17-MESON ELECTROMAGNETIC STRUCTURE

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1 I N S T I T U T E F O R H I G H E N E R G Y P H Y S I C S R. I.D z h e ly a d in, S.V.G o lo v k in, V.A.K a ch a n o v, A.S.K o n s ta n tin o v, V.F.K o n s t a n t in o v, V.P.K u b a r o v s k i, A. V. K u lik, L.G.L a n d s b e rg, V.M.L e o n t ie v, V.A.M u k h in, V.F.O b r a z ts o v, T. I. P e t r u n in a, Y u.d.p r o k o s h k in, V. A.V ik t o r o v, A.M.Z a its e v INVESTIGATION OF 17-MESON ELECTROMAGNETIC STRUCTURE IN Tj - n+fi у DECAY Serpukhov 1980

2 R.I.D z h e ly a d in. S.V.G o lo v k in, V.A.Kachanov, A.S.K o n s ta n tin o v, V.F.K o n s ta n tin o v, V.P.K u b a ro v s k i, A.V.K u lik, L.G.La n d sb e rg, V.M.L e o n tie v, V.A.M u khin, V.F.O b ra z ts o v, T. I. P e t r u n in a, Y u.d.p ro k o sh k in, V.A.V ik to r o v, A.M.Z a its e v INVESTIGATION OF 17-MESON ELECTROMAGNETIC STRUCTURE IN r\ - n+fi~ у DECAY Subm itted to Phys. L e t t, and Yad. F i z.

3 M - 24 Abstract Dehelyadin R.I., Golovkin S.V., Kachanov V.A., Konstantinov A.S., Konstantinov V.F., Kubarovskl V.P., Kulik A. V., Landsberg L.O., Leontiev V.M., Mukhin V.A., Obraztsov V.F., Petrunlna T.I., Prokoshkin Yu.D., Viktorov V.A., Zaitsev A.M. Investigation of >7-Meson Electromagnetic Structure la у Decay. Serpukhov, p. 9. (IHKP 80-58,SERP-B-134). Refs events of rare electromagnetic decay ij -p ц~y have been detected. They' branching ratio has been neaaured, BR ( r; * р*р~у) (3, ) <10"4. The i)x meson electromagnetic form-factor F( : 0) (1 - /Л2 has been defined, where Л» ( ) GeV/ca, which Is in a good agreement with the vector aeaon dominance. v Анвэтаиа Векторов B.A., ГЪловяш С.В., Джелядкв Р.И., Зайцев AiM,, Качанов В.А., Константинов А.С., К онстантинов В.Ф., КубаровсккЯ В.П., Кулпе А.В., Ландсберг Л.Г., Леонтьев В.М., Мухин В.А., Образцов В.Ф., Петрунина Т.И., Прокошкин Ю Д Исследование электромагнитной структуры» -мезона в распаде т) - ц V*У Серпухов, стр. с рас. (ИФВЭ 80-58,SKRP-E-134). Баблаогр Зфегкстрнроваю 000 событий редкого электр омагнятного распада Ч М м У. Измерена его относительная вероятность: BR (я у) ~ (3.1 1,0,4 )'1 0 ^. Определен електромагнитныа формфактор ч-мезона F( ; 0) - (1 -щ ^/А 2 ) *, где А - (0,72 ±.0,00) ГаВ/с2, что хорошо согласуется с моделью векторной доминантности.

4 In th e p re s e n t w ork we c o n tin u e d th e a tu d y o f th e r a r e e le c tr o m a g n e tic decay V * * H + H У (1) o b se rv e d e a r lie r ^ * ^. The f i n a l aim o f t h i s s tu d y was t o o b t a in i n fo r m a tio n on rj-m e so n e le c t r o m a g n e t ic s t r u c t u r e. The e x p e rim e n ts w ere p e rfo rm e d w it h th e " L e p t o n - G n s e t- u p t h a t a llo w e d t o d e t e c t e f f i c i e n t l y th e p ro c e s s e s w it h s im u lta n e o u s p r o d u c t io n o f muon p a i r /1 2/ and p h o to n s *. The s e t- u p c o n s is t e d o f in c id e n t p a r t i c l e d e t e c t o r s, th e t a r g e t ( L ih ) w it h g u ard.co u n te rs, p r o p o r t io n a l cham bers t o m easure p a r t i c l e s c o o r d in a t e s im m e d ia te ly a f t e r th e t a r g e t. To m easure th e c o o r d in a t e s and e n e r g ie s o f p h o to n s th e GAMS-type ho- d o sco p e s p e c tro m e te r w it h 64 le a d - g la s s c o u n te r s was u sed. Muon p a i r d e t e c t io n and m easurem ent o f t h e i r momenta w ere p e rfo rm e d by a w id e - a p e r t u r e m a g n e tic s p e c tro m e te r. B inary re a c tio n я~ Р * Tffl Ш 3

5 was used as 77-m eson s o u rc e in th e same way a s e a r l i e r. I t p r o v i ded r a t h e r fa v o u r a b le background c o n d it io n s f o r m easurem ents^1^. The e x p e rim e n ts w ere c a r r ie d o u t in th e 33 G ev/c seco n d a ry beam o f 6 th e 70 GeV IHEP a c c e le r a t o r. The beam i n t e n s i t y was (3-4 )-1 0 p e r c y c le. D u rin g th e ru n s th e w hole s e t- u p, and, in p a r t i c u l a r, i t s s p e c t r o m e t r ic ele m e n ts w ere r e p e a te d ly c a lib r a t e d. The r e s u lt s o f new e x p e rim e n ts have been com bined w it h th e e a r l i e r ones^ 1^. D u rin g a l l th e m easurem ents th e t o t a l e f f e c t i v e f l u x o f p io n s passed th ro u g h th e s e t- u p, w h ich c o rre s p o n d e d 7 t o th e p r o d u c t io n o f 2*10 ^-mesons in r e a c t io n ( 2). The d a ta a n a ly s is p ro c e d u re was a n a lo g o u s t o t h a t d e s c r ib e d / 1. 3, 4/ e a r l i e r. P a r t i c u l a r a t t e n t io n was p a id t o s u p p r e s s io n o f th e b ackground p r o c e s s e s. The s e t- u p e f f i c i e n c y was th o r o u g h ly in v e s t ig a t e d. A t th e i n i t i a l phase o f th e r e c o n s t r u c t io n, th e e v e n ts w it h two h ig h - e n e rg y muons ( E Г > 4.5 GeV) were s e le c t e d in th e m a g n e tic s p e c tro m e te r. F o r su ch e v e n ts we se a rch e d f o r muon t r a c k s i n p r o p o r t io n a l cham bers in o r d e r t o r e c o n s t r u c t th e in t e r a c t io n v e r te x. The re q u ire m e n t f o r th e v e r te x was t o be w it h in th e s e t-u p t a r g e t. We r e j e c t e d th e e v e n ts f o r w h ich, b e s id e s two muon t r a c k s, t h e r e was a t le a s t one a d d i t io n a l t r a c k em erging from th e i n t e r a c t io n v e r t e x. F u r t h e r on th e s e a rc h was p erfo rm e d f o r e le c tro m a g n e tic show ers in th e y - s p e c tro m e te r and e v e n ts w ere s e le c t e d w it h one pho- 4

6 t o n (Ey > 1.5 G ev). We a ls o s in g le d o u t th e e v e n ts w ith o u t p h o to n s i n th e f i n a l s t a t e. They p r o v id e d th e d a ta on r e a c t io n jt p -» p(«u)n, (3) U ц+ц" w h ic h was used f o r th e c r o s s s e c t io n n o r m a liz a t io n. F o r th e f i n a l a n a ly s is we s e le c t e d th e e v e n ts c o r r e s p o n d in g t o th e n p-*p+it yn r e a c t io n and s a t i s f y i n g th e re q u ire m e n t E (E. +E _+Ev) < 3.5 GeV, where E - i s th e in c id e n t p io n n~ ц F ' energy. The e f f e c t i v e mass sp ectru m o f th e fi~y system i s g iv e n i n f i g. 1. The peak i n th e spectru m c o rre s p o n d s t o d e ca y ( 1 ). I t c o n t a in s about 600 e v e n ts, th e b ackground l e v e l i s n o t h ig h e r th a n 8%. Fig. 1. I1 (i- у - maae spectrum. Peak corresponds to decay (1), The arrow points to the table value for ij mass. Thin histogram is for a ll events. Thick histogram is for the events with mjj < 0.24 GeV, used to define form-factor. 5

7 F o r direct evaluation of the branching ratio for decay (1) BR (q -* /х+/гу) = Г(ту ц+ у) / Г( i\ -* a l l ), we n o rm a liz e d f o r th e / 5 7 / number o f e v e n ts in r e a c t io n ( 3 ). In t h i s we used th e d a ta on th e c r o s s - s e c t io n s f o r r e a c t io n s (2) and (3) and th e v a lu e f o r /8/ BR[p(<u) -* p ft~ ] «= 4,6 ( 7.6 ) The b ra n c h in g r a t i o f o r dec a y (1 ) i s o b ta in e d to b e e q u a l t o BR(jj -* ц+{гу) = ( ) «10? The in d ic a t e d e r r o r was m a in ly due t o th e u n c e r t a in t ie s in e x p e r i- / 5-8 / m e n ta l d a ta Muon p a i r sp ectru m i n d e ca y (1) i s d e s c r ib e d w it h th e fo rm u la 4Г(т,» у) 2 g Г (т г у у ) (1 )3( 1+ ;0) lf(4 ) J j 3 * m 2 m 2 m 2 m2 W* йтцц 4 «* W* H ere m. i s th e muon p a i r m ass, m and m a re t h e m asses o f ЦЦ r ц 1} muon and 17-m eson, r e s p e c t iv e ly, F(m2 ; O) *= F( q^=m2 ; q2 =0) г ' l ^ 2 i s th e f o r m - f a c t o r o r th e v e r t e x f u n c t io n o f rj meson. I t i s par a m e t r iz e d u s u a lly in th e p o le a p p ro x im a tio n by: F(m2 ; 0). (I -m 2 / A2 f*. (5) Decay (1) i s d i r e c t l y lin k e d w it h th e ij-*yy d e c a y. One o f th e p h o to n s in d eca y ( 1 ) i s a v i r t u a l one and le a v e s th e mass s h e l l w h ic h a llo w s t o s tu d y th e dependence o f th e 17 f o r m - f a c t o r on th e t im e - l i k e momentum sq u a re d q2» ity^ [ fo r r/ *у у F(q2 =0 ; q 2 =0 ) =1]. F i g. 2 p r e s e n ts th e e x p e r im e n ta l d a ta on th e fo r m - f a c t o r s q u a re d F (m2 ; 0). F o r th e s e ^ ^ e g fvwts Si Ш Ъ Щ 6

8 Fig. 3. Data on ij-meson electromagnetic forefact or. Point* are experimental values for F (m* ; 0). Curve is the resu lt of fittin g with the dependence of the type К (1 -m 2 /Л 2 J -3, where Л - ( O.OfirTGeV/c2, co efficien t К takes into- account the experimental normalisation uncertainty. w ere d e f in e d a s th e r a t i o o f th e m easured sp e ctru m dn/dm^ г t o th e sp e ctru m, w h ic h was M o n te - C a rlo c a lc u la t e d u s in g fo rm u la (4) w it h F=1 and w it h a c c o u n t f o r th e a e t-u p e f f i c i e n c y. The c a lc u la te d sp e ctru m was n o rm a liz e d f o r th e t o t a l number o f q mesons, p ro d u ce d i n th e s e t- u p t a r g e t in r e a c t io n ( 2 ). The e x p e r im e n ta l d a ta on th e fo r m - fa c t o r w ere f i t t e d w it h th e p o le fo rm u la ( 5 ). The c h a r a c t e r i s t i c mass Л was fo u n d t o be e q u a l t o A = ( ) G e V /c2. (6) The s lo p e o f th e fo r m - f a c t o r a t = 0 i s df/dq* =* A 2 = s ( ) GeV с. Thus v a lu e i s i n a good agreem ent w it h th e p r e d i c t i o n s b ased on th e v e c t o r meson dom inance (VMD)^10^: 7

9 л " 2 * П т - 2 = 1,8 Gev"2c 4. VMD Р The b ra n c h in g r a t i o f o r decay (1) may be d e fin e d from o u r d a ta on 17 fo r m - f a c t o r w it h h ig h e r p r e c is i o n th fln in d i r e c t normal i z a t i o n on r e a c t io n ( 3 ), as was done a b o v e. U s in g th e experim e n t a l v a lu e s f o r th e c h a r a c t e r i s t i c mass A ( 6 ) and fo rm u la e (4) and (5) we o b t a in BRir, = ( ) (7) The r e s u lt s o f a l l m easurem ents o f t h e fo r m - fa c t o r s lo p e f o r n - and 77- o b ta in e d i n th e s t u d ie s o f th e d e ca y s я -»е+ e" у / И ~ 1 4 / tj -»e+e 'y and ( p re s e n t w ork) a re shown i n f i g. 3. A s i t i s c le a r fro m t h i s f i g u r e o n ly th e m easurem ents o f f o r n Fig. 3. Measurement results on electromagnetic form-factor slope of pseudoscalar mesons d F / d a 2 I 2 >, i, =0. «О * -. /11-14/ a is for n - у decay, O i s for i)-»e+ e y decay^* is for 1/ - p + ii~ у decay (present work). and o u r d a ta on rj have a p r e c is io n s u f f i c i e n t f o r th e s tu d y o f th e e le c t r o m a g n e t ic s t r u c t u r e o f n e u t r a l p s e u d o s c a la r m esons. I t s h o u ld be n o te d t h a t th e e x p e rim e n ts s t u d y in g th e d e ca y s w it h muon p a i r s in th e f i n a l s t a t e have o b v io u s a d v a n ta g e as compared w it h 8

10 th e p ro c e s s e s where e le c t r o n p a ir s a re e m itte d. T h is i s because o f r a d ia t iv e c o r r e c t io n s, w h ich a re in e s s e n t ia l f o r muon p a ir s, w h ile 2 in th e ca se o f e le c t r o n p a ir s they change tw ic e th e v a lu e f o r A. In c o n c lu s io n th e a u th o rs would l i k e to e x p re ss t h e ir g ra t it u d e to V.M.Budnev, G.V.E fim o v, A. T. F ilip p o v, A.Y u.k h o d za m iria n, E.N.P e rv u s h in, V.M.S h e khte r, M.K.V o lk o v f o r f r u i t f u l l d is c u s s io n s. R E F E R E N C E S 1. Y u.b.b u sh n in e t a l. Yad, F i z., 28, 1507 (1978); Phys. L e t t., 79B, 147 (1978). 2. Y u.b.b u sh n in e t a l. P r e p r in t IHEP 80-57, V.A.V ik to r o v e t a l. Yad. F i z., 29, 1513 (1979); P h y s. L e t t., 84B, 143 (1979). 4. V.A.V ik to r o v e t a l. P iz 'm a v JETPh, 30, 387 (1979). 5. О.I.D a h l e t a l. Phys. Rev. L e t t., 38, 54 (1977). 6. W.D.Apel e t al.. Yad. F i z., 29, 1519 (1979); N u c l. P h y s., B152, 1 (1979). 7. CERN/HERA C o m p illa tio n, 72-1, 1972; E sta b ro o k s e t a l. N u c l. P h y s., Ш, 70 (1974). 8. C.Bricm an e t a l. Rev. P a r t. P ro p. Phys. L e t t., 75B, 1 (1978), 9. R. H. D a lit z. P ro c. Phys. Soc. (London), A64, 667 (1951); N.M.K r o ll, W.Wada. Phys. R e v., 98, 1355 (1955). 10. M.G e ll Mann e t a l. Phys. Rev. L e t t., _8^ 261 (1962); C.Q uigg, J.D.J a c k s o n. UCRL (1968); V.M.Budnev, V.A.K arp ako v. P iz 'm a v JETPh, 29, 439 (1979); A.I.Iv a n o v, V.M.S hekher. Yad. F i z., 31, 530 (1980). 11. N.P.S am ios e t a l. Phys. R e v., 121, 275 (1961). 12. H.Kobrak e t a l. Nuovo Cim ento, 20, 115 (1961). 13. S.Devons. Phys. R e v., 184, 1356 (1969). 14. J. F is h e r e t a l. Phys. L e t t., 73B, 359 (1978). 15. A.K o tle w s k i. P r e p r in t Colum bia U n iv e r s it y (1973). 16. M.R.Ja n e e t a l. Phys. L e t t., 59B, 103 (1975). Received 8 May,

11 Институт физики высоких анергий, Издательская группа И Ф В Э. Заказ 494. Тираж ,4 уч.-иэд.л. Т Май, Редактор А.А.Антипова.