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Hadron and Nuclear Physics with ElectromagneticProbesK. Mamyamaand H. Okuno (Editors)9 2000 Elsevier Science B.V. All righ ts reserved. 271

L a s e r e l e c t r o n p h o t o n f a c i l i ty a t S P r i n g - 8T . H ot ta ~* , J .K . Ah n ~ , H . Ak imu ne b , Y . Asan o ~, W .C. Ch ang d , S . Da t (5~,M. Fuj iw ara a ,f , K . H icks g , K . Ima i i , T . Iw a ta h , T . I sh ikawa , H . Kaw ai ~ Z .Y . K im ,T . K i s h i m o t o m , N . K u m a g a i ~, S. M a k i n o p , T . M a t s u m u r a k 'r N . M a t s u o k a ~, T . M i b e ~,M. M iyab e i, Y . M iyachi h , T . Nakan @ , M. No ma chi a , Y . Oh ash i e , T . Oo ba ~H. O ok um a e , M. O osh im a f, C . R ang ach aryu lu 1'~, A . Sakagu chi m , T . Sasak i i, D . Seki i ,H . Sh im izu ~ , Y . Sug aya f , M. Su m iha m a re 'c , T . To oya m a h , H . T oyokaw a~, A. W aka i n ,C .W . W an g d , S .C . W ang d K . Yo neh ara b , T . Yo r i ta ~ , and M. Yosoia~Research Cente r fo r Nuc lea r Phys ics , Osaka Univer s i ty ,10-1 Mihogaoka , Iba rak i , Osaka567-0047 , Japanb D e p a r t m e n t o f P h y s i c s , K o n a n U n i v e r s it y , K o b e , H y o g o 6 5 8- 85 0 1, J a p a ncJ a p a n A t o m i c E n e r g y R e s e a r c h I n s t i tu t e , M i k a zu k i , H y o g o 6 7 9 -5 1 4 3 , J a p a nd I n s t i t u t e o f P h y s i c s , A c a d e m i a S i n i ca , T a i p e i 1 1 5 2 9 , T a i w a n~ J a p a n S y n c h r o t r o n R a d i a t i o n R e s e a r c h I n s t i tu t e , M i k a z u k i , H y o g o 6 7 9 -5 1 4 3 , J a p a nf A d v a n c e d S c i e n c e R e s e a r c h C e n t e r , J a p a n A t o m i c E n e r g y R e s e a r c h I n s t i t u t e , T o k a i ,I b a r a k i 3 1 9 - 1 1 9 5 , J a p a ng D e p a r t m e n t o f P h y s i c s , O h i o U n i v e r si ty , A t h e n s , O H 4 5 7 01 , U S Ah D e p a r t m e n t o f P h y s i c s , N a g o y a U n iv e r si ty , N a g o y a 46 4 -8 6 02 , J a p a ni D e p a r t m e n t o f P h y s i c s , K y o t o U n i v e rs i ty , K y o t o 6 06 -8 5 02 , J a p a nJD ep ar tm en t o f Phys ics , Seoul Na t iona l Univer s ity , Seoul 151-742 , Ko reak D e p a r t m e n t o f P h y s ic s , Y a m a g a t a U n i v er si ty , Y a m a g a t a , Y a m a g a t a 9 9 0-8 5 60 , J a p a n1 D e p a r t m e n t o f P h y s i c s , U n i v e r s i t y o f S a s k a t c h e w a n , S a s k a t o o n , S 7 N 5 E 2 , C a n a d am D e p a r t m e n t o f P h y s i c s , O s a k a U n i v e r si ty , T o y o n a k a , O s a k a 5 6 0 -0 0 43 , J a p a nn C e n t e r f o r I n t e g r a t e d R e s e a r c h i n S c i e n c e a n d E n g i n e e r i n g , N a g o y a U n i v e r s i t y , N a g o y a4 6 4 - 8 6 0 3 , J a p a n~ of Phys ics , C hib a Univer s i ty , I nage , C hiba 263-8522 , Jap anP W a k a y a m a M e d i c a l C o l l e g e , W a k a y a m a , W a k a y a m a 6 4 1 - 0 0 1 2 , J a p a n

A t S P r i n g - 8 , w e b u i l t a 2 . 4 G e V p h o t o n b e a m l i n e w i t h 3 5 0 n m l a s e r p h o t o n b a c k s c a t -t e r i n g o f f t h e c i r c u l a t i n g 8 G e V e l e c t r o n b e a m . A d e t e c t o r s y s t e m , o p t i m i z e d t o c a r r yo u t t h e p h o t o p r o d u c t i o n o f r m e s o n n e a r t h r e s h o l d , i s n e a r l y c o m p l e t e d .

1 . L A S E R E L E C T R O N P H O T O N F A C I L I T Y A T S P R I N G - 8H i g h e n e r g y p h o t o n b e a m s p r o d u c e d b y l a s e r - i n d u c e d b a c k w a r d C o m p t o n s c a t t e r i n g

of f the c i r cu la t ing e lec t rons ( l a se r e lec t ron ph oton ) i s u t i l i zed for nuc lea r ph ys ics s tud iesa t va r ious synchro t ron r ad ia t ion f ac i l i t i e s in the wor ld [1 ,2] . SPr ing-8 , the wor ld ' s h ighes te n e r g y t h i r d - g e n e r a t i o n s y n c h r o t r o n r a d i a t i o n f a c i l i t y , e n a b l e s u s t o p r o d u c e t h e h i g h e s t*Corresponding au tho r. E-m ail: hotta@rcnp.osaka-u.ac.jp

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F i g u r e 1 . T h e L a s e r E l e c t r o n P h o t o n f a c i li ty a t S P r i n g - 8 ( L E P S ) .

e n e r g y l a s e r e l e c t r o n p h o t o n b e a m . W e h a v e c o n s t r u c t e d t h e l a s e r e l e c t r o n p h o t o n ( L E P )f a c i li t y a t B L 3 3 L E P , o n e o f t h e 6 1 b e a m l i n e s ( F i g . 1 ). A t S P r i n g - 8 , 8 G e V e l e c t r o n b e a mis c i r c u l a t i n g w i t h / m a x = 1 00 m A . T h e l a s e r b e a m is i n j e c te d f r o m t h e L a s e r H u t c h t ot h e s t r a i g h t s e c ti o n . T h e C o m p t o n s c a t t e r i n g o f t h e l as e r p h o t o n w i t h a n 8 G e V e l e c t r o np r o d u c e s G e V p h o t o n s w h i c h c a n b e u s e d fo r t h e e x p e r i m e n t s o f q u a r k n u c l e a r p h y s ic s .

T h e m a x i m u m e n e r g y o f t h e L E P is d e t e r m i n e d b y th e e l e c tr o n e n e r g y a n d t h e l as e rw a v e l e n g th . T h e L E P S f ac il it y p r o d u c e s t h e m a x i m u m p h o t o n e n e r g y o f 2 .4 G e V w i t h3 5 0 n m A r l a s er . A t p r e s e n t , S P r i n g - 8 is t h e o n l y fa c il it y p r o v i d i n g L E P b e a m s a b o v e~ ,p ~ C p t h r e s h o l d . W h e n s h o r t e r w a v e l e n g t h (2 0 0 n m ) la s e r is u s e d in n e a r f u t u r e ,t h e m a x i m u m p h o t o n e n e r g y b e co m e s h i g h e r t h a n 3 G eV . O n e o f t h e a d v a n t a g e s o f L E Pb e a m is i ts f la t in t e n s i t y d i s t r ib u t i o n f or t h e C o m p t o n p h o t o n s . T h u s , e x p e r i m e n t s d o n o ts uf fe r fr o m h i g h i n te n s i ty l o w e n e r g y b a c k g ro u n d , a c o m m o n p r o b l e m w i t h B r e m s s t r a h l u n gb e a m s .

A n o t h e r f e a t u r e i s h i g h p o l a r i z a t i o n o f p h o t o n s . I f l a se r p h o t o n s a r e 1 0 0 % p o l a ri z e d , t h eL E P is a l so p o l a ri z e d a t t h e m a x i m u m e n e rg y . T h e p o l a r i z a t io n o f t h e p h o t o n d e c r e a s e sa t l o w e r p h o t o n e n e r g y. H o w e v e r , b y c h a n g i n g t h e l a se r w a v e l e n g t h , h i g h l y p o l a r i z e dp h o t o n s i n w i d e e n e r g y ra n g e c a n b e o b t a i n e d . T h e p o s i t i o n a n d p o l a r i z a t i o n o f t h e l a se rc a n b e m o n i t o r e d o n b o t h s i d es of t h e i n t e r a c t i o n r e g io n ; a t th e L a s e r H u t c h a n d t h e e n do f t h e b e a m l i n e .

W h e n t h e e l e c t r o n e n e r g y i s h i g h , L E P s a r e e m i t t e d i n a n a r r o w c o n e in t h e e l e c t ro nb e a m d i r e c t i o n . A t t h e L E P S f a c il it y , p h o t o n s i n t h e e n e r g y r e g i o n o f i n t e r e s t ( E ~ _>1 .5 G e V ) a r e w i t h i n t h e s c a t t e r e d a n g l e le ss t h a n 0 .1 5 m r a d , w h i c h re s u l t i n b e a m s iz e o fa b o u t 1 c m a t t h e t a r g e t p o i n t in th e E x p e r i m e n t a l H u t ch .

T h e e n e r g y o f t h e L E P is d e t e r m i n e d b y m e a s u r i n g re c o il e l e c tr o n w i t h a ta g g i n gs y s te m . T h e b e n d i n g m a g n e t o f t h e s t o ra g e r i n g is u s ed fo r a n a l y z in g t h e m o m e n t u mo f r e co i l e l e c tr o n . T h e t a g g i n g s y s t e m is a p o s i ti o n d e t e c t o r l o c a t e d i n s id e t h e r i n g a tt h e d o w n s t r e a m e n d ( a lo n g th e e l e c t ro n b e a m ) o f t h e b e n d i n g m a g n e t . I t c o n s i s ts o f t w ol a y e r s o f 5 0 0 # m t h i c k s i l ic o n s t r i p d e t e c t o r s w i t h 1 0 0 # m p i t c h a n d t w o l a y e r s o f p l a s t i cs c i n t i ll a t o r a r ra y . P h o t o n s o f 1.5 t o 3 .5 G e V c a n b e t a g g e d b y m e a s u r i n g r e co i l e l e c t ro n

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with the momentum range between 6.5 and 4.5 GeV. Simulation results show that thephoton energy resolution is expected to be around 15 MeV, and it is mainly determinedby energy and angular spread of incident electrons.

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ponents were modified to inject a laser beam against the electron, to extract a LEP,and to utilize the tagging system. The laser injection system was completed by the firstquarter of 1999. The first LEP beam at SPring-8 was produced on July 1st, 1999. APWO calorimeter was used for the measurement of the beam energy. Fig. 2 shows themeasured energy spectrum of the beam. The tagging system was also tested. Fig. 3 shows

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3 . P H Y S I C SA t t h e L E P S , m a n y e x p e r i m e n t s h a v e b e e n p r o p o s e d a n d d is c u s se d . O n e o f t h e e x p e r -

i m e n t t o b e d o n e a t t h e f i rs t s ta g e i s q5 p h o t o p r o d u c t i o n . T h e p h o t o p r o d u c t i o n o f v e c t o rm e s o n s ( V M s ) a r e d e s c r i b e d b y t h e p r o c e s s t h a t " y f l u c t u a t e s i n t o V M w h i c h is w e l l k n o w na s t h e v e c t o r m e s o n d o m i n a n c e . T h e n t h e V M i s s c a t t e r e d d i f fr a c ti v e ly b y P o m e r o n e x-c h a n g e [ 3 ]. I n th e f r a m e w o r k o f R e g g e th e o r y , P o m e r o n w a s i n t r o d u c e d t o d e s c r ib e t h eu n i v e r s a l ri s e o f h a d r o n i c c r o s s s e c t i o n a t h ig h e n e rg i e s . N o w a d a y s P o m e r o n e x c h a n g e c a nb e u n d e r s t o o d a s m u l t i- g l u o n e x c h a n g e p r o c e s s [ 4 ]. I n t h e ~b p h o t o p r o d u c t i o n , t h e m e s o ne x c h a n g e p r o c e s s i s s t r o n g l y s u p p r e s s e d b y O Z I r u l e a n d t h e c r o s s s e c t i o n f r o m t h r e s h -o l d t o H E R A e n e r g y re g i o n is m a i n l y d u e to P o m e r o n e x c h a n g e [5 ]. I t m e a n s t h a t t h eg l u o n e x c h a n g e p r o c e s s a t l o w e n e r g y c a n b e s t u d i e d o n l y b y q5 p h o t o p r o d u c t i o n . A s a ne x a m p l e , i t is s u g g e s t e d t h a t p r e c i se m e a s u r e m e n t o f u n p o l a r i z e d c r o s s s e c t i o n w i ll c l a r if y0 + g lu e b a l l c o n t r i b u t i o n [ 6 ]. B y u s i n g p o l a r i ze d p h o t o n a n d / o r p o l a r i z e d t a r g e t ; w e c a ns t u d y t h e c o n t r i b u t i o n s o f d i f fe r e n t p r o c e ss e s , i n c l u d i n g sg k n o c k o u t f r o m a n u c l e o n , i nt e r m s o f h e l i c i t y a m p l i t u d e s [7 ].4 . D E T E C T O R S

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T h e L E P S d e t e c t o r s e t u p i s s h o w n i n F ig . 4. T h e d e t e c t o r i s d e s i g n e d fo r t h e m e a -s u r e m e n t o f t h e r p h o t o p r o d u c t i o n i n t h e f o r w a r d d i r e c t io n . I t c o n s i s t s o f s il ic o n s t r i pv e r t e x d e t e c t o r s ( S V T X ) , m u l t i - w i r e d r i f t c h a m b e r s ( M W D C ) , a d i p o l e m a g n e t , a n d at i m e - o f - f l i g h t ( T O F ) w a l l .

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T h e d i p o le m a g n e t h a s 1 35 c m w i d e a n d 5 5 c m h i g h o p e n i n g a n d t h e l e n g t h o f t h e p o l ea l o n g th e b e a m is 6 0 c m . T h e f ie ld s t r e n g t h is 1 T a t t h e c e n t e r. T h e S V T X c o n s i s ts o f 2p l a n e s ( x a n d y ) o f s i n g l e - s id e d s il ic o n s t r ip d e t e c t o r s ( S S D s ) . T h e t h i c k n e s s o f e a c h S S Dis 3 0 0 # m a n d t h e s t r i p p i t c h is 1 20 # m . W i t h t h e S V T X w e m e a s u r e t h e p o s i t io n s a n de n e r g y lo s s f o r p a r t i c l e id e n t i fi c a ti o n . A M W D C ( D C 1 ) w i t h 5 p l a n e s ( x , x ' , y , y ' , u ) isl o c a t e d u p s t r e a m a n d a p a ir o f M W D C ( D C 2 a n d D C 3 ) a re d o w n s t re a m o f t h e m a g n e t .T h e s e n s i t iv e a r e a i s 8 0 c m w i d e x 3 0 c m h i g h f o r t h e D C 1 a n d 2 0 0 c m w i d e x 8 0 c mf o r t h e D C 2 a n d D C 3 . E a c h o f t h e D C 2 a n d D C 3 h a s 5 p l a n es ; x , x ', y , y ' a n d u ( fo r t h eD C 2 ) o r v (f o r t h e D C 3 ) .

T h e T O F w a l l is u s e d t o m e a s u r e t i m e s o f f li g ht o f p a r t ic l e s fr o m t h e t a r g e t . T h e T O Fw a l l c o n s i s t s o f 4 0 p l a s t i c s c i n t i l l a t o r b a r s o f d i m e n s i o n s : 4 c m t h i c k x 1 2 c m w i d e x 2 ml o ng . T h e t i m i n g r e s o l u t i o n o f b e t t e r t h a n a = 1 00 p s e c h a s b e e n a c h i ev e d .

T h e T O F s t a r t s i g n a l w i t h ~r = 1 2 p s e c is m a d e f r o m a n R F s ig n a l o f 8 G e V s t o r a g er in g . A s a s t a n d a r d s e t u p , t h e T O F w a l l is l o c a t e d a t 3 m f r o m t h e d ip o l e m a g n e t a n da fl ig h t l e n g t h o f a c h a r g e d p a r t i c l e i s a b o u t 4 m . I n t h i s c a se , K /T r s e p a r a t i o n o f u p t o2 G e V / c m o m e n t u m is e x p e c t ed a t b e t t e r t h a n 4 or le ve l. E a c h d e t e c t o r c o m p o n e n t s h a sb e e n t e s t e d s e p a r a t e ly b y u s i n g t h e L E P b e a m a n d B r e m s s t r a h l u n g ~, f ro m t h e r in g . A ni n t e g r a t e d t e s t o f t h e d e t e c t o r h a s b e e n j u s t s t a r t e d .

F o r o t h e r e x p e r i m e n t s , a p h o t o n c a l o r im e t e r , m a d e u p o f 2 5 2 l e a d s c in t i ll a t in g f i b erd e t e c t o r s o f 14 r a d i a t i o n l e n g t h s , c o v e r s f r o m 3 0 - 1 00 ~ i n l a b o r a t o r y s y s t e m . A l s o , as u p e r c o n d u c t i n g s o l e n o id m a g n e t i s b e i n g b u i l t fo r t h e f u t u r e d e v e l o p m e n t o f p o l a r i z e dt a r g e t .5 . S U M M A R Y

T h e l as e r e l e c tr o n p h o t o n b e a m w i th t h e m a x i m u m e n e rg y o f 2 .4 G e V h a s b e e n s u c-c e ss fu ll y p r o d u c e d a t a n ew l y d e v e l o p ed b e a m l in e , B L 3 3 L E P a t S P r in g - 8 . T h e b e a mi n t e n s i t y w il l b e i n c r e a s e d u p t o I x 1 0 7 / s e c a f t e r t h e b e a m c o m m i s s io n i n g . T h e d e t e c t o rs y s t e m h a s b e e n c o n s t r u c t e d a n d t h e t e s t s ar e u n d e r w a y . T h e f ir s t t e s t e x p e r i m e n t o f rp h o t o p r o d u c t i o n i s p l a n n e d f o r e a r l y 2 0 0 0 .R E F E R E N C E S1 . A . M . S a n d o r fi , J. L e V i n e , C . E . T h o r n , G . G i o r d a n o a n d G . M a t o n e , I E E E T r a n s .

N u c l . S c i . 3 0 , 3 0 8 3 ( 1 9 8 3 )2 . C . S c h a e r f , N u c l . P h y s . N e w s 2 ( 1 9 9 2 ) N o . 1 7 - 83 . T . H . B a u e r , R . D . S p i t a l , D . R . Y e n n i e a n d F . M . P i p k i n , R e v . M o d . P h y s . 5 0 , 2 61

( 1 9 7 8 )4 . A . D o n n a c h i e a n d P . V . L a n d sh o f f , P h y s . L e t t . B 2 9 6 , 2 2 7 ( 19 9 2 )5 . M . D e r r i c k e t a l . [ Z E U S C o l l a b o r a t i o n ] , P h y s . L e t t . B 3 7 7 , 2 5 9 ( 1 9 9 6 )6 . T . N a k a n o a n d H . T o k i , E x c i t in g P h y s i c s w i t h N e w A c c e l e r a to r F a c il it ie s , W o r l d

Sc i en t i f i c , 19977 . A . I . T i t o v , Y . O h a n d S . N . Y a n g , P h y s . R e v . L e t t . 7 9 , 1 6 3 4 ( 1 99 7 )