Kaon Photoproduction on Nuclei

7/28/2019 Kaon Photoproduction on Nuclei

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Hadron and Nuclear Physics with Electromagnetic ProbesK. Maruyamaand H. Okuno (Editors)2000 Elsevier Science B.V. All rights reserved. 101

K a o n p h o t o p r o d u c t i o n o n n u c le iK. M aeda ~*a Phys ics D epa r tm en t , G rad ua t e School of Sc ience , Tohoku Univers ity ,Senda i 9808587 , Japan

The pho to prod uc t ion s o f a A th roug h the 12C(7 , K +) and 3He(7, K ~ reac t ions weres tud ied near the th resho ld reg ion . The exper imen ta l da ta were compared wi th a numer-ical calcula t ion in term s of the quasifree A pho topro duc t ion . Th e cross sect ions werealso d iscussed by com parin g with the d ifferent mo del predict ions . We discuss the use ofpo la r ized pho to n b eam s fo r the (7 , K) reac t ions . I t enables us to s tudy the e le me n tarykaon pho toproduc t ion ampl i tudes in de ta i l .1 . I N T R O D U C T I O N

Ph oto nuc lear react ion s associa ted w ith a s t rang eness degree of freedom, such as (-) ,, K ~and (7 , K +) react ions , leave S = - 1 in nuclei convert ing a nucleon in to a hyper on. Ahyperon p roduc t ion in a nuc leus p rov ides a un ique oppor tun i ty to s tudy fundamenta lin fo rmat ion on the nuc leon-hyperon and the nuc leus -hyperon in te rac t ions . J1 ] They canbe s tud ied by use o f the quas if ree (QF) p rocess and the hyper -nuc leus spec troscopy.Un ti l now, these invest igat ions have been done us ing hador onic probes . In part ic ular ,the (Tr+ , K +) reac t ions have been used ex tens ive ly to s tu dy the p roduc t ion and decay o fA-hypernuclei. [2]

The A-pro duc ing (7 , K +) and (7 , K ~ reac t ions a re be l ieved to be com plem en tary to thehado ronic react ion . [3] Th e do min ance of the spin-f lip amp li tudes in (7 , K ) near th resh oldreg ion enab les us to s tudy the sp in depen den t behav io r o f a A in nuc lear po ten t ia l . S incethe m ean f ree pa th o f 3' in nuc lear me d ium i s much longer than any hadrons , we can p robeA in nucle i wi th l es s d i s to r t ion . In sp ite o f the im por tanc e o f (7 , K ) me asurem en ts onnuc lear t a rge t s , su i t ab le 7 beam s have no t been ava ilable . High du ty and h igh in tens i ty ~/beam s in GeV-reg ion a re requ i red to s tudy the A pho topr oduc t ions because o f the smal l(7 , K) c ross sec t ions an d the requ i remen ts o f the p ho ton tagg ing .

In th is ar t ic le , we de scribe the 12C(7 , K +) and 3He(7 , K ~ da ta near the thres holdreg ion . These me asurem en ts a re the f i rs t observa t ion o f the K + and K ~ pho to prod uc t ionon nuc le i u s ing the t agged pho ton beam . The expe r imen ta l da ta wi ll be in t roducedand co mp ared w i th theore t i ca l in te rp re ta t ions . F ina lly , we no t ice the impor ta nce o f thede ta i l ed s tud y o f the e lem en tary kaon p roduc t ion ampl i tudes , w h ich can be inves t iga tedb y u si n g t h e p o l a r iz e d p h o t o n b e a m s . Th e y c a n b e do n e a t th e La s e r -E l e c t ro n -P h o t o nfaci l ity a t S Pring8.[4] Th ey w ill g ive us a complete d at a set for the elem enta ry (7 , K )reac t ion ampl i tudes .*E-mail: maeda@ mail.cc.tohoku.ac.jp


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2. T A G X E X P E R I M E N T S2 .1 . /4 + p h o t o p r o d u c t i o n a t T A G X

Fig ure 1 . Dif ferent ial cross sect ion for12C('),,/(+) r eac t io n . T A G X d a t a w er e co m -p a r ed w i t h t h e e l em en t a r y c r o s s s ec t i o n an dt h e n u m er i ca l e s t i m a t e s i n t e r m s o f t h equas i f r ee p rocess .

F igure 2. Miss ing mass d i s t r i bu t io n fo r12C(~, K + ) r eac t io n i n th e p h o t o n en e r g yr an g e o f 1 .0 - 1 .1 G eV . L i nes r ep r e sen tthe r esu l t s o f t he M onte Car lo ca l cu l a t ion .The ar row ind ica t es t he ma ss o f t he k2Bg r o u n d s t a t e s .

T h e 12C(~, K +) r eac t ion was car r i ed ou t us ing the TAGX spec t rometer [5 ] a t t he 1 .3 -G eV T o k y o E l ec t r o n S y n ch r o t r o n L ab o r a t o r y . [6] T h e i n c id en t p h o t o n en e r g y r an g e w as E ~= 0 . 7 8 - 1.1 G eV . T h e ch a r g ed p a r ti c l e ev en ts w e re m o m e n t u m an a l y zed f r o m t h e h i ts o fS i -S t r ip Coun ter s and Cyl indr i ca l Dr i f t Chambers i n t he magnet i c f i e ld . The t iming andt r igger ing coun ter s were inner - and ou ter - sc in t i l l a t i on-coun ter hodoscopes , and a t ime-of -f li gh t wall . Th ey covered the ang u lar r ange s o f 10 to 40 degrees on the l e f t -hand s ideand 0 to 30 degrees on the r igh t -hand s ide . A typ ica l momentum reso lu t ion was 6% fo rp r o t o n s o f 0 .4 G e V / c .

F igure 1 shows the ob ta ined d i f fe ren t ia l c ross sec t ion . So l id squares show the TA GXd a t a co m p ar ed w i t h an av e r ag e o f t h e e l em en t a r y c r o s s s ec ti o n s m u l ti p l ied b y a f ac to ro f four. Th ic k curves show the M onte C ar lo ca l cu l a t ions , i n which the Q F process i sa s su m ed . T h e p r o t o n m o m en t u m d i s t r i b u t i o n w ere t ak en fr o m t h e 12 C(e, e'p) data[7].T h e d o t - d a sh ed cu r v es r ep r e sen t t h e co n t r i b u t i o n s o f t w o p r o t o n s i n th e p sh e ll an d f o u rpr oto ns in the s shell , respect ively . Th e th ick sol id l ine shows the su m of these twoco n t r i b u t i o n s . We can u n d e r s t a n d t h a t t h e n u c l ea r F e r m i m o t i o n cau ses t h e K + yi e ld tor i s e m o r e s l o w l y n ea r t h e t h r e sh o l d t h an t h e e l em en t a r y p r o ces s .

F igure 2 i s t he miss ing mass d i s t r i bu t ion fo r t he kaon even t s i n t he energy in t e rva l o f1.0 - I.I GeV . The curves sh ow the QF calculation. We can see a bro ad pea k structurecentered at 11.55 GeV/ c 2. It is rep rod uce d well by the sum of the kao ns fro m p and s


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s h e ll s t h r o u g h t h e Q F p r o c e s s . W e f in d a n ex c e ss o f t h e d a t a a b o v e t h e Q F c a l c u l a t i o ni n t h e m i s s i n g m a s s r e g i o n b e l o w 1 1 .4 G e V / c 2. T h e b o u n d A s t a t e s a r e e x p e c t e d i n t h i sr e g i on . T h e i n t e g r a t e d d i f fe r e n t ia l c r o s s s e c t i o n o v er t h i s r e g i o n is 0 .2 1 + 0 . 0 5 p b / s r . T h ee ff ec ti ve p r o t o n n u m b e r c a n b e i n t e r p r e t e d a s a n u m b e r o f t h e p a r t i c i p a t i n g n u c l e o nsi n t h e r e a c t i o n p r o c e s s . S i n c e a p r o t o n c h a n g e s t o a A , t h e e f f ec t iv e n u c l e o n n u m b e r i sr e p l a c e d b y a n e f f e ct iv e p r o t o n n u m b e r i n ( 7, K + ) p r o c e s s. O u r r e s u l t o f e f fe c ti ve p r o t o nnu m be r i s 4 .2 4 - 0 .6 . I t sugg es t t he (7 , K + ) i s a u se fu l t oo l t o p rob e the nuc le ar i n t e r io rw i t h l e s s d i s t o r t i o n .

T h e o r e t i c a l i n v e s t i g a t i o n s o f 12 C (7 , K + ) h a v e b e e n d o n e i n t e r m s o f t h e q u a s i - fr e e p r o -c e ss l e a d i n g t o a n u n b o u n d A w i t h a t h r e e b o d y m o d e l , J 8,9 ] a n d a n i n t r a n u c l e a r c a s c a d emodel [10 ] . Lee e t a l . [8] p re d ic t e d bo th the inc lu s ive an d exc lus ive 12C(7 , K + ) c ro sss e c ti o n s b y u s i n g t h e r e c e n t ly d e v e l o p e d a m p l i t u d e s [ I l l o f 7 P ~ K + A a n d w a v e fu n c -t i o n s fr o m t h e r e l a t iv i s t i c m e a n - f i e l d t h e o ry [ 1 2 ]. T h e y o b t a i n e d 0 . 1 9 p b / s r f o r t h e s u mo f t h e c r o s s s e c t io n s f o r a ll b o u n d A s t a t e s , w h i c h i s c lo s e t o t h e e x p e r i m e n t a l v a l u e0 .2 1 + 0 . 0 5 p b / s r . T h e i n c l u s i ve c r o s s s e c t i o n in th e e n e r g y r e g io n u p t o E ~ = 1 . 1 G e V w a sc a l c u l a t e d b a s e d o n th e t h r e e b o d y m o d e l . T h e i r n u m e r i c a l e s t i m a t e s a r e s h o w n in F ig -u r e 1. T h e t h i n - d o t t e d l in e i s t h e c a l c u l a t e d c r o s s s e c ti o n o f t h e e x c l u si v e 1 2 C (7 , K + ) ~ 2 B .T h e t h i n - d a s h e d c u r v e i s t h e c r o s s s e c t i o n o f t h e Q F 1 2 C (7 , K + ) X p r o c e s s . T h e t h i n - s o l i dc u r v e is t h e s u m o f t h e s e t w o c o n tr i b u t io n s . T h e Q F A p r o d u c t i o n d o m i n a t e s t h e ( 7, K + )p r o c e s s i n t h e t h r e s h o l d r e g i o n . A r e d u c t i o n f a c t o r ~ 2 .2 w a s u s e d t o f it t h e d a t a . I t i sm a i n l y d u e t o t h e m e d i u m e f f e c t s o n 7 , K + a n d A . W h e n w e u s e t h e e x p e r i m e n t a l v a l u eo f t h e t o t a l 7 N a n d K + N cross sec t ion , t he r edu c t io n f ac to r bec om e R ~ 1 .6 . I t sugg es tt h a t t h e n u c l e a r m e d i u m e f f e c t s o n t h e A p r o p a g a t i o n i n n u c l e i m u s t b e s i g n i f i c a n t .

A n o t h e r n u m e r i c a l a p p r o a c h i s a n i n t r a n u c l e a r c a s c a d e m o d e l. J1 0 ] I t d e s c r ib e s t h e 7 + Ar e a c t i o n s i n t h e f r a m e w o r k o f a tw o - s t e p c a s c a d e p r o c e s s . T h e f ir s t s t e p i s a r a p i d p r o c e s s .T h e i n t r a n u c l e a r c a s c a d e d e v e l o p s t h r o u g h t h e b i n a r y c o ll is io n s . T h e s e c o n d s t e p i s t h ed e c a y p r o c e s s of t h e e x c i t e d n u c le u s . T h e y i n c lu d e b o t h t h e m e s o n p r o d u c t i o n c h a n n e la n d m u l t i c o l li s i o n a l i n t r a n u c l e a r c a s c a d e p r o c e s s . T h e g e n e r a l b e h a v i o r o f t h e c a s c a d ec a l c u l a t io n i s i n g o o d a g r e e m e n t w i t h T A G X d a t a . T h e s u b t h r e s h o l d c r o s s s e c t io n c a n b ee x p l a i n e d b y F e r m i m o t i o n a n d e f f ec t iv e m a s s o f b o u n d n u c l e o n s .

2 . 2 . K ~ p h o t o p r o d u c t i o n a t T A G XT h e K ~ p h o t o p r o d u c t i o n o n 3 H e w e r e m e a s u r e d u s i n g t h e T A G X s p e c t r o m e t e r . T h e

i n v a r i a n t m a s s s p e c t r u m c o n s t r u c t e d f r o m t w o p i o n e v e n t s i s s h o w n i n F i g u r e 3 , w h e r et h e p h o t o n e n e r g y r a n g e is a b o v e t h e r e a c t i o n t h r e s h o l d f or th e e l e m e n t a r y K ~ p h o t o -p r o d u c t i o n a n d t h e v e r t e x c u t w a s e m p l o y e d o u t s i d e o f t h e t a r g e t c e ll . W e c a n i d e n t i f yt h e K ~ s i g n a t u r e a t ~ 0 .5 G e V / c 2 i n t h e i n v a r i a n t m a s s s p e c t r u m . I n o r d e r t o e s t i-m a t e 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 , t h e T A G X M o n t e C a r l o w a s u s e d t o d e t e r m i n e t h ed e t e c t i o n e ff ic ie nc y.[ 5] W e o b t a i n e d t h e c r o s s s e c t i o n a = 0 . 6 9 # b f o r K ~ p h o t o p r o -du c t io n on 3H e. Th ere f o re th e a~ ,/~o bec om es to ab ou t 1 .4pb , w h ich i s s imi l a r t o a~ ,/~+at E~ ~, , 1 GeV.T h e d i f f e re n t i a l c r o s s s e c t io n i s s h o w n i n F i g u r e 4 . A l t h o u g h t h e K + c r os s s e c t io n s a r es t r o n g l y p e a k e d i n f o r w a r d d ir e c ti o n , t h e T A G X d a t a d o e s n o t s ho w t h e f o r w a r d p e a k in g .I t is u n d e r s t o o d b y a l a ck o f t h e t - c h a n n e l d i a g r a m i n t h e e l e m e n t a r y K ~ p h o t o p r o d u c t i o n .T h e c u r v e s i n F i g u r e 4 a r e t h e n u m e r i c a l c a l c u l a t i o n u s i n g a c o m p u t e r c o d e p r o v i d e d


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Figure 3. Invar iant mass spe ctrum con-stru cted f rom two pion events. Theshadow indicates the K ~ events a t M0 .5Ge V /c 2.

Figu re 4. Differential cross section foraHe(7, K~ Lines show the numerical est i-mates with dif ferent models.

by Sotona .[13] Input pa ram ete rs for the p roduc t ion ampl i tudes a re obta ined f rom K +ph oto pro du ctio n models.[14-16] I t is repo rted t ha t the pred icted (7, K +) cross sect ionusing these dif ferent models show rather similar behavior in the 1 GeV energy region.In Figure 4, dif ferent model predict ions denoted by AS1114], C4115] a n d AW4[16] arecom pared w i th exper im enta l da ta . The numer ica l va lues of the (7, K~ c ross sec tions a res t rongly mo de l depen dent in cont ras t to the K + photoproduc t ion .

3. K A O N P H O T O P R O C U C T I O N W I T H P O L A R I Z E D P H O T O N B E A M SIn the A produc ing kaon photoproduc t ion , a sp in 0 kaon is photoproduced on a sp in

1/2 nucleon leading to a spin 1/2 A. There are e ight possible spin sta tes in the system.There fore the e lementa ry kaon photoproduc t ion process can be expressed wi th four in-dep end ent amp li tudes. Th e defined exper im ental observables in (7, K ) are differentia lc ross sec t ions , three s ingle pola r iza t ion asy mm etr ies of P( recoi l) , E(b eam ) and T ( ta rg e t ) ,and twelve double polar izat ion asymmetr ies. I t is suff ic ient to measure e ight observablesamong them to comple te the da t a se ts for the ana lys is of e lementa ry N(3 ', K) A processes.The Laser -Elec t ron-Photon fac i l i ty a t SPr ing8 provides a lmost 100% pola r ized photonbeams.[4] The photon beams are tagged in an enegy range from 1.5 to 2.4 GeV with theenergy spread of 15 MeV. The average intensi ty is 107/see which is l imited by the l ightsource operat ion. A large acceptan ce charged par t ic le spectrom eter will be comple ted in2000. In this condit ion, we have a chance to meas ure the e lem entary A producin g kaonphotoproduc t ion observables , da/dFt, P, E and T in an energy range of E~ = 1 . 5 - 2 .4GeV. I t must be the most impor tant informat ion to s tudy the kaon photoproduc t ion onnuclei.


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4 . S U M M A R YWe m easu red th e differential cross sections of th e 12C(~/, K + ) and 3H e(7, K ~ reactio n in

the threshold region at 1.3-GeV Tokyo electron synchrotron laboratory. Th e 12C(7 K +)cross sections are comp ared with numerical es t ima tes and theoretical interpretations. Wefound tha t the accurate s tudy of the elem entary (7, K) process and nuclear mediumeffects of 7, kaons, A propag ation mu st be im portan t for the future investigation of kaonpho toprod uctio n on nuclei. We noted tha t the me asurements of the spin observables in(7, K) is very effective to unde rstand the kaon photoproduction. They can be done atnew Laser-Ba ckw ard-Com pton facili ty in the near future. They will give us impo rtantinformation to s tudy the A-nucleus interaction and th e s truc ture of the A -hypernuclei .R E F E R E N C E S1. S. S. Hsiao and S. R. Cotan ch, Phy s. Rev. C28, (1983 ) 1668, M. Sotona, e t a l . A 5 4 7

(1992) 63c, and C. Bennhold, Nuc l. Phys. 5 47 (1992) 79c.2. O.H ashim oto, S.Ajimura, K.Aoki, H.Bha ng, T .End o, Y .Fujii, O .Hotchi,E.Hungerford, J .H.Kim, Y.D.Kim, T.Kishimoto, K.Koshino, K.Kubota, K.Maeda,T.Nagae, H.Noumi, Y.Ohta, K.Omata, H.Outa, H.Park, Y.Saito, T.Saito, Y.Sato,M.Sekimoto, T.Shibata, T.Takahashi, T.Tamagawa, H.Tamura, L.Tang, H.Tanita,M.Youn, Nucl.Phys. A 62 9, (1998) 405c and references therein.3. C .B . Dover, Nucle. Phys. A 54 7 (1992) 27c.

4. T.N akano , H.Ejiri, M.Fujiwa ra, T.H otta , K.Takanashi, H.Toki, S.Hasegawa, T.Iw ata,K.Okamoto, T.Murakami, J .Tamii , K.Imai, K.Maeda, K.Maruyama, S.Date,M .M.Ob uti , Y.Ohashi, H.Ohku ma, N.Kum agai, Nucl.Phys. A62 9, (1998) 559c5. TAG X Collaboration: Nucl. Instrm and Methods A 37 6 (1996) 335.6. K.Maeda, e t a l , Nuclear Physics A5 77 (1994) 227c, and H .Yamazaki,, e t a l , Physical

Review C 52, Rl157 (1995).7. J. Mougey, M. Bernheim , A. Bussi~ re, A. Gillebert, Ph an X uan HS, M Priou, D.Royer, I. Sick and G. J. Wagn er, Nucl. Phys. A 26 2, 461 (1976).

8. T.S. L ee , Z.Y . Ma, B. Saghai and H. Toki, Phys. Rev. C58 (1998) 1558.9. E. Ya. Parye v, private com mu nication.10. S. de Pina, E. C. de Oliveira, E. L. Medeiros, S. B. Duarte and M. Gonalves, PhysicsLet ters B4 34 (1998) 1.11. J. C. David, C F ayad, G. H. La m ot and B. Saghai, Phys. Rev. C 53 (1996) 2613.12. Z. Ma, J. Sp eth, S. Krewald, B. Chen, and A. Ruber, N ulc. Phys. A 60 8 (1996) 305.

v13. M. Sotona, K. Itonag a, T. M otoba, O. Richter and J. Zofka, Nucl. Phys. A 54 7, 63c(1992), and M. Soton a and S. Frullani, Prog. Theor. Phys. 117 (1994) 151.14. R. A. Adelseck and B. Saghi, Phys. ReV. C42 (1990) 108.15. R. Williams, C h. R. Ji and S. R. Cotanch, Phys. ReV. C 46 (1992) 1617.16. R. A. Adelseck and L.E. W right, Phys. ReV. C3 8 (1988) 1965.

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Kaon Photoproduction on Nuclei