Quarks and nuclei: An experimentalist's point of view

Quarks and Nuclei: An Experimentalist's Point of View B. POVH Max-Planek-lnstitut far Kernphysik, Heidelberg, F. R. Germany

ABSTRACT Strong i n t e r a c t i o n at low energies which is relevant f o r nuclear properties is a complex phenomenon. Understanding of t h i s i n t e r a c t i o n and i t s r e l a t i o n to the quark dynamics of the hadrons can therefore be obtained only i f a s u f f i c i e n t l y large amount of the experimental data on hadron-hadron i n t e r a c t i o n is a v a i l a b l e . This is at present not the case. In p a r t i c u l a r , experiments with strange p a r t i c l e s and antiprotons on nucleons and nuclei are needed as constraints of the theor e t i c a l models of the strong i n t e r a c t i o n . Deep i n e l a s t i c scattering of electrons on nuclei gives a s e n s i t i v e measure of the short-range i n t e r a c t i o n of nucleons in nuclei. KEYWORDS Strong i n t e r a c t i o n : Discussed: strange p a r t i c l e and antiproton i n t e r a c t i o n with nucleons and nuclei. Deep i n e l a s t i c scattering of electrons on nuclei. INTRODUCTION Quarks at low energies are almost e x c l u s i v e l y a domain of t h e o r i s t s . This is quite in contrast to high energy physics where experiment and theory go hand in hand. Is t h i s because everything of importance has been measured at low energies and we have to wait f o r the theoreticians to f i n d a proper d i c t i o n a r y to t r a n s l a t e our old picture of the low energy hadron-hadron i n t e r a c t i o n into the quark language or because the important experiments relevant f o r quark description have not been performed yet and the t h e o r i s t s feel safe in applying t h e i r imagination without much experimental constraint? A tremendous amount of low energy data on hadron-hadron i n t e r action has been collected in the l a s t 30 years; nevertheless I believe the truth is closer to the second a l t e r n a t i v e than to the f i r s t . I t is true that the strong i n t e r a c t i o n is involved and there is not a single cruc i a l c l e a r - c u t experiment which would t e l l us how to describe the low energy i n t e r action. What I would l i k e to object to in our a t t i t u d e is that we do not even have very clear ideas which experiments are s e n s i t i v e to the substructure of hadrons at low energies.

326

B. Povh

There is only one example I know of in which the physicists f e l t one could make some progress in the problem of the quark properties at low energies; t h i s is the pp i n t e r a c t i o n . Some years ago one decided at CERN to build a low enerqy antiproton f a c i l i t y (LEAR) which would increase the i n t e n s i t y of p at low energies for orders of magnitudes. This decision was made on e s s e n t i a l l y two grounds. F i r s t l y , f o r an experimentalist i t is appealing to obtain a tool which enables him to improve the accuracy of the experiment for orders of magnitude hoping to find something new and unexpected, and secondly, at t h i s time one had numerous indications of the baryonium states. Baryonium states are in the language of nuclear physics pp bound states and in the language of quarks diquark-diantiquark states. One can hardly overemphasize the importance of spectroscopy type data to make the models. We look enviously at the heavy quarks f o r which charmonium and botomium o f f e r an excellent case to t r e a t them in a very simple way as a system of two heavy quarks i n t e r a c t i n g with a simple force and which can be treated n o n r e l a t i vistically. I f baryonium spectroscopy can be performed, that i s , i f baryonium states can be experimentally investigated, the connection between the quark picture and the boson-exchange picture of the hadron i n t e r a c t i o n could e a s i l y be established. The evidences f o r existence of narrow baryonium states have, however, hardly survived and we should recognize that pp experiments w i l l give much less i n formation on the quark-quark i n t e r a c t i o n at low energies than we had hoped a few years ago. The main part of my lecture w i l l be discussion of the information we can obtain from the pp system at low energies. In f a c t the LEAR f a c i l i t y w i l l be able to give a large amount of very detailed information on pp i n t e r a c t i o n , c e r t a i n l y much more than we w i l l be able to obtain on any other systems l i k e strange p a r t i c l e interaction with nucleons. Nevertheless, I believe i t would not be wise to bet only on the pp system. Even more, NN and NN systems are the most complicated two-hadron systems to be treated t h e o r e t i c a l l y that we can imagine. Therefore even less abundant information on i n t e r a c t i o n with strange p a r t i c l e s may be as useful as the f u l l analysis of the pp system. Let me quote, f o r example, the K÷p system. In the K+p i n t e r a c t i o n the ~ quark is i n e r t - as we w i l l discuss l a t e r - so we have to consider £nly four-quark i n t e r a c t i o n . I find the K+p system so much simpler than the NN or NN that we should not forget i t in our considerations when we t a l k about the quarks at low energies. I w i l l s t a r t my lecture therefore with some remarks on s p i n - o r b i t coupling in systems with S = -I and +I before turning to the pp system. F i n a l l y I would l i k e to comment on the p o s s i b i l i t i e s offered by electromagnetic i n t e r a c t i o n to learn about the quarks in nuclei. SYSTEM WITH STRANGENESS Let me i l l u s t r a t e as an example why the i n t e r a c t i o n with strange p a r t i c l e s would be of great i n t e r e s t . As is well known, the confinement volume of quarks is one of th( central issues of the low energy i n t e r a c t i o n . The bag model is one of the attempts to introduce quarks into the low energy hadron-hadron i n t e r a c t i o n . As discussed extensively at t h i s school, there are two versions of t h i s model on the market. The small bag model conserves to a large extent our old picture of nucleon-nucleon i n t e r a c t i o n which is dominated f o r distances greater than 0.5 fm by boson exchange. In contrast to nucleons, the strange p a r t i c l e s have in large and small bag models the same size of I fm. In the small bag model one should f i n d , therefore, quite a difference between the nucleon-nucleon i n t e r a c t i o n which is e s s e n t i a l l y of the boson exchange type and the nucleon-strange p a r t i c l e i n t e r a c t i o n which is already at large distances of the quark-quark type. With information on low energy i n t e r action of strange p a r t i c l e s with nucleons of s i m i l a r q u a l i t y as nucleon-nucleon and very soon antinucleon-nucleon data, the problem would be already solved. Unfortu-

Quarks and Nuclei: An Experimentalist's Point of View

327

nately the experiments that could perform such a task are not anticipated. In fact only s wave interaction between the strange particles and protons has been studied in bubble chambers. The next b i t of information for strange particles is at energies of a few GeV. In this situation also some scattered data on the interaction of strange particles with nuclei may be of great interest. Hypernuclei Let me start with the by now probably already f a m i l i a r spectra (BrUckner and coworkers, 1978) (Fig. I) demonstratina that the effective spin-orbit force for A g 2mq

MHY-MA

180 ]~r'' I /

~

190 I

200 1

I

M,,,- Mn

210 I

1

220 I

i

.,/k.

230 i

}I=O.j:O

12C(K- i%-}IA~ c

t50

(a) [Ip~,115~) An

^N

U }CORE

N~

100 0

I=J=S=O

.0

g

N

A s

2rns 20

-20

-10

tO

-30

__

B^[MeV]

} I=O, j=O

3011

(b) 2~

~

('~'@^ 10(

NO

(Is~,Ip~lAn ~

20

10

0

I -10

-~

I

I -30

-40

B^ [M,V] Fig. I. Spectra obtained from the (K-,~-) reaction on (a) z2C and (c) ~60 at a kaon momentum of 715 MeV/c plotted as a function of the transformation energy MHy MA in the strangeness-exchange reaction. The A-Beutron mass difference MA - Mn is also indicated. In addition, the A binding energy BA is plotted for each spectrum. From the energy s p l i t t i n g of the

I ON

CORE I=J=S=O

Fig. 2. Schematic of the interaction of the A particle with the one of the nucleon. (a) As the up and down quarks are antiparallel, their contribution to the spin-orbit interaction cancels out. (b) The contribution of the strange quark to the spin-orbit interaction is reduced because of its large mass as compared to the up and down quark.

(1P3/2,1P~2)An and (lp~/2, lP~2)An states in ~GO the spin-orbit strength in the A-nucleus interaction has been deduced. particles in nuctei is close to zero. The s p l i t t i n g between the (P~#2,P3/2)nA and (P~2,Pl/2)nA states is 6 MeV as is also the s p l i t t i n g between (P#~2,sl/2)nA and (PT~,s~/2)nA states in ~0 and the s p l i t t i n g between the P3/2 and Pz/2 hole states in bO. I w i l l need these spectra so as to compare them to the few so far measured

328

B. Povh

on z hypernuclei which w i l l action.

enable me to draw some conclusions on ~ nucleus i n t e r -

A weak s p i n - o r b i t coupling in A hypernuclei is no surprise and can be reproduced in the boson-exchange model (Brockman and Weise, 1977). In the quark model, however, i t comes out n a t u r a l l y ( P i r n e r , 1979). The A p a r t i c l e has the up and the down quark coupled to a spin and isospin s i n g l e t . Therefore the s p i n - o r b i t i n t e r action due to the exchange of up and down quarks cancels out. The strange quark exchange is hindered as the nuclear core does not have a strange quark (Fig. 2). Let us turn now to z hypernuclei. I t was quite a surprise to find out that the l i g h t z hypernuclear states show up in a ( K - , ~ - ) reaction as w e l l - d e f i n e d peaks ( B e r t i n i and co-workers 1981). In an experiment j u s t s t a r t i n g next month we hope to be able to confirm these findings and to learn more about when and why the p a r t i c l e conversion into z p a r t i c l e s in nuclei is hindered. MHy- MA (MeV)

MHy-M A (MeV)

175 200 225 250 275 300 325 350 .

grA

.

.

.

.

.

= 57 M e V / c

.

.

9Be(K.~ ) 9Be

175 200 225 250 275 300 325 350 12o L

.

grA

.

.

.

.

57 MeV/c

12C (K.~-) 1 C

300 100

I

250 g~A= 67 >

8° F

200

r

~n 150

8

C~A = 67 MeV/c

MeV/c

20

~ , I

C~o= 129 ~eV/c

i

i

i

0 -20 -40

BA(MOV)

112c K: . J c~~.=129 MeV/c

20

t

-25 -50 -75 -100-125 -150 -175 BA (MeV) 25

r

20 40

~

t

,

BA(Mev)

1%e(K:÷)~.ae1

100

50

0 -20 -40

i

i

I

i

I

0 -25 -50 -75 -100 B~o(MeV )

Fig. 3. Comparison of the A production on a beryllium target at a t r a n s f e r momentum of 57 MeV/c and 67 MeV/c to the production on the same t a r g e t at a t r a n s f e r momentum of 129 MeV/c.

0 ~

25

0

. . . . -25 -50 -75 -100 -125 -150 -175 B^ (MeV) J ~ 25 0 -25 -50 -75 -100 B~.(MeV)

Fig. 4. Same as Fig. 3, but for the 12C t a r g e t .

We published our ~Be and i~C data but never commented on them as the r e s u l t s seemed hard to b e l i e v e . Nevertheless I w i l l sketch my personal i n t e r p r e t a t i o n of these data. I f we compare A and s spectra in the "binding energy" scale BA and BZ we observe that the continuum states in z hypernuclei (here only P3/2 states) are s h i f t e d about 3 MeV to higher e x c i t a t i o n s (Figs. 3 , 4 ) . Even i f we assume that the s p i n - o r b i t potential is zero as in the case of a A p a r t i c l e , t h i s would mean that

329

Quarks and Nuclei: An Experimentalist's Point of View

the average z potential must be weaker than the A. I f the quark model arguments about the s p i n - o r b i t coupling f o r A are any good the s p i n - o r b i t coupling f o r z part i c l e s must be stronger than f o r nucleons. The z p a r t i c l e has up and down quarks coupled to the spin and isospin t r i p l e t state and should r e s u l t in a strong spino r b i t term. I f the s p i n - o r b i t term had the same sign as f o r the nucleon the spino r b i t force would push the continuum l e v e l s ( r e c o i l l e s s ~ production) to lower exc i t a t i o n s and we would end up with an extremely shallow potential depth f o r s part i c l e s in the nuclei. In the spectrum of Fig. 4 we observe a weak peak at Bs ~ 10 MeV which could be the ground state of I~C and would be j u s t at the binding energies expected f o r equal potential depth of s and A p a r t i c l e s in nuclei. The simplest way to i n t e r p r e t the data, providing that one takes them s e r i o u s l y , is to give the s p i n - o r b i t term of the s p a r t i c l e in the nucleus the sign opposite to thai of the nucleon. I t is a matter of simple arithmetics to show that the absolute magnitude of the s p i n - o r b i t term f o r z is the same as f o r nucleon-nucleus interact i o n . The p o s i t i v e sign of the s p i n - o r b i t term in z nucleus i n t e r a c t i o n cannot be e a s i l y reconstructed in the boson-exchange model. The vector boson exchange between the baryons w i l l always push the states J = 1 + I / 2 down in the e x c i t a t i o n . As G. E. Brown w i l l show you (Brown, 1981) in a simple version of the n o n r e l a t i v i s t i c quark model, providing that one manipulates the signs of matrix elements corr e c t l y , the s p i n - o r b i t term has the opposite sign from that in the boson-exchange model. I f my i n t e r p r e t a t i o n of the z hypernucleon data turns out to be correct i t w i l l have a quite important influence on our picture of low energy i n t e r a c t i o n of hadrons. My i n t e n t i o n , however, was not to speculate about the data but rather to show how s e n s i t i v e a t e s t of the models may be obtained by such simple information as s p i n - o r b i t i n t e r a c t i o n of strange p a r t i c l e s in nuclei. Quite some e f f o r t was necessary to make a l l these experiments requiring intense low momentum K- beams. The development of the state of the a r t in the l a s t 10 years can be best demonstrated by Fig. 5, which shows the l a s t three K- beams used

I I

1974: [ 900 MeV/c] Prod. Target

~ Seporotor-Stufe I

I

1977 :

1/ Koonen// Spektrometer / / /

i/

/

1981: [500 MeV/c]

I

~

I

i-~]~,/II i

T
/ z

/

/I/I I

/

//

"~

I

/11

I

PionenSpektrometer

iI I I

I

5m

I

Fig. 5. Kaon beams at CERN in the l a s t decade show a c l e a r tendency to shortening t h e i r lengths as they increase the kaon i n t e n s i t y at low momenta.

330

B. Povh

in hypernucleon experiments. In fact the f i r s t intense K- beam at CERN, b u i l t in 1969 by the Heidelberg Group, was only 12 meters long and has been used f o r kaons of 430 MeV/c momentum. I t s optical properties were, however, good only to use i t as a stopped kaon beam. The beams shown in Fig. 5 give about the same i n t e n s i t y of about 10" K-/burst on the target at I GeV/c, 700 MeV/c and 450 MeV/c, respect i v e l y . The main progress in kaon beams applicable f o r spectroscopy has been made by learning how to tag shorter and shorter beams with counters as close as 5 meters from the production target. In spite of t h i s information obtained from hypernuclei i t would be of much more use, at least for the subject discussed here, to have the scattering data of A, ; , Z and maybe even ~-on protons. S o f a r o n l y t h e scattering data of A and z on protons up to 20 MeV k i n e t i c energy have been measured in bubble chamber experiments. Why does nobody make more extensive measurements on these i n t e r e s t i n g systems? They are hard. In contrast to pions and kaons, the strange baryons have a l i f e t i m e of about I0 -z° sec and at low momenta t h e i r free path is on the order of a few c e n t i meters. To make a low energy beam of A's and ; ' s is therefore not easy. I f one r e a l l y f e l t that the A-p and z-p data at low momenta are of great importance for understanding the quark structure of the baryons, such experiments are possible to perform by use of present experimental techniques. Figure 6 shows schematically a hybrid system f o r scattering experiments of hyperons. A K- beam of a few GeV/c and 106 K-/sec h i t s an external hydrogen target. A spectrometer ident i f i e s the ( K - , ~ - ) , (K-,~+), or (K-, K+) reaction on hydrogen. In conjunction with the measured angle of the recoil z+, ~-, or ~- the twobody reaction can be picked out. The momentum of the recoil p a r t i c l e and i t s p o l a r i z a t i o n can be experiment a l l y adjusted. The p a r t i c l e enters a rapid cycling bubble chamber of not more than 10 cm in diameter. Bubble chambers capable of being triggered 30 times in a second are in operation at present. The hybrid systems are becoming of i n t e r e s t , in p a r t i c u l a r because of the p o s s i b i l i t y of applying holographic pictures in experiments with s h o r t - l i v e d part i c l e s . For the measurements d i s cussed here, a standard technique is f u l l y adequate.

~W ~ W

Hz PRODUCTION

~ '~ ~

TARGET i ~ ' i < , , . .

METER

\w

~,w._ w"/"~:;-

K-~ MONT ,OR

..-

w

w

H2 TAROET ~ " / ~

OR RCBC

\ Z

W-

-~--

/

/I

Fig. 6. Schematic of hybrid setup f o r measuring the hyperon-nucleon cross sections.

The t r i g g e r f o r the bubble chamber requires a pion or kaon of proper momentum in the spectrometer, selection of the recoil p a r t i c l e so as to s a t i s f y the two-body production, and the i d e n t i f i c a t i o n of the hyperon decay. This t r i g g e r is the most r e s t r i c t i v e i f we want to detect e l a s t i c scattering. Because of the short l i f e t i m e of the hyperons only a few centimeters of the hydrogen in the bubble chamber acts as the target. Only 10-3 hyperons scatter within t h i s path. All data on low energy hyperon scattering have been collected in the bubble chambers using the hyperon production following the stopped K- capture in

Quarks and Nuclei: An Experimentalist's

Point of View

331

hydrogen. The main d i f f i c u l t y of these measurements was the tremendous i n e f f i c i ency of the scanning of the pictures. For a single scattering event a thousand pictures have to be analyzed. For scattering at higher energies in spite of select i v e t r i g g e r i n g of the bubble chamber the e f f i c i e n c y of the selection of the proper events has not improved. I t is questionable whether with present labor costs such a research program would be f e a s i b l e . I t was pointed out by V. Hepp (1981) that charge-exchange reaction can be studied more e f f e c t i v e l y . In f a c t the charge-exchange reactions give as valuable information as does the e l a s t i c s c a t t e r i n g . The two reactions could be investigated: z- + p ÷ zo + n and ~- + p ÷ ~o + n. The t r i g g e r would now require i d e n t i f i c a t i o n of ~o or ~o and thus pick out only the proper events. I t is quite possible that such an experiment can be done without a bubble chamber. The resolution of the wire chambers is probably so good that the angular d i s t r i b u t i o n s of the ~o and ~o can be measured with s u f f i c i e n t accuracy so as to determine the i n t e r a c t i o n parameters. Furthermore, f o r the production target as well as for the s c a t t e r e r , one could use polarized hydrogen targets. Such experiments would be d i f f i c u l t and time consuming, but they are f e a s i b l e . I t is r e a l l y the question of how one estimates the importance of the information which would decide whether the hyperon-nucleon data should be investigated experimentally. K+p System There are quite some data collected on this system. The phase s h i f t analysis is done f o r kaon momenta up to 2 GeV/c. Polarization measurements have been well done and successfully used to resolve the ambiguities in phase s h i f t analysis. I f of i n t e r e s t K+p experiments can be easily refined using present high i n t e n s i t y beams. K+

p

N

N

~A

.~ u

u

u

Fig. 7. The lowest order diagrams f o r quark-gluon exchange in K+N and NN i n t e r a c t i o n . The K+p strongly resembles the NN system. In Fig. 7 the quark exchange in K+p and pp is shown. Both diagrams are nonplanar only! Most of the K+p data have been collected because of the search f o r Z* resonances, an analogue of the dibaryon

332

B. Povh

state in the pp system._ There is no evidence f o r Z*. Therefore the K+p system is extremely simple. The s quark i s i n e r t ; i t cannot be exchanged with up and down quarks in the i n t e r a c t i o n . I t contributes to the gluon exchange; in other respects i t behaves, however, as a spectator. The K+p p o t e n t i a l is r e p u l s i v e ; the s wave s c a t t e r i n g can be approximated by a s c a t t e r i n g on a hard sphere of 0.3 fm radius. In the e f f e c t i v e range approximation the s wave phase s h i f t s can be w r i t t e n as

k cot 60, I

_

rO,l I + ~ k2 ao,1

(I)

For isospin I = 0 the a0 ~ 0 whereas f o r I = I , al ~ -0.3 fm and the range of i n t e r a c t i o n r~ = 0.5 fm. The p wave amplitudes s t a r t f i r s t above 500 MeV/c of kaon momentum. This i l l u s t r a t e s how short range the KN i n t e r a c t i o n i s . The p wave phase s h i f t s can be i n t e r p r e t e d as r e s u l t i n g e n t i r e l y from the s p i n - o r b i t i n t e r a c tion. In the I = I channel the sign of the s p i n - o r b i t i n t e r a c t i o n is the same as t h a t f o r NN in the same isospin channel. I f i n d t h i s extremely e x c i t i n g . The KN i n t e r a c t i o n at small energies behaves as a pure hard core i n t e r a c t i o n expected in NN i n t e r a c t i o n . The extremely short range of the KN i n t e r a c t i o n (~0.5 fm) makes t h i s system an exc e l l e n t t e s t f o r the quark d e s c r i p t i o n of the i n t e r a c t i o n . I t would be hard to b e l i e v e t h a t the boson-exchange p i c t u r e could have much v a l i d i t y at such small d i s tances. In p a r t i c u l a r the p o l a r i z a t i o n in K+p systems should be i n v e s t i g a t e d in d e t a i l to f i n d out i f i t f o l l o w s a t channel as in the boson-exchange model or channel dependence as in the quark model (Brown, 1981). I t seems to me to be cruc i a l to f i n d out i f the boson-exchange and quark-exchange pictures are j u s t a d i f f e r e n t way of describing the same physical p i c t u r e or i f they describe two a l t e r n a t i v e behaviors in long- and short-range hadron i n t e r a c t i o n . THE NN INTERACTION AT LOW ENERGIES General Properties of the T o t a l , E l a s t i c , and A n n i h i l a t i o n Cross Sections Let me b r i e f l y summarize the dominant features of the pp system at low energies without going i n t o any i n t e r p r e t a t i o n of the data. Angular d i s t r i b u t i o n s of pp have been measured f o r several momenta above 0.6 GeV/c. Typical d i s t r i b u t i o n s are shown in Fig. 8. The d i f f r a c t i v e s t r u c t u r e of the d i s t r i b u t i o n is obvious. This and the r e s t of the d i s t r i b u t i o n can be reasonably well reproduced by the s c a t t e r ing on a black disk of a ~ 1.2 fm. The s e n s i t i v i t y to the s c a t t e r i n g on the real p o t e n t i a l can be observed only weakly in the amplitude of the second maximum. In f a c t the gross features of the t o t a l e l a s t i c and a n n i h i l a t i o n cross section below 2 GeV/c can be well approximated by j u s t assuming a strong absorption f o r d i s tances smaller than 1.2 fm. In Figs. 9 and 10 the t o t a l and the s c a t t e r i n g cross sections are p l o t t e d in dependence on PLab, s, I/pLVP-Laba b. The most remarkable feature is the very large value of the cross section a t the lowest measured momentum (~300 MeV/c) where aT reaches 300 mb (Oel ~ 80 mb). With increasing PLab the cross section decreases r a p i d l y . I t has, however, s t i l l oT ~ 80 mb at 2.5 GeV/c, a value twice t h a t f o r opp. The main features of the pp cross section are: a)

Below I GeV/c the t o t a l pp cross section can be well approximated by

Quarks and Nuclei: An Experimentalist's Point of View

333

d~

m~sr! ~00

.''

0 li9 GeV/c

J

0"?9 GIVIc

.p,.

~,.,~

10 0,1

J'

J":'

j. ~."

.-

.......

L'#~'I~'I~'

'" .............

~

0-01 100 0 0"86 GcVtc

10-0

,'J ./..J" /.Y"

10

0"99 GeVlc

../" "" ..J

01

t

0.01 100 0 1 "09 GeVIc

tO.O

I 1/, GcV/c

'"/

""

.,/"

10

/-

01

','H~uN;m,,~

001 100 0 1 23 GeV/c

10"0

....

I "30GtVI¢

"

" /

10 .,,.,............ ,,, ,,:" 0'1 0 01 1'360eV/c

100

./ ,~

1 43GcV/¢

.Y

.."

1'0

."

:

/

0'1 0"01 1000 10"0

I ' 5 0 GcVI¢

:Y

I-0

~"

1 60 GeVlc

/'

:/

o.1 r"'.'-,..,.........~"".............."'Y

:'

"".,-..,,~.~.,."'" ............""

0.01 -1.0

-0 $

O0

05

."

1.0 -1.0

-0.5

0.0

0.5

1.0

Co$0"

Fig. 8. The pp d i f f e r e n t i a l beam momenta.

~p~ = 66 + 2--~-6 mb Pcm

cross section at d i f f e r e n t

(Pcm : CMS momentum in GeV/c) .

(2)

b) The main d i f f e r e n c e between the t o t a l and e l a s t i c cross section - below the pion threshold only the charge exchange which i s only -10% of e l a s t i c s c a t t e r ing has to be taken i n t o account also - i s due to the a n n i h i l a t i o n . The a n n i h i l a t i o n dominates t~e low energy pp i n t e r a c t i o n . The rapid increase of the a n n i h i l a t i o n cross section can be understood in the I / v behavior of exothermic r e a c t i o n s , v a l i d i f the m a t r i x element i s f i n i t e f o r v ÷ O. c) Performing a p a r t i a l wave a n a l y s i s o f low energy a n n i h i l a t i o n , i t i s found t h a t a t low energy the p a r t i a l wave a n n i h i l a t i o n cross section saturates the unitarity limit

B. P o v h

334 ~p

:rosm

tot~

Ecw

IlllDt£~:

~icklon

CtO:HP

Ecw

(GeV)

h~

~

m~ctton

(GeV)

,

~

~'

;

;.

: . ; ." I ' . . . . .

b

Io,

,

J

, , ,,,,~

, Pb*~

J

, , J,J,l v

i

......

ioo

(GeV/c)

p ~

(GeV/c)

I

1 1

,

,

,

, , , , ,iio S

, , , ~L

• • °I |

llo

|o

( G e V 2)

5

,

. . . .

(GeV t )

| !

4~I--

'

L

°o

'

'

1

I ,I,

I ,,I-g" os

1o

1~

zu

'

'

'

1 / ~P~

Fig. 9. The t o t a l pp cross section as a f u n c t i o n of three d i f f e r e n t v a r i a bles, PLab, s, I//PL'a' b.

(1)n = (21 + I)4T~ 2 Oan d) ai n

,

b

~'

'

'

~o'

'

'

~,'

'

'

'

io'

'

'

1

1/~/P~,m

Fig. 10. The pp e l a s t i c cross section as a f u n c t i o n of three d i f f e r e n t v a r i ables PLab, s, I/V'#La b.

(3)

The r a t i o 1.8-

1.5 ,

(4)

°el which decreases with increasing energy, is c l e a r l y l a r g e r than the r a t i o f o r the black sphere which is Oin/Cel I , c l e a r l y demonstrating an important role of the real part of the pp p o t e n t i a l as well as d) Charge exchange pp ÷ nn which is small (~10% of the e l a s t i c cross section) but i t o r i g i n a t e s from the real part of the pp i n t e r a c t i o n . The t o t a l , e l a s t i c and charge-exchange cross sections, t h e i r magnitude and energy dependence can be well reproduced by a real p o t e n t i a l d i r e c t l y obtained from the pp p o t e n t i a l and applying the G p a r i t y transformation when used f o r the pp system and then a strong imaginary p o t e n t i a l which has to already have at I fm a value of 100 MeV.

Quarks

and N u c l e i :

An Experimentalist's

Point

of V i e w

335

In s p i t e of the dominance of the a n n i h i l a t i o n in the °DP and i t s f l a t momentum dependence, most of the experiments on pp systems have been performed in order to search f o r narrow s t r u c t u r e s . This search has been stimulated p a r t i a l l y by theor e t i c a l arguments p r e d i c t i n g baryonium states and by some experimental i n d i c a t i o n of narrow peaks in °D6" There i s no good evidence f o r such narrow states at present. In a recent exh~riment by the Heidelberg-Saclay group working at CERN which has so f a r c o l l e c t e d the highest s t a t i s t i c s f o r pp cross sections no i n d i c a t i o n of the resonances in the e x c i t a t i o n f u n c t i o n have been observed. Only i n d i r e c t arguments of possible pole or poles close to the pp threshold can be used as the r a t i o of the real to imaginary amplitude approaches 0 at 500 MeV/c momentum (Grein, 1977). General Properties of A n n i h i l a t i o n As we have seen, a n n i h i l a t i o n represents the major c o n t r i b u t i o n to the t o t a l pp cross section at low energies. The a n n i h i l a t i o n products c o n s i s t of pions or of resonances decaying i n t o pions. At r e s t 95% of the a n n i h i l a t i o n cross section cont a i n s only pions, the remaining part c o n s i s t i n g e s s e n t i a l l y of strange p a r t i c l e s plus pions. The average m u l t i p l i c i t y i s such t h a t the number of pions produced i s much smaller than the f i g u r e allowed by energy conservation. At r e s t the average m u l t i p l i c i t y = 5.0±0.15. I t slowly increases with energy reaching = 6.7±0.3 at PLab = 7 GeV/c. In t h a t range of energy, the m u l t i p l i c i t y of charge pions can be well represented by = a + byes. These f i g u r e s must be compared with the maximum possible number of pions, given by nLa b = #s/m . For PLab = 0 the #'s = 1.88 and f o r PLab = 7 GeV/c the ~ = 3.85GeV. Thus f o r PLab = O, nLa b = 13 and f o r PLab = 7 GeV/c nLa b = 27. This means t h a t a large f r a c t i o n of the energy -

f

-


V~

>m ~

~60% ~

at PLab

=

0

-

(5) ~75% at PLab = 7 GeV/c

goes i n t o the k i n e t i c energy of the pions produced. This f r a c t i o n is l a r g e , but i t i s s t i l l much smaller than the corresponding f i g u r e f o r production in nucleon-nucleon i n t e r action a t high energy, where only about 10% of the a v a i l a b l e k i n e t i c energy i s used f o r the masses of the p a r t i c l e produced. Therefore we can conclude t h a t the m u l t i p l i c i t y of a n n i h i l a t i o n i s r e l a t i v e l y b i g , and t h a t , at l e a s t at low energy, a n n i h i l a t i o n i s quite d i f f e r e n t from p a r t i c l e production. The m u l t i p l i c i t y d i s t r i b u t i o n i s q u i t e narrow (Fig. 11), narrower than a Poisson d i s t r i b u t i o n . At r e s t , where = 5, the branching r a t i o s are (Fig. 11) ~45% f o r n = 5, ~20% f o r n = 4, and 6-8% f o r n = 3

Annihilation rote %

~0

p~ ~ nTl: 30

R(n) =

R° (n -%)~ * A 2

Ro = ~3

! \

20

10

J i

1

no = S A= 1

.

2

i

/*

5

6

7

8

n

Fig. 11. T h e _ m u l t i p l i c i t y d i s t r i b u t i o n pions in the pp a n n i h i l a t i o n at r e s t .

of

336

B. Povh

and 7 and <1% f o r n = 2. I t is important to stress t h a t the a n n i h i l a t i o n and 2K is smaller than I%.

in 2~

I t is also important to observe t h a t the t o t a l two-body a n n i h i l a t i o n pp ÷ MzM2, where M~,M2 are p a r t i c l e s or resonances (p, ~, A2, . . . ) adds up only to about 7% of the observed a n n i h i l a t i o n cross s e c t i o n . A n n i h i l a t i o n is t h e r e f o r e not at a l l a two-body or a quasi-two-body phenomenon. These statements we have to take, however, w i t h some p r e c a u t i o n , as the a n a l y s i s of the many-body a n n i h i l a t i o n in the broad resonances is by no means s t r a i g h t f o r w a r d . The best studied channels of the a n n i h i l a t i o n s are the ~+~- and K+K- . The angular d i s t r i b u t i o n s can be analyzed in terms of broad resonances at 2150, 2310, and 2380 MeV having I Pc = 2++ , 3 - - , 4++ , r e s p e c t i v e l y . This is not s u r p r i s i n g as the x+~channel has very r e s t r i c t i v e quantum numbers I Pc = 0++, I - - , 2++ , and so on. I f we s e l e c t 9uantum numbers of the pp system very r e s t r i c t i v e l y , broad resonances appear in the pp cross s e c t i o n . The r e p r e s e n t a t i v e a n n i h i l a t i o n channels are, however, those f o r which the pp goes i n t o three bosons. At r e s t about 50% of decays are in the channels: ~ + ~ - , ~2~ °, p+p-~0, pop%0, a l l of these e v e n t u a l l y decaying i n t o f i v e pions. The a n n i h i l a t i o n t h e r e f o r e seems to be j u s t a p a i r i n g of the three quarks of the proton w i t h the antiquarks of the a n t i p r o t o n . But an_attempt to describe the decay in a quark-recombination model, in which a l l the qq combinations are projected i n t o boson s t a t e s , does not reproduce the a n n i h i l a t i o n channels. I t gives the average number of pions = 5, but the d i s t r i b u t i o n i s much broader than the measured one. As the m u l t i p l i c i t y d i s t r i b u t i o n s have been well measured, there is no doubt t h a t the dynamics of the a n n i h i l a t i o n i s much more i n v o l v e d than assumed in the q u a r k - r e a r rangement model. At momenta of a few GeV/c, at l e a s t f o r the 37 channel a leading p a r t i c l e can be observed in the a n n i h i l a t i o n . I f this_phenomenon is re§ponsible f o r the a n n i h i l a t i o n the process goes s u c c e s s i v e l y by pp ~ qq + X (X ÷ qq + qq). The problem, however, is t h a t at 0.8 GeV/c a new i n e l a s t i c channel, the pion emission, opens, and a systematic knowledge of the a n n i h i l a t i o n should be c o l l e c t e d so as to decide whether the leading p a r t i c l e is due to the a n n i h i l a t i o n r a t h e r than to the i n e l a s t i c s c a t t e r i n g f o l l o w e d by the a n n i h i l a t i o n . In my o p i n i o n , the outstanding problem of the a n n i h i l a t i o n at low energies is i t s long range. In the pre-quark time the range of the a n n i h i l a t i o n had been r e l a t e d to the nucleon-exchange process and had a range o f 0.1 fm. To o b t a i n 100 MeV s t r e n g t h of the imaginary p o t e n t i a l at I fm distance a p o t e n t i a l depth of at l e a s t 10 GeV was assumed. Such p o t e n t i a l s have h a r d l y any physical s i g n i f i c a n c e . I feel t h a t i t is much more l i k e l y t h a t the large a n n i h i l a t i o n range has an i n t i m a t e connection to the f i n a l dimensions of the confinement volumes of the proton and a n t i proton. For the connection between the a n n i h i l a t i o n range and the confinement v o l ume, however, the dynamics of the a n n i h i l a t i o n has to be understood. Let me summarize the main features of the pp i n t e r a c t i o n at low energies: I ) At distances r < I fm dominates the a b s o r p t i o n . The imaginary p a r t of the p o t e n t i a l has to have at l e a s t a value o f 100 MeV at I fm. A l l t r i c k s i n v e n t i n g smaller absorption so as to e x p l a i n narrow resonances, f o r the existence of which we do not have any experimental proofs, but s t i l l reproducing the main gross features of the pp i n t e r a c t i o n , seem to be s u p e r f l u o u s . 2) At distances r > I fm the real p o t e n t i a l dominates. The study of i t gives us an e x c e l l e n t p o s s i b i l i t y to compare i t to the pp one and to check whether our

Quarks and Nuclei: An Experimentalist's Point of View

337

present picture of the pp i n t e r a c t i o n is good down to I fm internucleon distances. For t h i s , however, a f u l l analysis of the phase s h i f t s in pp is necessary. 3) For a n n i h i l a t i o n the quark-antiquark i n t e r a c t i o n is presumably responsible. Thus study of the a n n i h i l a t i o n can give us valuable information on the quark properties at low energies. The new f a c i l i t y LEAR, we h£pe, is a good tool for investigating f u r t h e r the real and imaginary parts of the pp p o t e n t i a l . LEAR The low energy antiproton ring (LEAR) is one of the spin-offs of the large pp coll i d e r project at §ERN. Antiprotons produced at 3.5 GeV/c, where this is the o p t i mum momentum f o r p production at 25 GeV/c energy of the PS (proton r e g u l a t i o n ) , are collected in the antiproton accumulator. The p beam is cooled s u f f i c i e n t l y so as to have an emittance acceptable f o r PS. For the c o l l i d e r and f o r the i n t e r s e c t i n g storage ring the antiprotons are accelerated in the PS; for the low energy experiments they are decelerated down to 0.6 GeV/c. At 0.6 GeV they are injected into the low energy antiproton ring. In the ring they are f u r t h e r manipulated e i t h e r to increase or to decrease t h e i r energy. Antiprotons are transferred from the antiproton accumulator about once an hour with an i n t e n s i t y of about I0 z° antiprotons. The LEAR takes care of providing a slow ejection f o r the external targets. The beam available f o r shearing has an i n t e n s i t y of about 106 p/sec. The LEAR beam means tremendous improvements in the i n t e n s i t y , many orders of magnitude for the p below I GeV/c. But i t means also a tremendous improvement in the beam q u a l i t y , the energy resolution (Ap/p ~ I 0 - " ) , and beam size (-I mm2). The experimental technique with such a beam can be l a r g e l y s i m p l i f i e d as compared to the standard secondary beams. But in order to perform experiments supplying data which could be compared to the pp data, one needs a polarized p beam. The great v e r s a t i l i t y of the LEAR f a c i l i t y promises to give us the chance in the next few years to learn most of the information on pp at low energies. But only i f we succeed in obtaining polarized p in LEAR w i l l the wealth of information compete with that of NN that was obtained in cyclotrons using polarized sources. Research Program with LEAR I w i l l b r i e f l y sketch the LEAR program relevant to the question treated here, i . e . , quarks at low energies. I) Two groups, with a complementary program, are looking into the e l a s t i c scattering (Bailey and co-workers, 1980) and into the charge exchange (Braune and co-workers, 1980) in p on p. Both experiments are planned to be done with a l l of the possible obstacles measuring the d i f f e r e n t i a l cross sections in a l l possible polarizations. Let me comment on some simple features of the e l a s t i c and charge-exchange cross section following these expressions. The amplitudes AI= 0 and AI= I can be decomposed into two parts, one representing d i f f r a c t i v e scattering due to the imaginary pp potential and the second due to the scattering in the real pp p o t e n t i a l . The d i f f r a c t i v e scattering is equal for I = 0 and I = I i f the imaginary potential is independent of the isospin. In this case f o r the charge-exchange amplitude the contribution from the d i f f r a c t i v e scattering cancels out exactly. As we have seen, the e l a s t i c scattering is dominated by d i f f r a c t i o n . Therefore i t is not surprising to find that the charge-exchange cross section is only about 10% of the e l a s t i c

338

B. Povh

one. The pp system is and I = I states. The metric combination for scattering is therefore in isospin amplitudes A~p+~p : -

not an eigenstate of isospin but an equal pure isospin states are antisymmetric for I = I of pp and ~n states. The amplitude j u s t the projection of the pp system on

mixture of I = 0 I = 0 and a symf o r e l a s t i c pp i t s e l f , expressed

AI= I + AI= 0 2

(6)

The charge-exchange amplitude is obtained by projecting the pp state on the nn; thus App+nn =

AI= I - AI= 0 2

(7)

In order to explain the r a t i o 10 for e l a s t i c and charge-exchange cross section the two amplitudes A~D_~BD/ABD+~n ~ 3, which means that the AI_ and AI_ 0 have about twice as much d i ~ r ~ t i g e c o n t r i b u t i o n as that for the real part of the pp potential I f , however, there are baryonium states in the pp system that degenerate in the isospin, the imaginary pp potential should be isospin dependent. The charge exchange should be extremely sensitive to these differences. The effect should be seen in the charge-exchange angular d i s t r i b u t i o n by the appearance of an inteference between the d i f f r a c t i v e peak and the boson-exchange contribution. This effect may have been observed already by Bogdanski and co-workers (1976) (Fig. 12) in a measurement at 700 MeV/c laboratory momentum of p. The measurement is done at a single momentum and thus cannot be interpreted unambiguously. Both measurements of e l a s t i c and chargeexchange d i f f e r e n t i a l cross section i f measured in selected p o l a r i z a t i o n states w i l l be much more sensitive to the possible structure in the e x c i t a t i o n function than the total cross section measurements available so far. There is another i n t e r e s t i n g feature i f one compares j u s t the charge exchange and the e l a s t i c scattering on the real part of the pp p o t e n t i a l . In the boson-exchange model the contributions of the isoscalar exchange are obviously independent of the isospin and thus add up for the e l a s t i c and cancel out for the charge-exchange channel. Opposite is the case for the exchange of the isovector bosons, the effect of which cancels for the e l a s t i c scattering and adds up for charge exchange; thus, f o r charge exchange only the contributions

I|

4

I

~

[

1

o) ~ ~

3

-°~ ~ b ~

2L-'-'~

~ ~ +

pf", I I

0

I

, 0.5

~~i°~-°~ 9ooc,oo ~ 0 9 ~O-od 0 -0,5 -I.O COS

I

I

N

~ • l

b)

,oo E

~ 5O b Oo I

0

o l

00000

~0000000000000

0.2 03 04 -t (GeVz ) Fig. 12. The d i f f e r e n t i a l cross section for the reaction pp+nn: a) as a function of cos 0", where 0* is the c.m. angle and b) as a function of t (Bogdanski and co-workers, 1976).

339

Quarks and Nuclei: An Experimentalist's Point of View

of the ~ and p exchange are important. I t is not necessary to stress that the charge exchange should have a v i o l e n t spin-dependent cross section. Therefore we believe to have in the charge-exchange process an e x c e l l e n t t e s t of the boson-exchange model down to distances of I fm. So far only integrated charge-exchange cross section has been measured using the counter technique. The single angular d i s t r i b u t i o n has been done in a bubble chamber experiment. The Heidelberg group is planning to measure angular d i s t r i b u t i o n s in the charge exchange by a system of calorimeters shown in Fig. 13. I t is rather obvious that this apparatus is much more involved than the standard apparatus for e l a s t i c scattering measurements. Elastic s c a t t e r i n g , on the other hand, should show only weak p o l a r i z a t i o n dependence.

FH MWPC FC target

.

1L TCB

elevation

WLall v l : w

Fig. 13. Experimental setup f o r a measurement of the d i f f e r e n t i a l cross section f o r the reaction pp÷nn as proposed in Braune and co-workers (1980). The meaning of the abbreviations is as follows: SD p defining detector MWPC multiwire proportional TCB target calorimeter box chambers AC s c i n t i l l a t o r slabs FC forward calorimeter FH forward hodoscope PM photomul t i p l i e r I f the comparison between the NN and NN i n t e r a c t i o n should be used as a sensitive t e s t of the boson-exchange model the f u l l phase s h i f t analysis of the NN system should be done. This is not a modest undertaking as in addition to the real phases as in the NN system we have additional imaginary phases because of absorption. It is f a r too premature to judge on the success of this program at LEAR. ANNIHILATION There is only one apparatus, ASTERIX (Armenteros and co-workers, 1980), planned for the time being at LEAR which can study the a n n i h i l a t i o n in d e t a i l . I t has, how-

340

B. Povh

ever, in contrast to i t s g a l l i c r e l a t i v e in name, developed i t s fabulous strength only at rest. Being adopted from an apparatus used in c o l l i d i n g beam experiments, i t has the highest s e n s i t i v i t y f o r detecting the p a r t i c l e s at 90 ° with respect to the beam. I t is therefore well adapted f o r the study of the a n n i h i l a t i o n at rest. The acceptance f o r charge and n e u t r a l _ p a r t i c l e s is about 25% and 7% of the 47, res p e c t i v e l y . But already at moderate p momenta a v a i l a b l e at LEAR of I GeV/c the missing s e n s i t i v i t y in the forward d i r e c t i o n makes t h i s apparatus less s u i t a b l e to study the a n n i h i l a t i o n in f l i g h t . Nevertheless I feel that at present a detailed study of a n n i h i l a t i o n at rest is the most urgent problem to solve. The previous experiments, done in the bubble chamber, are lacking s t a t i s t i c s so as to look in any d e t a i l of the a n n i h i l a t i o n process. In addition, ASTERIX w i l l be able to sel e c t the i n i t i a l state of pp by selecting the atomic X ray during the p cascade in the pp atom. Let me b r i e f l y discuss the a l t e r n a t i v e s - a n n i h i l a t i o n at rest versus a n n i h i l a t i o n in f l i g h t . The study of a n n i h i l a t i o n is a clear analogy to the study of the reaction mechanism. Therefore one should choose the kinematical conditions in such a way as to select the part of the phase space in which a single reaction mechanism w i l l dominate. A n n i h i l a t i o n at rest is obviously not such a case. On the other hand, to study the a n n i h i l a t i o n below the pion threshold (p < 0.8 GeV/c) does not seem to be any improvement as compared to the a n n i h i l a t i o n in f l i g h t . In the current models the quarks have a Fermi momentum of about 0.6 GeV/c._ I f the momentum of incoming quarks should be l a r g e r than the Fermi momentum the p beam should have at least 3 GeV/c. In f a c t the experimental i n d i c a t i o n of a leading p a r t i c l e phenomenon in the a n n i h i l a t i o n s t a r t s f i r s t at 3 GeV/c incoming momentum. But at these momenta the f u l l complications a r i s i n g from numerous i n e l a s t i c channels come into play. Therefore at least f o r experiments on LEAR a study of the a n n i h i l a t i o n at rest seems to me s u f f i c i e n t . I t is another question, however, i f one believes i t possible to cope with the problems of pp i n t e r a c t i o n at energies of a few GeV and one can select the relevant part f o r the a n n i h i l a t i o n out of i t . There are many very i n t e r e s t i n g experiments planned at LEAR that I have not mentioned. They may turn out eventually to be of great importance f o r understanding of the behavior of quarks at low energies, but t h e i r primary i n t e r e s t is oriented at the moment to some other matter. Concluding the pp t o p i c , I would l i k e to make t h i s remark. The dominance of the a n n i h i l a t i o n makes the study of the real part of the pp potential more complicated as is the case in the NN system, being i t s e l f not a t r i v i a l case. A n n i h i l a t i o n i t s e l f , which may have many i n t e r e s t i n g phenomena on the quark-quark i n t e r a c t i o n , is involved because we are dealing with the analysis of many-body decays and because there are no simple kinematical conditions to single out the dominant properties of a n n i h i l a t i o n . I t is r e a l ! y the experimental tool LEAR which gives us the poss i b i l i t y to investigate the pp system in d e t a i l that gives us f a i t h that with the pp system we may improve our knowledge of the quark i n t e r a c t i o n at low energies. INTERACTION OF ELECTRONS WITH NUCLEI Electrons have played an important role in revealing the quark structure of the nucleons. Deep i n e l a s t i c scattering gives not only the number of quarks in the nu cleon but, via the structure function, also information about the quark dynamics in the nucleon. I t is therefore straightforward to ask i f we can use electrons al so to learn about the role quarks play in the nucleus. Let me i l l u s t r a t e the s i t u a t i o n by comparing i t to that in atomic physics. The analogy of the deep i n e l a s t i c scattering is in atomic physics the Compton e f f e c t on bound electrons. The Fermi motion of electrons in the atoms gives some smearing of the two-body kinematics in the photoelectron scattering. This smearing is

Quarks and Nuclei: An Experimentalist's Point of View

341

related to the dynamical properties of the system. I f atoms are bound in molecules the motion of the electrons which are shared by more than one atom w i l l be changed. Consequently also the smearing of the Compton kinematics w i l l be changed. I f quarks in nuclei c l u s t e r to groups of more than three objects t h e i r Fermi motion w i l l be changed as compared to that in the free nucleon. In the deep i n e l a s t i c scattering the quark Fermi motion is taken into account by the structure function. The question arises therefore whether we can measure the difference between the structure function of the free and the bound nucleons. High momentum components of the nucleons in the nuclei may be related to the question of multiquark clusters as I w i l l t r y to show below. In nuclei the average distance between the nucleons is about 1.g fm. At this distance the nucleon-nucleon i n t e r a c t i o n is well described by phenomenological potent i a l s which are obtained by the analysis of the nucleon-nucleon scattering data and using the boson-exchange model f o r avoiding ambiguities in the phase-shift analysis. These potentials can be r e l i a b l y used down to distances of 0.8 fm (Vinh Mau, 1979). Opening of the i n e l a s t i c channel in the nucleon-nucleon scattering at the pion threshold makes the analysis of the scattering data too complex f o r a r e l i a ble construction of the potential at small distances. At small distances the nucleon-nucleon potential is repulsive. In the boson-exchange model a strong repulsion at small distances is expected due to the exchange of vector bosons. But a q u a n t i t a t i v e treatment of boson-exchange at small distances would hardly be considered as s u f f i c i e n t l y r e l i a b l e in helping a construction of the nucleon-nucleon p o t e n t i a l . The behavior of the nucleons in the nucleus at small r e l a t i v e distances, usually denoted as short-range c o r r e l a t i o n s , has never been properly understood. The short-range behavior of the nuclear force determines, however, many nuclear properties such as high momentum components of the Fermi motion and, together with the long-range a t t r a c t i o n , such a fundamental quantity as nuclear density. A revival of the i n t e r e s t f o r short-range correlations in the nucleus at present is obvious. At some point f o r small r e l a t i v e distances i t becomes inappropriate to t a l k about an i n t e r a c t i o n between the two nucleons but rather to consider an i n t e r action of six quarks. Or, formulating more p i c t o r i a l l y , the nucleus is not only a composition of independent clusters of three quarks, being well approximated by nucleons, but in a small f r a c t i o n also a composition of clusters of six or even more quarks. In the jargon today one usually talks about bags with three quarks, six or even more rather than the quark c l u s t e r . We, however, would l i k e to avoid the use of the bag concept in the present paper so as not to i n f e r s p e c i f i c bag model predictions possibly relevant f o r the following discussion. One of the clearest demonstrations of the overshoot in the high momentum components in nuclei can be observed in the i n e l a s t i c electron scattering on 3He (Day and coworkers, 1979). For the three-body system the ground state wave function can be calculated using the " r e a l i s t i c " nucleon-nucleon forces by applying the Fadeev method. A comparison with experiment shows, however, that the " r e a l i s t i c " nucleonnucleon potentials do not give s u f f i c i e n t high momenta (Side, Day and McCarthy, 1980) in the 3He. In a recent paper Pirner and Vary (1981) suggest a model in which the high momentum components r e s u l t from the regions in the nucleus where six quarks overlap so as to exchange t h e i r momenta. In t h e i r model 16% of the quarks in ~He have to be in clusters of six rather than three quarks so as to reproduce the high momentum t a i l of the Fermi motion in 3He. Recently Th. Walcher and mys e l f (1981) suggested introducing another measure of the p r o b a b i l i t y f o r six-quark clusters i n . t h e nuclei by measuring the simultaneous production of K+, and z+ can be produced simultaneously only i f two protons in the nucleus are s u f f i c i e n t l y close together as to rearrange t h e i r quarks (Fig. 14) and providing that the f i n a l state i n t e r a c t i o n can be neglected. This point, however, we have to discuss l a t e r in more d e t a i l .

342

B.

Povh

~

"Yv

u

S } K÷

CLUSTER , ~

n

Fig. 14. Schematic showing associated K+,z + production on a six-quark c l u s t e r . The suggestion made is t h a t the six-quark c l u s t e r s should be named the space-time regions of nuclei f o r which the branching r a t i o s f o r the decay, when bringing them on the energy s h e l l , are obtained by simple p r o j e c t i o n of the six-quark s t a t e i n t o the decay channels. Under these assumptions the cross section f o r associated K+,z + production in the deep i n e l a s t i c s c a t t e r i n g on nuclei is d4q(e + A ÷ e' + K+ + z + + A') _

d4~(e + p ÷ e' + K+ + sO)

-

dQ2dsdtd#

(8)

dQ2dsdtd#

I t means t h a t i t is proportional to the cross section f o r replacing an up quark in a proton by a strange one (Fig. 14). The cross section f o r K+,Z ° production in deep i n e l a s t i c s c a t t e r i n g on protons is u s u a l l y given in dependence on the momentum t r a n s f e r Q and the energy loss s of the e l e c t r o n . The v a r i a b l e t and 6 determine the kinematics of K÷. In equation (8) I Z(Z

= ~ A(A where P(6) f a c t o r Z(Z the f a c t o r i n t o K+,z +

-

I)

P(6)

I)

(9)

is the p r o b a b i l i t y of f i n d i n g any six-quark c l u s t e r in the nucleus, the - I ) / A ( A - I ) counts the p r o b a b i l i t y of h i t t i n g a two-proton c l u s t e r and I / 3 is the p r o j e c t i o n of the six-quark state with I s , 3u and 2d quarks channel.

F i n a l l y we have to consider the f i n a l state i n t e r a c t i o n which could obscure the a n a l y s i s of the associated K+,z + production on n u c l e i . The only f i n a l state i n t e r action simulating the associated K+,z + production is the charge exchange of s ° on protons t r a v e r s i n g the nucleus. The cross section f o r simultaneous K+,z + product i o n via the f i n a l state is again proportional to the K+,s ° production on the proton times the p r o b a b i l i t y of converting z ° i n t o s+ which we approximate by W(S0 + z+) ~ °CH~p (I - exp(-OlN~)) .

(i0)

°iN~

Here ~CH is the charge-exchange cross s e c t i o n , °IN the t o t a l i n e l a s t i c cross sect i o n , ~ the nucleon d e n s i t y and pD the proton density in the nucleus. In the approximation (10) we replaced the ~ntegral over the z ° path by an average path . I n s e r t i n g the numbers i n t o formulas (8), (9) and (10) f o r d i f f e r e n t nuclei one soon finds t h a t the only chance to observe a clean signature f o r K+,E+ associated production on nuclei is on SHe and 4He. For P(6) = 0.2, 10% of K+ are associated by s+ production in SHe and only 5% on 4He. On the other hand the f i n a l s t a t e i n t e r -

Quarks and Nuclei: An Experimentalist's Point of View

343

action contributes in both nuclei the same; about 5% of the ~0 are converted into z+. I t seems therefore that the measurement of the associated K+,z+ production on 3He and 4He could give us extremely interesting information on possible existence of six-quark clusters in the nuclei. CONCLUDING REMARKS Strong interaction as manifested in the nuclear force has always been considered to be a complex phenomenon and thus hard to treat theoretically in simple mathematical models. Introducing quarks and gluons into the game will hardly simplify the problem of the nuclear force. I t is not the simplicity but the new physical content that challenges us when we introduce the quarks into nuclear physics. The question seems to be whether the nuclear force is mediated by boson exchange or by quark exchange. Maybe by both, one responsible for the long-range and the second for the short-range force. The question is far from being answered; we do not even know whether the question as such has been properly formulated. In the present lecture I have tried to demonstrate that i t is up to the experiments to produce the answers to the open questions. In the strong interaction - because of i t s complexity - i t is not a small number of crucial experiments which makes up the answer but rather many small contributions, like a mosaic which, i f patiently put together out of small pieces, emerges eventually as a completed picture. The experimental techniques have been developed in the last years so we can extend our experiments far beyond the mostly studied nucleon-nucleon and pion-nucleon systems. Interaction of strange particles and antiparticles with nucleons and nuclei can be studied with such sophistication as we are accustomed to in nuclear physics. Experiments on electron-nucleon interaction which have been very successful in investigating the nucleon structure can be profitably extended to the electron-nucleus system. REFERENCES Armenteros et al. (1980). CERNProposal, CERN/PSCC/80-101, PSCC/P28. Bailey et al. (1980). CERNProposal, CERN/PSCC/80-76, PSCC/P16. Bertini, R., Bing, 0., Birien, P., BrUckner, W., Catz, H., Chaumeaux, A., Durand, J.M., Faessler, M.A., Garreta, D., Ketel, T.J., Kilian, K., Mayer, B., Pietrzyk, B., Povh, B., Ritter, H.G., Uhrmacher, M., and Walcher, T. (1980). Phys.Lett., 90B, 375. Bogdanski, M., Emura, T., Ganguli, S.N., Gurtu, A., Hamada, S., Hamatsu, R., Jeannet, E., Kita, I . , Kitamura, S., Kishiro, J., Kohno, H., Komatsu, M., Malhotra, P.K., Matsumoto, S., Mehtani, U., Montanet, L., Raghavan, R., Subramanian, A., Takahashi, K., and Yamagata, T. (1976). Phys.Lett., 62B, 117. Braune et al. (1980). CERNProposal, CERN/PSCC/80-85, PSCC/P20. Brockman, R., and Weise, W. (1977). Phys.Lett., 69B, 167. Brown, G.E. (1981). Lecture notes in this volume. BrUckner, W., Faessler, M.A., Ketel, T.J., Kilian, K., Niewisch, J., Pietrzyk, B., Povh, B., Ritter, H.G., Uhrmacher, M., Birien, P., Catz, H., Chaumeaux, A., Durand, J.M., Mayer, B., Thirion, J., Bertini, R., and Bing, O. (1978). Phys. Lett., 79B, 157. Day, D., McCarthy, J.S., Sick, I . , Arnold, R.G., Chertok, B.T., Rock, S., Szalata, Z.M., Meching, B.A., and Tamas, G. (1979). Phys.Rev.Lett., 43, 1143. Eisenhandler, E., Gibson, W.R., Hojurat, C., Kalmus, P.I.P., Lee, L.C.Y., Pritchard, T.W., Usher, E.C., Williams, D.T., Harrison, M., Range, W.H., Kemp, M.A.R., Rush, A.D., Woulds, J.N., Arrison, G.T.Y., Astbury, A., Jones, D.P., and Parson, A.S.L. (1976). Nucl.Phys., BII3, I.

344

B. Povh

Grein, W. (1977). Nucl.Phys., B131, 255. Hepp, V. (1981). Private communication. Pirner, H.J. (1979). Phys.Lett., 85B, 190. Pirner, H.J., and Vary, J.P. (1981). Phys.Rev.Lett., 46, 1376. Povh, B., and Walcher, Th. (1981). To be published in the Proceedings of the IX International Conference on High Energy Physics and Nuclear Structure. Side, I . , Day, D., and McCarthy, J.S. (1980). Phys.Rev.Lett., 45, 871. Vinh Mau, R. (1979). In Mesons in Nuclei (M. Rho and D.H. Wilkinson, eds.). North-Holland, Amsterdam.