- Email: [email protected]
Measurement of the partial width of the decay φ → e+ + e−
Volume27B, number 2
MEASUREMENT
PHYSICS
OF
THE
PARTIAL
LETTERS
WIDTH
1 0 J u n e 1968
OF
THE
DECAY
~b --~ e + + e -
D . M . B I N N I E , A. D U A N E , A . R . F A R U Q I , J . P . H O R S E Y , W . G . J O N E S $ , M . E . K A Y , D. C. MASON, P . J . N I C H O L S O N , I. U. R A H M A N , J . W A L T E R S 55, J . G . WII_SON
Department of Physics, Imperial College, London, UK and P. P A L I T Rutherford Laboratory, Chilton, Berkshire, UK Received 13 May 1968
Evidence is p r e s e n t e d for the p r o c e s s ~ - + p'-* ~b + n followed by ~b-'~e+ + e - . The b r a n c h i n g r a t i o (~b ~ e + + e-)/(¢p--~ total) is found to be (7.2 ~-3.9) × 10 -4, corresponding to a p a r t i a l width for ~b---*e + + e of 2.4 ± 1.5 keV/c 2 and a coupling c o n s t a n t T ~ / 4 f r of 1.9+2:97 .
An e x p e r i m e n t d e s i g n e d to d e t e r m i n e t h e b r a n c h i n g r a t i o (~b ~ e + + e - ) / ( ¢ ~ K + + K - ) a n d h e n c e t h e p a r t i a l w i d t h of ¢ -~ e + + e - h a s b e e n performed at the 7 GeV proton synchrotron ' N i m r o d ' of t h e R u t h e r f o r d L a b o r a t o r y . I t c o n s i s t e d of two p a r t s . T h e f i r s t w a s a m e a s u r e m e n t of t h e n u m b e r of ~b m e s o n s p r o d u c e d i n t h e r e a c t i o n ~ - + p ~ q~ + n a t 1.58 GeV/c, w h e r e t h e ¢ m e s o n s w e r e d e t e c t e d t h r o u g h t h e K+K - d e c a y mode, and the second was a search, using the same reaction at the same momentum, for the leptonic decay. As it is intended to publish a fuller account e l s e w h e r e , o n l y a n o u t l i n e of t h e p r o d u c t i o n cross-section measurement is presented here. T h e a p p a r a t u s i s s k e t c h e d i n fig. 1. T h e t e c h n i q u e u s e d i s s i m i l a r in p r i n c i p l e t o t h a t a d o p t e d i n e a r l i e r e x p e r i m e n t s [1,2] i n w h i c h n e u t r o n time-of-flight spectra were recorded for various incident pion momenta close to the threshold m o m e n t u m f o r t h e r e a c t i o n . A s y s t e m of h o d o s cope counters in conjunction with a quadrupole triplet and bending magnet was employed, so t h a t s e v e r a l m o m e n t u m c h a n n e l s , e a c h of Ap/p = = ~°/o, c o u l d b e d e f i n e d a n d u s e d i n p a r a l l e l . T h e neutron counters have already been described [1]. T h e K+K - m o d e w a s s e l e c t e d b y d e m a n d i n g at least two coincident signals from the scintillation counters El_ 4 and no signal from the $ P r e s e n t a d d r e s s : Synchrotron Laboratory, C a l i f o r nia Institute of Technology. $$ Now at Physical L a b o r a t o r i e s , U n i v e r s i t y of Manchester. 106
6
-,
<.~
Spark chambers
E1-4
Neutron counters
Fig. 1. The detection of ~b in m e s o n s in ?r- + p---* ~b + n via the K+K - decay mode a t a b e a m m o m e n t u m just above threshold. The ~b is e m i t t e d at a few d e g r e e s to beam direction in the l a b o r a t o r y s y s t e m . The coincidence between any two of scintillation counters E l _ 4 accepts about half of all ~b --. K+K - decays. threshold water ~erenkov counter. F i g . 2(a) s h o w s a n e u t r o n t i m e - o f - f l i g h t spectrum obtained at the optimum momentum f o r d e t e c t i o n of ¢ m e s o n s . T h i s s p e c t r u m i s i n s u f f i c i e n t i n i t s e l f t o g i v e a m e a s u r e of t h e n u m b e r of ¢ m e s o n s p r o d u c e d , a s a n a p p r e c i a b l e b a c k g r o u n d of K+K - e v e n t s c o u l d b e p r e s e n t . H o w e v e r a n a n a l y s i s of t h e v a r i a t i o n i n t h e y i e l d of n e u t r o n s i n a n a p p r o p r i a t e t i m i n g g a t e a s a f u n c t i o n of b e a m m o m e n t u m , t h e ' y i e l d c u r v e ' , fig. 2(b), r e s o l v e d t h e ¢p f r o m t h e b a c k g r o u n d . I n a d d i t i o n , f o r a s a m p l e of t h e e v e n t s , s p a r k c h a m b e r p h o t o g r a p h s w e r e t a k e n of t h e K m e s o n trajectories so that complete reconstructions could be made and decay angular distributions
I
Volume 27B, number 2
L~
PHYSICS LETTERS
10 June 1968
Shower
, • region =
n
Lead Scintillator
50
15
20 25 30 35 Neutron Time of Flight, ns.
40
(b)
8O
(a)
lelectron (b)
Fig. 3. (a) The arrangement of lead plates, scintillation counters and thin foil spark chambers used to select and identify electrons. Each lead plate is about 1.1 radiation length thick. (b) An enlarged section of one of the counter, lead, spark chamber sandwiches.
E li,
|IT~ 0
0
s
t;oo
"';~5o
,~
Beam momentum, MeV/c
Fig. 2. (a) A typical neutron time-of-flight spectrum over 4m using the equipment sketched in fig. 1. (b) The 'yield curve' for the reaction 7r- + p--, n + K+ + + K-. This shows the total number of counts, in a narrow gate chosen to include the ~, as a function of beam momentum. The characteristic rapid rise near 1560 MeV/c is evidence of ~ production, as opposed to other sources of K pairs. The continuous line shows the best fit to the data for a ~ m e s o n cross-section rising li nearly with c.m.s, momentum, superimposed on nonresonant kaon and constant pion backgrounds. The dashed line shows the contribution from ~ meson production. obtained. A n a l y s i s to d a t e i n d i c a t e s that the data can be a d e q u a t e l y d e s c r i b e d by a s i m p l e S w a v e m o d el in which t h e t o t a l c r o s s - s e c t i o n n e a r t h r e s h o l d r i s e s l i n e a r l y with the final s ta te c . m . s , m o m e n t u m p*, and the production and decay a n g u l a r d i s t r i b u t i o n s a r e i s o t r o p i c . Using t h i s model, the t o t al c r o s s - s e c t i o n was found to be (0.28 ~ 0.07)p* ~b, w h e r e p* is m e a s u r e d in MeV/c. T h i s d e s c r i p t i o n i s c o n s i s t e n t with the data of H e s s et al. [3]. To i n v e s t i g a t e the e+e - d e c a y of the ~, the K + K - d e t e c t i o n s y s t e m w a s r e p l a c e d by the a r r a n g e m e n t of thin f o i l o p t i c a l s p a r k c h a m b e r s , l e a d p l a t e s and s c i n t i l l a t i o n c o u n t e r s s k e t c h e d in fig. 3. E l e c t r o n s w e r e i d e n t i f i e d by the c h a r a c t e r i s t i c s h o w e r s they p r o d u c e d in the lead p l a t e s . T h e r e w e r e 5 p l a t e s on e a c h s i d e of the t a r g e t , each about 1.1 r a d i a t i o n length thick• Each s p a r k
c h a m b e r module contained four a c t i v e gaps which in g e n e r a l made it p o s s i b l e to identify individual s e c o n d a r y t r a c k s in the s h o w e r s . Th e i n n e r m o s t c h a m b e r s of each side w e r e u s e d mainly f o r m e a s u r e m e n t s of i n c i d e n t e l e c t r o n t r a j e c t o r i e s , although they a l s o played a v a l u a b l e r o l e in r e ducing c e r t a i n b a c k g r o u n d s . C o u n t e r s p l a c e d i m m e d i a t e l y beyond the i n n e r s p a r k c h a m b e r s d e t e c t e d the a r r i v a l of c h a r g e d p a r t i c l e s , and f u r t h e r c o u n t e r s j u s t b e f o r e the t h i r d and fifth lead p l a t e s g a v e i n f o r m a t i o n on the s i z e of any s h o w e r s a f t e r two and four r a d i a t i o n lengths r e s p e c tively. W h i l e individual e l e c t r o n e n e r g i e s could v a r y between 400 and 900 MeV, the c o m b i n e d e n e r g y , being that of the ~, was a l m o s t constant. T h e r e f o r e the gains of the four s h o w e r c o u n t e r s w e r e m a t c h e d , and f o r the e l e c t r o n i c s e l e c t i o n a t h r e s h o l d r e q u i r e m e n t was p l a c e d on the c o m bined signal. The d e t a i l e d p e r f o r m a n c e of this a r r a n g e m e n t , including the design of c r i t e r i a to s e l e c t e l e c t r o n s ef f i ci en t l y w h i l e r e j e c t i n g b a c k ground p r o c e s s e s , was b a s e d on an a n a l y s i s of photographs of the s h o w e r s p r o d u c e d by c a l i b r a tion e l e c t r o n s taken b e f o r e the e x p e r i m e n t p r o p e r was s t a r t e d . I d e a l l y , the b r a n c h i n g r a t i o would be d e t e r m i n ed f r o m a c o m p a r i s o n of the y i e l d c u r v e s f o r the e+e - and K+K - d ecay m o d e s . H o w e v e r , b e c a u s e of the low v a l u e a n t i c i p a t e d f o r this r a t i o it was only p r a c t i c a b l e to m e a s u r e the y i e l d at two points, one below t h r e s h o l d and the o t h e r n e a r the m a x i m u m of the y i e l d c u r v e . T h e signal
10'/
Volume 27B. number 2
PHYSICS LETTERS
' r u n s ' w e r e made only by c o m b in in g t h r e e m o m e n t u m channels at 1574, 1582, and 1590 MeV/c, and the ' b ack g ro u n d r u n s ' by combining four f r o m 1526 to 1550 MeV/c. F o r the signal ru n s , a 5.5 ns wide n e u tr o n t i m e - o f - f l i g h t gate was p o s i t i o n e d f o r o p t i m u m ~b y i e l d a s shown by the peak in the K+K - channel. The l i m i t a t i o n s thus i m p o s e d on n e u tr o n v e l o c i t y , n e u t r o n angle and b e a m m o m e n t u m i m p l i e d that the m i s s i n g m a s s of events s e l e c t e d by the e l e c t r o n i c s was c e n t r e d on the @ m a s s and had a full width at half height of 15 MeV/c 2. T h e r a n g e of m i s s i n g m a s s a c c e p t e d by the 11 ns gate u s e d in the background runs was c e n t r e d on 1003 MeV/c 2 with a full width at half height of 20 MeV/c 2. A p p r o x i m a t e l y 220 000 photographs of the s h o w e r c h a m b e r s w e r e taken using a bias on the s h o w e r c o u n t e r s c a l c u l a t e d to be m o r e than 95% e f f i c i e n t for e+e - p a i r s . A f t e r c o r r e c t i o n s f o r the v a r i o u s l o s s e s , and a d j u s t m e n t s to take into account the v a r i o u s gate s e t t i n g s used, the total e f f e c t i v e n u m b e r of pions incident on the hydrogen t a r g e t was e s t i m a t e d at 5.2 ± 0.3 × 1010 during the s i g nal run s , and 7.6 ± 1.0 × 1010 during the b a c k ground r u n s . Ini t i al l y the photographs w e r e scanned u s i n g v e r y weak c r i t e r i a and y i e l d e d 4 700 events. A l m o s t all of t h e s e w e r e c l e a r l y background p r o c e s s e s , mainly a r i s i n g f r o m photon s h o w e r s s u p e r i m p o s e d on ~ m e s o n s , i n t e r n a l or e x t e r n a l c o n v e r s i o n of y r a y s f r o m yo d e c a y s , and the c a s u a l s u p e r p o s i t i o n of u n a s s o c i a t e d t r a c k s . More stringent c r i t e r i a were therefore designed in o r d e r to r e d u c e the n u m b e r of a c c e p t e d e v e n t s , and w e r e f i r s t c h e c k e d a g a i n s t the c a l i b r a t i o n e l e c t r o n s . They w e r e b a s e d on the n u m b e r of i n t e r a c t i o n s in the ' s h o w e r ' , (multiple t r a c k s , gaps, l a r g e angle s c a t t e r s ) but a l s o r e q u i r e d : e v i d e n c e of i n t e r a c t i o n b e f o r e the t h i r d r a d i a t i o n length; i n c o m p a t i b i l i t y with a single o r i g i n f o r all s e c o n d a r i e s ; and a d e g r e e of s y m m e t r y in the show er . S h o w e r s that showed c o l l i n e a r t r a c k s in at l e a s t the f i r s t four c h a m b e r s w e r e p e n a l i z e d and e v e n t s with two c l o s e l y spaced, a l m o s t p a r a l l e l , t r a c k s in the i n n e r c h a m b e r w e r e r e j e c t e d. The 255 e v e n t s s u r v i v i n g the a p p li c a ti o n of t h e s e c r i t e r i a w e r e m e a s u r e d at l e a s t t w i c e to obtain c o n s i s t a n t r e s u l t s , and those k i n e m a t i c a l l y i n c o n s i s t e n t with the p r o c e s s 7r- + p e + + e - + n w e r e r e j e c t e d . P r i n t s w e r e made of the r e m a i n i n g 80 c a n d i d a t e s and, to avoid b i a s e s , i d e n t i f i c a t i o n m a r k s on the p r i n t s w e r e c o n c e a l e d . It was s t i l l evident that many of the photographs w e r e not e x a m p l e s of e+e -. A f u r t h e r study was t h e r e f o r e made of non-fitting e ve nts , and two final c r i t e r i a w e r e d e v e l o p e d 108
10June 1968
and applied to the c a n d i d a t e s . One of t h e s e s p e c i fied e v i d e n c e of i n t e r a c t i o n s beyond the t h i r d l e a d plate, and the o t h e r was an ex t en si o n of the s y m m e t r y r e q u i r e m e n t s on the s h o w e r s . T h e r e s u l t i n g 27 c a n d i d a t e s w e r e l a t e r i d e n t i f i e d and c o m p r i s e d 17 events f o r the 5.2 × 1010 incident pions in the signal runs and 10 events f o r the 7.6 × 1010 pions in the background r u n s. A f t e r n o r m a l i s a t i o n this y i e l d e d an e x c e s s of 10.1 ± 4.8 events in the signal r u n s . P r i o r to i d e n t i f i c a t i o n , the 27 p r i n t s had been r e s c r u t i n i s e d and a s e l e c t i o n m a d e of t h o se events c o n s i d e r e d to be wholly s a t i s f a c t o r y e x a m p l e s of e l e c t r o n p a i r s . This s u b j e c t i v e s e l e c tion y i e l d e d 9 events of which 8 w e r e l a t e r found to be f r o m the signal runs and 1 f r o m the b a c k ground. Although the efficiency of this s e l e c t i o n was unknown, the r e s u l t was si g n i f i can t in that the p r o b a b i l i t y f o r a c h a n c e a s s y m e t r y of this magnitude was l e s s than 0.1%. A ~b decay mode o t h er than e+e -, for e x a m p l e q~ -~ ~+lr-g °, could have co n t r i b u t ed to the signal events, but we p r e d i c t l e s s than 0.1 event f r o m this s o u r c e . We conclude t h e r e f o r e that we have o b s e r v e d e x a m p l e s of ~ ~ e + + e - . As we cannot make a r e l i a b l e e s t i m a t e of the l o s s of genuine events in the s u b j e c t i v e s e l e c t i o n , we e s t i m a t e the b r a n c h i n g r a t i o by c o n s i d e r i n g t h e 27 events r e s u l t i n g f r o m the application of the final c r i t e r i a . Using the s i m p l e S wave model, the f r a c t i o n of 4 -~ e + + e - e v e n t s e n t e r i n g the s p a r k c h a m b e r f i d u c i a l v o l u m e was c a l c u l a t e d to be 22%. Th e c o m b i n e d efficiency of al l the s h o w e r and k i n e m a t i c c r i t e r i a f o r the two e l e c t r o n s w as e s t i m a t e d at 69 ± 6%. Th e p r e s e n t v a l u e s of the b r an ch i n g r a t i o (4 -* K + + K - ) / @ total) is 48 + 3%, although this may be r e v i s e d as f u r t h e r decay m o d e s a r e detected. We u s e this v a l u e and find that the t o t al n u m b e r of p o t e n t i a l 4 ~ e + + e - events was 14000 • 2200, which g i v e s a b r a n c h i n g r a t i o of (4 ~ e + + + e - ) / ( 4 ~ t o t a l ) of (7.2~-3.9) x 10 -4. F r o m the c u r r e n t e s t i m a t e of 3.4 i 0.8 MeV/c 2 f o r I~. [4], we deduce a p a r t i a l width f o r $ ~ e + + e - of:2.4 ± ± 1.5 k e V / c z, giving a v al u e f o r the coupling 2-~ ,~+2.9
Y(O/a~ = 1"v-0.7"
Th e b r a n c h i n g r a t i o found is c o n s i s t e n t with the e s t i m a t e of between (1.8 and 7.0) × 10 -4 of Wehmann et al. [5] (the main u n c e r t a i n t y in this e x p e r i m e n t a r i s e s f r o m lack of knowledge of the production c r o s s - s e c t i o n ) and of A s t r a c a t u r o v et al. [6] who find (6.6+4: 4) x 10 -4. In the l a t t e r n u m b e r , h o w e v e r , it should be noted that no a l l o w a n c e has been made f o r the s u b s t a n t i a l u n c e r t a i n t y in the 4 production c r o s s - s e c t i o n . E v i d e n c e f o r ~b ~ e + + e - has a l s o been r e p o r t e d by A s b u r y et al. [7].
PHYSICS
Volume 27B, n u m b e r 2
LETTERS
T h e p r e c i s i o n of o u r m e a s u r e m e n t i s u n f o r t u n a t e l y t o o l o w t o a l l o w a n y d e t a i l e d c h e c k of the relationship between the vector meson coupli n g c o n s t a n t s . T h e c o m p i l a t i o n b y T i n g [8] s u g g e s t s a v a l u e f o r T ~ / 4 ~ of a b o u t 0 . 4 4 , w h i c h , c o ~ n b i n e d w i t h t h e SU(6) p r e d i c t i o n [9] of T~ : ~ 2 : y-2~b = 9 : 1 : 2, w o u l d g i v e ~ / 4 ~ of a b o u t 2.0, i n g o o d a g r e e m e n t w i t h o u r v a l u e . H o w e v e r t h e e s t i m a t e s of S a k u r a i et a l . [10] a n d of D a s e t a l . [11] w h i c h i m p l y t h a t w e s h o u l d h a v e s e e n 6 a n d 4 e x a m p l e s r e s p e c t i v e l y , of ~ -~ e + + e - a r e also consistent with our result.
$
W e w i s h t o t h a n k D. G. M i l l e r , R. F. H o b b s , D. J . S c h o l e s a n d R . J . G r a y f o r t h e i r i n v a l u a b l e w o r k i n t h e c o n s t r u c t i o n of t h e a p p a r a t u s , a n d also the Nimrod support staff and operating crew for their continued assistance.
References 1. D . M . B i n n i e , A.Duane, M . R . J a n e , W . G . J o n e s , D. C. Mason, J . A . Newth, D. C. P o t t e r , I.U. Rahman, J. W a l t e r s , 13. Dickinson, R . J . Ellison, A . E . H a r c k ham, M. Ibbotson, R. M a r s h a l l , R. F. T e m p l e m a n and A . J . W y n r o e , P h y s . L e t t e r s 18 (1965) 348.
10 June 1968
2. W . G . J o n e s , D . M . B i n n i e , A.Duane, J . P . H o r s e y , D. C. Mason, J . A . Ne~¢h, I . U . R a h m a n , J. Walters, N. Horwitz and P. Palit, Phys. L e t t e r s 23 (1966) 597. 3. R . I . H e s s , O.I.Dahl, L . M . H a r d y , J . K i r z a n d D . H. Miller, Phys. Rev. L e t t e r s 17 (1966) 1109. 4. A . H . R o s e n f e l d , N. B a r a s 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. S~ding, W . J . Willis and C.G. Wohl, UCRL - 8030, September, 1967. 5. A . A . W e h m a n , E . E n g e l s , C.M.Hoffman, P . G . I n nocenti, R. Wilson, W.A. Blanpied, D . J . Drickey, L. N. Hand and D.G. Stairs, Phys. Rev. L e t t e r s 20 (1968) 748. 6. R.G. A s t r a c a t u r o v , M.A. Azimov, A.M. Baldin, A. S. Belousov, I.V. Chuvilo, J . Hladky, V.I. Ivanov, M. N. Khachaturyan, M.S. Khvastunov, A . T . Matyushin, V . T . Matyushin, L . N . Shtarkov and L.I. Zhuravleva, P r e p r i n t JINR E1-3770 (1968). 7. J . G . A s b u r y , U . B e c k e r , W.K. B e r t r a m , P . J o o s , M.Rohde, A . J . S . Smith, C . L . J o r d a n and S . C . C . Ting, Phys. Rev. L e t t e r s 19 (1967) 869. 8. S . C . C . Ting, Intern. Syrup. on E l e c t r o n and photon i n t e r a c t i o n s at high e n e r g i e s , Stanford U n i v e r s i t y (1967). 9. P . G . O . F r e u n d , Nuovo Cimento 44A (1966) 411; C.A. Levinson, H . J . Lipkin and S.Meshkov, Phys. L e t t e r s 7 (1963) 81; H . H a r a r i , P h y s . Rev. 155 (1967) 1565. 10. R . J . O a k e s and J . J . S a k a r a i , Phys. Rev. L e t t e r s 19 (1967) 1266. 11. T . D a s , V . S . M a t h u r and S.Okubo, Phys. Rev. L e t t e r s 19 (1967) 470.
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MEASUREMENT
PHYSICS
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LETTERS
WIDTH
1 0 J u n e 1968
OF
THE
DECAY
~b --~ e + + e -
D . M . B I N N I E , A. D U A N E , A . R . F A R U Q I , J . P . H O R S E Y , W . G . J O N E S $ , M . E . K A Y , D. C. MASON, P . J . N I C H O L S O N , I. U. R A H M A N , J . W A L T E R S 55, J . G . WII_SON
Department of Physics, Imperial College, London, UK and P. P A L I T Rutherford Laboratory, Chilton, Berkshire, UK Received 13 May 1968
Evidence is p r e s e n t e d for the p r o c e s s ~ - + p'-* ~b + n followed by ~b-'~e+ + e - . The b r a n c h i n g r a t i o (~b ~ e + + e-)/(¢p--~ total) is found to be (7.2 ~-3.9) × 10 -4, corresponding to a p a r t i a l width for ~b---*e + + e of 2.4 ± 1.5 keV/c 2 and a coupling c o n s t a n t T ~ / 4 f r of 1.9+2:97 .
An e x p e r i m e n t d e s i g n e d to d e t e r m i n e t h e b r a n c h i n g r a t i o (~b ~ e + + e - ) / ( ¢ ~ K + + K - ) a n d h e n c e t h e p a r t i a l w i d t h of ¢ -~ e + + e - h a s b e e n performed at the 7 GeV proton synchrotron ' N i m r o d ' of t h e R u t h e r f o r d L a b o r a t o r y . I t c o n s i s t e d of two p a r t s . T h e f i r s t w a s a m e a s u r e m e n t of t h e n u m b e r of ~b m e s o n s p r o d u c e d i n t h e r e a c t i o n ~ - + p ~ q~ + n a t 1.58 GeV/c, w h e r e t h e ¢ m e s o n s w e r e d e t e c t e d t h r o u g h t h e K+K - d e c a y mode, and the second was a search, using the same reaction at the same momentum, for the leptonic decay. As it is intended to publish a fuller account e l s e w h e r e , o n l y a n o u t l i n e of t h e p r o d u c t i o n cross-section measurement is presented here. T h e a p p a r a t u s i s s k e t c h e d i n fig. 1. T h e t e c h n i q u e u s e d i s s i m i l a r in p r i n c i p l e t o t h a t a d o p t e d i n e a r l i e r e x p e r i m e n t s [1,2] i n w h i c h n e u t r o n time-of-flight spectra were recorded for various incident pion momenta close to the threshold m o m e n t u m f o r t h e r e a c t i o n . A s y s t e m of h o d o s cope counters in conjunction with a quadrupole triplet and bending magnet was employed, so t h a t s e v e r a l m o m e n t u m c h a n n e l s , e a c h of Ap/p = = ~°/o, c o u l d b e d e f i n e d a n d u s e d i n p a r a l l e l . T h e neutron counters have already been described [1]. T h e K+K - m o d e w a s s e l e c t e d b y d e m a n d i n g at least two coincident signals from the scintillation counters El_ 4 and no signal from the $ P r e s e n t a d d r e s s : Synchrotron Laboratory, C a l i f o r nia Institute of Technology. $$ Now at Physical L a b o r a t o r i e s , U n i v e r s i t y of Manchester. 106
6
-,
<.~
Spark chambers
E1-4
Neutron counters
Fig. 1. The detection of ~b in m e s o n s in ?r- + p---* ~b + n via the K+K - decay mode a t a b e a m m o m e n t u m just above threshold. The ~b is e m i t t e d at a few d e g r e e s to beam direction in the l a b o r a t o r y s y s t e m . The coincidence between any two of scintillation counters E l _ 4 accepts about half of all ~b --. K+K - decays. threshold water ~erenkov counter. F i g . 2(a) s h o w s a n e u t r o n t i m e - o f - f l i g h t spectrum obtained at the optimum momentum f o r d e t e c t i o n of ¢ m e s o n s . T h i s s p e c t r u m i s i n s u f f i c i e n t i n i t s e l f t o g i v e a m e a s u r e of t h e n u m b e r of ¢ m e s o n s p r o d u c e d , a s a n a p p r e c i a b l e b a c k g r o u n d of K+K - e v e n t s c o u l d b e p r e s e n t . H o w e v e r a n a n a l y s i s of t h e v a r i a t i o n i n t h e y i e l d of n e u t r o n s i n a n a p p r o p r i a t e t i m i n g g a t e a s a f u n c t i o n of b e a m m o m e n t u m , t h e ' y i e l d c u r v e ' , fig. 2(b), r e s o l v e d t h e ¢p f r o m t h e b a c k g r o u n d . I n a d d i t i o n , f o r a s a m p l e of t h e e v e n t s , s p a r k c h a m b e r p h o t o g r a p h s w e r e t a k e n of t h e K m e s o n trajectories so that complete reconstructions could be made and decay angular distributions
I
Volume 27B, number 2
L~
PHYSICS LETTERS
10 June 1968
Shower
, • region =
n
Lead Scintillator
50
15
20 25 30 35 Neutron Time of Flight, ns.
40
(b)
8O
(a)
lelectron (b)
Fig. 3. (a) The arrangement of lead plates, scintillation counters and thin foil spark chambers used to select and identify electrons. Each lead plate is about 1.1 radiation length thick. (b) An enlarged section of one of the counter, lead, spark chamber sandwiches.
E li,
|IT~ 0
0
s
t;oo
"';~5o
,~
Beam momentum, MeV/c
Fig. 2. (a) A typical neutron time-of-flight spectrum over 4m using the equipment sketched in fig. 1. (b) The 'yield curve' for the reaction 7r- + p--, n + K+ + + K-. This shows the total number of counts, in a narrow gate chosen to include the ~, as a function of beam momentum. The characteristic rapid rise near 1560 MeV/c is evidence of ~ production, as opposed to other sources of K pairs. The continuous line shows the best fit to the data for a ~ m e s o n cross-section rising li nearly with c.m.s, momentum, superimposed on nonresonant kaon and constant pion backgrounds. The dashed line shows the contribution from ~ meson production. obtained. A n a l y s i s to d a t e i n d i c a t e s that the data can be a d e q u a t e l y d e s c r i b e d by a s i m p l e S w a v e m o d el in which t h e t o t a l c r o s s - s e c t i o n n e a r t h r e s h o l d r i s e s l i n e a r l y with the final s ta te c . m . s , m o m e n t u m p*, and the production and decay a n g u l a r d i s t r i b u t i o n s a r e i s o t r o p i c . Using t h i s model, the t o t al c r o s s - s e c t i o n was found to be (0.28 ~ 0.07)p* ~b, w h e r e p* is m e a s u r e d in MeV/c. T h i s d e s c r i p t i o n i s c o n s i s t e n t with the data of H e s s et al. [3]. To i n v e s t i g a t e the e+e - d e c a y of the ~, the K + K - d e t e c t i o n s y s t e m w a s r e p l a c e d by the a r r a n g e m e n t of thin f o i l o p t i c a l s p a r k c h a m b e r s , l e a d p l a t e s and s c i n t i l l a t i o n c o u n t e r s s k e t c h e d in fig. 3. E l e c t r o n s w e r e i d e n t i f i e d by the c h a r a c t e r i s t i c s h o w e r s they p r o d u c e d in the lead p l a t e s . T h e r e w e r e 5 p l a t e s on e a c h s i d e of the t a r g e t , each about 1.1 r a d i a t i o n length thick• Each s p a r k
c h a m b e r module contained four a c t i v e gaps which in g e n e r a l made it p o s s i b l e to identify individual s e c o n d a r y t r a c k s in the s h o w e r s . Th e i n n e r m o s t c h a m b e r s of each side w e r e u s e d mainly f o r m e a s u r e m e n t s of i n c i d e n t e l e c t r o n t r a j e c t o r i e s , although they a l s o played a v a l u a b l e r o l e in r e ducing c e r t a i n b a c k g r o u n d s . C o u n t e r s p l a c e d i m m e d i a t e l y beyond the i n n e r s p a r k c h a m b e r s d e t e c t e d the a r r i v a l of c h a r g e d p a r t i c l e s , and f u r t h e r c o u n t e r s j u s t b e f o r e the t h i r d and fifth lead p l a t e s g a v e i n f o r m a t i o n on the s i z e of any s h o w e r s a f t e r two and four r a d i a t i o n lengths r e s p e c tively. W h i l e individual e l e c t r o n e n e r g i e s could v a r y between 400 and 900 MeV, the c o m b i n e d e n e r g y , being that of the ~, was a l m o s t constant. T h e r e f o r e the gains of the four s h o w e r c o u n t e r s w e r e m a t c h e d , and f o r the e l e c t r o n i c s e l e c t i o n a t h r e s h o l d r e q u i r e m e n t was p l a c e d on the c o m bined signal. The d e t a i l e d p e r f o r m a n c e of this a r r a n g e m e n t , including the design of c r i t e r i a to s e l e c t e l e c t r o n s ef f i ci en t l y w h i l e r e j e c t i n g b a c k ground p r o c e s s e s , was b a s e d on an a n a l y s i s of photographs of the s h o w e r s p r o d u c e d by c a l i b r a tion e l e c t r o n s taken b e f o r e the e x p e r i m e n t p r o p e r was s t a r t e d . I d e a l l y , the b r a n c h i n g r a t i o would be d e t e r m i n ed f r o m a c o m p a r i s o n of the y i e l d c u r v e s f o r the e+e - and K+K - d ecay m o d e s . H o w e v e r , b e c a u s e of the low v a l u e a n t i c i p a t e d f o r this r a t i o it was only p r a c t i c a b l e to m e a s u r e the y i e l d at two points, one below t h r e s h o l d and the o t h e r n e a r the m a x i m u m of the y i e l d c u r v e . T h e signal
10'/
Volume 27B. number 2
PHYSICS LETTERS
' r u n s ' w e r e made only by c o m b in in g t h r e e m o m e n t u m channels at 1574, 1582, and 1590 MeV/c, and the ' b ack g ro u n d r u n s ' by combining four f r o m 1526 to 1550 MeV/c. F o r the signal ru n s , a 5.5 ns wide n e u tr o n t i m e - o f - f l i g h t gate was p o s i t i o n e d f o r o p t i m u m ~b y i e l d a s shown by the peak in the K+K - channel. The l i m i t a t i o n s thus i m p o s e d on n e u tr o n v e l o c i t y , n e u t r o n angle and b e a m m o m e n t u m i m p l i e d that the m i s s i n g m a s s of events s e l e c t e d by the e l e c t r o n i c s was c e n t r e d on the @ m a s s and had a full width at half height of 15 MeV/c 2. T h e r a n g e of m i s s i n g m a s s a c c e p t e d by the 11 ns gate u s e d in the background runs was c e n t r e d on 1003 MeV/c 2 with a full width at half height of 20 MeV/c 2. A p p r o x i m a t e l y 220 000 photographs of the s h o w e r c h a m b e r s w e r e taken using a bias on the s h o w e r c o u n t e r s c a l c u l a t e d to be m o r e than 95% e f f i c i e n t for e+e - p a i r s . A f t e r c o r r e c t i o n s f o r the v a r i o u s l o s s e s , and a d j u s t m e n t s to take into account the v a r i o u s gate s e t t i n g s used, the total e f f e c t i v e n u m b e r of pions incident on the hydrogen t a r g e t was e s t i m a t e d at 5.2 ± 0.3 × 1010 during the s i g nal run s , and 7.6 ± 1.0 × 1010 during the b a c k ground r u n s . Ini t i al l y the photographs w e r e scanned u s i n g v e r y weak c r i t e r i a and y i e l d e d 4 700 events. A l m o s t all of t h e s e w e r e c l e a r l y background p r o c e s s e s , mainly a r i s i n g f r o m photon s h o w e r s s u p e r i m p o s e d on ~ m e s o n s , i n t e r n a l or e x t e r n a l c o n v e r s i o n of y r a y s f r o m yo d e c a y s , and the c a s u a l s u p e r p o s i t i o n of u n a s s o c i a t e d t r a c k s . More stringent c r i t e r i a were therefore designed in o r d e r to r e d u c e the n u m b e r of a c c e p t e d e v e n t s , and w e r e f i r s t c h e c k e d a g a i n s t the c a l i b r a t i o n e l e c t r o n s . They w e r e b a s e d on the n u m b e r of i n t e r a c t i o n s in the ' s h o w e r ' , (multiple t r a c k s , gaps, l a r g e angle s c a t t e r s ) but a l s o r e q u i r e d : e v i d e n c e of i n t e r a c t i o n b e f o r e the t h i r d r a d i a t i o n length; i n c o m p a t i b i l i t y with a single o r i g i n f o r all s e c o n d a r i e s ; and a d e g r e e of s y m m e t r y in the show er . S h o w e r s that showed c o l l i n e a r t r a c k s in at l e a s t the f i r s t four c h a m b e r s w e r e p e n a l i z e d and e v e n t s with two c l o s e l y spaced, a l m o s t p a r a l l e l , t r a c k s in the i n n e r c h a m b e r w e r e r e j e c t e d. The 255 e v e n t s s u r v i v i n g the a p p li c a ti o n of t h e s e c r i t e r i a w e r e m e a s u r e d at l e a s t t w i c e to obtain c o n s i s t a n t r e s u l t s , and those k i n e m a t i c a l l y i n c o n s i s t e n t with the p r o c e s s 7r- + p e + + e - + n w e r e r e j e c t e d . P r i n t s w e r e made of the r e m a i n i n g 80 c a n d i d a t e s and, to avoid b i a s e s , i d e n t i f i c a t i o n m a r k s on the p r i n t s w e r e c o n c e a l e d . It was s t i l l evident that many of the photographs w e r e not e x a m p l e s of e+e -. A f u r t h e r study was t h e r e f o r e made of non-fitting e ve nts , and two final c r i t e r i a w e r e d e v e l o p e d 108
10June 1968
and applied to the c a n d i d a t e s . One of t h e s e s p e c i fied e v i d e n c e of i n t e r a c t i o n s beyond the t h i r d l e a d plate, and the o t h e r was an ex t en si o n of the s y m m e t r y r e q u i r e m e n t s on the s h o w e r s . T h e r e s u l t i n g 27 c a n d i d a t e s w e r e l a t e r i d e n t i f i e d and c o m p r i s e d 17 events f o r the 5.2 × 1010 incident pions in the signal runs and 10 events f o r the 7.6 × 1010 pions in the background r u n s. A f t e r n o r m a l i s a t i o n this y i e l d e d an e x c e s s of 10.1 ± 4.8 events in the signal r u n s . P r i o r to i d e n t i f i c a t i o n , the 27 p r i n t s had been r e s c r u t i n i s e d and a s e l e c t i o n m a d e of t h o se events c o n s i d e r e d to be wholly s a t i s f a c t o r y e x a m p l e s of e l e c t r o n p a i r s . This s u b j e c t i v e s e l e c tion y i e l d e d 9 events of which 8 w e r e l a t e r found to be f r o m the signal runs and 1 f r o m the b a c k ground. Although the efficiency of this s e l e c t i o n was unknown, the r e s u l t was si g n i f i can t in that the p r o b a b i l i t y f o r a c h a n c e a s s y m e t r y of this magnitude was l e s s than 0.1%. A ~b decay mode o t h er than e+e -, for e x a m p l e q~ -~ ~+lr-g °, could have co n t r i b u t ed to the signal events, but we p r e d i c t l e s s than 0.1 event f r o m this s o u r c e . We conclude t h e r e f o r e that we have o b s e r v e d e x a m p l e s of ~ ~ e + + e - . As we cannot make a r e l i a b l e e s t i m a t e of the l o s s of genuine events in the s u b j e c t i v e s e l e c t i o n , we e s t i m a t e the b r a n c h i n g r a t i o by c o n s i d e r i n g t h e 27 events r e s u l t i n g f r o m the application of the final c r i t e r i a . Using the s i m p l e S wave model, the f r a c t i o n of 4 -~ e + + e - e v e n t s e n t e r i n g the s p a r k c h a m b e r f i d u c i a l v o l u m e was c a l c u l a t e d to be 22%. Th e c o m b i n e d efficiency of al l the s h o w e r and k i n e m a t i c c r i t e r i a f o r the two e l e c t r o n s w as e s t i m a t e d at 69 ± 6%. Th e p r e s e n t v a l u e s of the b r an ch i n g r a t i o (4 -* K + + K - ) / @ total) is 48 + 3%, although this may be r e v i s e d as f u r t h e r decay m o d e s a r e detected. We u s e this v a l u e and find that the t o t al n u m b e r of p o t e n t i a l 4 ~ e + + e - events was 14000 • 2200, which g i v e s a b r a n c h i n g r a t i o of (4 ~ e + + + e - ) / ( 4 ~ t o t a l ) of (7.2~-3.9) x 10 -4. F r o m the c u r r e n t e s t i m a t e of 3.4 i 0.8 MeV/c 2 f o r I~. [4], we deduce a p a r t i a l width f o r $ ~ e + + e - of:2.4 ± ± 1.5 k e V / c z, giving a v al u e f o r the coupling 2-~ ,~+2.9
Y(O/a~ = 1"v-0.7"
Th e b r a n c h i n g r a t i o found is c o n s i s t e n t with the e s t i m a t e of between (1.8 and 7.0) × 10 -4 of Wehmann et al. [5] (the main u n c e r t a i n t y in this e x p e r i m e n t a r i s e s f r o m lack of knowledge of the production c r o s s - s e c t i o n ) and of A s t r a c a t u r o v et al. [6] who find (6.6+4: 4) x 10 -4. In the l a t t e r n u m b e r , h o w e v e r , it should be noted that no a l l o w a n c e has been made f o r the s u b s t a n t i a l u n c e r t a i n t y in the 4 production c r o s s - s e c t i o n . E v i d e n c e f o r ~b ~ e + + e - has a l s o been r e p o r t e d by A s b u r y et al. [7].
PHYSICS
Volume 27B, n u m b e r 2
LETTERS
T h e p r e c i s i o n of o u r m e a s u r e m e n t i s u n f o r t u n a t e l y t o o l o w t o a l l o w a n y d e t a i l e d c h e c k of the relationship between the vector meson coupli n g c o n s t a n t s . T h e c o m p i l a t i o n b y T i n g [8] s u g g e s t s a v a l u e f o r T ~ / 4 ~ of a b o u t 0 . 4 4 , w h i c h , c o ~ n b i n e d w i t h t h e SU(6) p r e d i c t i o n [9] of T~ : ~ 2 : y-2~b = 9 : 1 : 2, w o u l d g i v e ~ / 4 ~ of a b o u t 2.0, i n g o o d a g r e e m e n t w i t h o u r v a l u e . H o w e v e r t h e e s t i m a t e s of S a k u r a i et a l . [10] a n d of D a s e t a l . [11] w h i c h i m p l y t h a t w e s h o u l d h a v e s e e n 6 a n d 4 e x a m p l e s r e s p e c t i v e l y , of ~ -~ e + + e - a r e also consistent with our result.
$
W e w i s h t o t h a n k D. G. M i l l e r , R. F. H o b b s , D. J . S c h o l e s a n d R . J . G r a y f o r t h e i r i n v a l u a b l e w o r k i n t h e c o n s t r u c t i o n of t h e a p p a r a t u s , a n d also the Nimrod support staff and operating crew for their continued assistance.
References 1. D . M . B i n n i e , A.Duane, M . R . J a n e , W . G . J o n e s , D. C. Mason, J . A . Newth, D. C. P o t t e r , I.U. Rahman, J. W a l t e r s , 13. Dickinson, R . J . Ellison, A . E . H a r c k ham, M. Ibbotson, R. M a r s h a l l , R. F. T e m p l e m a n and A . J . W y n r o e , P h y s . L e t t e r s 18 (1965) 348.
10 June 1968
2. W . G . J o n e s , D . M . B i n n i e , A.Duane, J . P . H o r s e y , D. C. Mason, J . A . Ne~¢h, I . U . R a h m a n , J. Walters, N. Horwitz and P. Palit, Phys. L e t t e r s 23 (1966) 597. 3. R . I . H e s s , O.I.Dahl, L . M . H a r d y , J . K i r z a n d D . H. Miller, Phys. Rev. L e t t e r s 17 (1966) 1109. 4. A . H . R o s e n f e l d , N. B a r a s 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. S~ding, W . J . Willis and C.G. Wohl, UCRL - 8030, September, 1967. 5. A . A . W e h m a n , E . E n g e l s , C.M.Hoffman, P . G . I n nocenti, R. Wilson, W.A. Blanpied, D . J . Drickey, L. N. Hand and D.G. Stairs, Phys. Rev. L e t t e r s 20 (1968) 748. 6. R.G. A s t r a c a t u r o v , M.A. Azimov, A.M. Baldin, A. S. Belousov, I.V. Chuvilo, J . Hladky, V.I. Ivanov, M. N. Khachaturyan, M.S. Khvastunov, A . T . Matyushin, V . T . Matyushin, L . N . Shtarkov and L.I. Zhuravleva, P r e p r i n t JINR E1-3770 (1968). 7. J . G . A s b u r y , U . B e c k e r , W.K. B e r t r a m , P . J o o s , M.Rohde, A . J . S . Smith, C . L . J o r d a n and S . C . C . Ting, Phys. Rev. L e t t e r s 19 (1967) 869. 8. S . C . C . Ting, Intern. Syrup. on E l e c t r o n and photon i n t e r a c t i o n s at high e n e r g i e s , Stanford U n i v e r s i t y (1967). 9. P . G . O . F r e u n d , Nuovo Cimento 44A (1966) 411; C.A. Levinson, H . J . Lipkin and S.Meshkov, Phys. L e t t e r s 7 (1963) 81; H . H a r a r i , P h y s . Rev. 155 (1967) 1565. 10. R . J . O a k e s and J . J . S a k a r a i , Phys. Rev. L e t t e r s 19 (1967) 1266. 11. T . D a s , V . S . M a t h u r and S.Okubo, Phys. Rev. L e t t e r s 19 (1967) 470.
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