Production of the kappa meson in K−-p interactions

Volume 5, number 4

PHYSICS LETTERS

PRODUCTION

OF

THE

KAPPA

MESON

IN K--p

15 July 1963

INTERACTIONS

S. G. WOJCICKI, G.R. KALBFLEISCH and M a r g a r e t H. ALSTON

Lawrence Radiation Laboratory, University of California, Berkeley, Calif .oy'nia Received 17 June 1963

The r e c e n t work at B e r k e l e y 1) on ~-p i n t e r a c tions has given evidence f o r the existence of an excited state in the K~r s y s t e m at ~ 725 MeV which the a u t h o r s c h o s e to call the x meson. To s e a r c h for this effect in K-p i n t e r a c t i o n s we have undertaken a s y s t e m a t i c a n a l y s i s of the r e a c t i o n

In the study we used two-prong events with an a s s o c i a t e d Vo, i . e . K ° decaying via the c h a r g e d K1° mode. The events w e r e p r o c e s s e d through our t r a c k r e c o n s t r u c t i o n and kinematical fitting p r o g r a m Package. In about 98% of the c a s e s , examples of r e a c t i o n (1) w e r e s e p a r a t e d unambiguously f r o m other r e a c t i o n s p o s s e s s i n g the s a m e topology. The K- + p - K ° + T r - + p (1) ambiguous events could always be r e s o l v e d by inover the r a n g e of m o m e n t a f r o m 1.0 to 1.7 GeV/c. spection of the ionisation of the two positive t r a c k s . We find a significant enhancement c e n t e r e d at The n u m b e r of events obtained at each momentum 7 2 3 ± 3 MeV, with a width l e s s than 12 MeV. We can- is given in table 1. not r u l e out the possibility that the o b s e r v e d width is The combined m a s s s p e c t r u m of the ~o~- s y s t e m due entirely to the experimental resolution. B e c a u s e f o r all m o m e n t a in this experiment is shown in of insufficient s t a t i s t i c s we a r e unable to draw any fig. la. It is c l e a r that the 890 MeV K* dominates conclusions as to the spin or p a r i t y of this r e s o n a n c e . r e a c t i o n (1) at all momenta. Inspection of the Dalitz Some t h e o r e t i c a l implications of the x a r e conplots (not shown) indicates that neither the 1238 sidered. MeV/4* r e s o n a n c e nor the r e c e n t l y d i s c o v e r e d The exposure was taken in the L a w r e n c e Radia1660 MeV YI* 4) a p p e a r s to play a prominent r o l e tion L a b o r a t o r y ' s 72-inch hydrogen bubble c h a m b e r , in the reaction. The c u r v e drawn o v e r the data in the s e p a r a t e d K- beam designed under the d i r e c r e p r e s e n t s the expected distribution if at all motion of H. K. Ticho 2). The amount of film taken and m e n t a the r e a c t i o n p r o c e e d s 75% of the time via the number of events obtained at each m o m e n t u m the mode setting a r e s u m m a r i z e d in table 1. The path length K- + p -. K*- (890 MeV) + p, K*- -. K ° + lr" (2) was obtained by (a) counting the n u m b e r of T d e c a y s and (b) n o r m a l i z i n g the total number of i n t e r a c t i o n s and the other 25% of the time by the t h r e e - b o d y to the known total c r o s s sections 3). The two m e t h mode (1). It a p p e a r s , that the c u r v e r e p r o d u c e s ods a g r e e d to within 10% at each momentum. The the data quite well except in the region of 723 MeV, typical s p r e a d in the m o m e n t u m of the beam at each w h e r e a significant enhancement is o b s e r v e d (33 setting was of the o r d e r of + 3%. events above the background c u r v e of 45). The bin s i z e of 14 MeV in fig. l a was chosen to localize Table 1 all of the effect in one bin. We henceforth limit Path lengths and numbers of events. o u r s e l v e s to a d i s c u s s i o n of the effect at 723 MeV. Incident K- momentum Number of Path length We f i r s t turn to the question of the statistical (GeV/c) l~° p~- observed (events/~b) significance of this enhancement. The K ° r m a s s region of i n t e r e s t (680 to 800 MeV) is displayed in 1.08 109 0.4 fig. lb in 3 MeV intervals. The dashed c u r v e r e 1.22 601 1.2 1.33 p r e s e n t s a rough e s t i m a t e of the background. The 604 1.5 1.43 362 0.8 experimental resolution in this region of K°~ - m a s s 1.51 2212 5.2 is approximately + 3 MeV. We would not expect to 1.70 0.7 T 408 see any r e a l s t r u c t u r e with a full width of l e s s than 6 MeV. The data can be i n t e r p r e t e d in one of two ways : either we see the effect of the kappa in the region f r o m 719 to 731 MeV with a downward flucThis number corresponds to the path length in which the reaction K-p ~ KOu-p was analyzed. The total path * This work was done under the auspices of the U.S. length here is about 1.1 events/~b. Atomic Energy Commission. 283

Volume 5, number 4

PHYSICS

t u a t l o n n e a r 727 MeV, o r we s e e it f r o m 719 to 725 MeV with an u p w a r d f l u c t u a t i o n n e a r 730 MeV. T h e p r o b a b i l i t y that the o b s e r v e d e n h a n c e m e n t i s due to a s t a t i s t i c a l a c c i d e n t i s s m a l l e r t h a n one c h a n c e i n 106 f o r e i t h e r i n t e r p r e t a t i o n of the d a t a (if we a s s u m e that the d a s h e d c u r v e i s the t r u e r e p r e s e n t a -

700 (e)

6O0

500 z

> 4O0

"300

~2oo

i S

0

600

~0

700 I

>e 25

I

I

800 I

I

I

I 900 I

I000 I

I

I I00

I

I

I

I

I

I

~-~ rresolutlofl

(b)

2c

i ,° 0

i 5 S Z 0

680

I

I

I

710 Mess

l

I

I

I

I

740 of K " 7r- system

7'70

800

(MW)

Fig. 1. (a) Plot of the l~°n- mass of the 4296 events from the reaction K- + p ~ l~°n-p at 1.0-1.7 GeV/c in 14 MeV intervals. The curve represents the phase space for the reaction proceeding 75% of the time through K-p ~ l~*-p and 25% through K-p - ]~O~-p directly. The K s mass is taken as 890 MeV, and its width F as 50 MeV. An enhancement is seen at 723 MeV of 33 events above the background curve or 45 (or 42 events above the average of the two neighbouring bins). (b) Plot of the 680 to 800 MeV mass region in 3 MeV intervals to show the structure of the distribution around the ~ mass. The dashed curve represents a rough estimate of the background. The resolution is 6 MeV in this region. 284

15 July 1963

t i o n of the b a c k g r o u n d ) . If we r a i s e the e s t i m a t e of the b a c k g r o u n d i n t h i s r e g i o n to - 10 e v e n t s / 3 MeV i n t e r v a l (which a p p e a r s to b e i n c o m p a t i b l e with the data) the odds i n c r e a s e to one p a r t i n 104 . In q u o t i n g t h e s e o d d s , we h a v e t a k e n the p o i n t of view that the r e g i o n n e a r 725 MeV h a s b e e n s p e c i f i e d b y the ~ - e x p e r i m e n t . If we w e r e to t r e a t o u r exp e r i m e n t i n d e p e n d e n t l y of the ~ - d a t a , we w o u l d h a v e to m u l t i p l y t h e s e odds b y the n u m b e r of b i n s i n o u r s p e c t r u m (~ 30). W e c o n c l u d e t h a t the e n h a n c e m e n t n e a r 725 MeV i s s t a t i s t i c a l l y s i g n i f i c a n t , e v e n if n o p r e v i o u s e x p e r i m e n t had s u g g e s t e d evidence for a r e s o n a n c e in this region. W e now c o n s i d e r the p r o p e r t i e s of the k a p p a . In view of the low s t a t i s t i c s a n d the v a r i o u s w a y s of i n t e r p r e t i n g the d a t a we c a n at b e s t e s t i m a t e the c e n t r a l v a l u e * of the m a s s of the r e s o n a n c e to b e 723 • 3 MeV a n d t h e f u l l width r to b e l e s s than 12 MeV. W e c a n n o t , h o w e v e r , e x c l u d e the p o s s i b i l i t y of a z e r o n a t u r a l width. T h e f a c t that i n no c a s e d o e s the x h a v e a m e a s u r a b l e l e n g t h s e t s a n u p p e r l i m i t of ~ 10-12 s e c on t h e l i f e t i m e . T o i n v e s t i g a t e the effect of s y s t e m a t i c e r r o r s ( m a g n e t i c f i e l d , t u r b u l e n c e , etc. ) on the m a s s of the ~ o ~ - s y s t e m , we f i t t e d the e v e n t s g i v i n g a s a t i s f a c t o r y X2 to r e a c t i o n (1) to the h y p o t h e s i s K - + p -~ ~1- + ~2 + + ~ 3 - + P ,

I00

~

LETTERS

(3)

w h e r e t h e f i r s t two p i o n s a r e t h e d e c a y p r o d u c t s of l~° . S u b s e q u e n t l y we c a l c u l a t e d the m a s s of the (~1-, ~2 +) s y s t e m . T h e r e s u l t s of t h i s p r o c e d u r e f o r a l l e x a m p l e s of r e a c t i o n (1) with M(i~°~- ) < 800 MeV a r e i l l u s t r a t e d i n fig. 2. In a d d i t i o n , we e m p i r i c a l l y t e s t e d the r e l a t i v e s e n s i t i v i t y of the m a s s of the i~° a n d the i~° - ~- s y s t e m to the v a r i o u s s y s t e m a t i c e r r o r s . W e h a v e f o u n d t h a t on the a v e r a g e the 1~° m a s s i s m o r e s e n s i t i v e t h a n the ~ o _ ~r- m a s s b y a b o u t a f a c t o r of two to v a r i o u s c h a n g e s in the i n p u t data. T h i s p r o c e d u r e g i v e s u s a n u p p e r l i m i t of 1 MeV f o r p o s s i b l e s y s t e m a t i c s h i f t s i n the x m a s s , which i s s m a l l c o m p a r e d with the s t a t i s t i c a l u n c e r t a i n t y . F u r t h e r m o r e , t h e r e s h o u l d b e no a p p r e c i a b l e b r o a d e n i n g of the m a s s d i s t r i b u t i o n due to s u c h s y s t e m a t i c e r r o r s . A r o u g h e s t i m a t e of the e x c i t a t i o n f u n c t i o n i s g i v e n i n fig. 3. T h e n u m b e r of x ' s a t each m o m e n t u m w a s d e t e r m i n e d by p l o t t i n g the 1~°~ - m a s s e s i n the 14 MeV b i n s of fig. l a a n d s u b t r a c t i n g the a v e r a g e n u m b e r of e v e n t s in the two n e i g h b o u r i n g • The usual formula for the e r r o r on the central value is 5 M = a/JN, where ~ is the width of the distribution, either experimental or natural or both and N the number of events in the sample. In this formula it is assumed that we can determine which events b e long to the distribution under consideration. This is not true in the case of a reasonance superimposed upon a high background. An additional uncertainty due to this background is introduced which is not taken into account in the above formula.

Volume 5, n u m b e r 4 50 1

,

PHYSICS LETTERS

I

i

I

I

Accepted

value

I

s p i n - p a r i t y a s s i g n m e n t f o r t h i s m e s o n (for s p i n l e s s than 2). We h a v e e x a m i n e d the d e c a y a n g u l a r d i s t r i b u t i o n s of the x a n d f i n d t h a t , w i t h i n s t a t i s t i c s , they a r e a l l c o m p a t i b l e with i s o t r o p y . H o w e v e r , we f e e l that t h i s f a c t s h o u l d not be t a k e n a s e v i d e n c e a g a i n s t the 1- a s s i g n m e n t b e c a u s e of the i n t r i n s i c d i f f i c u l t i e s a s s o c i a t e d with the r e l a t i v e l y high b a c k g r o u n d , low s t a t i s t i c s , a n d wide r a n g e of i n c i d e n t momenta. A s s u m i n g that the r e a c t i o n

I

5O0 499

4 9 Z 9 +-0.6

'

497 o

:E

15 July 1963

496

K- +p-x-

+p

(4)

495 494 I

I

1.0

I

I

1.2

I

I

1.4

]

1.6

PK- ( S e V / c

1.8

)

Fig. 2. Plot of the I[° mass at each incident momentum as deteLmined by fitting the K-p -* i~°~-p events as K-p (~1-, ~2 +)~-p [i. e . , without constraining the (~1-, ~2+) to be a ~o]. All events having a satisfactory X2 for the reaction K-p - K°n-p and having a l~c~- mass below 800 MeV were used. The average mass of the (~1-, ~2 +) from the fit is plotted as the ordinate. I

i

i

i

120

i

l

K-÷ P --.' K'+ p

I00 80 60 ,Q =L 4 0 b 20 I

0 -20 -40

I

1.0

I

I

1.2

I

I

1.4 Px-

( GeV/c

I

1.6

l

1.8

)

Fig. 3. Excitation function for the reaction K- + p ~ x- + p from 1.0 to 1.7 GeV/c. The cross sections were determined under the assumption that the z has isotopic spin I = ½ and that the rate of decay v i a electromagnetic transitions is negligible. If the isotopic spin of the x turns out to be I = I , then the ordinate scale should he doubled. bins f r o m the n u m b e r in the central bin. T h e ordinate scale (pb) w a s obtained on the assumption of an I = ½ assignment for the x. T h e scale should be doubled if the isotopic spin of the x turns out to be ~. W e a s s u m e that there is no appreciable rate of decay via electromagnetic transitions. A n y systematic shifts in the x m a s s as a function of K - m o m e n t u m are small c o m p a r e d to 14 M e V and would not influence this plot. T h e K ~ decay m o d e of the x indicates a 0 + or 1-

p r o c e e d s v i a the o n e - p i o n e x c h a n g e d i a g r a m [as o r i g i n a l l y s u g g e s t e d f o r r e a c t i o n (2) by Beg a n d De C e l l e s 5)] one c a n r e l a t e the width of the x to the c r o s s s e c t i o n f o r r e a c t i o n (4) f o r e i t h e r a s c a l a r (0 +) or v e c t o r (1-) a s s i g n m e n t f o r the ×. T h e d a t a a r e i n s u f f i c i e n t to t e s t to what e x t e n t the o n e - p i o n e x c h a n g e d i a g r a m d o m i n a t e s the r e a c t i o n . A s s u m i n g h o w e v e r that the o n e - p i o n e x c h a n g e d i a g r a m i s the d o m i n a n t one, a n d that the I - s p i n of the x i s 1 ~, we o b t a i n a width of the o r d e r of 1 to 3 MeV f o r the s c a l a r h y p o t h e s i s , a n d about 10 to 100 keV f o r the v e c t o r a s s u m p t i o n . N e i t h e r v a l u e i s r u l e d out by the e x p e r i m e n t . F u r t h e r m o r e , the o n e - p i o n exc h a n g e d i a g r a m y i e l d s a s l i g h t f o r w a r d p e a k i n g of the ang,!~r distribution of the x for the scalar hypothesis. S o m e e n h a n c e m e n t in the forward direction is indeed observed, but it m a y be due to the background. It would be of interest to ascertain whether the decay m o d e x- -* K - + 7 exists, as its observation would rule out the 0 + assignment. H o w e v e r , this decay m o d e is inaccessible in our experiment. W e can determine only the upper limit on another decay m o d e x- -* I~° + ~- + 7 to be 6 % of the I~°~ - m o d e *. W e find that w e are unable to d r a w any conclusions as to the spin or parity of the x ** * W e observe no events which fit the hypothesls K- + p - R°~-yp, with the energy of the y ray>50 MeV, and the mass of the K°~-y system in the x mass band. If we assume that the energy spectrum of the ¥ rays is determined purely by phase-space considerations, the observation of one such event would give an upper limit of 6% for the branching ratio. ** The strong decay of K*(890 MeV) - x + ~ is forbidden for a 0+ kappa. As indicated by Glashow 6), the ratio R = rate (K~ -* ~ + ~)/rate (K* - K + ~) can be calculated for a 1 - assignment (similar to that done by Rosenfeld, Carmony and Van de Walle 7)for p ~ ~ + u). We obtain R ~ 2(f/E) 2 (q/p)3 , where q and p are the decay momenta of a and K, and f a n d g are coupling constants for K* to x~ and K~, respectively, and the factor of two arises from spin considerations. For f of the order of g, R is .-- 5%. An experimental value of R less than 0.1%, say, would suggest a 0 + kappa. A preliminary value of R can be obtained from the reactions K-+ p~ K°~-prr° and K-~-~+p at 1.22 G e V / c . Examination of a film sample containing 1300 K*-reveals no KO~-prrO events and six K-~-~+p events which might be interpreted as K *-~ a ° +~, giving R < 1%. We can draw no conclusions at this time. 285

Volume 5, n u m b e r 4

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LETTERS

15 July 1963

I t i s i n t e r e s t i n g to c o n s i d e r s o m e of t h e t h e o r e b l e c h a m b e r a n d of t h e B e v a t r o n , a n d to o u r s c a n t i c a l i m p l i c a t i o n s of t h e x m e s o n . T h e SU 3 u n i t a r y ning and measuring staff, without whose help this s y m m e t r y s c h e m e p r o p o s e d b y G e l l - M a n n 8) a n d experiment would not have been possible. N e' e m a n O) has been used with considerable success r e c e n t l y in c l a s s i f y ~ g t h e known m e s o n s a n d b a r y o n s into m u l t i p l e t s . It i s i m p o r t a n t to n o t e t h a t no o t h e r known m e s o n s c o u l d f o r m a u n i t a r y m u l t l p l e t w i t h t h e x m e s o n . I t w i l l b e i n t e r e s t i n g to s e e if s u c h Refe~'ences " u n i t a r y p a r t n e r s " e x i s t . T h e r e a r e p r e s e n t l y two p o s s i b l e a s s i g n m e n t s f o r t h e x t h a t do not r e q u i r e 1) G.Alexander et a l . , Phys. Rev. Letters 8 (1962) 447. G. Alexander et a l . , Proc. Intern. Conf. on High Energy any " p a r t n e r s " . T a k e d a h a s p o i n t e d out a t h e o r y of Phys. (CERN, Geneva, 1962), p. 320. t h e w e a k i n t e r a c t i o n s w h i c h i s c o m p a t i b l e with a l l D. H. M i l l e r et a l . , Phys. Letters 5 (1963) 279. p r e s e n t l y k n o w n e x p e r i m e n t a l data, b u t w h i c h a l 2) H.K. Ticho et a l . , Lawrence Radiation Laboratory, l o w s t h e p o s s i b i l i t y of copious p r o d u c t i o n of t h e i n Berkeley (unpublished work). t e r m e d i a t e v e c t o r b o s o n s 11). A l s o N a m b u a n d 3) V. Cook et a l . , Phys. Rev. 123 (1961) 320. 4) L . W . A l v a r e z et a l . , Phys. Rev. Letters 10 (1963) S a k u r a i h a v e c o n s i d e r e d t h e h y p o t h e s i s t h a t the K* 184. (890 MeV) m e s o n s a r e c o u p l e d to s t r a n g e n e s s 5) M. Baqi B~g and P . D e Celles, Phys. Rev. Letters 6 changing currents which are conserved "as exactly (1961) 145, 428 (E); see also C. Chan, Phys. Rev. a s p o s s i b l e " 12). T h i s h y p o t h e s i s s u g g e s t s t h e e x Letters 6 (1961) 383 and W. G.Wagner and D. H. Sharp, i s t e n c e of a Y = + 1, T = ½, J = 0 + m e s o n w h o s e Phys.Rev. 128 (1962) 2899. 6) S. Glashow, private communication. c o u p l i n g to o t h e r s t r o n g l y i n t e r a c t i n g p a r t i c l e s v a n 7) A. Rosenfeld, D. Carmony and R. Van de Walle, Phys. i s h e s in t h e l i m i t of e x a c t u n i t a r y s y m m e t r y . P r e s Rev. Letters 8 (1962) 293. e n t e x p e r i m e n t a l d a t a do n o t a l l o w u s to d r a w a n y 8) M.Gell-Mann, Phys.Rev. 125 (1962)1067; California c o n c l u s i o n s a s to t h e a s s i g n m e n t of t h e x m e s o n to Institute of Technology Report CTSL-20, 1961 (una n y of t h e s e s c h e m e s . Any d i s c u s s i o n of a s s i g n published). 9) Y. Ne'eman, Nuclear Phys. 26 (1961) 222. m e n t s a t t h i s s t a g e m u s t b e c o n s i d e r e d to b e p u r e l y S. L. Glashow and A. H. Rosenfeld, Phys. Rev. Letters IO) speculative. 10 (1963) 192. 11) G.Takeda, Phys.Rev. Letters 10 (1963) 167. W e w i s h to t h a n k P r o f e s s o r L u i s W. A l v a r e z f o r 12) Y. Nambu and J. J. Sakurai, K*(725) and the Strangeness h i s e n c o u r a g e m e n t a n d s u p p o r t in t h i s e x p e r i m e n t . Changing Currents of Unitary Symmetry, Enrico W e a r e i n d e b t e d to t h e o p e r a t o r s of t h e 7 2 - i n c h b u b F e r m i Institute Report 63-26, April 1963 (to be pub* F o r a review of the experimental situation see r e f s . 4, 10). lished).

$$*$$

SOME R E M A R K S

ABOUT

A SECOND

POMERANCHUK

TRAJECTORY

V. I. L E N D Y E L * a n d J. M A T H E W S

California Institute of Technology, Pasadena, California Received 17 June 1963

R e c e n t l y , a t t e n t i o n h a s b e e n d r a w n to t h e p o s s i b i l i t y of a s e c o n d P o m e r a n c h u k t r a j e c t o r y . It w a s p o i n t e d out by I g i 1) t h a t t h e a s s u m p t i o n of no t r a j e c t o r y ~(t) w i t h ~(0) > 0 e x c e p t t h e P o m e r a n c h u k t r a j e c t o r y (~p(0) = 1) l e a d s to a s u m r u l e f o r t h e non-isotopic-spin-flip scattering length which disagrees seriously with the experimental value. Igi f u r t h e r o b s e r v e d 1, 2) t h a t by i n c l u d i n g a s e c o n d v a c u u m t r a j e c t o r y ~ p , (t), w i t h ~ p , (0) ~ 0.5, t h e c o r rect scattering length can be obtained. * On leave of absence from Uzhgorod University, USSR.

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On t h e o t h e r h a n d , i t h a s b e e n p o i n t e d out 3) t h a t h i g h e n e r g y pp a n d p~ t o t a l c r o s s s e c t i o n d a t a a l s o s e e m to r e q u i r e a P'. T h e n e a r l y c o n s t a n t or__ p and rapidly decreasing a~ have been fitted by S~p a n d W a g n e r 3), who ~ g g e s t O~p,(0) ~. 0.3, w h i c h e s sentially agrees with Igi's result. It w o u l d s e e m of i n t e r e s t to d e t e r m i n e a p , (0) a s a c c u r a t e l y a s p o s s i b l e , u s i n g b o t h of t h e a b o v e a p p r o a c h e s . If o n e a s s u m e s t h a t no c u t s e x i s t b e t w e e n cx = 0 a n d ~ = 1, t h e n t h e two m e t h o d s s h o u l d g i v e t h e s a m e r e s u l t . On t h e o t h e r h a n d , if a d i s c r e p a n c y