions, energetic photons and electrons) more experimental phenomena have been found which cannot be explained consistently within the framework of the IPM. ~ o m e typical examples are: the ab- sorption rates of bound pions, the photo- absorption cross section at intermediate photon energies (50 MeV < E < 150 MeV),

Y

the quasideuteroneffect and the form factor for elastic electron scattering in the region of large momentum transfer.

Al1 those various experiments - and prob- ably others- seem to exhibit a common feature: the modification of the IPM at small internucleonic distances, generally called "short range correlations" (SRC).

In this talk the S R C f s are explained in detail when discussing the quasideuteron- effect, i.e. the (y,pn)-reaction: a one body operator induces a two body transition thereby indicating that nucleons do not move independently whenever they are close to each other (r < 1 fm).

From a discussion of those reactions we are faced with the following problem: the IPM is a sufficiently well established basis for a description of the low energy properties of the nucleus. tiow can one conserve this property and at the same time introduce explicitely the effects of the short ranged nucleon-nucleon interac- tion? There are several attempts to intro- duce SRC's from so called "first princip- les"; so far, however, only more pheno- menological approaches have been applied to the analysis of experimental data.

Most frequently used is Jastrow's method of correlated basis wave functions (1):

by introducing a correlation factor with the proper boundary properties SRC's are effectively simulated. The method and its

physical properties are discussed in more detail for example in Ref. (2).

This technique has been applied to the analysis of pion- and photoinduced reacti- ons. In this talk only photonuclear reac- tions are reviewed in detail; in particular the quasi'deuteronprocess (y ,pn) (3) , ^the

(y,p)- and the (y,n)-reactions (4) and the elastic scattering of electrons (5,6) are discussed; the influence of the SUC'S on the cross sections is exhibited, the results are compared with the experimental data available so far.

From the comparison between the calcula- tions and the experiments it can be con- cluded that it is possible to explain the magnitude and even details (such as the angular distribution of photoprotons (7)) consistently on the basis of an IPM modi- fied by SRC's, whereas the pure IPM pre- dictions generally fail to explain the magnitude of the experimental data.

In this connection it should be mentioned that using the same assumptions about the SKC's it is possible to explain not only those photonuclear data but also the magnitude of the absorption rate of bound pions (8).

Also it seems that the introduction of SRC's improves the description of the nuc- leus appreciably it should be pointed out that at present the analysis is only very crude. There are many open problems which are not expected to change the picture drastically, but which may affect the details of the analysis; the most important ones are:

1 . the interplay between long and short

range correlations;

2. the effects of baryon resonances which- to some extend - are expected to be simulated by the presence of SRC's;

3. the effects of exchange currents, which have to be investigated in more detail.

In order to arrive at definite conclusions

THE INFLUENCE O F SHORT RANGE CORRELATIONS ON PHOTONUCLEAR REACTIONS

concerning those questions and in order to pin down more precisely the details of the structure of the nucleus at small inter- nucleonic distances it is mandatory to have more precise experimental data on photonuclear reactions. Apart from the reactions already mentioned the quasiela- stic scattering of electrons, i.e. (e,e'p) and (e,e1d) seem to be a very attractive test for model assumptions about the short range structure of the nucleus (9). In this connection it should also be pointed out that the elastic scattering of electrons provides a powerful test to any model of the nucleus. Especially data in the region of large momentum transfer (q > 2 fm-') are sensitive to the details of the wave function at small internucleonic distances.

Finally there is a comment to be made about a question which is frequently dis- cussed: "Do Short Range Correlations

really exist?" This question, however, does not secm to be particularly interesting:

SRC's denote a theoretical concept, which -

as pointed out already - is always appro- ximate in character. Therefore there are no experiments which can test such an assumption, there is no "experimentum crucis" that can decide whether or not there are SRC's (there is, for instance, no test to prove that there are indepen- dently moving nucleons in a nucleus). The question that has to be asked, however, is rather different: "1s the concept of SRC's useful for the description of nuclear pro- perties?" 1.e.: 1s it possible by the in- troduction of SRC's to improve the IPW description of the nucleus in such a way that the various experimental phenornena observed in the intermediate energy range

(y-absorption, n-absorption, etc.) can be understood consistently in the same pic- ture which also is successful for the explanation of the low energy properties of the nucleus?

answer to this question: From the analysis available so far it seems to be necessary to modify the IPM wave function at small internucleonic distances by the introduc- tion of SRC's. Those modifications effec- tively simulate the scattering of other- wise independently moving nucleons due to the short ranged nucleon-nucleon inter- action. The corresponding momentum to be exchanged turns out to be of the order of some 300 MeV/c. This result is obtained for light nuclei (A

,

REFERENCES

(1) Jastrow (R.), Phys. Rev. ,1955,1479.

(2) Iluber (M.G.), Ann. de Phys. ,1970,5,239.

(3) Weise (W.) et al., Z. Phys. ,1970,236, 176.

(4) Weise (W.) et al., Nucl. Phys. ,1971, A162, 330.

(5) degli Atti (C.) , Phys. Rev. ,197O,C1, 809.

(6) Wright (L.E.) et al., Nuovo Cim. Lett., 1970,3,253.

(7) Weise (W.), private communication.

(8) Chung (K.N.) et al., Z. Phys. ,1970, 240,195.

(9) Hofmann ( H . M . ) , Weise (W.), private communication.

~t pres.ent there is only a preliminary