- Email: [email protected]
c K−p interactions
PHYSICS
Volume 27B, number 8
NEUTRAL
NON-STRANGE IN 5.5 GeV/c R. DAVIS, Northwestern
LETTERS
16 September
MESONS NEAR 960 K-p INTERACTIONS
MeV *
1968
PRODUCED
R. AMMAR and J. MOTT
University,
Evanston,
Illinois,
USA
S. DAGAN 1, M. DERRICK and T. FIELDS t$ Argonne
National Laboratoy,, Received
Argonne,
Illinois,
USA
27 July 1968
A peak observed near 1 GeV in the 7~‘71-71’mass spectrum in events fitting the reaction K-p - A8’r-i~~ is found to arise from the decay of the X0(959) into i7”?‘-7 rather than the Ho. The decay is predominantly via pay and the decay angular distribution of the p” agrees well with the sin20 expectation. A branching ratio X0 -+POY = 0.31 + 0.15 is measured. X0 - nng
Two neutral non-strange mesons (X0 and Ho) have been reported with masses near 960 MeV [1,2]. We present the results of a search for and a study of these mesons in reactions of the type K- + p -A + ?T++ 7r- + neutral(s). The experiment is based on an analysis of 400000 pictures taken at an incident beam momentum of 5.5 GeV/c in the 30-inch ANL hydrogen bubble chamber, corresponding to - ‘7 events/pb. Further details on the exposure and analysis may be found elsewhere [3-51. The nfn-7] mass spectrum for events fitting the hypothesis An+n-77 shows a narrow peak near 960 MeV, arising from the decay X0 - n+n-q as may be seen in fig. l(a). An enhancement is also observed in the ~+~-~” invariant mass distribution at a mass of about 1 GeV, for events fitting the hypothesis An+n-r”. The p”no mass distribution is enhanced around 1 GeV, as compared with the p-r+ and p+n- channels t. This latter final state is particularly sensitive to the presence of p+ from decay of a higher resonance, because a p+ cannot be produced directly in a single meson exchange interaction from an incident Kmeson. An I = 0 meson such as the proposed H meson [2] would decay with equal relative rates * Work supported by the National Science Foundation and the U. S. Atomic Energy Commission. % Presently at the University of Tel-Aviv, Tel-Aviv, Israel. Tf Also at Northwestern University. t It must be remembered that the three p bands all lie in the s%-s~ Dalitz plot for the r+871° mass in the region of 1 GeV [e.g. 61.
532
25
E 2
0.4 0
IL
1.1
I.4
1.7
21)
0.6
0.6
M (n+rr-1.
2.3
LO GeV
2o-(b) IO-
o-
:
0.6 M
0.8
4.0
4.2
(r+n-y) ,GcV
MM’
from
A a+a-.GeV’
Fig. 1. (a) 7J+7J-?j mass distribution for 571 events of the reaction K- + p -f A + a’+ + 7’r- + 77, where the missing mass recoiling from the An%- system lies in the range 0.5 -0.6 GeV. (b) i’r+?‘r-y mass distribution for 337 peripheral events of the reaction K- + p + A + ?lf + ?‘r- + y. A peripheral event is taken to be one in which the cosine of the angle between the target proton and the A direction in the K-p center of mass, lies between 0.8 and 1.0. The ef0.640 C iW(?T’ir-) s 0.880 GeV, fect of requiring ap’, is shown shaded. (c) i~f- mass distributions for 34 events in (b) with 0.938 C M(7’r+?l-r) C 0.978 GeV. (d) Distribution of the missing mass squared (MM2) recoiling from the ATI%- system for 31 of the events in (c). The effect of requiring ape as in (b) is shown shaded. (e) MM2 distribution for 51 events of the reaction K- + ; p - A + nc + T- + 7f” with 0.763 C M(?~+lr-i7’) C 0.803 GeV.
Volume 27B. n u m b e r 8
PHYSICS
i n t o p + ~ - , pOlO, p - ~ + c o n t r a r y to o u r o b s e r v a t i o n . On t h e o t h e r h a n d , s i n c e s u c h e v e n t s a r e g e n e r a l ly a m b i g u o u s w i t h t h e A ~ + ~ - y h y p o t h e s i s , t h e e n h a n c e m e n t m a y a l s o r e p r e s e n t t h e ~ + ~ - y d e c a y of t h e X °, i n w h i c h t h e m i s s i n g :~ h a s b e e n m i s i n t e r p r e t e d a s a ~o. In o r d e r to i n v e s t i g a t e t h i s a m b i g u i t y f u r t h e r , all events with a mass recoiling against the A in t h e r a n g e 660 to 1260 M e V , w e t r i e d f o r t h e h y p o t h e s i s A~+~-~ '. T h o s e e v e n t s w h i c h f i t t h e 4 c o n s t r a i n t h y p o t h e s e s A~+~ - a n d AK+K - w e r e removed from the sample. Fig. l(b) shows the ~+~r-y m a s s d i s t r i b u t i o n f o r t h e r e m a i n i n g e v e n t s w h i c h f i t t h e A~+~r-y h y p o t h e s i s a n d f o r w h i c h t h e A is peripherally produced. The enhancement at 1 GeV is sharper when the missing neutral is i n t e r p r e t e d a s a y. T h e ~ + ~ - m a s s d i s t r i b u t i o n f o r a l l e v e n t s in t h i s e n h a n c e m e n t i s g i v e n i n fig. l ( c ) a n d s h o w s a p e a k a t t h e pO m a s s . T h e e f f e c t on t h e ~r+~-y m a s s d i s t r i b u t i o n of r e q u i r i n g t h e ~ + u - m a s s t o l i e i n t h e pO r e g i o n i s s h o w n s h a d e d in fig. l ( b ) . H e r e t h e b a c k g r o u n d h a s b e e n d r a s t i c a l l y r e d u c e d , w h i l e t h e p e a k a r o u n d 960 M e V h a s b e e n r e d u c e d f r o m ~ 20 e v e n t s a b o v e b a c k g r o u n d to ~ 14 e v e n t s a b o v e b a c k g r o u n d . T h e enhancement is now very narrow, consistent with our mass resolution. M a n y of t h e e v e n t s s h o w n in fig. l ( b ) r e a l l y b e l o n g to t h e f i n a l s t a t e A~+~r-~ ° a l t h o u g h t h e y give a satisfactory fit to the A~+~-y hypothesis. T h i s a m b i g u i t y w a s i n v e s t i g a t e d by e x a m i n i n g t h e m i s s i n g m a s s r e c o i l i n g f r o m t h e Au+~ - s y s t e m a s s h o w n in fig. l ( d ) f o r t h o s e e v e n t s w i t h t h e m a s s of t h e m e s o n s y s t e m a r o u n d 960 M e V . T h e e f f e c t of r e q u i r i n g a pO i s s h o w n s h a d e d . T h o s e e v e n t s l y i n g i n t h e X ° r e g i o n , w h i c h do n o t h a v e a pO, have a broad missing mass distribution recoiling from the A~+Tr- system, (the unshaded events in fig. l(d)), which differs from the case where a pO is required. For events lying in the X° region and having a ~+~r- invariant mass appropriate to the pO, the hypothesis A~+Tr-7 is clearly preferred, whereas for the neighboring regions the missing mass is in better agreement with a 7r°. For comparison fig. l(e) shows the missing mass for those events fitting the hypothesis A~+Tr-~° with the ~+~-7~° mass lying in the ¢o region. The distribution peaks near the mass of the ~o, as expected. The cosine of the production angle in the K-p center of mass lies in the range 0.9 - 1.0 for two classes of events: (a) X° ~ ~r+~-~ events from the final state A?r+~-~7, and (b) X° ~ pOy. Events from the final state A~+~-V having the ~r+~r-7 invariant mass of the X° but with the y+y- mass not in the p band are less sharply peaked and lie
LETTERS
16 September 1968
i n t h e r a n g e 0.8 - 1.0. A s i m i l a r s i t u a t i o n o b t a i n s f o r e v e n t s f r o m a c o n t r o l r e g i o n in w h i c h t h e ~ + ~ - y m a s s l i e s on e i t h e r s i d e of t h e X °. T h i s supports the conclusion reached from missing m a s s p l o t s , n a m e l y , t h a t in t h e f i n a l s t a t e A~+~-T w e h a v e a c l a s s of e v e n t s r e p r e s e n t i n g t h e d e c a y X ° ~ p°T. From the preceding considerations we conc l u d e t h a t t h e e n h a n c e m e n t a t 960 M e V s h o u l d b e identified with the X ° rather than the H° meson. T h e D a l i t z p l o t f o r t h e X ° ~ po~ d e c a y m o d e shows the depopulation at the kinematic boundaries consistent with the prediction for the dec a y of a 0 - p a r t i c l e v i a a n M1 t r a n s i t i o n . T h e Dalitz plot for the remaining events (without a pO b u t w i t h t h e ~ + ~ - ~ m a s s i n t h e X ° r e g i o n ) d o e s not exhibit such a depopulation at the kinematic boundaries. F o r t h e d e c a y m o d e X ° -~ t h e po p o l a r i zation is fixed since the y has helicity + 1 along i t s d i r e c t i o n of p r o p a g a t i o n . T h i s g i v e s f o r t h e d e c a y a n g u l a r d i s t r i b u t i o n of t h e pO in i t s own r e s t f r a m e , r e f e r r e d t o t h e d i r e c t i o n of t h e r e -
pOT,
c o i l i n g ~, a n a n g u l a r d i s t r i b u t i o n
iY~ 1
sin2~)
1: Such a distribution could also be produced by pO decay following pO~.O S-wave decay of the H ° produced by K* exchange.
5 {a)
,
I 50
i
25 O'
o
-t.1
0
I.o - L o
J
o COS 8
'
I.o
0 ILO
0
1.0
7 ÷
Fig. 2. The p o decay angular distribution, defined as shown, in the g*~- c e n t e r of m a s s for: (a) 17 p e r i p h e r a l events of the reaction K- + p ~ A + * ~* + ~'- + ¥ with the effective ~÷Y-T m a s s lying in the X ° region and with the effective g ~ ' - m a s s lying in the pO region. 0.640-0.880 GeV. (b) 17 p e r i p h e r a l events of the reaction K- ÷ p ~ A ~ ~ + ~'- + y with the effective ~'+~-y m a s s lying in the X ° region but with the effective Tf+~"- m a s s lying outside the po region. (c) 281 p e r i p h e r a l events of the reaction K- + p ~ A + y~ ~ ~'- * }~ with the effective ~÷~-~ m a s s lying in the control region on e i t h e r side of the X °, in the range 0 . 7 - 0 . 9 3 8 GeV or in the range 0.978 -1.216 GeV. 533
Volume 27Bo number 8
PHYSICS
T h e e x p e r i m e n t a l d i s t r i b u t i o n i s s h o w n in f i g . 2 (a), t o g e t h e r w i t h a n o r m a l i z e d s i n 2 0 c u r v e , w h i c h i s s e e n t o g i v e a g o o d fit t o t h e d a t a . T h e c o r r e s p o n d i n g d i s t r i b u t i o n s f o r the e v e n t s in the X° r e g i o n without a po, and for n e i g h b o r i n g r e g i o n s , a r e s h o w n i n f i g . 2(b~, (c), r e s p e c t i v e l y . T h e s e do n o t e x h i b i t a sin~O d e p e n d e n c e . W e e s t i m a t e t h e r e a r e 14 e v e n t s w i t h X ° -* p O w i t h o u r pO s e l e c t i o n . W e h a v e 31 e v e n t s w i t h X ° ~ 7r+~-~ a n d ~? ~ a l l n e u t r a l s . U s i n g t h e m o s t r e c e n t v a l u e s [6] f o r 77 b r a n c h i n g r a t i o s , a n d c o r r e c t i n g f o r t h e w i d t h of t h e pO s e l e c t i o n u s e d , w e obtain a value
LETTERS
2.
X O __~pO~ R
-
-
X 0 ~
0.31 :~ 0.15
.
~?T?7
Using this branching ratio one obtains a total c r o s s s e c t i o n f o r t h e r e a c t i o n K - p --" A X ° of 20 ~ 4 ~ b a t an i n c i d e n t m o m e n t u m of 5.5 G e V / c .
3.
T h i s e x p e r i m e n t would not have been p o s s i b l e w i t h o u t t h e h e l p of t h e ZGS a n d b u b b l e c h a m b e r c r e w s , a s w e l l a s t h e c a r e f u l w o r k of o u r s c a n ners and measurers at Argonne and Northwestern. T o a l l of t h e m w e e x t e n d o u r t h a n k s . 4.
References 1. G.R. Kalbfleisch, L.W. Alvarez. A. B a r b a r o - G a l t i e r i , O.I.Dahl, P . E b e r h a r d , W . E . H u m p h r e y , J . S . Lindsey, D.W. M e r r i l l . J . J . Murray, A. Rittenberg, R. R. Ross. J . B . sharer. F . T . Shively, D.M. Siegel, G. A. Smith and R. D. Tripp, Phys. Rev. L e t t e r s 12 (1964) 527; M. Goldberg, M. Gundzik, J. L e i t n e r , M. P r i m e r , P. L. Connolly, E. L. Hart, K.W. Lai, G.W. London, N. P. Samios and S. S. Yamamoto. Phys. Rev. L e t t e r s 13 (1964) 249;
534
5.
6.
16 September 1968
G. R. Kalbfleisch, O.I. Dahl and A. Rittenberg, Phys. Rev. L e t t e r s 13 (1964) 349a; P. M. Dauber, W . E . Slater, L . T . Smith, D.H. Stork and H.K. Ticho, Phys. Rev. L e t t e r s 13 (1964) 449; J . B a d i e r , M. Demoulin, J . G o l d b e r g , B . P . G r e g o r y , C . P e l l e t i e r , A.Rouge. M.Ville, R. Barloutaud, A. Leveque, C. Louedec, J . M e y e r , P. Schlein, A. V e r glas, Do J. Holthuizen, W. Hoogland, J . C . Kluyver and A . G . T e n n e r , Phys. L e t t e r s 17 (1965) 337; G.W. London, R . R . Rau, N . P . Samios, S . S . Y a m a moto, M.Goldberg, S. Lichtman, M. P r i m e r and J . L e i t n e r . Phys. Rev. 143 (1966) 1034. G. Benson. E. Marquit, B. Roe, D. Sinclair and J . VanderVelde, Phys. Rev. L e t t e r s 17 (1966) 1234. This l e t t e r gives e a r l i e r r e f e r e n c e s to observations of the H meson in ~ p interactions. The only r e ported observation in KN interactions is in a paper p r e s e n t e d to the Heidelberg Conference, ~Evidence for a T= 0 t h r e e pion r e s o n a n c e at M ~ 990 MeV", G. B. Chadwick, Z . G . T . Guiragossian. E. Pickup, A. B a r b a r o - G a l t i e r i . M . J . Matison and A. Rittenberg. Details of the 7° beam at the ZGS are given in, T. H. F i e l d s , E. L. Goldwasser and U. E. Kruse, Design for a 6 GeV/¢ s e p a r a t e d K- beam. T H F / E L G / UEK-I. Argonne National Laboratory, 1961 (unpubli shed) ; R.Ammar. T.H.Fields. M.Derrick. E.L.Goldw a s s e r , M.L.Good. U . E . Kruse, D. Reeder, F. Schweingruber and J. D. Simpson, P r o c . 166 Intern. Conf. on Instrumentation for high energy physics, Stanford USAEC Conf. 660916, p. 620. Results on the final s t a t e s containing two charged p a r t i c l e s and a visible A are m o r e fully r e p o r t e d in, J. Mott, R. A m m a r , R. Davis, W. Kropac, D. Slate, B . W e r n e r . S. Dagan. M . D e r r i c k . T . F i e l d s , J. Loken and F.Schweingruber, submitted to Phys. Rev. Results f r o m the s a m e topology but where the neutral particle is a K ° have been previously r e p o r t e d in, F . S c h w e i n g r u b e r . M . D e r r i c k . T. Fields, D. Griffiths, L.Hyman, R . J . J a b b u r . J . L o k e n , R. Amm a r , R. Davis, W. Kropac and J. Mott, Phys. Rev. 166 (1968) 1317. A.H. Rosenfeld, N. B a r a s c h - S c h m i d t , A. B a r b a r o Galtieri, W . J . P o d o l s k y , L . R . P r i c e , M. Roos, P. Soding, W . J . W i l l i s and C.G. Wohl, UCRL-8030, r e vised September 1967.
Volume 27B, number 8
NEUTRAL
NON-STRANGE IN 5.5 GeV/c R. DAVIS, Northwestern
LETTERS
16 September
MESONS NEAR 960 K-p INTERACTIONS
MeV *
1968
PRODUCED
R. AMMAR and J. MOTT
University,
Evanston,
Illinois,
USA
S. DAGAN 1, M. DERRICK and T. FIELDS t$ Argonne
National Laboratoy,, Received
Argonne,
Illinois,
USA
27 July 1968
A peak observed near 1 GeV in the 7~‘71-71’mass spectrum in events fitting the reaction K-p - A8’r-i~~ is found to arise from the decay of the X0(959) into i7”?‘-7 rather than the Ho. The decay is predominantly via pay and the decay angular distribution of the p” agrees well with the sin20 expectation. A branching ratio X0 -+POY = 0.31 + 0.15 is measured. X0 - nng
Two neutral non-strange mesons (X0 and Ho) have been reported with masses near 960 MeV [1,2]. We present the results of a search for and a study of these mesons in reactions of the type K- + p -A + ?T++ 7r- + neutral(s). The experiment is based on an analysis of 400000 pictures taken at an incident beam momentum of 5.5 GeV/c in the 30-inch ANL hydrogen bubble chamber, corresponding to - ‘7 events/pb. Further details on the exposure and analysis may be found elsewhere [3-51. The nfn-7] mass spectrum for events fitting the hypothesis An+n-77 shows a narrow peak near 960 MeV, arising from the decay X0 - n+n-q as may be seen in fig. l(a). An enhancement is also observed in the ~+~-~” invariant mass distribution at a mass of about 1 GeV, for events fitting the hypothesis An+n-r”. The p”no mass distribution is enhanced around 1 GeV, as compared with the p-r+ and p+n- channels t. This latter final state is particularly sensitive to the presence of p+ from decay of a higher resonance, because a p+ cannot be produced directly in a single meson exchange interaction from an incident Kmeson. An I = 0 meson such as the proposed H meson [2] would decay with equal relative rates * Work supported by the National Science Foundation and the U. S. Atomic Energy Commission. % Presently at the University of Tel-Aviv, Tel-Aviv, Israel. Tf Also at Northwestern University. t It must be remembered that the three p bands all lie in the s%-s~ Dalitz plot for the r+871° mass in the region of 1 GeV [e.g. 61.
532
25
E 2
0.4 0
IL
1.1
I.4
1.7
21)
0.6
0.6
M (n+rr-1.
2.3
LO GeV
2o-(b) IO-
o-
:
0.6 M
0.8
4.0
4.2
(r+n-y) ,GcV
MM’
from
A a+a-.GeV’
Fig. 1. (a) 7J+7J-?j mass distribution for 571 events of the reaction K- + p -f A + a’+ + 7’r- + 77, where the missing mass recoiling from the An%- system lies in the range 0.5 -0.6 GeV. (b) i’r+?‘r-y mass distribution for 337 peripheral events of the reaction K- + p + A + ?lf + ?‘r- + y. A peripheral event is taken to be one in which the cosine of the angle between the target proton and the A direction in the K-p center of mass, lies between 0.8 and 1.0. The ef0.640 C iW(?T’ir-) s 0.880 GeV, fect of requiring ap’, is shown shaded. (c) i~f- mass distributions for 34 events in (b) with 0.938 C M(7’r+?l-r) C 0.978 GeV. (d) Distribution of the missing mass squared (MM2) recoiling from the ATI%- system for 31 of the events in (c). The effect of requiring ape as in (b) is shown shaded. (e) MM2 distribution for 51 events of the reaction K- + ; p - A + nc + T- + 7f” with 0.763 C M(?~+lr-i7’) C 0.803 GeV.
Volume 27B. n u m b e r 8
PHYSICS
i n t o p + ~ - , pOlO, p - ~ + c o n t r a r y to o u r o b s e r v a t i o n . On t h e o t h e r h a n d , s i n c e s u c h e v e n t s a r e g e n e r a l ly a m b i g u o u s w i t h t h e A ~ + ~ - y h y p o t h e s i s , t h e e n h a n c e m e n t m a y a l s o r e p r e s e n t t h e ~ + ~ - y d e c a y of t h e X °, i n w h i c h t h e m i s s i n g :~ h a s b e e n m i s i n t e r p r e t e d a s a ~o. In o r d e r to i n v e s t i g a t e t h i s a m b i g u i t y f u r t h e r , all events with a mass recoiling against the A in t h e r a n g e 660 to 1260 M e V , w e t r i e d f o r t h e h y p o t h e s i s A~+~-~ '. T h o s e e v e n t s w h i c h f i t t h e 4 c o n s t r a i n t h y p o t h e s e s A~+~ - a n d AK+K - w e r e removed from the sample. Fig. l(b) shows the ~+~r-y m a s s d i s t r i b u t i o n f o r t h e r e m a i n i n g e v e n t s w h i c h f i t t h e A~+~r-y h y p o t h e s i s a n d f o r w h i c h t h e A is peripherally produced. The enhancement at 1 GeV is sharper when the missing neutral is i n t e r p r e t e d a s a y. T h e ~ + ~ - m a s s d i s t r i b u t i o n f o r a l l e v e n t s in t h i s e n h a n c e m e n t i s g i v e n i n fig. l ( c ) a n d s h o w s a p e a k a t t h e pO m a s s . T h e e f f e c t on t h e ~r+~-y m a s s d i s t r i b u t i o n of r e q u i r i n g t h e ~ + u - m a s s t o l i e i n t h e pO r e g i o n i s s h o w n s h a d e d in fig. l ( b ) . H e r e t h e b a c k g r o u n d h a s b e e n d r a s t i c a l l y r e d u c e d , w h i l e t h e p e a k a r o u n d 960 M e V h a s b e e n r e d u c e d f r o m ~ 20 e v e n t s a b o v e b a c k g r o u n d to ~ 14 e v e n t s a b o v e b a c k g r o u n d . T h e enhancement is now very narrow, consistent with our mass resolution. M a n y of t h e e v e n t s s h o w n in fig. l ( b ) r e a l l y b e l o n g to t h e f i n a l s t a t e A~+~r-~ ° a l t h o u g h t h e y give a satisfactory fit to the A~+~-y hypothesis. T h i s a m b i g u i t y w a s i n v e s t i g a t e d by e x a m i n i n g t h e m i s s i n g m a s s r e c o i l i n g f r o m t h e Au+~ - s y s t e m a s s h o w n in fig. l ( d ) f o r t h o s e e v e n t s w i t h t h e m a s s of t h e m e s o n s y s t e m a r o u n d 960 M e V . T h e e f f e c t of r e q u i r i n g a pO i s s h o w n s h a d e d . T h o s e e v e n t s l y i n g i n t h e X ° r e g i o n , w h i c h do n o t h a v e a pO, have a broad missing mass distribution recoiling from the A~+Tr- system, (the unshaded events in fig. l(d)), which differs from the case where a pO is required. For events lying in the X° region and having a ~+~r- invariant mass appropriate to the pO, the hypothesis A~+Tr-7 is clearly preferred, whereas for the neighboring regions the missing mass is in better agreement with a 7r°. For comparison fig. l(e) shows the missing mass for those events fitting the hypothesis A~+Tr-~° with the ~+~-7~° mass lying in the ¢o region. The distribution peaks near the mass of the ~o, as expected. The cosine of the production angle in the K-p center of mass lies in the range 0.9 - 1.0 for two classes of events: (a) X° ~ ~r+~-~ events from the final state A?r+~-~7, and (b) X° ~ pOy. Events from the final state A~+~-V having the ~r+~r-7 invariant mass of the X° but with the y+y- mass not in the p band are less sharply peaked and lie
LETTERS
16 September 1968
i n t h e r a n g e 0.8 - 1.0. A s i m i l a r s i t u a t i o n o b t a i n s f o r e v e n t s f r o m a c o n t r o l r e g i o n in w h i c h t h e ~ + ~ - y m a s s l i e s on e i t h e r s i d e of t h e X °. T h i s supports the conclusion reached from missing m a s s p l o t s , n a m e l y , t h a t in t h e f i n a l s t a t e A~+~-T w e h a v e a c l a s s of e v e n t s r e p r e s e n t i n g t h e d e c a y X ° ~ p°T. From the preceding considerations we conc l u d e t h a t t h e e n h a n c e m e n t a t 960 M e V s h o u l d b e identified with the X ° rather than the H° meson. T h e D a l i t z p l o t f o r t h e X ° ~ po~ d e c a y m o d e shows the depopulation at the kinematic boundaries consistent with the prediction for the dec a y of a 0 - p a r t i c l e v i a a n M1 t r a n s i t i o n . T h e Dalitz plot for the remaining events (without a pO b u t w i t h t h e ~ + ~ - ~ m a s s i n t h e X ° r e g i o n ) d o e s not exhibit such a depopulation at the kinematic boundaries. F o r t h e d e c a y m o d e X ° -~ t h e po p o l a r i zation is fixed since the y has helicity + 1 along i t s d i r e c t i o n of p r o p a g a t i o n . T h i s g i v e s f o r t h e d e c a y a n g u l a r d i s t r i b u t i o n of t h e pO in i t s own r e s t f r a m e , r e f e r r e d t o t h e d i r e c t i o n of t h e r e -
pOT,
c o i l i n g ~, a n a n g u l a r d i s t r i b u t i o n
iY~ 1
sin2~)
1: Such a distribution could also be produced by pO decay following pO~.O S-wave decay of the H ° produced by K* exchange.
5 {a)
,
I 50
i
25 O'
o
-t.1
0
I.o - L o
J
o COS 8
'
I.o
0 ILO
0
1.0
7 ÷
Fig. 2. The p o decay angular distribution, defined as shown, in the g*~- c e n t e r of m a s s for: (a) 17 p e r i p h e r a l events of the reaction K- + p ~ A + * ~* + ~'- + ¥ with the effective ~÷Y-T m a s s lying in the X ° region and with the effective g ~ ' - m a s s lying in the pO region. 0.640-0.880 GeV. (b) 17 p e r i p h e r a l events of the reaction K- ÷ p ~ A ~ ~ + ~'- + y with the effective ~'+~-y m a s s lying in the X ° region but with the effective Tf+~"- m a s s lying outside the po region. (c) 281 p e r i p h e r a l events of the reaction K- + p ~ A + y~ ~ ~'- * }~ with the effective ~÷~-~ m a s s lying in the control region on e i t h e r side of the X °, in the range 0 . 7 - 0 . 9 3 8 GeV or in the range 0.978 -1.216 GeV. 533
Volume 27Bo number 8
PHYSICS
T h e e x p e r i m e n t a l d i s t r i b u t i o n i s s h o w n in f i g . 2 (a), t o g e t h e r w i t h a n o r m a l i z e d s i n 2 0 c u r v e , w h i c h i s s e e n t o g i v e a g o o d fit t o t h e d a t a . T h e c o r r e s p o n d i n g d i s t r i b u t i o n s f o r the e v e n t s in the X° r e g i o n without a po, and for n e i g h b o r i n g r e g i o n s , a r e s h o w n i n f i g . 2(b~, (c), r e s p e c t i v e l y . T h e s e do n o t e x h i b i t a sin~O d e p e n d e n c e . W e e s t i m a t e t h e r e a r e 14 e v e n t s w i t h X ° -* p O w i t h o u r pO s e l e c t i o n . W e h a v e 31 e v e n t s w i t h X ° ~ 7r+~-~ a n d ~? ~ a l l n e u t r a l s . U s i n g t h e m o s t r e c e n t v a l u e s [6] f o r 77 b r a n c h i n g r a t i o s , a n d c o r r e c t i n g f o r t h e w i d t h of t h e pO s e l e c t i o n u s e d , w e obtain a value
LETTERS
2.
X O __~pO~ R
-
-
X 0 ~
0.31 :~ 0.15
.
~?T?7
Using this branching ratio one obtains a total c r o s s s e c t i o n f o r t h e r e a c t i o n K - p --" A X ° of 20 ~ 4 ~ b a t an i n c i d e n t m o m e n t u m of 5.5 G e V / c .
3.
T h i s e x p e r i m e n t would not have been p o s s i b l e w i t h o u t t h e h e l p of t h e ZGS a n d b u b b l e c h a m b e r c r e w s , a s w e l l a s t h e c a r e f u l w o r k of o u r s c a n ners and measurers at Argonne and Northwestern. T o a l l of t h e m w e e x t e n d o u r t h a n k s . 4.
References 1. G.R. Kalbfleisch, L.W. Alvarez. A. B a r b a r o - G a l t i e r i , O.I.Dahl, P . E b e r h a r d , W . E . H u m p h r e y , J . S . Lindsey, D.W. M e r r i l l . J . J . Murray, A. Rittenberg, R. R. Ross. J . B . sharer. F . T . Shively, D.M. Siegel, G. A. Smith and R. D. Tripp, Phys. Rev. L e t t e r s 12 (1964) 527; M. Goldberg, M. Gundzik, J. L e i t n e r , M. P r i m e r , P. L. Connolly, E. L. Hart, K.W. Lai, G.W. London, N. P. Samios and S. S. Yamamoto. Phys. Rev. L e t t e r s 13 (1964) 249;
534
5.
6.
16 September 1968
G. R. Kalbfleisch, O.I. Dahl and A. Rittenberg, Phys. Rev. L e t t e r s 13 (1964) 349a; P. M. Dauber, W . E . Slater, L . T . Smith, D.H. Stork and H.K. Ticho, Phys. Rev. L e t t e r s 13 (1964) 449; J . B a d i e r , M. Demoulin, J . G o l d b e r g , B . P . G r e g o r y , C . P e l l e t i e r , A.Rouge. M.Ville, R. Barloutaud, A. Leveque, C. Louedec, J . M e y e r , P. Schlein, A. V e r glas, Do J. Holthuizen, W. Hoogland, J . C . Kluyver and A . G . T e n n e r , Phys. L e t t e r s 17 (1965) 337; G.W. London, R . R . Rau, N . P . Samios, S . S . Y a m a moto, M.Goldberg, S. Lichtman, M. P r i m e r and J . L e i t n e r . Phys. Rev. 143 (1966) 1034. G. Benson. E. Marquit, B. Roe, D. Sinclair and J . VanderVelde, Phys. Rev. L e t t e r s 17 (1966) 1234. This l e t t e r gives e a r l i e r r e f e r e n c e s to observations of the H meson in ~ p interactions. The only r e ported observation in KN interactions is in a paper p r e s e n t e d to the Heidelberg Conference, ~Evidence for a T= 0 t h r e e pion r e s o n a n c e at M ~ 990 MeV", G. B. Chadwick, Z . G . T . Guiragossian. E. Pickup, A. B a r b a r o - G a l t i e r i . M . J . Matison and A. Rittenberg. Details of the 7° beam at the ZGS are given in, T. H. F i e l d s , E. L. Goldwasser and U. E. Kruse, Design for a 6 GeV/¢ s e p a r a t e d K- beam. T H F / E L G / UEK-I. Argonne National Laboratory, 1961 (unpubli shed) ; R.Ammar. T.H.Fields. M.Derrick. E.L.Goldw a s s e r , M.L.Good. U . E . Kruse, D. Reeder, F. Schweingruber and J. D. Simpson, P r o c . 166 Intern. Conf. on Instrumentation for high energy physics, Stanford USAEC Conf. 660916, p. 620. Results on the final s t a t e s containing two charged p a r t i c l e s and a visible A are m o r e fully r e p o r t e d in, J. Mott, R. A m m a r , R. Davis, W. Kropac, D. Slate, B . W e r n e r . S. Dagan. M . D e r r i c k . T . F i e l d s , J. Loken and F.Schweingruber, submitted to Phys. Rev. Results f r o m the s a m e topology but where the neutral particle is a K ° have been previously r e p o r t e d in, F . S c h w e i n g r u b e r . M . D e r r i c k . T. Fields, D. Griffiths, L.Hyman, R . J . J a b b u r . J . L o k e n , R. Amm a r , R. Davis, W. Kropac and J. Mott, Phys. Rev. 166 (1968) 1317. A.H. Rosenfeld, N. B a r a s c h - S c h m i d t , A. B a r b a r o Galtieri, W . J . P o d o l s k y , L . R . P r i c e , M. Roos, P. Soding, W . J . W i l l i s and C.G. Wohl, UCRL-8030, r e vised September 1967.