P o l a r i z a t i o n O b s e r v a b l e s i n K + - m e s o n P h o t o - p r o d u c t i o n a t L E P S High-energy γ -rays, called “Inverse Compto n γ - rays,” are generated from collisions between 8 Ge V electrons and laser photons at beamlin e BL33LEP . The γ -ray can attain a maximum energy of 2.4 Ge V at maximum. T hese γ -rays have excellent properties regarding directivity and polarization, and provid e a good means to study t he behavior of quar k movements inside nucleon s and nuclei since one of the best way to investigate the inside of hadrons is to use electromagnetic probes. This is thanks to t he reduced theoretical complexity of the photonuclear r eaction with r eal-photons. For this purpose, we constructed a detector system called t he L EPS Experimental hutch Fig. 1. Top view of the spectrometer system (LEPS) in the experimental hutch. The spectromete r m ainly c onsists of a dipole m agnet, drift-chambers, and scintillation c ounters for TOF measurement. H igh-energy γ -rays coming from the left side are stopped at the beam dump (o n the right side of the figure). A typical event of K + + K – pairs for φ meson production is shown. s pectrometer to analyze c harged particles. Th e LEPS spectrometer shown in Fig. 1 mainly consists of one dipole magnet with a large aperture, th re e drift chamber s, and TOF (Time of Flight) scintillation counters. T he dipole magnet is used to bend th e c harged particles produced by photonuclear r eactions. O ne of t he three drift c hambers is locate d between the target and the dipole magnet . A ll t he drift c hambers are used to determine th e particle trajectories. T he ray-tracing technique is fully employed to determine t he trajectories of c harged particles, and t he TOF scintillation counters serve to identify the particle mass through 122 0 2 0 0 0 4 0 0 0 6 0 0 0 8 0 0 0 1 0 0 0 0 1 2 0 0 0 1 1 . 5 2 C o u n t s M i s s i n g m a s s γ + p → K + + X ( G e V ) ( γ + p → π + + n ) b a c k g r o u n d Λ ( 1 1 1 6 ) Λ ( 1 4 0 5 ) Σ 0 ( 1 1 9 2 ) Σ 0 ( 1 3 8 5 ) Λ ( 1 5 2 0 ) t h e m e a s u r e m e n t o f t h e t i m e - o f - f l i g h t ( T O F ) . I n t h e c a s e o f φ m e s o n p h o t o - p r o d u c t i o n , a φ - m e s o n d e c a y s i n t o K + + K – p a i r s w h e n i t i s g e n e r a t e d f r o m t h e γ + p → p + φ r e a c t i o n , f o r e x a m p l e . I n f o r m a t i o n o n t h e m e a s u r e d m a g n e t i c f i e l d s i s e m p l o y e d i n t h e R u n g e - K u t t a m e t h o d t o o b t a i n t h e p a r t i c l e t r a j e c t o r i e s f o r t h e r a y t r a c i n g . I n t h i s s h o r t r e p o r t , w e s h o w a t y p i c a l e x p e r i m e n t u s i n g p o l a r i z e d γ - r a y s . I t i s w e l l k n o w n t h a t a p r o t o n c o n s i s t s o f q u a r k s w i t h a “ u u d ” c o n f i g u r a t i o n , e x c h a n g i n g g l u o n s ( t h e o r i g i n o f “ s t r o n g f o r c e ” ) t o c o m b i n e t h e m . W h e n a p r o t o n a b s o r b s a h i g h - e n e r g y γ - r a y i n t h e p h o t o - r e a c t i o n , a K + - m e s o n ( a p a i r o f a n u - q u a r k a n d a n a n t i - s t r a n g e s - q u a r k ) i s c r e a t e d , l e a v i n g t h r e e q u a r k s w i t h a c o n f i g u r a t i o n o f “ u d s . ” T h i s s y s t e m o f h a d r o n s i s e x p e c t e d t o p r o v i d e u s w i t h a n n e w a s p e c t o f s t u d y i n g t h e q u a r k b e h a v i o r , s i n c e i t c o n t a i n s “ s t r a n g e q u a r k s , ” w h i c h d o e s n o t a p p e a r i n t h e n o r m a l w o r l d a t l o w t e m p e r a t u r e . W h e n p o l a r i z e d h i g h - e n e r g y γ - r a y s c r e a t e t h e s - s q u a r k p a i r a s a r e s u l t o f t h e i n t e r a c t i o n w i t h t h e q u a r k - g l u o n f i e l d f r o m t h e n u c l e o n , t h e c r e a t e d p a r t i c l e s t e n d t o k e e p t o t h e p o l a r i z e d a x i s a n d a r e e m i t t e d a l o n g t h e p o l a r i z a t i o n a x i s o f t h e γ - r a y s . T h i s i s p h y s i c a l l y n a t u r a l . T h e q u a r k a n t i - q u a r k p a i r i s m o s t l y g e n e r a t e d i n t h e v a c u u m b r e a k i n g p r o c e s s t h r o u g h a g l u o n e x c h a n g e p r o c e s s . T h e d i s t r i b u t i o n o f c r e a t e d K + m e s o n s h a v e a b a s i c p a t t e r n w i t h r e s p e c t t o t h e l i n e a r p o l a r i z a t i o n a x i s o f γ - r a y s , w h i c h i s p r e d i c t e d o n t h e b a s i s o f q u a n t u m p h y s i c s a n d g i v e n a s ( d σ / d Ω ) p o l = ( d σ / d Ω ) u n p o l ( 1 + P Σ c o s ( 2 φ ) ) , w h e r e P i s t h e p o l a r i z a t i o n o f t h e γ - r a y b e a m , Σ i s t h e a s y m m e t r y p a r a m e t e r c o m m o n l y c a l l e d Σ p a r a m e t e r , a n d φ i s t h e d e v i a t i o n a n g l e f r o m t h e p o l a r i z a t i o n d i r e c t i o n o f t h e γ - r a y . W e h a v e s u c c e e d e d i n o b s e r v i n g s u c h p a t t e r n s . F i g u r e 3 F i g . 2 . M i s s i n g m a s s s p e c t r u m f o r t h e γ + p → K + + Λ ( Σ 0 ) r e a c t i o n . N a r r o w p e a k s f o r Λ ( 1 1 1 6 ) , Σ 0 ( 1 1 9 2 ) , Λ ( 1 5 2 0 ) p a r t i c l e s a n d t h e r a t h e r b r o a d p e a k s f o r Λ ( 1 4 0 5 ) a n d Σ 0 ( 1 3 8 5 ) p a r t i c l e s c a n b e d i s t i n g u i s h e d . A s m a l l c o n t a m i n a t i o n f o r t h e γ + p → π + + n r e a c t i o n i s a l s o p r e s e n t . F i g u r e 2 s h o w s a m a s s i d e n t i f i c a t i o n s p e c t r u m f r o m p h o t o n u c l e a r r e a c t i o n s o n p r o t o n s , w h i c h d e m o n s t r a t e s t h a t v a r i o u s k i n d s o f b a r y o n s a r e c r e a t e d w i t h a n “ u d s ” q u a r k c o n f i g u r a t i o n c o u p l e d t o v a r i o u s s p i n s a n d i s o s p i n s . T h i s s p e c t r u m h a s b e e n o b t a i n e d b y m e a s u r i n g a K + m e s o n w i t h t h e L E P S m a g n e t i c s p e c t r o m e t e r ( s e e F i g . 1 ) . A K + m e s o n h a s a m a s s o f 4 9 3 . 7 M e V , a n d m a i n l y c o n s i s t s o f a u - q u a r k a n d a n a n t i - s t r a n g e s - q u a r k . W h e n a h i g h - e n e r g y γ - r a y c r e a t e s a n s - s q u a r k p a i r b y i n t e r a c t i n g w i t h p r o t o n , a n a n t i - s t r a n g e q u a r k p i c k s u p t h e u - q u a r k f r o m t h e p r o t o n w i t h a u u d c o n f i g u r a t i o n , m a k i n g a K + m e s o n a n d l e a v i n g t h e Λ a n d Σ p a r t i c l e s w i t h a n u d s c o n f i g u r a t i o n . T h e s e a r e n a i v e e x p l a n a t i o n s f o r t h e r e a s o n w h y w e c a n o b s e r v e v a r i o u s k i n d s o f Λ a n d Σ 0 p a r t i c l e s i n h i g h - e n e r g y p h o t o r e a c t i o n . F i g u r e 2 s h o w s t h e m i s s i n g m a s s s p e c t r u m f o r Λ a n d Σ p a r t i c l e s p r o d u c e d v i a t h e p r o c e s s e s o f γ + p → K + + Λ , K + + Σ 0 . T h e Λ a n d Σ 0 b a r y o n s w i t h m a s s e s o f 1 1 1 6 , 1 1 9 2 , 1 4 0 5 , 1 3 8 5 , 1 5 2 0 M e V a r e c l e a r l y i d e n t i f i e d i n t h e s p e c t r u m . 123 – 0 . 5 – 0 . 4 – 0 . 3 – 0 . 2 – 0 . 1 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 2 4 6 Σ 0 ( 1 1 9 2 ) φ ( r a d ) ( N v – k N h ) / ( N v + k N h ) – 0 . 5 – 0 . 4 – 0 . 3 – 0 . 2 – 0 . 1 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 2 4 6 φ ( r a d ) Λ ( 1 1 1 6 ) ( N v – k N h ) / ( N v + k N h ) s h o w s t h e m e a s u r e d a s y m m e t r y f o r t h e Λ ( 1 1 1 6 ) a n d Σ 0 ( 1 1 9 2 ) p r o d u c t i o n s . T h e K + m e s o n p r o d u c t i o n i s f o u n d t o b e s p a t i a l l y a s y m m e t r i c a n d i s w e l l d e s c r i b e d u s i n g a “ c o s i n e ” c u r v e . I t s a m p l i t u d e d e p e n d s s l i g h t l y o n t h e t y p e o f p a r t i c l e p r o d u c t i o n a n d i n c i d e n c e e n e r g y o f t h e γ - r a y s . T h e a m p l i t u d e o f t h e o b t a i n e d “ c o s i n e ” c u r v e d e p e n d s o n t h e i n t e r a c t i o n s t r e n g t h s a s s o c i a t e d w i t h t h e K + m e s o n c r e a t i o n a n d o n t h e d e t a i l s o f r e a c t i o n m e c h a n i s m s ( i . e . , r e s o n a n c e s t a t e s i n v o l v e d i n t h e r e a c t i o n p r o c e s s ) . T h e s e e x p e r i m e n t a l r e s u l t s r a i s e a s o m e w h a t c o n t r o v e r s i a l q u e s t i o n , c a l l i n g f o r t h e o r e t i c a l c h a l l e n g e s t o d e s c r i b e a s y m m e t r i e s f o r Λ a n d Σ 0 p r o d u c t i o n s . M a m o r u F u j i w a r a a , b ( a ) R e s e a r c h C e n t e r f o r N u c l e a r P h y s i c s ( R C N P ) , O s a k a U n i v e r s i t y ( b ) 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 E R I ( T o k a i ) E - m a i l : f u j i w a r a @ r c n p . o s a k a - u . a c . j p F i g . 3 . A n g l e a n d e n e r g y - i n t e g r a t e d a s y m m e t r y p l o t s f o r t h e γ + p → K + + Λ ( 1 1 1 6 ) r e a c t i o n ( l e f t ) a n d γ + p → K + + Σ 0 ( 1 1 9 2 ) r e a c t i o n ( r i g h t ) . 124
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