Retrospect and prospect of Λ hypernuclear physics

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Hadron and N uclear Physics with ElectromagneticProbesK. M aruyamaand H. Okuno (Editors)9 2000 Elsevier Science B.V . All rights reserved. 137

Retrospect and prospect of A hypernuclear physicsO s a m u H a s h i m o t o ~~ D e p a r t m e n t o f P h y s i c s , T o h o k u U n i v e r si ty , S e n d a i 9 8 0 -8 5 7 8E - m a i l: h a s h i m o t @ l a m b d a . p h y s . t o h o k u , a c. j p

R e c e n t p r o g r e s s a n d d e v e l o p m e n t o f h y p e r n u c l e a r p h y s i c s i n t h e S = - I r e g im e w a s re -v i ew e d . W i d e v a r i e ty o f s p e c t r o s c o p y a s a t o o l t o i n v e s ti g a te A h y p e r n u c l e a r s t r u c t u r e a n dA N i n t e r a c t i o n is d i s c u s s e d r e f e rr i n g re c e n t s p e c t r o s c o p i c d a t a , w h i c h h a v e b e e n o b t a i n e dt h r o u g h t h e ( T r+ , K + ) r e a c t i o n u s i n g a l a rg e a c c e p t a n c e h i g h r e s o l u t i o n s p e c t r o m e t e r (S K S ) a t K E K 1 2 G e V P S . V e r y r e c e n t r e s u l t o f h y p e r n u c l e a r -y r a y s p e c t r o s c o p y i s a l s op r e s e n t e d , w h e r e p r e c is i o n i n f o r m a t i o n o n t h e s p in d e p e n d e n t A N i n t e r a c t i o n c a n b e e x -t r a c t e d . N e w h i g h r e s o lu t i o n r e a c t io n s p e c t r o s c o p y b y t h e ( e ,e ' K + ) r e a c ti o n u n d e r w a ya t J l a b i s d e s c r i b e d , t o o . F u t u r e p r o s p e c t o f h y p e r n u c l e a r p h y s i c s is d i s c u s s e d b a s e d o nt h e s e r e c e n t d e v e l o p m e n t .1 . I N T R O D U C T I O N

T h e A h y p e r n u c l e i p r o v i d e u n i q u e o p p o r t u n i t i e s i n t h e i n v e s t i g a t i o n o f h a d r o n i c m a n y -b o d y s y s t e m w i t h s t r a n g e n e s s - I a n d n e w a s p e c t s o f t h e s t r o n g a n d w e a k i n t e r a c t i o n i nn u c l e i i n v o l v i n g t h e s t r a n g e n e s s d e g r e e o f f r e e d o m . R e c e n t p r o g r e s s d e m o n s t r a t e d t h es i g n i f i c a n c e o f A h y p e r n u c l e a r s p e c t r o s c o p y a n d e s t a b l i s h e d n e w e x p e r i m e n t a l p a t h w a y st o w a r d s t h e s e s t u d i e s i n a q u a n t i t a t i v e w a y .

T h e r e a r e s e v e r a l a s p e c t s t h a t h y p e r n u c l e a r i n v e s t i g a t i o n p l a y s k e y r o l e s i n s h e d d i n gl i g h t s o n s t r a n g e n e s s n u c l e a r p h y s i c s . F i r s t l y , s i n c e a h y p e r o n c a r r i e s a s t r a n g e n e s s d e g r e eo f f r e e d o m w i t h a s t r a n g e q u a r k , i t b e h a v e s d i f f e r e n t l y f r o m n u c l e o n s i n a n u c l e a r s y s t e m .B e i n g f r e e f r o m P a u l i e x c l u s i o n p r i n c i p l e , a h y p e r o n c a n b e a p r o b e t o s t u d y d e e p l y b o u n ds t a t e s . I t i s a l s o p o i n t e d o u t t h a t i n v e s t i g a t i o n o f A h y p e r n u c l e a r s t r u c t u r e m a y a n s w e rt h e f u n d a m e n t a l q u e s t i o n w h e t h e r a A h y p e r o n d e e p i n s i d e a n u c l e u s k e e p s i t s i d e n t i t y a s ab a r y o n o r n o t . [ I ] S e c o n d l y , u n i q u e s t r u c t u r e s o f h a d r o n i c m a n y - b o d y s y s t e m w i l l m a n i f e s tt h e m s e l v e s w i t h t h e n e w q u a n t u m n u m b e r , s t r a n g e n e s s , s i n c e t h e i n t e r a c t i o n b e t w e e na n u c l e o n a n d a h y p e r o n h a s n e w c h a r a c t e r i s t i c s d i f f e r e n t f r o m t h o s e o f t h e n u c l e o n -n u c l e o n i n t e r a c t i o n . T h i r d l y , w e a k i n t e r a c t i o n i n n u c l e a r m e d i u m c a n b e w e l l i n v e s t i g a t e dt h r o u g h h y p e r n u c l e a r w e a k d e c a y . T h e w e a k d e c a y o f a A h y p e r o n i n a n u c l e u s s h o w su n i q u e f e a t u r e s c o m p a r e d w i t h t h o s e i n f r e e s p a c e . I n p a r t i c u l a r , n o n m e s o n i c w e a k d e c a y ,A N - - , N N , i s o n e o f t h e b e s t p l a c e t o s t u d y b a r y o n - b a r y o n w e a k i n t e r a c t i o n i n n u c l e i . I ti n v o l v e s l a r g e m o m e n t u m t r a n s f e r a n d i s d o m i n a n t o v e r t h e m e s o n i c w e a k d e c a y , A - - ~ N T r ,i n h e a v i e r A h y p e r n u c l e i . R e c e n t d a t a o n t h e n o n m e s o n i c w e a k d e c a y p o s e s a p u z z l ec e n c e r n i n g i t s d e c a y w i d t h s a n d i t i s u n d e r i n t e n s i v e i n v e s t i g a t i o n b o t h e x p e r i m e n t a l l y


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and theo re t ica l ly , though we wi l l no t go in to the de ta i l in the p resen t paper .I t c a n b e s a id t h a t A h y p e r n u c l e a r i n v e s t i g a t i o n h a s a l r e a d y u n d e r g o n e t h r e e s t a g e s o f

dev e lop m en t s ince the f i r st d i scovery o f the hy pernuc le us in 1953 . T he f i r s t s tage ma y bec a l le d a s "e m u l s io n e r a " , w h e n i t w a s e s t a b l i sh e d t h a t t h e A p o t e n t i a l d e p th i s a b o u t 2 /3 o fth e n u c l e o n c a s e t h r o u g h th e m e a s u r e m e n t s o f t h e b in d in g e n e rg i e s o f l i g h t A h y p e r n u c le i .T h e s e c o n d s t a g e b e g a n i n e a r ly 7 0 ' s w h e n c o u n te r e x p e r im e n t s b e c a m e p o s s ib l e w i thK - b e a m s a t C E R N a n d s o o n l a t e r a t B N L . T h e r e a c t i o n u s e d f o r t h e h y p e r n u c l e a rin v e s t i g a t i o n t h a t t im e w a s m o s t ly t h e ( K - , 7 r- ) r e a c ti o n . I t w a s c la im e d th a t t h e s p in -o rb i t sp l i t t ings in p - she l l hypernuc le i a r e much smal le r than the nuc leon case . P rog ress o fthe f i r s t and second s tages was r ev iewed in var ious l i t e r a tu res . [2 ] The th i rd s tage s ta r ted inmid 80 ' s a t BNL u t i l i z ing the (Tr+ , K + ) r e a c t io n t o p o p u la t e A h y p e r n u c l e a r b o u n d s t a t e s ,bu t i t took somet ime befo re new exper imen ta l f ac i l i t i es became fu l ly ava i lab le . S ince la te8 0 ' s u n t i l n o w , t h e s u p e r c o n d u c t in g k a o n s p e c t r o m e te r ( S K S ) a t K E K 1 2 G e V P S p l a y e da key ro le in exp lo r ing A hype rnuc le ar spe c t roscopy by the (Tr+,K +) r eac t ion . W eakd e c a y o f A h y p e r n u c l e i i s a ls o b e in g s tu d i e d w i th f a r b e t t e r q u a l i t y t h a n e v e r o b t a in e d .Fur thermore , new exper imen ta l too ls a r e r ecen t ly deve loped , such as hypernuc lear " ) , - r ays p e c t r o s c o p y a n d a l so e l e c t r o m a g n e t i c p r o d u c t io n o f s t r a n g e n es s . T h e s e w i ll b e a l s o t h es u b j e c t o f t h e p r e s e n t p a p e r .

2 . A H Y P E R N U C L E A R S P E C T R O S C O P YA A hypernuc lear s ta te i s fo rmed in the r eac t ion as a nuc leon-ho le A-par t ic le s ta te . As

sche m at ica l ly shown in F ig .1 fo r a 2~ ta rg e t as an exam ple , A b ind ing energ ies a r eu s u a l ly m u c h l a r g e r t h a n t h o s e fo r a p r o to n o r a n e u t r o n . T h e r e f or e , t h e h y p e r n u c l e a rs t a t e s i n w h ic h a A h y p e r o n is b o u n d a b o v e t h e p - s he l l o r b i t a l a r e m o s t ly n u c l e o n u n b o u n da n d d e c a y b y e m i t t i n g n u c l e o n s. H o w e v er , th e s p r e a d in g w id th s o f s u c h h ig h -ly in g s t a t e s

neutronor proton4/ B^ -~ Na rrow spreading widths

~ < a f ew 10 0 k e V\ I Likar,Rosina,Povh.~an d o , Mo to b a ,Yamamoto- ~ neutron2 O 7 ^ T 1 B ~ - . . . . . .

* : : : 7 2 ~?

2o8^pbW eak d ecaynonmesonicmesonic

F i g u r e I . A h y p e r n u c l e a r s t a t e s a n d t h e i r d e c a y m o d e s .


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a r e c a l c u l a t e d t o b e n a r r o w e r t h a n a f e w 1 0 0 k e V e v e n t h e y a r e e x c i t e d a b o v e t h e n u c l e o nemis s ion th re s ho ld s , which i s in con t ra s t to the case o f an o rd ina r y nuc leus .[3 ,4 ] W he nt h e h y p e r n u c l e u s d e e x c i t e s b e l o w t h e p a r t i c l e e m i s s i o n t h r e s h o l d , 7 d e c a y s t a k e p l a c e .E v e n t u a l l y , t h e A h y p e r n u c l e u s u n d e r g o e s w e a k d e c ay s fr o m i ts g r o u n d s t a t e . F o r t h o s es t a t e s t h a t e m i t ~ /- ra ys , p r e c is i o n 7 - r a y s p e c t r o s c o p y c a n b e c a r r i e d o u t a s w i ll b e d e s c r i b e dl a t e r. R e a c t i o n s p e c t r o s c o p y w i ll b e a p p l i e d t o a l l t h e s t a t e s r e g a r d l e s s o f t h e e x c i t a t i o nene rgy .

I n a d d i t i o n t o t h e n a r r o w n e s s o f t h e s p r e a d i n g w i d t h s , p e r t u r b a t i o n b y th e p r e s e n c e o fa A h y p e r o n i n th e n u c l e a r s y s t e m i s m o d e l a t e b e c a u s e o f t h e w e a k n e s s of t h e i n t e r a c t i o nb e t w e e n a n u c l e o n a n d a A h y p e r o n . T h e r e f o r e , i n f o r m a t i o n o n A h y p e r n u c l e a r s t r u c t u r ec a n b e r e l a t e d t o t h e A N i n t e r a c t io n r a t h e r s t r a ig h t f o rw a r d l y . T h u s , t h e A h y p e r n u c l e a rs p e c t r o s c o p y i s i n d i s p e n s a b l e f or t h e i n v e s t i g a t i o n o f t h e A N i n t e r a c t i o n . S u c h c h a r a c -t e r i s t ic s a r e q u i t e u n i q u e f e a t u r e s r e a li z e d in A h y p e r n u c l e i a n d a r e h a r d l y e x p e c t e d i no t h e r h y p e r n u c l e i s u c h a s E a n d E h y p e r n u c le i . I n t h i s r e g a r d s , t h e s p e c t r o s c o p y o f Ahypernuc le i i s in a un ique pos i t ion in the s t rangeness nuc lea r phys ic s .

T h e Y N , Y Y e f f e c t i v e i n t e r a c t i o n s a r e c o n s t r u c t e d t h r o u g h G - m a t r i x c a l c u l a t i o n , s t a r t -i n g f r o m p h e n o m e n o l o g i c a l i n t e r a c t i o n s i n fr ee sp a c e. A n a l y t i c a l f u n c t i o n s o f t h e e f-f e c ti v e p o t e n t i a l s c a n b e g i v e n in t h e f o r m o f t h r e e r a n g e g a u s s i a n , V A N ( r ) = } 2 ( a i +b i k f + c i k } ) e x p ( - r 2 / b ~ ) . [ 5 ] I t i s a l so poss ib le to expres s the e f fec t ive AN in te rac t ion a s ,V A N = V ( r ) + V ~ ( r ) a h . CrN + V A ( r ) L . a A + V N ( r ) L . crN + V T S A N , w i t h o u t r e f f e r i n g t o t h ee l e m e n t a r y i n t e ra c t i o n . I n t e g r a t i o n o f t h e r a d i a l d e p e n d e n t p o t e n t i a ls c a n b e p e r f o r m e dw i t h a p p r o p r i a t e w a v e f u n c ti o n s . F o r t h e p - s h e ll A h y p e r n u c l e i , t h e s e i n t e g r a ls w e r e as -s u m e d t o b e c o n s t a n t a n d d e s i g n a t e d a s A , SA , S N a n d T . [6 ] W i d e v a r i e t y o f h y p e r n u c l e a rp r o p e r t i e s , s u c h a s l e v e l s t r u c t u r e s , s p l i t t i n g s d u e t o t h e s p i n - d e p e n d e n t i n t e r a c t i o n , r e -a c t i o n c r o ss s e c ti o n s e tc . a r e t h e n c a l c u l a t e d . T h e s e c a l c u l a t i o n s a r e r e a s o n a b l y r e l ia b l ep a r t l y b e c a u s e t h e A N i n t e r a c t i o n i s m u c h w e a k e r t h a n t h e N N i n t e r a c t i o n a n d a n t i s y m -m e t r i z a t i o n w i t h n u c l e o n s n e e d n o t b e c o n si d e re d . T h e r e f o r e , o n ce t h e i n f o r m a t i o n o n t h eh y p e r n u c l e a r s t r u c t u r e a r e o b t a in e d e x p e r im e n t a ll y , w e ca n t r a c e th e p r o c e d u r e b a c k w a r da n d c a n i n v e s t i g a t e t h e fr ee a n d e f fe c ti ve h y p e r o n - n u c l e o n i n t e r a c t io n s . H y p e r n u c l e a rs p e c t r o s c o p i c d a t a a r e d i r e c t l y c o m p a r e d a n d t h e s p i n - s p i n , s p i n - o r b i t a n d t e n s o r i n t e r -a c t i o n b e t w e e n a A h y p e r o n a n d a n u c le o n a r e r e a d i l y in v e s t i g a te d . I t is o n e o f t h e m o s ti m p o r t a n t a s p e c t o f A h y p e r n u c l e a r i n v e s t ig a t i o n , p a r t i c u l a r l y b e c a u s e i t is n o t e a s y t od i r e c t l y s t u d y t h e h y p e r o n - n u c l e o n i n t e r a c t i o n t h r o u g h h y p e r o n s c a t t e r i n g e x p e r i m e n t s .F o r t h e s e s t u d i e s , h i g h q u a l i t y s p e c t r o s c o p i c d a t a w h i c h w e r e r e c e n t l y a c c u m u l a t e d p r o -v i d e v a l u a b l e i n f o r m a t i o n o n t h e A N i n t e r a c t i o n , b e i n g c o m b i n e d w i t h t h e s e t h e o r e t i c a lefforts .3 . T H E (Tr+ , K + ) R E A C T I O N S P E C T R O S C O P Y A N D T H E S K S S P E C T R O M -

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T h e s p e c t r o s c o p y b y t h e (7r + , K + ) r e a c t i o n w a s f ir s t a p p l i e d t o a c a r b o n t a r g e t a t B N L ,a n d l a t e r t o h e a v i e r o n e s b o t h a t B N L a n d K E K . [ 7 - 1 0 ] T h e h y p e r n u c l e a r c r o s s s e c ti o n s a r eo f t e n a b o u t 2 o r d e r s o f m a g n i t u d e s m a l l e r t h a n t h o s e b y t h e ( K - , 7 r -) r e a c ti o n . H o w e v e r,t h e s m a l l c r o s s s e c t i o s a r e w e l l c o m p e n s a t e d b y h i g h i n t e n s i t y p i o n b e a m s a t 1 . 0 6 G e V / cw h e r e t h e c r o s s se c t io n o f t h e e l e m e n t a r y p r o c e s s r e a c h e s a m a x i m u m . T h e r e a c t i o n h a s


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T a b le 1H y p e r n u c l e a r p h y s i c s e x p e r i m e n t s w i t h t h e S K S s p e c t r o m e t e r

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System atic spectroscopy of A hypernuclei1nOB, ~2C, 2hSSi 891i, ~ 3 9 L a ~ O S p bNonmesonic weak decay of polarized 5He [13]

Lifetimes and weak decay wid ths of [14]light and medium - heavy A hypernucleiLight A hypernuclear spectroscopy7 L i , 9 B e , ~2C, ~3C, 160 [15]Sp ec troscopy of sA9Y [16]7 ray spec trosco py of ~Li [17]

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E 4 3 8(Noumi, KEK )E 4 9 0(Outa, KEK)

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a n a d v a n ta g e t o f a v o r a b ly e x c i t e s p in - s t r e t c h e d s t a t e s d u e t o i t s l a r g e m o m e n tu m t r a n s f e ra n d i s s u t a b l e t o p o p u la t e h y p e r n u c l e a r b o u n d s t a te s . W i th a n i n t e n s io n t o t a k e fu lladv an ta ge o f the (Tr+ , K + ) r eac t ion fo r the hype rnuc le ar spec t roscopy , a su perc ond uc t ingk a o n s p e c t r o m e te r ( S K S ) w a s c o n s t r u c t e d a t K E K 1 2 G e V P S . [ 1 1 , 1 2 ]

T h e S K S s p e c t r o m e t e r h a s g o o d r e s o l u t i o n o f 1 . 5 - 2 M e V F W H M a n d s i m u l t a n e o u s l ycover s a la rge so l id ang le o f 100 msr , acce p t ing ab ou t 60 % of ~2C g ro und - s ta te y ie ld inthe (~r+ , K + ) r eac t ion. [9 ] Th e la rge acce p tance o f the SKS spe c t ro m ete r a l so a l lows usto per fo rm ef f ic ien t co inc idence exper im en ts such as s tud y o f hyp ernu c lear w eak decayand 3' r ay spec t roscopy . A se ries o f expe r ime n ts fo r the inve s t iga t ion o f hypernu c le iu s in g t h e S K S s p e c t r o m e te r h a s b e e n c a r r i e d o u t i n t h e p a s t y e a r s a n d t h e y a r e l i s t e d i nTab le 1. A m ong the m , the fou r expe r ime n ts , E140a , E336 , E369 and E419 were fo r ther e a c t i o n s p e c t r o s c o p y o f A h y p e r n u c l e i a n d s tu d i e d A h y p e r n u c l e a r s t r u c tu r e a n d t h e A Nin t e r a c t i o n t h r o u g h th e m e a s u r e m e n t o f h ig h q u a l i t y sp e c t r o s c o p ic d a t a .

Th e E140a , the f i rs t o f the se r ies o f exper im en ts , in te nded to inv es t iga te the n a tu re o fa A hyp ero n dee p ly bou nd to heavy nuc lei as well as ligh t ones . Th e b in d ing energ ieso f a A hyp ero n bou nd to a wide r ange o f hypernuc le i , up to sgy , )t39La and ~~ wered e d u c e d f r o m th e e x c i t a t i o n e n e r g y s p e c tr a . I t w a s r e v e al e d t h a t t h e s i n g l e -p a r t ic l e n a tu r eo f a A hyperon per s i s t s even in heavy nuc le i . J10 ] Much improved s ta t i s t ics and r eso lu t ionwere ob ta ined fo r s9y in the mos t r ecen t E369 exper imen t . J16 ] In the E336 exper imen tw h ic h w a s c a r ri e d o u t w i th t h e u p g r a d e d S K S s p e c t r o m e te r , t h e p - s h e ll A h y p e r n u c l e ia s li s t e d in T a b le 1 w e r e in v e s t i g a t e d w i th h ig h - q u a l i t y s p e c t r a . I t is n o t e d t h a t t h ey i e ld r a t e o f t h e k 2C g r o u n d s t a t e i s a b o u t 5 e v e n t s /g / c m 2 /1 0 9 p io ns , t h a t i s, a b o u t1 00 0 ~2C g r o u n d s t a t e s a r e o b s e r v e d f or a l g / c m 2 t h i c k t a r g e t b y t h e S K S s p e c t r o m e te rp e r d a y. I n t h e ~ 2C s p e c t r u m o f t h e K E K E 3 3 6 e x p e r im e n t , tw o s a t e l l it e p e a k s ( # 2 a n dr w e r e o b s e r v e d a s f ir s t r e p o r t e d b y E 1 4 0 a n e a r t h e tw o p r o m in e n t p e a k s ( # 1 a n dr Th e @1 an d r pea ks are or ig ina ted f rom con f igura t ions of yp3-/12 A s 1 ~ 2


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- 1a n d u p 3 ~ : | A p l / : , a / : , r e s p e c ti v e ly . T h e s a t e ll i te p e a k s c a r r y a b o u t 1 / 1 0 c r o s s s e c t i o n sa n d w e r e i n t e r p r e t e d a s o r i g i n a t e d f r o m a n Sl /~ . A h y p e r o n c o u p l e d t o e x c i t e d s t a t e s o ft h e 11C c o r e. A l t h o u g h t h e s e e x c i t a t i o n e n e r g i e s a r e e x p e c t e d t o b e c lo s e t o t h o s e o f t h ec o r e n u c l e i i n t h e w e a k c o u p l i n g l im i t , t h e e x c i t a t i o n e n e r g y o f t h e # 3 p e a k , 6 . 9 M e V ,is c o n s i d e r a b l y g r e a t e r t h a n t h e c o r r e s p o n d i n g c o re e x c i t a ti o n e n e r g y o f 4 .8 M e V . I t w a sd i s c u s se d t h a t t h e d e v i a t i o n o f t h e e x c i t a ti o n e n e rg i es c a r ri es i n f o r m a t i o n o n t h e A Ns p i n - s p i n i n t e r a c t i o n . [ 9 ] A l t h o u g h i t i s q u a l i t a t i v e l y s o , t h e d a t a o f f e r e d s e r i o u s p r o b l e mt o t h e o r e t i c a l i n t e r p r e t a t i o n a n d t h e r e a r e a r g u m e n t s t h a t t h e p e a k i s n o t n e c e s s ar il y d u eo n l y t o a s i m p l e 1 1 C ( 4 . 8 M e V ) x s A c o n f i g u r a t i o n b u t c o u l d h a v e s o m e c o n t r i b u t i o n f r o mi n t e r s h e l l c o n f i g u r a t i o n m i x in g . J1 9 , 2 0] F u r t h e r i n v e s t i g a t i o n o n n e w e x c i t a t i o n m o d e o ft h e A h y p e r n u c l e u s i s r e q u i r e d .

E x c i t a ti o n e n e r g y [ M e V ]- 1 0 - 5 0 5 1 0 1 5 2 0 2 5250 " ' ; ~ ' " : ' " ! ~ ' : " : ' " ' i ' " ' ; ' : " ~ " ''~

2 o o :- 2~ ...... _ ~. ~,,- --: . . . . 1: E 3 3 6 : : - I" ~ , : , ? J

d " Prelim inary ~ : ~:i1 : , . I~ ~lt, . . , : - . . , . . , . . , . . , ~ , 9

5 0 - i ~i+ ~' ,~{i : ..{ :K E K -S K S]0 - - : ; -' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 7 0 1 7 5 1 8 0 1 8 5 1 9 0 1 9 5 2 0 0 2 0 5 2 1 0

M w z P - M A [ M e V ]

lZc(rr+, K+)lZAc P,T=I.06 e V / cz 0 ~ . . . I . . . . I . . . . I . . . . I . . . . I ' " qI 4

~ I1 5 ~ , , , Pr el imi na ry

9 I . . . . I . . .0 I . . . . . . . . . . . . . . . . . . . . . . . . . . .0 . 0 2 . 5 5 . 0 7 . 5 1 0 . 0 1 2 . 5 1 5 . 0

e5 f ' b i ' ~ 4 ' i s J ; ; ' e ' + ~ + 04 l i ' ' ' I ' ' ' '~

0 . . . . . . . . . . . .. I . . . i . . . . . . . . . . . . . . . ~ l0 . 0 2 . 5 5 . 0 7 . 5 1 0 . 0 1 2 . 5 1 5 . 0

01ab [deg.] 0 ~ b [deg.]

0 . 0 2 . 5 5 . 0 7 . 5 1 0 . 0 1 2 . 5 1 5 . 0 0 . 0 2 . 5 5 . 0 7 . 5 1 0 .0 1 2 . 5 1 5 . 0o , . b [ d e g . ] e , . b [ d e g . ]

F i g u r e 2 . H i g h s t a t i s t i c s e x c i t a t i o n s p e c t r u m o f ~ : C o b t a i n e d b y 1 2 C (T r+ ,K + )~ t2 C r e a c t i o nw i t h t h e S K S s p e c t r o m e t e r s y s t e m . T h e s p e c t r u m h a s s ta t i s ti c s 5 t i m e s b e t t e r t h a n t h ep r e v i o u s E 1 4 0 a e x p e r i m e n t .

T h e h i g h - q u a l i t y a n g u l a r d i s t r i b u t i o n s w e r e f o r t h e f i r s t t i m e o b t a i n e d b y f i t t i n g t h ee x c i t a t i o n s p e c t r a i n 2 d e g r e e b in s a n d a r e a ls o s h o w n in F i g . 2 . I n t h e f ig u r e, t h eD W I A c a l c u l a t i o n b y I t o n a g a is c o m p a r e d w i t h t h e d a t a b y s ol id li ne s. J2 1 ] T h e o v e r a l la g r e e m e n t w a s o b s e r v e d a n d i t w i ll i m p l y t h a t t h e p r e s e n t D W I A c a l c u l a ti o n a r e r el ia b l ei n p r e d i c t i n g t h e a b s o l u t e c r o s s s e c t i o n s b y t h e ( ~ r+ , K + ) r eac t i on .

T h e ~ : C s p e c t r u m d e m o n s t r a t e s a g o o d e x a m p l e o f ( T r+ , K + ) r e a c t i o n s p e c t r o s c o p y a n dt h e e x c i t a t i o n f u n c t i o n o f t h e o t h e r A h y p e r n u c l e i l i s t e d in T a b l e 1 w e r e a ls o m e a s u r e d .A p a r t o f t h e r e s u l t w i ll b e f o u n d i n re f. [1 5 ] H e r e , it is o n l y e m p h a s i z e d t h a t c o m p a r i s o no f s p e c t r a b y t h e ( K - , 7 r -) a n d (Tr+ , K + ) r e a c t i o n s p r o v i d e s v a l u a b l e i n f o r m a t i o n o n t h eA h y p e r n u c l e a r l e ve l s t r u c t u r e , s in c e t h e y p o p u l a t e d i ff e re n t s t a t e s o f s p i n p a r t n e r s e t c.


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4 . H Y P E R N U C L E A R 3` R A Y S P E C T R O S C O P YA s d e s c r i b e d i n t h e i n t r o d u c t i o n , t h e "7 r a y s p e c t r o s c o p y o f f er s t h e b e s t e n e r g y r e s o l u t i o n

f o r t h o s e h ' d e c a y i n g h y p e r n u c l e a r s t a t e s . H o w e v e r , i t h a s b e e n t h o u g h t d i f f ic u l t t o p e r f o r m3` r a y s p e c t r o s c o p y f o r A h y p e r n u c l e i , s i n ce t h e h y p e r n u c l e a r y i e l d i s v e r y m u c h l i m i t e dd u e t o s m a l l c ro s s se c t i o n a n d / o r l ow in t e n s i t y o f k a o n a n d p i o n b e a m s . T h e 3` r a ys p e c t r o s c o p y w i t h G e d e t e c t o r s h a s b e e n t h o u g h t e v e n m o r e d i f f i c u l t t h a n t h o s e w i t h N a Is c i n t i l l a t o r s s i n c e t h e p h o t o - p e a k e f f i c i e n c y i s u s u a l l y l o w c o m p a r e d t o N a I d e t e c t o r s a n da l s o i t i s n o t e a s y t o o p e r a t e G e d e t e c t o r s u n d e r t h e e n v i r o n m e n t t h a t t h e b a c k g r o u n da s s o c i a t e d w i t h s e c o n d a r y b e a m s i s s e ve r e. A l t h o u g h h y p e r n u c l e a r 3` r a y s w e r e d e t e c t e d b yP b g l a ss e s a n d N a I s c i n t il l a to r s , n o e x p e r i m e n t s w e r e s u c ce s s fu l in d e t e c t i n g h y p e r n u c l e a r"7 r a y s b y G e d e t e c t o r s .

T h e K E K E 4 1 9 e x p e r i m e n t ( S p o k e s p e r s o n : H . T a m u r a ) w a s c a r r ie d o u t b y i n s ta l li n g t h eH y p e r b a l l s y s t e m w h i c h c o n s i s ts o f 1 4 G e d e t e c t o r s ( 6 0 % ) in th e t a r g e t r e g i o n of t h e S K Ss p e c t r o m e t e r s y s t e m . T h e o v e r a ll p h o t o p e a k d e t e c t i o n e f f ic ie n cy o f a b o u t 2 .5 % f o r I M e V3` r a y s w a s a c h i e v e d w i t h t h e s o l id a n g l e o f 1 5 % . T h e B G O c o u n t e r s s u r r o u n d i n g e a c hG e d e t e c t o r w e r e u s e d to s u p p r e s s b a c k g r o u n d c o m i n g f r o m 7 r~ a n d C o m p t o n 3` r a y s .T w o h y p e r n u c l e a r 3` r a y s f r o m 7 L i , w h i c h w e r e e x c i t e d t h r o u g h 7 L i (T r + ,K + ) r e a c t i o n ,w e r e o b s e r v e d a s s h o w n i n F i g. 3. O n e i s t h e M 1 t r a n s i t i o n b e t w e e n 3 / 2 + 4 1 / 2 + a n d

7 L i ( r r + , K + y ) E 4 1 9~ , . ; : i : : : : i : ~ , > , , : , . . . , . , : ' , ' 1 .2 0 0 F 5 0 ~ , ] D : ~ .~ e r ~ ~ 2 0 ~ l ~ -~ , ~ " t ~ ' ~ , , , j I1 o o r r .o t ~ , , I . ~ , o L I , q - I :

> ~ ' ~ ~ I o t i ' , ~ . ~ ~ ~ ~ 6 - ~ ' , ~ . ~ % ~ ~ ,~1 1 5 0 % I l o o o , 2 o 0 o 2 2 o o , -" , ~ . . I " , , . . ' ~ "o ' ~ . _ 1 ~ I ~ i ~ , ,_1 0 h i . - - - ~ ' 1 ~ I " " . . ~ 7 v ; ~ .N i l , ~ ,~ M I ( 3 / 2 + ~ I / 2 + ) ' ^ ~ . , -

o ~ " U ~ I , " ~ ' ~ ' , J ~ " I * - - . ' , E 2 ( 5 / 2 " - , " 1 / 2 " ) ' -~ o , ~ . ~ j . , v : o , ~ ' , ~ , =', 1 , ' :

l / "0 ' ' ' ' ' ' ' '0 5 0 0

IH c +d...3.943.56"-. .' -0? . . . . . . . . [" l '" l~ 3 .88"=' M ~ [ T - - 1

~+ I 7 n2 " 1 8 " ...... s a + 2 .o ,( M o V ) _ _ h . + . ...... !* . * ! 3 / 2 0 . 69~ .. . . . . . . _ 1 _ _ _ ~ , ~ o ~ 1 7 6Iai present

1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0E 7 ( k e V )

1 . 0 5 C ~ V I c(x+, +)o (0 = 0 - 1 5 o )0.600.081.23

0.131.21 (~ )

Hiyama t al.

F i g u r e 3 . h ' r a y s p e c t r u m m e a s u r e d b y H y p e r b a l l i n t h e 7 L i( T r+ , K + ) ~ L i re a c t i o n .[ 1 7 ] Ar e l e v a n t d e c a y s c h e m e o f t h e 6 Li a n d ~ L i t o g e t h e r w i t h c a l c u l a t e d c r o ss s e c t io n s i s s h o w no n t h e r i g h t .t h e e n e r g y w a s d e t e r m i n e d 6 9 1 . 7 + 0 . 6 ( , t a t ) + 1 . 0 ( s y s ) k e V . T h e "7 e n e r g y w e r e d i r e c t l yc o m p a r e d w i t h t h e r e c e n t c a l c u l a t i o n b a s e d o n a c l u s t e r m o d e l a n d t h e m a g n i t u d e o fs p i n - s p i n i n t e r a c t i o n w a s d is c us s ed .[ 17 ] T h e 2 05 0 k e V p e a k w h i c h c o r r e s p o n d s t o t h e5 / 2 + ~ 1 / 2 + E 2 t r a n s i t i o n h a s a b r o a d e r p e a k s h a p e d u e t o t h e D o p p l e r e f f ec t of t h er e c o i l i n g h y p e r n c l e i . A f t e r li n e s h a p e f i t t in g t a k i n g i n t o a c c o u n t t h e s l o w d o w n p r o c e s so f [ L i , t h e l i f e ti m e o f t h e 5 / 2 + s t a t e w a s d e r i v e d t o b e T = 5 .8 + o9o:7(s ta t ) + 0 . 7 ( s y s )p s . [ 1 8 ] I t w a s f o u n d t h a t t h e c h a r g e d i s t r i b u t i o n o f ~ L i s h r i n k s b y 2 0 % c o m p a r e d w i t h


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th at of 6Li due to the p resence of a A hyp eron base d on the B (E2) value of the t rans i t ionwhich is s trong ly depen den t on the charge distr ibu tion of the hypernucleus. Th e shr inkageis consistent with theoret ical predict ions that c la imed a glue l ike role of A hyperon byMotoba[22] an d also by a recent clu ster calculation.J23] Hy pern ucle ar -y-ray spe ctrosc opyby the Hy perb all was carr ied out recently a lso for the 9 Be (K- , 7r-)~Be reaction, movingthe hyperball system to the D l ine of BNL AGS. I t is expected to provide precisioninform ation on the spin-orbit interac t ion th roug h the spli t t ing of the ls par tne r levels,3/2 + and 5/2 +, which w ere not resolved in the previous BNL experim ent using N aIdetectors.J24] I t is emp hasized tha t the hypern uclear 3` ray spectroscopy w ith Ge detectorsbecame possible with the large solid-angle high eff ic iency Ge arrays that can handle largebackground. I t i s p lanned to extend the measurement to o ther l ight hypernuc le i wi th anintension to der ive ma gni tud es of spin depe nde nt intera ct ion quali ta t ively. An effor t todouble the effic iency of the hype rball is unde r way at the sametime.Th e 3` ray spectroscop y with use of N aI sc int i l la tor array was very recently appliedin order to observe the 3 ' t ransi t ion s f rom A pa/2 and pl/2 sta tes to the groun d s ta te oflaC Th e two 3` rays abo ut 10 MeV were c lear ly detected and the en ergy difference wil lA 9give the ma gn itud e of spin-orbit interact ion. This exper im ent is the second example tha tde m on stra tes the pow erfulness of the 3` ray spectroscopy.[25]5. H Y P E R N U C L E A R S T U D Y W I T H E L E C T R O N B E A M S

Figure 4. Hypernuclear Spectrometer system in Jlab Hall C. The exper iment is scheduledin the spr ing of 2000.

Strangeness production in the elementary processes with electromagnetic probes has


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been s tud ied for var ious react ion ch annels unt i l now. However , there a re few invest iga-t ions on the s t range ness p ro duc t ion wi th the nuc lea r t a rge t . Only recen t ly , quas if ree Ap r o d u c t i o n o n a c a r b o n t a r g e t b y t h e t a g g e d p h o t o n b e a m a t T a n a sh i - E S w a s r e p o r t e d a spresen ted in th i s symposium,J26] though spec t roscp ic s tudy for the hypernuc lea r bounds ta t e s by the t agged photon beam would be d i f f i cu l t s ince the beam in tens i ty i s l ow .

On the o the rhand , an e l ec t ron beam in the GeV reg ion i s a un ique too l fo r the hyper -nuc lea r s tudy . In con t ra s t t o reac t ions w i th 7r, K meson beam s which have been mo st lyused fo r the hy pernu c lea r ex pe r iment s , t he (e , e'K +) reac t ion has l a rge sp in-f l ip am pl i tudeeven a t 0 degrees. Thus, i t exci tes both spin f l ip and spin non-f l ip hypernuclear s ta tes s i -mu l taneou lsly . In add i t ion, co ntra ry to the (Tr+ , K +) and (K - , 7 r -) r eac t ions , t he reac t ionconver t s a p ro ton to a A hyperon and produces A hypernuc le i which a re no t access ib le bysuch reac t ions . I sosp in m ul t ip l e t s t a t e s w i ll be inves t iga ted by comp ar i son of these reac -t ion spec t ra . Pa r t i cu la r ly in the l igh t mass reg ion , the (e , e'K +) reac t ion t en ds to p opu la teneu t ron r i ch A hypernuc le i . For example , t he (e , e 'K +) reac t ion on the 7Li t a rg e t ma ke i tposs ib le to s tudy a ~He hypernuc leus , which would be the neu t ron ha l lo A hypernuc leus .F rom the expe r imen ta l p o in t o f v i ew , the (e , e'K +) reac t ion h as a po ten t i a l power th a t w i llr ea li ze hyp ernuc lea r reac t ion spec t roscopy w i th sub-MeV ene rgy re so lu t ion . Even a few100 keV reso lu t ion , which i s comparab le to th a t o f NaI -y de tec tors , can be ach ieved whenan app rop r i a t e spec t rom ete r sy s t em is ava il ab le . Such h igh re so lu t ion becomes poss ib lebecause the p r imary e l ec t ron beam has exce l l en t beam emi t t ance . A sma l l beam spo t s i zeof the o rde r o f 0 .1 mm a t the t a rge t and h igh beam cu r ren t a re eas i ly ob ta ined . Thus , wecan employ sma l l and th in t a rge t s , even enr i ched i so topes .

There a re e f fo r t s fo r the A hypernuc lea r spec t roscopy a t Ha l l A and Ha l l C of Je f fe r sonLab ora tory . In Ha l l C where the exp e r imen t i s scheduled to run in the sp r ing of 2000 ,the expe r imenta l se tup was des igned to ca r ry ou t in the "0 degree t agg ing" conf igura t ion .[27] S ince the v i r tua l pho ton f lux i s ve ry much forward peaked( nea r ly a t ze ro degrees)and kaon angula r d i s t r ibu t ions a re a l so fo rward peaked , hype rnuc lea r p roduc t ion wi l l bemost e f f i c i en t ly de tec ted by measur ing sca t t e red e l ec t rons a ssoc ia t ed wi th v i r tua l pho tonsand kaon s bo th a t 0 degrees . In the pe resen t se tup in Ha l l C , a sp l i t t e r ma gne t isins t a l l ed imm edia te ly do wn st ream of the t a rge t a s schemat i ca l ly shown in F ig .4 . Sca t t e re de lec t rons and pos i t ive kaons a re ben t to the oppos i t e d i rec t ions and gu ided to the ENGEsp e c t r o m e t e r t h a t t a g s e le c tr o n s f ro m 2 3 0-3 40 M e V / c a n d t o th e S O S sp e c t r o m e t e r ( 6 m s r )t h a t d e t e c t s k ao n s a r o u n d 1 G e V / c w h e n t h e b e a m m o m e n t u m is 1 .6 45 G e V / c . A l t h o u g hi t is know n tha t the (e , e 'K +) reac t ion is inva luab le to the hyp ernuc lea r s tudy , i t hasbecome feas ib le on ly recen t ly a t Je f fe r son Labora tory where h igh in t ens i ty CW e lec t ronbea m s a re acce le ra ted . In add i t ion to a good re so lu t ion kaon spec t ro me te r , i t i s a lsorequ i red to have a sca t t e red e l ec t ron spec t romete r tha t has good ene rgy re so lu t ion andcan de tec t h igh ra t e e l ec t rons which a re a ssoc ia t ed wi th b remst rah lung f rom the t a rge t .Th e Ha l l C se tup wi th a s il icon s t r ip de tec tor a r ray th a t covers the 70 cm foca l p l ane of thee lec t ron spec t ro me te r works we ll und e r such envronm ent . Exp er im ent fo r the p - she ll Ahyp ernu c lea r spec t roscopy(E89 -09: L . Tang and E . Hunger ford) i s i n p rogress w i th t a rg e t sof aLi, 7Li , 9Be, l~ 11B and 12C. I t is a lso planne d to ex tend the inv est iga t ion by the(e , e 'K +) reac t ion to a heav ie r t a rge t (E97-08:O . Hash imoto and L . Tang) and exp lore theposs ib i l i ty to ca r ry ou t h igh- reso lu t ion h ypernu c lea r spec t roscopy of heav ie r A hypernuc le iw i t h e l e c t r o n b e a m s .


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6 . S U M M A R Y A N D F U T U R E P R O S P E C T ST h r o u g h t h e r e c e n t p r o gr e ss , w e h a v e l e a rn e d t h a t t h e A h y p e r n u c l e a r sp e c t r o sco p y

has g rea t s ign if i cance fo r the inves t iga t ion of the A hypern uc lea r s t ruc tu re and AN in-t e r a c ti o n . N o w , q u a n t a t i v e d a t a , w h i ch w e re b e y o n d t h e d r e a m so m e y e ar s ag o , c a n b eobta in ed t ak ing adv antages o f new exper im enta l oppor tun i t i e s . In pa r t i cu la r , t he (Tr+, K +)r e a c t io n sp e c t r o sc o p y w i t h t h e S K S sp e c t r o m e t e r h a s e s t a b li sh e d t h e v a lu e o f t h e A h y p e r -nuc lea r spec t roscopy wi th i t s good re so lu t ion and h igh de tec t ion e f f i c i ency . Hypernuc lea r7 ray spec t roscopy wi th the Ge de tec tor a r ray proved i t s unpa ra l l e l ed ene rgy re so lu t ion ,g iv ing c ruc ia l i n forma t ion on the sp in-dependent AN in te rac t ion . [17] A new spec t roscopics tud y of A hypernuc le i w i th the (e , e 'K +) reca t ion is in p rogress a t Je f fe r son Lab ora tory ,which i s exp ec ted to revea l s t ruc tur e o f neu t ron r ich A hypernuc le i .

Tab le 2C o m p a r i so n o f A h y p e r o n p r o d u c t i o n r e a c t io n s

A Z = 0 A Z = - 1ne u t ron t o A p ro t on t o A comment

( ~ + , K + ) ( ~ - , K ~in-flight ( K -, 7r-)s topped (K- , 7 r - )

in-flight (K - , 7r~s topped (K - , 7r~

( e , e ' K ~ ( e , e ' K + )( p , p ' K ~ ( p , p ' K + )(p,K o) (p,K+ )

stretched,high spinsubsti tut ional at low momentumlarge yield, via atomic statesspin fl ip,unnatural pari ty, virtual (7,K)virtual ( lr ,K)very large momentum transfer

In the nea r fu tu re , A hypenuc lea r spec t roscopy wi l l be exp lored wi th w ide va r i e tyof reac t ions a s l is t ed in Table 2. Hy pernu c lea r s t a t e s and hy pernuc le i to be s tud iedwi ll be e xpa nde d grea t ly . Am ong th em , the (e ,e 'K +) reac t ion wi ll p l ay a s ign if i can tro le and wi ll r evea l new aspec t s o f A hypernuc lea r s t ruc tu re w i th i t s sub-M eV or even2-300 ke V( FW H M ) ene rgy re so lu t ion . We expec t the 7 ray spec t roscopic s tud ies w il lbe fu r th e r ex ten ded wi th m uch h ighe r e f fi ci ency of a new Ge de tec tor a r ray a nd w i llp rov ide key in form a t ion on the sp in depe nde nt p a r t s o f the AN in te rac t ion . Th e reac t ionspec t roscopy , which a l lows us access to the hyp ernuc lea r s t a t e s above the nuc leon em iss ionthreshold requires higher energy resolut ion keeping high detec t ion eff ic iency, whi le the 7-ray spec t roscopy needs much l a rge r de tec t ion e f f i c i ency .

Al tho ugh the p res en t pa pe r dea l s most ly w i th spec t roscopy of A hypernuc le i , i nves tiga -t ion of E hypernuc le i which had impor tan t p rogress in the l a s t f ew yea rs w i l l be fu r the rexplpored , fo r exam ple , by th e (Tc -,K +) reac t ion . Beyon d tha t , i t is eage rly wa i t ed th a thypernuc lea r spec t roscopy in the S=-2 reg ime become poss ib le when h igh in t ens i ty kaonbeams a re ava i l ab le in p l anned accce le ra to r f ac i l i t i e s such a s the 50 GeV PS of JHF .

The au tho r i s t han kfu l fo r the use ful d iscussions w i th the co l l abora tors o f K EK -PSE140a , E336 , E369 , E419 and J l ab E89-09 , E97-08 , pa r t i cu la r ly w i th P rofessors H .


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Ta m ura , L . Ta ng and E . Hunge r fo rd . Di scuss ions wi th Pro f . T . Motoba , K . I t onag a ,D . J . M i l l en e r an d M . S o t o n a o n t h e t h eo r e t i ca l a s p ec t s a r e a l s o g r ea t l y ap p r ec i a t ed .R E F E R E N C E S

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9.10.11 .12 .13 .14 .15.16.

17 .

18.19.20.21.22.23.24.25.26.27.

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Retrospect and prospect of Λ hypernuclear physics

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