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THE NUCLEUS OF THE FUTURE
D. Wilkinson
To cite this version:
D. Wilkinson. THE NUCLEUS OF THE FUTURE. Journal de Physique Colloques, 1972, 33 (C5),
pp.C5-207-C5-210. �10.1051/jphyscol:1972516�. �jpa-00215118�
JOURNAL DE PHYSIQUE C o l l o q u e C 5 , suppldment a u no 8-9, Tome 33, Aofit-Septembre 1972, p a g e C5-207
THE NUCLEUS OF THE FUTURE
D.H. WILKINSON
N u c l e a r P h y s i c s L a b o r a t o r y , O x f o r d
A tremendous e f f o r t h a s gone i n t o t h e s t u d y of t h e a t o m i c n u c l e u s and we know a n enormous amount a b o u t it. Our t h e o r i e s have met w i t h b r i l - l i a n t s u c c e s s and o u r c o n v e n t i o n a l programmes, b o t h e x p e r i m e n t a l and t h e o r e t i c a l , l e a d t o g r e a t e r and g r e a t e r c o n f i d e n c e i n t h e e s s e n t i a l r i g h t n e s s of o u r i d e a s . B u t a l l t h i s knowledge i s , i n a c e r t a i n s e n s e , s u p e r f i c i a l because it d o e s n o t r e f e r t o t h e b u l k of t h e n u c l e o n s i n t h e n u c l e u s as I s h a l l re- mark i n a moment ; t h e n u c l e o n s c o n c e r n e d i n most of o u r t h e o r y and e x p e r i m e n t a t i o n a r e t h e f a v o u r e d few v a l e n c e p a r t i c l e s a t t h e t o p of t h e Fermi s e a who r e a r r a n g e t h e m s e l v e s a n d promote t h e m s e l v e s m o d e s t l y i n t o n e a r b y s h e l l s , They form a c o s y c l u b a n d a r e , of c o u r s e , v e r y i m p o r t a n t b u t t h e y owe t h e i r p o s i t i o n and i m p o r t a n c e t o t h e i r u n d e r p i n n i n g by t h e b u l k o f t h e A n u c l e o n s a b o u t which we remain a s t o n i s h i n g l y i g n o r a n t . A l l t h a t w e know a b o u t them i s t h a t t h e y a r e t h e r e , and w e a r e n o t e v e n comple- t e l y c e r t a i n a b o u t t h a t ,
T h e n u c l e u s o f t h e f u t u r e may l o o k r a t h e r d i f f e r e n t from t h a t of t o d a y
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o r i t may n o t . Today I j u s t want t o a s k a f e w q u e s t i o n s , t h a t may n e v e r h a v e a n s w e r s,
a b o u t o u r knowledge o f t h e n u c l e u s a n d t o m e n t i o n o n e o r two less u s u a l ways i n which we may l e a r n a b o u t t h e n u c l e u s a s a whole. I s h a l l n o t a t t e m p t a n y t h i n g r e s e m b l i n g a r e v i e w o f o u r knowledge and i g n o r a n c e n o r s h a l l I t r y t o mention even a l l t h e n o v e l and e x o t i c ways i n which some o f t h e i g n o r a n c e i s b e i n g d i s p e l l e d . B e f o r e we c l a i m t o u n d e r s t a n d t h e n u c l e u s we s h o u l d presumably b e a b l e t o g i v e a n a c c o u n t of i t s s i z e and s h a p e and o f t h e d i s t r i b u t i o n w i t h i n i t of i t s c o n s t i t u e n t p a r t i c l e s and t h e i r m o t i o n s ; we s h o u l d a l s o b e a b l e t o p r e d i c t t h e s p e c t r u m of e i g e n s t a t e s and t h e de- t a i l s of t h e v a r i o u s t r a n s i t i o n s l i n k i n g them ; a l l t h i s s h o u l d b e done on t h e b a s i s of i n t e r a c t i o n s t h a t a r e t h e m s e l v e s u n d e r s t o o d f i e l d t h e o r e t i c a l l y .We a r e , of c o u r s e , v e r y f a r from s u c h a n u n d e r s t a n d i n g . The phenomena of c u r r e n t n u c l e a r s p e c t r o s c o p y u s u a l l y i n v o l v e o n l y t h e topmost v e r y few n u c l e o n s of t h e F e r m i s e a and o u r t h e o r i e s u s u a l l y i n v o l v e v e r y few p a r t i a l e s and h o l e s , Even i n f i s s i o n , which would seem t o i n v o l v e a l l t h e nu- c l e o n s of t h e n u c l e u s , w e have l e a r n t from C l a u d e B l o c h a n d o t h e r s t h a t t h e d e t a i l s a r e dominated by
s h e l l e f f e c t s and s o t h e p r o c e s s i s e s s e n t i a l l y a s u p e r f i c i a l one. Even when we i n v o l v e s o few n u c l e - o n s we c a n o n l y c h e c k t h e t h e o r e t i c a l p r e d i c t i o n s i n r e l a t i o n t o t h e l o w e s t s t a t e s o f t h e model
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I f we a d d i t i o n a l l y p r q o t e a few n u c l e o n s from t h e 4 0 ~ a c o r e t h e e x p l o s i o n o f s t a t e s becomes even more absurd. B u t is t h e 4 0 ~ a c o r e i n f a c t c l o - s e d ? What d o t h e d e p t h s of t h e Fermi s e a l o o k l i k e ? If w e express B r u e c k n e r t h e o r y c o n f i g u r a t i o n a l l y what deep h o l e s h a s t h e h a r d c o r e opened up and i n t o what e x c i t e d c o n f i g u r a t i o n s h a v e t h o s e p a r t i - c l e s gone ? But d o e s t h e NN h a r d c o r e p e r s i s t i n n u c l e a r m a t t e r ? Where d o w e see its e f f e c t s ? And, i n d e e d , i s t h e n u c l e t r s made o f neutronfi and p r o t o n s i n t h e f r e e s e n s e a t a l l o r s h o u l d we r a t h e r e x p e c t n u c l e i t o b e b e t t e r d e s c r i b e d a s p i e c e s of h a d r o n i c m a t t e r of c e r t a i n b a r y o n i c and c h a r g e number t h a t may u n d e r S u i t a b l e c o n d i t i o n s b e h a v e t o some d e g r e e a s though t h e y c o n t a i n e d same i n d i v i d u a l e n t i t i e s a s y m p t o t i c a l l y i d e n t i f i a b l e w i t h n u c l e o n s ? W e t r a - d i t i o n a l l y d i s c u s s n u c l e i from t h e l o w - d e n s i t y view- p a i n t : we b r i n g " r e a l " n u c l e o n s c l o s e r t o g e t h e r and m e s s t h a t by t h e t i m e n u c l e a r d e n s i t i e s a r e r e a c h e d t h e mesonic e x c h a n g e s t h a t a r e r e s p o n s i b l e f o r t h e b i n d i n g a r e s t i l l r e l a t i v e l y weak p e r t u r b a t i o n s on t h e n u c l e o n s ' own s t r u c t u r e s o t h a t i n t h a t s e n s e n u c l e i a r e made o u t of v e r y - n e a r l y - n e u t r o n s and v e r y - n e a r l y - p r o t o n s . But t h e r e i s a l s o t h e high-Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972516
C5-208 D.H. WILKINSON
d e n s i t y v i e w p o i n t i n which we s t a r t from b l a c k - h o l e m a t t e r a t e x c e e d i n g l y h i g h p r e s s u r e s o t h a t c h a r g e , b a r y o n i c number and s p i n a r e a l l t h a t we h a v e t o c h a r a c t e r i z e i t ; e v e n t u a l l y , a s t h e p r e s s u r e i s l o w e r e d , m a s s i v e d n t i t i e s , p r o b a b l y unknown t o u s now, t h e q u a r k ' s q u a r k s , emerge f o l l o w e d a t l e n g t h by t h e f a m i l i a r p a r t i c l e s i n a l l t h e i r compl-xity and r i c h n e s s , h i g h l y e x c i t e d hyperon r e s o n a n c e s and s o on and t h e n f i n a l l y when t h e h i g h Fermi e n e r g y a s s o c i a t e d w i t h t h e low mass c a n b e a f f o r d e d we g e t down t o n e u t r o n s and p r o t o n s . A t some p o i n t a l o n g t h i s l i n e we p a s s t h r o u g h n u c l e a r d e n s i t i e s
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b u twhere ? I t i s n o t c l e a r t h a t by t h e t i m e n u c l e a r d e n s i t i e s have been r e a c h e d t h e h a d r o n i c m a t t e r h a s become overwhelmingly n e u t r o n s and p r o t o n s and d o e s n o t i n f a c t r e t a i n v e r y s i g n i f i c a n t memories of i t s r i c h e r a n c e s t r y .
W i l l t h e n , t h e n u c l e u s o f t h e f u t u r e p e r h a p s b e d e s c r i b e d i n t e r m s v e r y d i f f e r e n t from t h a t o f t o d a y ? Do w e i n f a c t h a v e r e a l e v i d e n c e t h a t t h e n u c l e u s i s made o u t of ( v e r y n e a r l y ) neu- t r o n s and p r o t o n s ? The argument t h a t s h e l l model c a l c u l a t i o n s of l e v e l schemes, m a g n e t i c moments, e t c . d o p r e t t y w e l l i s n o t c o n v i n c i n g b e c a u s e even " r e a - l i s t i c " c a l c u l a t i o n s admit of a f a i r amount of em- p i r i c a l a d j u s t m e n t a n d , i n any c a s e , a s a l r e a d y r e - marked, r e f e r t o o n l y a s m a l l f r a c t i o n of t h e n u c l e a r c o n t e n t s . I n p o i n t of f a c t t h e e v i d e n c e t h a t t h e n u c l e u s c a n be d i s c u s s e d i n l o w - d e n s i t y l a n g u a g e i s now becoming q u i t e s t r o n g , a s a r e s u l t of e x p e r i m e n t s a t i n t e r m e d i a t e and h i g h e n e r g i e s . One l i n e of e v i - d e n c e c o n c e r n s p i o n s c a t t e r i n g e x p e r i m e n t s i n t h e 3 , 3 r e s o n a n c e r e g i o n
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t h e f i r s t n u c l e o n i s o b a r . Such e x p e r i m e n t s r e v e a l t h a t t h e r e s o n a n c e-
t h e Ap a r t i c l e
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i s formed i n s i d e t h e n u c l e u s i n v e r y much t h e same way a s i n t h e f r e e s t a t e . I n p a r t i c u l a r , i n t h e e l a s t i c s c a t t e r i n g of p i o n s from 1 2 c,
done by t h e B r u x e l l e s 4 r s a y g r o u p , a v e r y d e e p minimum i s s e e n i n t h e a n g u l a r d i s t r i b u t i o n a t a c e r t a i n an- g l e when t h e p i o n e n e r g y i s i n t h e b r e g i o n a n d t h i s , a s p o i n t e d o u t by T o r l e i f E r i c s o n , would n o t b e found w i t h o u t t h e v a n i s h i n g of t h e r e a l p a r t of t h e XN s c a t t e r i n g a m p l i t u d e i n s i d e t h e n u c l e u s j u s t a s i n t h e f r e e s t a t e . So 12c a t l e a s t seems t o c o n t a i n n u c l e o n s . But what o f heavy n u c l e i ? We know n o t h i n g d i r e c t l y a b o u t t h e low-lying n u c l e o n s i n heavy nu- c l e i , t h e i n n e r s h e l l s , and we a r e f r e e t o doubt w h e t h e r t h e y e x i s t a t a l l a s such. A q u i t e s t r o n gi n d i c a t i o n t h a t n u c l e o n s a r e , i n f a c t , a l s o t h e c o n s t i t u e n t s of heavy n u c l e i comes from t h e i n c l u - s i v e r e a c t i o n p
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a n y t h i n g c a r r i e d o u t by a CERN-Rome c o l l a b o r a t i o n a t 24 GeV and a n a l y z e d by~ofoed- an sen
w i t h a z e r o p a r a m e t e r G l a u b e r a p p r o a c h u s i n g a s i n p u t d a t a t h e e l e m e n t a r y f r e e p a r t i c l e c r o s s s e c t i o n s . Agreement i s e x c e l l e n t o v e r a t r e - mendous r a n g e of c r o s s s e c t i o n -about 6 d e c a d e s-
and o n e must f e e l t h a t i n d e e d t h e model and i t s c e n t r a l a s s u m p t i o n t h a t 2 0 8 ~ b c o n s i s t s o f t h e a s s u - med 126 n e u t r o n s and 82 p r o t o n s must b e v e r y l a r g e l y t r u e . P r e c i s e l y t h e same a n a l y s i s g a v e e q u a l l y good agreement f o r ' ~ e t o c o n f i r m t h e n e u t r o n - p r o t o n p i c t u r e .
I t t h e r e f o r e seems t o m e l i k e l y t h a t t h e n u c l e u s of t h e f u t u r e w i l l c o n t i n u e t o c o n s i s t ba- s i c a l l y of n e u t r o n s and p r o t o n s . But we s h a l l c e r - t a i n l y have t o a d m i t o t h e r t h i n g s . The v a r i o u s me- s o n exchanges t h a t t a k e p l a c e between n u c l e o n s f i r s t l y modify t h e n u c l e o n s t r u c t u r e somewhat s o t h a t , f o r example, t h e i n t r i n s i c m a g n e t i c moments w i l l change s l i g h t l y a s a l s o t h e a x i a l c o u p l i n g c o n s t a n t and, s e c o n d l y , c o n s t i t u t e c u r r e n t s i n t h e i r own r i g h t t h a t f u r t h e r modify v a r i o u s n u c l e a r p r o - p e r t i e s by t h e m s e l v e s c o u p l i n g t o t h e e l e c t r o m a g n e - t i c and o t h e r f i e l d s . A f u r t h e r c o n s e q u e n c e of t h e mesonic exchanges i s t h a t n u c l e o n s c a n be r a i s e d i n t o e x c i t e d s t a t e s ( s u c h a s t h e h ) which we must t h e r e f o r e now admit a s s i g n i f i c a n t c o n s t i t u e n t s o f t h e n u c l e a r ground s t a t e . Some of t h e s e e f f e c t s a r e a l r e a d y s u s c e p t i b l e of r e a s o n a b l y a c c u r a t e c a l c u - l a t i o n a s h a s been shown by Rho, Green and o t h e r s and we h a v e s e e n t h a t t h e y c a n p r o d u c e c h a n g e s of t h e o r d e r of 10% t o m a g n e t i c moments and @decay r a t e s . T h e s e s h i f t s a r e comparable w i t h c o n f i g u r a - t i o n m i x i n g e f f e c t s a n d we m y e x p e c t t o h e a r much more a b o u t A s e t c . . i n t h e n u c l e u s o f t h e f u t u r e . A s Danos h a s emphasized, t h e v e r y s t r o n g c o u p l i n g of c e r t a i n of t h e i s o b a r s may have i m p o r t a n t c o n s e - q u e n c e s f o r t h e e n e r g y b a l a n c e w i t h i n n u c l e i and we may e x p e c t t o f i n d i s o b a r e f f e c t s c r e e p i n g i n t o t h e mass f o r m u l a e . We c a n n o t a f f o r d t o admit A s a s permanent n u c l e a r c o n s t i t u e n t s w i t h t h e i r own quan- tum s t a t e s
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a n d i n d e e d t h e s h e l l model c l a s s i f i - c a t i o n o f ground s t a t e s p i n s t e l l s u s t h a t t h i s i s s o-
b u t t h e p r o b a b i l i t y t h a t a n u c l e d n be i n i t s A s t a t e c a n s t i l l b e s e v e r a l p e r c e n t which i s elror- mous. I t i s c l e a r t h a t t h e i n t r o d u c t i o n of t h e ATHE NUCLEUS O F THE FUTURE
i n t o s h e l l model c a l c u l a t i o n s i s a b o u t t o b e g i n . W e m u s t a l s o n o t l o s e s i g h t o f t h e mesons.
T h e y t o o may h a v e to b e a l l o t t e d a more e x p l i c i t r o l e i n o u r f u t u r e p i c t u r e of n u c l e a r s t r u c t u r e . I t i s c o n c e i v a b l e t h a t t h e r e may e x i s t l a r g e scale o r g a n i z a t i o n of m e s o n i c m o t i o n s w i t h t h e m e s o n i c o r b i t a l s e n c o m p a s s i n g many n u c l e o n s . T h i s i n t u r n m i g h t g i v e rise t o c o l l e c t i v e m e s o n i c d e g r e e s o f f r e e d o m a n d t o j o i n t meson-nucleon c o l l e c t i v e mo- t i o n s of t h e " g i a n t r e s o n a n c e " k i n d . ( E r i c h Vogt h a s p o i n t e d o u t t h a t i n h e a v y n u c l e i we m i g h t e x p e c t a s i g n i f i c a n t " r e a l " I s
7;
a m d l i t u d e t h a t c a n be t h o u g h t o f a s t h e Coulomb m i x i n g i n o f t h e a p p r o - p r i a t e p i o n i c a t o m ) ,B u t e v e n i f we a d m i t t h a t t h e n u c l e u s i s c h i e f l y n e u t r o n s a n d p r o t o n s how d o w e know what t h e y a r e d o i n g ? Do we r e a l l y & n s t h a t t h e s t r o n g s h o r t - r a n g e c o r r e l a t i o n s d u e t o t h e h a r d N N c o r e e x i s t a s t h e y s h o u l d ? iVe c e r 5 i i n l y d o n o t ; t h e y o r e most e l u s i v e a n d s o f a r i t h a s p r o v e d i m p o s s i - b l e t o r u n t h e m down. P e r h a p s t h e b e s t c u r r e n t h o p e i s f r o m h i g h e n e r g y s c a t t e r i n g o f t h e t y p e j u s t m e a t i o n e d , I n d e e d Kofoed-Ransen's G l a u b e r a n a l y s i s
s h 3 w s i t s l a r g e s t d i s c r e p a n c y w i t h e x p e r i m e n t i n a r e g i o n t h a t t h e model s a y s s h o u l d b e d o m i n a t e d by d o ~ b l e s c a t t e r i n g w i t h o u t h a d r o n p r o d u c t i o n a n d t h e d i s c r e p a n c y c a n b e n i c e l y removed by p u t t i n g i n t o t h e c o m p u t a t i o n s n u c l e o n c o r r e l a t i o n s of a b o u t t h e e x p e c t e d m a g n i t u d e . I t is, however, t o o e a r l y t o make a p o s i t i v e c l a i m . B u t u n t i l we h a v e a d e c e n t m a p p i n g o f n u c l e o n d i s t r i b u t i o n a n d b e h a v i o u r a s a f u n c t i o n of d e p t h i n t h e F e r m i s e a we s h a l l n o t b e a b l e t o s a y t h a t o u r p i c t u r e o f t h e n u c l e u s i s f u l l y c o n f i r m e d
.
@ h e g r e a t d i f f i c u l t y i n p r o b i n g t h e F e r m i s e a i s t h a t it i s f u l l : we c a n n o t s e n d a n o t h e r n u c l e o n down i n t o i t t o f i n d o u t what i t i s l i k e a n d i f we p u l l o u t e x i s t i n g n u c l e o n s t h e u n c e r t a i n - t i e s a s s o c i a t e d w i t h e x t r a c t i n g d e t a i l e d i n f o r m a - t i o n f r o m t h e e x p e r i m e n t a l c r o s s s e c t i o n s seem a l - most too d i f f i c u l t t o o v e r c o m e
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t h e q u a n t i t a t i v e i n f o r m a t i o n c o m i n g f r o m ( p , 2 p ) , ( e , e p ) e x p e r i m e n t s , f o r e x a m p l e , i s s t i l l e x t r e m e l y r e s t r i c t e d . A h o p e f o r t h e f u t u r e is t o u s e t h e A0 as o u r p r o b e . A ho p u t i n t o t h e F e r m i sea a t t h e t o p w i l l s i n k t o t h e b o t t o m , e x p E o r i n g i t s s i n g l e - p a r t i c l e l e v e l s a s i t g o e s , I t w i l l map o u t t h e h o - n u c l e u s p o t e n t i a l i nd e t a i l a n d t h e e n e r g y l e v e l s t h a t i t o c c u p i e s w i l l r e f l e c t t h e p r o p e r t i e s o f t h e n u c l e o n d i s t r i b u t i o n t h r o u g h w h i c h i t p a s s e s . T h e f u t u r e f i e l d o f h y p e r - n u c l e a r spectroscopy, j u s t now b e g i n n i n g , s h o u l d e v e n t u a l l y b r i n g u s r i c h i n f o r m a t i o n a b o u t the o l d - f a s h i o n e d h o s t n u c l e i e v e n more t h a n a b o u t t h o h O i t s e l f .
I t is a p p r o p r i a t e , i n t h i n k i n g a b o u t t h e f u t u r e , t o p a u s e o n o t h e r a s p e c t s o f t h e e l e m e n t a r y p a r t i c l e
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n u c l e a r s t r u c t u r e s y m b i o s i s . N u c l e i a r e w o n d e r f u l l i t t l e l a b o r a t o r i e s i n which t o c a r r y o u t e l e m e n t a r y p a r t i c l e e x p e r i m e n t s a n d i t i s bound t o b e a two-way t r a f f i c . A r e c e n t v e r y b e a u t i f u l i l l u s - t r a t i o n o f t h i s i s t h e c o h e r e n t p h o t o p r o d u c t i o n o f n e u t r a l v e c t o r mesons c a r r i e d o u t i n s e v e r a l l a b o - r a t o r i e s w h i c h h a s g i v e n n u c l e a r s t r u c t u r e p h y s i - c i s t s t h e i r b e s t i n f o r m a t i o n i n t h e s p a t i a l d i s t r i - b u t i o n o f n e u t r o n s i n t h e n u c l e u s a n d h a s g i v e n p a r t i c l e p h y s i c i s t s t h e v a l u e o f t h epi
s c a t t e r i n g cross s e c t i o n . S i m i l a r l y ;r-,
37t c o h e r e n t p r o d u c - t i o n i n t h e mass r a n g e of t h e A 1 meson h a s g i v e n t h e A 1 N s c a t t e r i n g cross s e c t i o n (CERN-ETH-Imperial C o l l e g e - M i l a n o c o l l a b o r a t i o n ) . T h i s same e x p e r i m e n t a l s o s t u d i e d t h e .rr. -t 5a p r o c e s s a n d p r o d u c e d a n a s t o n i s h i n g r e s u l t t h a t p o i n t s t o a n i m p o r t a n t ex- t e s l s i o n o f t h e s y m b i o s i s . T h e r e s u l t is t h a t t h e c r o s s s e c t i o n f o r %,N s c a t t e r i n g i s a p p a r e n t l y s i g n i f i c a n t l y s m a l l e r t h a n i h e c r o s s s e c t i o n f o r K N s c a t t e r i n g : 5 p i o n s a r e s m a l l e r t h a n one. Van Hove h a s s u g g e s t e d t h a t t h e e x p l a n a t i o n may l i e i n t h e f a c t t h a t t h e p r o d u c t i o n p r o c e s s i t s e l f h a s a n i n t r i - s i c t i m e s c a l e of o r d e r %/M2 i n n a t u r a l u n i t s whe- re E i s t h e i n c i d e n t e n e r g y a n d M i s t h e p r o d u c e d mass. T h e c o r r e s p o n d i n g f l i g h t d i s t a n n e is o f o r d e r 0.3 E fm f o r M 3 1 GeV w i t h E i n GeV.
T h e p r o - d u c e d s t a t e i s t h e r e f o r e e v o l v i n g q u i t e s l o w l y to- w a r d s i t s f i n a l l y o b s e r v e d f o r m as i t p a s s e s t h r o u g ht h e n u c l e u s a n d many s u c c e s s i v e n u c l e o n c o l l i s i o n s are p o s s i b l e w h i l e t h i s i s h a p p e n i n g , u n l i k e p r o - d u c t i o n i n c o l l i s i o n w i t h s i n g l e n u c l e o n s . T h e s u c - c e s s i v e n u c l e o n c o l l i s i o n s t h e n r e p r e s e n t s t e p s i n t h e e v o l u t i o n trf t h e f i n a l s t a t e r a t h e r t h a n t h e r e m o v a l o f t h e f i n a l s t a t e f r o m t h e c o h e r e n t a m p l i - t u d e ; t h e a p p a r e n t s c a t t e r i n g c r o s s s e c t i o n c a n t h e n b e q u i t e low. From t h i s phenomenon t h e h i g h e n e r g y p h y s i c i s t may h o p e t o l e a r n a b o u t t h e tempo- r a l e v o l u t i o n of h a d r o n i c s t a t e s a n d t h e n u c l e a r
C5-210 D.H. WILliINSON
structure physicist about the spatial arrangement of nucleons in the nucleus.
Of course, nuclei may help particle phy- sics in many other ways : nuclei carry intense electric fields so that Coulomb excitation such as K + K* w i U effectively measure the inverse K* -+ K+y like processes ; the intense electric fields may eventually yield the electric polariza- bility of negatively-charged hadrons such as n
- ,
- -
K
,
p,
C-, etc.. ; the mast intense fields availa- ble from superheavy elements or in heavy ion colli- sions, where " Z U>
1 " may teach us how to handle large coupling constants ; the charge-asymmetry of the nuclear force that wlll probably establish it- self in the next year or two may teach us the rela- tive phases of x - ~ anj +-a mixing which would be a real plum ; an understanding of multiple scatte- ring, if only empirically, may illuminate the parton structure of the hadrons-
a comparison of heavy ion- heavy ion scattering with hadron-hadron scatte- ring should tell us whether, for example, protons really are highly complex structures...
And so one could go on. It is not always clear what there is in it for nuclear structure but it is most unlikely that we shall learn nothing about the nucleus by subjec- ting it to new forms of interaction.I
have suggested that our knowledge of the nucleus really only scrapes the sbrface. We do not yet know that our traditional views are correct.W e do not know, for example, much about nuclear sub- structure and whether, in particular, an alpha-par- ticle-like substructure may not be a literal reality at s m e level. I think that it is perfectly possible that light nuclei are very largely substructured and that heavy nuclei ~nly be also at least superficially.
Such things may be unlikely &t they cannot be ruled
out. Let me repeat that we do not yet know what the nucleons are doing deep inside the nucleus and that until we do we must not be too sure about anything.
Recent studies on possible hollow and toroidal nuclei are very interesting and show that other points of view than the traditional can be seriously maintained.
T o me one of the big att ractims of heavy ion physics is the possibility of putting nuclear matter into new kinds of state : the tremendous upheaval of two
large nuclei in head-on collision is bound on occa- s i m t e G u a n expanded state of nuclear matter which
will collapse back, perhaps rather slowly if it is at the saddle point, into a conventional sort of nucleus. Of course, we have put large amounts oi energy into nuclei before, but never in a way that could spread the excitation more or less uniformly.
It may all be too rare and too difficult to reco- gnize but some sort of complex molecule of a nucleus with a number of separated sizeable fragments and a number of free nucleons swarming among them must have at least a transitory existence.
How is the nucleus of the future to be approached ? My o m feeling is that the two areas that must be rigorously explored are the high ener- gy approaches and the heavy ion approaches. High
energies will look at the spatial structure of the nucleus in fine detail and that we must do before we claim a full account. Heavy ions will excite the the nucleus in new ways and will involve its com- pressibility and viscosity for the first time to a serious degree. The high energy approach is, or soon will be, well provided for. Heavy ion prepara- tions lag seriously but it is to be hoped that the situation will remedy itself.
The nucleus has constantly provided us with surprises and will only stop doing so when we
lose our willingness to l m k at it with an open mind.
