- Email: [email protected]
Energy dependence of the structure in high energy proton-proton elastic scattering
Volume 34B, number 5
PHYSICS L E T T E R S
ENERGY DEPENDENCE
15March 1971
OF THE S T R U C T U R E
IN HIGH E N E R G Y P R O T O N - P R O T O N
ELASTIC SCATTERING
J. V. ALLABY. A. N. DIDDENS. R. W. ROBINSON. A. KLGVNING*, J. LITT, L.S. ROCHESTER**, K. SCHLUPMANN, A. M. WETHERELL C E R N , Goner'a, S u ' i t z e r l a n d
and U. A M A L D I , R. B I A N C A S T E L L I ,
C. BOSIO a n d G. M A T T H I A E
P h y s i c s Laboralo~:v , I s l i t u l o S u p e r i o r e di Saniti~ and I N F N , S o l l o s e z i o n e Sanilg~, R o m e , Italy Received
25 J:mu3ry
1971
The anguhlr distribution of proton-proton el~stic sc~ttering h~isbeen measured for incident be~m momentaof i0.0. 12.0. 14.2 ~md 24.0 GeV/c over ~l r~mge of labor3tory se~itteringangles from 12 to "tbout 140 mrad. The results are compared with the fourth power of the electromngnetic ferm factor of the proton.
Proton-proton elastic scattering has been s t u d i e d by d e t e c t i n g t h e f o r w a r d s c a t t e r e d p r o ton in a h i g h r e s o l u t i o n m a g n e t i c s p e c t r o m e t e r . T h e t e c h n i q u e u s e d w a s s i m i l a r to t h a t d e s c r i b e d in r e f . [1]. A c o m p l e t e d i s c u s s i o n of t h e a p p a r a t u s a n d t h e d a t a a n a l y s i s will a p p e a r in a f u t u r e p a p e r , t h u s only a b r i e f d e s c r i p t i o n will b e g i v e n here. The slow ejected beam from the CERN proton s y n c h r o t r o n (for n o r m a l 24 GeV. c o p e r a t i o n a b o u t 1012 p r o t o n s in a 350 m s b u r s t ) w a s u s e d to b o m b a r d a 10 o r a 20 c m long l i q u i d h y d r o g e n t a r g e t . T h e s c a t t e r e d p r o t o n s w e r e d e t e c t e d in a magnetic spectrometer. 80 m long, which had two loci, the f i r s t ( F l ) d i s p e r s i v e , and the second (F2) achromatic. Scattering angles within the range 12 - 147 mrad were selected by varying the positions and currents of two bending magnets, placed immediately after the target, and the current of a fixed bending magnet on the spectrometer axis. An.array of nine scintillation counters (each accepting a momentum bite Ap p = +0.1%) placed at F I provided nine contiguous momentum channels. A second a r r a y was placed at F 2 and used to improve the momentum resolution at large angles (where the finite length of * P r e s e n t a d d r e s s : Dep,~rtment of P h y s i c s . U n i v e r s i ty of B e r g e n , B e r g e n , N o r w a y . ** S u p p o r t e d by a g r a n t f r o m the Nntion,~t -geienc~ Found,~ tion.
t h e t a r g e t c a u s e d a d e t e r i o r a t i o n of the r e s o l u t i o n ) by d i v i d i n g the i m a g e of t h e t a r g e t i n t o f i v e z o n e s . T h e i n f o r m a t i o n p r o v i d e d by t h e c o i n c i d e n c e s b e t w e e n t h e two a r r a y s of c o u n t e r s a l l o w e d a m o m e n t u m r e s o l u t i o n of ~0.2% to b e m a i n t a i n e d at a l l a n g l e s . T h e c o m p u t e d s o l i d a n g l e of t h e s p e c t r o m e t e r w a s 1.3 ~< 10 -5 s t e r a d : it c o u l d b e r e d u c e d by m e a n s of a v a r i a b l e d i a m e t e r c o l l i m a t o r in o r d e r to k e e p e l e c t r o n i c s d e a d - t i m e l o s s e s at a t o l e r a b l e l e v e l . A s m a l l o n - l i n e c o m p u t e r w a s u s e d to c a l c u l a t e a n d s e t t h e c u r r e n t s of t h e e l e m e n t s of t h e s p e c t r o m e t e r . T h e c o m p u t e r w a s a l s o u s e d to c h e c k a n d t r i m t h e s e t t i n g s of t h e m a g n e t i c e l e m e n t s a n d to r e c o r d a n d plot t h e d a t a . T h e p r i m a r y b e a m i n t e n s i t y w a s m o n i t o r e d by a s e c o n d a r y e m i s s i o n c h a m b e r i n s e r t e d in t h e b e a m u p s t r e a m of t h e t a r g e t , a n d by two c o u n t e r t e l e s c o p e s w h i c h d e t e c t e d p a r t i c l e s p r o d u c e d in t h e t a r g e t . T h e a b s o l u t e c a l i b r a t i o n of t h e b e a m w a s o b t a i n e d by m e a s u r i n g the 2 4 N a a c t i v i t y i n d u c e d in i r r a d i a t e d aluminium foils. For the 24Na activation cross s e c t i o n t h e v a l u e 8.6 m b w a s u s e d [2 I. A n g u l a r d i s t r i b u t i o n s of p r o t o n - p r o t o n e l a s t i c s c a t t e r i n g h a v e b e e n m e a s u r e d at 10.0, 12.0, 14.2 a n d 24.0 G e V "c. F o r t h e t h r e e l o w e r e n e r gies the slow ejectedFroton beam was operated in a p a r a s i t i c m o d e r e s u l t i n g i n a s h o r t e r e f f e c tiv_e b u r s t l e n g t h a n d l o w e r i n t e n s i t y . S o m e d a t a h a v e a l s o b e e n t a k e n at 19.2 G e V c to o v e r l a p with t h e m e a s u r e m e n t s of ref. [1]. T h e e l a s t i c 431
Volume 34B. number 5
PHYSICS
c r o s s s e c t i o n s h a v e b e e n o b t a i n e d by i n t e g r a t i n g the a r e a u n d e r the e l a s t i c peak. E m p t y t a r g e t s u b t r a c t i o n s w e r e b e t w e e n 1(#~,and 8%. d e p e n d i n g on the s c a t t e r i n g a n g l e , and t h e c o r r e c t i o n a p p l i e d f o r p r o t o n a b s o r p t i o n along the s p e c t r o m e t e r w a s a b o u t 10~. At the s m a l l e s t a n g l e s d e a d t i m e and a c c i d e n t a l c o r r e c t i o n s up to 10% w e r e applied. B e c a u s e of the f i n i t e t a r g e t l e n g t h , p r o t o n s w h i c h w e r e s c a t t e r e d t w i c e in the t a r g e t g a v e an a p p r e c i a b l e c o n t r i b u t i o n to the m o m e n t u m s p e c t r u m at i n t e r m e d i a t e a n g l e s . Due to the g o o d momentum resolution, these protons, whose mom e n t u m d i s t r i b u t i o n p e a k s a b o v e t h a t of the s i n gly s c a t t e r e d p r o t o n s c o u l d be s u b t r a c t e d . T h e a c c u r a c y of t h i s s u b t r a c t i o n w a s c h e c k e d by c o m p a r i n g d a t a t a k e n w i t h 10 c m and 20 c m t a r gets. S t a t i s t i c a l e r r o r s w e r e a l w a y s s m a l l e r than t h e s y s t e m a t i c e r r o r s . F r o m a c o n s i d e r a t i o n of the s p r e a d in v a l u e of the p o i n t s m e a s u r e d s e v e r a l t i m e s , it was c o n c l u d e d that i n d i v i d u a l s y s t e m a t i c e r r o r s of ±10q~ s h o u l d be a s s i g n e d to e a c h m e a s u r e d d a t a point. It is b e l i e v e d that an i m p o r t a n t c o n t r i b u t i o n to t h i s s y s t e m a t i c e r r o r a r o s e f r o m the u n c e r t a i n t y and i r r e p r o d u c i b i l i t y in the i n c i d e n t b e a m d i r e c t i o n and t h u s in the a b s o l u t e v a l u e of the s c a t t e r i n g angle. In a d d i t i o n at 24 GeV c t h e r e is an o v e r a l l s c a l e e r r o r of ±10(~ m a i n l y b e c a u s e of the ±7% u n c e r t a i n t y in the 24Na a c t i v a t i o n c r o s s s e c t i o n u s e d in the a b s o l u t e n o r m a l i z a t i o n . At 10.0, 12.0, and 14.2 GeV c the s c a l e e r r o r is e s t i m a t e d to be ±15C~. The m e a s u r e d angular d i s t r i b u t i o n s are shown in fig. 1 t o g e t h e r with o t h e r r e p r e s e n t a t i v e d a t a [ 1 , 3 , 4 1 in the f o r m of d ~ d/ p l o t t e d a g a i n s t the invariant f o u r - m o m e n t u m t r a n s f e r squared, it. T h e new d a t a t a k e n at 19.2 GeV c a g r e e well with the p r e v i o u s l y m e a s u r e d c r o s s s e c t i o n s [1 ]. T h e s h o u l d e r - l i k e s t r u c t u r e a l r e a d y o b s e r v e d at 19.2 GeV c in the r e g i o n of f o u r - m o m e n t u m t r a n s f e r s q u a r e d !/¢- 1.5 GeV 2 is e v i d e n t i n a l l t h e newly measured angular distributions. This structure s e e m s to b e c o m e m o r e p r o n o u n c e d a s the e n e r g y i n c r e a s e s in the r a n g e c o v e r e d by the p r e s e n t e x p e r i m e n t . On the o t h e r h a n d . the 5 and 7 GeV c d a t a of C l y d e [31 a r e s m o o t h and s t r u c t u r e l e s s . T h i s is c o n f i r m e d by the m o r e r e c e n t e x p e r i m e n t of B r a b s o n et al. [5 I w h i c h e x t e n d s up to 5.5 GeV c. T h e c o n c l u s i o n is that the s t r u c t u r e d e v e l o p s r a t h e r r a p i d l y b e t w e e n 7 and 10 GeV c. T h a t s o m e c h a r a c t e r i s t i c of pp s c a t t e r i n g changes rapidly around 8- 9 GeV c has already b e e n n o t e d in s o f a r a s the e n e r g y d e p e n d e n c e of the 90 ° c r o s s s e c t i o n e x h i b i t s a s h a r p b r e a k [6[ a n d that the s l o p e of the a n g u l a r d i s t r i b u t i o n s 432
LETTERS
15 March 1971
102c
PROTON-PROTON ELASTIC SCATTERING MOMENTA (GeVIc) 101
- CLYDE
(1966) &0,5.0,70,71
(1967) 14 2 (/9~) 1 71,101,121
ALLABY eta(
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Fig, 1. Anguiar distributions for proton-proton elastic scqttering. The new results r e p o r t e d in this letter are those marked All'tby et hi. (1971). The data at the same incident momentum have been joined by a smooth curve. The symbol qt the end of the curves corresponding t() the low-energy data indicates 90 ° c e n t r e - o f - m a s s . The curve Inbelled G4(l) indicates the fourth power of the electromagnetic form factor of the proton. n e a r to 90 ° i n c r e a s e s a b r u p t l y [4]. It s e e m s e v i d e n t that t h e s e o b s e r v a t i o n s at l a r g e s c a t t e r i n g a n g l e s and the s t r u c t u r e at s m a l l e r s c a t t e r i n g a n g l e s a r e m a n i f e s t a t i o n s of the s a m e p h e n o m e non. S e v e r a l t h e o r e t i c a l d i s c u s s i o n s {71 h a v e s u g g e s t e d t h a t the a s y m p t o t i c pp d i f f e r e n t i a l c r o s s s e c t i o n s h o u l d be p r o p o r t i o n a l to G4(/), the f o u r t h p o w e r of the e l e c t r o m a g n e t i c f o r m f a c t o r of the p r o t o n . T h i s f u n c t i o n , n o r m a l i z e d to the o p t i c a l t h e o r e m v a l u e at 20 GeV c , is s h o w n in fig. 1, t o g e t h e r with the m e a s u r e d d i s t r i b u t i o n s F o r s i m p l i c i t y , the d i p o l e f o r m : G(I) - (1+ !/I 0-71) - 2 h a s b e e n u s e d to r e p r e s e n t G(I), s i n c e the d e v i a t i o n s of the e x p e r i m e n t a l d a t a on G(I) f r o m t h e d i p o l e f o r m m a k e no e s s e n t i a l d i f f e r e n c e f o r t h i s
l
8
Volume 34B. number 5
PHYSICS L E T T E R S
comparison. The present data show that as the e n e r g y i n c r e a s e s t h e pp a n g u l a r d i s t r i b u t i o n at m o d e r a t e m o m e n t u m t r a n s f e r (1l! ~ 1 G e V 2) d r o p s b e l o w G4(/) by a f a c t o r ~ 1 0 a s t h e s t r u c t u r e d e v e l o p s , b u t r i s e s to a b o v e G4(/) at l a r g e r momentum transfer. This and other details are b e t t e r d i s p l a y e d in fig. 2 w h i c h s h o w s a c o m p i l a t i o n of t h e a v a i l a b l e d a t a [ 1 , 3 , 4, 81 in t h e f o r m of X G4(I), w h e r e X is t h e r a t i o of t h e d i f f e r e n t i a l c r o s s s e c t i o n , dcr d/. to the c a l c u l a t e d o p t i c a l t h e o r e m v a l u e in the f o r w a r d d i r e c t i o n . Fig. 2 s h o w s , in a v e r y s t r i k i n g way, t h e d e v e l o p m e n t of t h e s t r u c t u r e a s a f u n c t i o n of t h e incident momentum. The most prominent feature is t h e s h a r p dip a t ]11 = 1.2 G e V 2, w h i c h d e v e l o p s at i n c i d e n t m o m e n t a a b o v e 7 G e V / c . T h i s dip
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entiai c r o s s sections and G4(/). The qunntitv X is the ratio of the differential c r o s s section, dff/dl, to the calculated optical theorem value in the forward direction~ The figure shows X divided by G4(I) as a function of 1 i. The lines drawn nre m e r e l y to link the points c o r r e sponding to a fixed incident momentum which is indicated at the end of each line.
15 March 1971
c o r r e s p o n d s to t h e r a p i d c h a n g e of t h e s l o p e of t h e a n g u l a r d i s t r i b u t i o n s e e n at t h e e n d of t h e d i f f r a c t i o n p e a k in fig. 1. In a d d i t i o n , fig~ 2 s h o w s at a r o u n d ill : 4 G e V 2 a f l a t t e n i n g of t h e q u a n t i t y X G 4 ( / ) s t a r t i n g at 10 G e V c a n d b e c o m ing m o r e p r o n o u n c e d with i n c r e a s i n g i n c i d e n t momentum. This additional feature corresponds to t h e p o i n t of i n f l e x i o n a t t h e e n d of t h e s h o u l d e r in t h e a n g u l a r d i s t r i b u t i o n s d i s p l a y e d in fig. 1. It a p p e a r s t h a t t h e s t r u c t u r e p r e s e n t in the pp e l a s t i c s c a t t e r i n g is q u a l i t a t i v e l y d i f f e r e n t f r o m t h a t a l r e a d y e x h i b i t e d at l o w e r i n c i d e n t m o m e n t a i n ~ i p , K - p a n d ~ p e l a s t i c s c a t t e r i n g f o r I/i " 2 G e V 2. T h e s e l a t t e r s t r u c t u r e s , w h i c h t e n d r o d e c r e a s e with e n e r g y , h a v e b e e n d i s c u s s e d in v a r ious ways and a strong connection has been made [91 with t h e o p e r a t i o n of d i r e c t c h a n n e l r e s o n a n c e s w h i c h a r e a b s e n t in t h e pp s y s t e m . An e l a b o r a t i o n of t h e i d e a s of Wu a n d Yang [7 I h a s b e e n p r o p o s e d [101 w h i c h t r e a t s t h e two p r o t o n s a s e x t e n d e d d i s t r i b u t i o n s of m a t t e r , w h o s e s h a p e i s r e l a t e d to t h e e l e c t r o m a g n e t i c f o r m f a c t o r G(/). T h i s m o d e l , w h i c h is s u p p o s e d t o h o l d a s y m p t o t i c a l l y a t i n f i n i t e e n e r g y , r e s u l t s in a n angular distribution which contains strong structure. Without further assumptions, this model c a n n o t p r e d i c t t h e o n s e t of t h e s t r u c t u r e a s a f u n c t i o n of e n e r g y . O t h e r m o d e l s h a v e b e e n s u g g e s t e d [11] w h i c h a t t e m p t to d e s c r i b e t h e e n e r g y d e p e n d e n c e of t h e e l a s t i c c r o s s s e c t i o n . Howe v e r , at t h e p r e s e n t s t a g e of d e v e l o p m e n t of t h e s e m o d e l s , no q u a n t i t a t i v e c o m p a r i s o n is p o s s i b l e . All t h e m o d e l s p r e d i c t t h a t a s e c o n d m i n i m u m at ![i ~ 5 G e V 2 s h o u l d d e v e l o p at h i g h e r e n e r g i e s . It is c o n c e i v a b l e t h a t the k n e e d e v e l o p ing a t !1! ~ 4 G e V 2 in fig. 2 will e v e n t u a l l y b e c o m e a s e c o n d d i p a s the e n e r g y i s i n c r e a s e d A c o m m o n f e a t u r e of t h e s e m o d e l s is t h a t t h e y m a y be f o r m a l l y i n t e r p r e t e d {121 in t e r m s of a multiple scattering mechanism. Qualitatively, o n e t r i e s to u n d e r s t a n d t h e pp s t r u c t u r e a s due to t r a n s i t i o n s b e t w e e n d i f f e r e n t o r d e r s of s c a t t e r ing ( s i n g l e to d o u b l e ) e t c . ) by a n a l o g y with the s c a t t e r i n g of c o m p o s i t e s y s t e m s ( n u c l e u s - n u c l e u s o r h a d r o n - n u c l e u s ) . In t h i s i n t e r p r e t a t i o n one w o u l d e x p e c t s u c h s t r u c t u r e to b e a c o m m o n p r o p e r t y of a l l h a d r o n - h a d r o n s c a t t e r i n g p r o c esses. A similar behaviour should therefore be s e e n i n ~ i p , K i p a n d ~p s c a t t e r i n g at h i g h e r e n ergies than have so far been explored. In c o n c l u s i o n , it m a y b e s a i d t h a t w h i l e t h e s t r u c t u r e in t h e pp a n g u l a r d i s t r i b u t i o n s e t s in r a t h e r s u d d e n l y b e t w e e n 7 a n d 10 G e V c i t s f u r t h e r d e v e l o p m e n t up to 30 G e V c is not v e r y r a p id t h u s i n d i c a t i n g a r a t h e r s l o w a p p r o a c h to a l i m i t i n g d i s t r i b u t i o n , if s u c h e x i s t s . F u r t h e r e x 433
Volume 34B. number 5
PHYSICS L E T T E R S
p e r i m e n t a l w o r k in t h e 7 - 10 GeV c t r a n s i t i o n r e g i o n is c l e a r l y d e s i r a b l e t o g e t h e r with e x t e n s i o n s to e n e r g i e s a b o v e 30 GeV in p a r t i c u l a r to s t u d y the p o s s i b l e d e v e l o p m e n t of a s e c o n d s t r u c t u r e a r o u n d i/i ~ 4 GeV 2. A c o m p a r i s o n with the s c a t t e r i n g of o t h e r h a d r o n i c s y s t e m s at h i g h e r e n e r g i e s will a l s o be m o s t i m p o r t a n t in d e t e r m i n i n g w h e t h e r o r not t h e r e i s a c o m m o n s c a t t e r i n g m e c h a n i s m at l a r g e m o m e n t u m t r a n s f e r s a n d v e r y high e n e r g i e s . T h e a u t h o r s w i s h to a c k n o w l e d g e the e x c e l l e n t t e c h n i c a l s u p p o r t p r o v i d e d by R. D o n n e t , M. F e r r a t and C. S. S t f i h l b r a n d t . T h e e f f i c i e n t o p e r a t i o n of the s l o w e j e c t e d p r o t o n b e a m and the i n s t a l l a t i o n of the s p e c t r o m e t e r w e r e p r o v i d e d by t h e MPS d i v i s i o n ( P r o t o n S y n c h r o t r o n ) . P. S c h a r f f - H a n s e n g a v e c o n s i d e r a b l e h e l p with the p r o g r a m m i n g of the o n - l i n e c o m p u t e r and I. J a r s t o r f f c a r r i e d out the m e a s u r e m e n t s on the activated aluminium foils. The s e c o n d a r y e m i s s i o n m o n i t o r w a s p r o v i d e d by V. A g o r i t s a s , and the s p e c i a l liquid h y d r o g e n t a r g e t w a s s u p p l i e d by G. B r a n d and L. M a z z o n e . F i n a l l y . the a u t h o r s would like to t h a n k G. S h e r i n g who p a r t i c i p a t e d in the e a r l y p h a s e of the e x p e r i m e n t .
Rc:fel.'g3tlC¢,s [1] d.V,Allaby, F, Binon. A.N. Diddens. P. Duteil, A. Klovning, R, Meunier. d . P . Peigneux. E.J.S:~chnridis. K. Sehli~pmann. M. Spighel, d . P . Stroot. A.M. Thorndikc andA:qM.Wetherell. Phys. Letters 28B (1968) 67. [2] J . B . Cumming, J. Hudis. A.M. Poskanzer andS. Kuuffmann. Phys. Rev. 128 (1962) 2392; J. B. Cumming. Ann. Rex'. Nu,el. Science 13 (1963) 261. [31 A. R. Clyde. UCRL-16275 (1966): J . V . A l l a b y . A.N. Diddens. A.Kiovning, E . L i l l e -
434
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thun, E.J. Sacharidis. K. Schltipmann andA.M. Wethere[|. Phys.Letters 27B (1968) 49: C.M.Ankenbrandt, A.R. Clark. BoCork. T.Elioff, L.T.Kerth and W.A.Wenzel. Phys.Rev. 170 (1968) 1223. [4] J.V.Allaby. G. Coeconi. A.N. Diddens. A.Klovning G. Matthiae. E.J.Saeharidis andA.M. Wetherell. Phys. Letters 25B (1967) 156. [51 B.B.Branson, R.R. Crittenden, R.M. Heinz. R.C. Kammerud. H.A.Neal, H.W,Paik. R.A.Sidwell and K.F.Suen. Phys.Rex,. Letters 23 (1969) 1306; B.B.Brnbson. R.R. Crittenden. R.M. Heinz. R.C. Kammerud. H.A.Neal, H.W. Paik andR.A.Sidwell. Physics Dept.. Indiana University. Bloomington, Int. Report COO-2009-18. [61 C,W. Akerlof. R.H. Hieber. A.D, Krisch. K.W. Edwards. L,G. Ratner andK, Ruddick. Ph\'s.Rev.
L e t t e r s 17 (1966) 1105: Phys. Rex'. 1 59 (1967) 1138. [7[ T.T. Wu and C.N. Yang. Phys. Rev. 137 (1965) B708, L. van Hove. Proc. Stoney-Brook Conf, on High Energy Two-body Refections (April 1966) and in: Particle interactions "It high e n e r g i e s (Scottish Universities Summer School 1966). eds. T.W. P r e i s t and L . L . V i c k (Oliver and Boyd, 1967! p. 63. sect. 4.4. R . A . C n r r i g a n , R.M. Edelstein. N.C. Hien. T . J . [8] McMahon. I. Nadelh~t. E.W. Anderson. E . J . Bleser. G.B. Collins. T. Fujii. J . M e n e s and F. Turkot. Phys. Rev. L e t t e r s 24 (1970) 683: G. Coceoni. V.T. Cocconi. A.D. Kriseh. J . O r e a r . R. Rubinstein, D.B. Sctiri. W. F. Baker. E.W. Jenkins and A . L . Read. Phys. Rev. L e t t e r s 11 (1963) 499: Phys. Rev. 138 (1965) B165. [9] H. Harari, submitted to Annuls of Physics. [10] L. Durand a n d R . Lipes. Phys. Rev. L e t t e r s 20 (1968) 637: T.T. Chou and C.N.Yang. Phys. Rex'. L e t t e r s 20 (1968) 1213. [11] S. Frautschi and 13. Margolis, Nuovo Cimento 56A (1968) 1155: C.B. Chiu a n d J , Finkelstein. Nuovo Cimento 57A (1968) 649: 59A (1969) 92. [12] J.D. Jackson. Rev, Mod. Phys. 42 (1970) 12,
PHYSICS L E T T E R S
ENERGY DEPENDENCE
15March 1971
OF THE S T R U C T U R E
IN HIGH E N E R G Y P R O T O N - P R O T O N
ELASTIC SCATTERING
J. V. ALLABY. A. N. DIDDENS. R. W. ROBINSON. A. KLGVNING*, J. LITT, L.S. ROCHESTER**, K. SCHLUPMANN, A. M. WETHERELL C E R N , Goner'a, S u ' i t z e r l a n d
and U. A M A L D I , R. B I A N C A S T E L L I ,
C. BOSIO a n d G. M A T T H I A E
P h y s i c s Laboralo~:v , I s l i t u l o S u p e r i o r e di Saniti~ and I N F N , S o l l o s e z i o n e Sanilg~, R o m e , Italy Received
25 J:mu3ry
1971
The anguhlr distribution of proton-proton el~stic sc~ttering h~isbeen measured for incident be~m momentaof i0.0. 12.0. 14.2 ~md 24.0 GeV/c over ~l r~mge of labor3tory se~itteringangles from 12 to "tbout 140 mrad. The results are compared with the fourth power of the electromngnetic ferm factor of the proton.
Proton-proton elastic scattering has been s t u d i e d by d e t e c t i n g t h e f o r w a r d s c a t t e r e d p r o ton in a h i g h r e s o l u t i o n m a g n e t i c s p e c t r o m e t e r . T h e t e c h n i q u e u s e d w a s s i m i l a r to t h a t d e s c r i b e d in r e f . [1]. A c o m p l e t e d i s c u s s i o n of t h e a p p a r a t u s a n d t h e d a t a a n a l y s i s will a p p e a r in a f u t u r e p a p e r , t h u s only a b r i e f d e s c r i p t i o n will b e g i v e n here. The slow ejected beam from the CERN proton s y n c h r o t r o n (for n o r m a l 24 GeV. c o p e r a t i o n a b o u t 1012 p r o t o n s in a 350 m s b u r s t ) w a s u s e d to b o m b a r d a 10 o r a 20 c m long l i q u i d h y d r o g e n t a r g e t . T h e s c a t t e r e d p r o t o n s w e r e d e t e c t e d in a magnetic spectrometer. 80 m long, which had two loci, the f i r s t ( F l ) d i s p e r s i v e , and the second (F2) achromatic. Scattering angles within the range 12 - 147 mrad were selected by varying the positions and currents of two bending magnets, placed immediately after the target, and the current of a fixed bending magnet on the spectrometer axis. An.array of nine scintillation counters (each accepting a momentum bite Ap p = +0.1%) placed at F I provided nine contiguous momentum channels. A second a r r a y was placed at F 2 and used to improve the momentum resolution at large angles (where the finite length of * P r e s e n t a d d r e s s : Dep,~rtment of P h y s i c s . U n i v e r s i ty of B e r g e n , B e r g e n , N o r w a y . ** S u p p o r t e d by a g r a n t f r o m the Nntion,~t -geienc~ Found,~ tion.
t h e t a r g e t c a u s e d a d e t e r i o r a t i o n of the r e s o l u t i o n ) by d i v i d i n g the i m a g e of t h e t a r g e t i n t o f i v e z o n e s . T h e i n f o r m a t i o n p r o v i d e d by t h e c o i n c i d e n c e s b e t w e e n t h e two a r r a y s of c o u n t e r s a l l o w e d a m o m e n t u m r e s o l u t i o n of ~0.2% to b e m a i n t a i n e d at a l l a n g l e s . T h e c o m p u t e d s o l i d a n g l e of t h e s p e c t r o m e t e r w a s 1.3 ~< 10 -5 s t e r a d : it c o u l d b e r e d u c e d by m e a n s of a v a r i a b l e d i a m e t e r c o l l i m a t o r in o r d e r to k e e p e l e c t r o n i c s d e a d - t i m e l o s s e s at a t o l e r a b l e l e v e l . A s m a l l o n - l i n e c o m p u t e r w a s u s e d to c a l c u l a t e a n d s e t t h e c u r r e n t s of t h e e l e m e n t s of t h e s p e c t r o m e t e r . T h e c o m p u t e r w a s a l s o u s e d to c h e c k a n d t r i m t h e s e t t i n g s of t h e m a g n e t i c e l e m e n t s a n d to r e c o r d a n d plot t h e d a t a . T h e p r i m a r y b e a m i n t e n s i t y w a s m o n i t o r e d by a s e c o n d a r y e m i s s i o n c h a m b e r i n s e r t e d in t h e b e a m u p s t r e a m of t h e t a r g e t , a n d by two c o u n t e r t e l e s c o p e s w h i c h d e t e c t e d p a r t i c l e s p r o d u c e d in t h e t a r g e t . T h e a b s o l u t e c a l i b r a t i o n of t h e b e a m w a s o b t a i n e d by m e a s u r i n g the 2 4 N a a c t i v i t y i n d u c e d in i r r a d i a t e d aluminium foils. For the 24Na activation cross s e c t i o n t h e v a l u e 8.6 m b w a s u s e d [2 I. A n g u l a r d i s t r i b u t i o n s of p r o t o n - p r o t o n e l a s t i c s c a t t e r i n g h a v e b e e n m e a s u r e d at 10.0, 12.0, 14.2 a n d 24.0 G e V "c. F o r t h e t h r e e l o w e r e n e r gies the slow ejectedFroton beam was operated in a p a r a s i t i c m o d e r e s u l t i n g i n a s h o r t e r e f f e c tiv_e b u r s t l e n g t h a n d l o w e r i n t e n s i t y . S o m e d a t a h a v e a l s o b e e n t a k e n at 19.2 G e V c to o v e r l a p with t h e m e a s u r e m e n t s of ref. [1]. T h e e l a s t i c 431
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PHYSICS
c r o s s s e c t i o n s h a v e b e e n o b t a i n e d by i n t e g r a t i n g the a r e a u n d e r the e l a s t i c peak. E m p t y t a r g e t s u b t r a c t i o n s w e r e b e t w e e n 1(#~,and 8%. d e p e n d i n g on the s c a t t e r i n g a n g l e , and t h e c o r r e c t i o n a p p l i e d f o r p r o t o n a b s o r p t i o n along the s p e c t r o m e t e r w a s a b o u t 10~. At the s m a l l e s t a n g l e s d e a d t i m e and a c c i d e n t a l c o r r e c t i o n s up to 10% w e r e applied. B e c a u s e of the f i n i t e t a r g e t l e n g t h , p r o t o n s w h i c h w e r e s c a t t e r e d t w i c e in the t a r g e t g a v e an a p p r e c i a b l e c o n t r i b u t i o n to the m o m e n t u m s p e c t r u m at i n t e r m e d i a t e a n g l e s . Due to the g o o d momentum resolution, these protons, whose mom e n t u m d i s t r i b u t i o n p e a k s a b o v e t h a t of the s i n gly s c a t t e r e d p r o t o n s c o u l d be s u b t r a c t e d . T h e a c c u r a c y of t h i s s u b t r a c t i o n w a s c h e c k e d by c o m p a r i n g d a t a t a k e n w i t h 10 c m and 20 c m t a r gets. S t a t i s t i c a l e r r o r s w e r e a l w a y s s m a l l e r than t h e s y s t e m a t i c e r r o r s . F r o m a c o n s i d e r a t i o n of the s p r e a d in v a l u e of the p o i n t s m e a s u r e d s e v e r a l t i m e s , it was c o n c l u d e d that i n d i v i d u a l s y s t e m a t i c e r r o r s of ±10q~ s h o u l d be a s s i g n e d to e a c h m e a s u r e d d a t a point. It is b e l i e v e d that an i m p o r t a n t c o n t r i b u t i o n to t h i s s y s t e m a t i c e r r o r a r o s e f r o m the u n c e r t a i n t y and i r r e p r o d u c i b i l i t y in the i n c i d e n t b e a m d i r e c t i o n and t h u s in the a b s o l u t e v a l u e of the s c a t t e r i n g angle. In a d d i t i o n at 24 GeV c t h e r e is an o v e r a l l s c a l e e r r o r of ±10(~ m a i n l y b e c a u s e of the ±7% u n c e r t a i n t y in the 24Na a c t i v a t i o n c r o s s s e c t i o n u s e d in the a b s o l u t e n o r m a l i z a t i o n . At 10.0, 12.0, and 14.2 GeV c the s c a l e e r r o r is e s t i m a t e d to be ±15C~. The m e a s u r e d angular d i s t r i b u t i o n s are shown in fig. 1 t o g e t h e r with o t h e r r e p r e s e n t a t i v e d a t a [ 1 , 3 , 4 1 in the f o r m of d ~ d/ p l o t t e d a g a i n s t the invariant f o u r - m o m e n t u m t r a n s f e r squared, it. T h e new d a t a t a k e n at 19.2 GeV c a g r e e well with the p r e v i o u s l y m e a s u r e d c r o s s s e c t i o n s [1 ]. T h e s h o u l d e r - l i k e s t r u c t u r e a l r e a d y o b s e r v e d at 19.2 GeV c in the r e g i o n of f o u r - m o m e n t u m t r a n s f e r s q u a r e d !/¢- 1.5 GeV 2 is e v i d e n t i n a l l t h e newly measured angular distributions. This structure s e e m s to b e c o m e m o r e p r o n o u n c e d a s the e n e r g y i n c r e a s e s in the r a n g e c o v e r e d by the p r e s e n t e x p e r i m e n t . On the o t h e r h a n d . the 5 and 7 GeV c d a t a of C l y d e [31 a r e s m o o t h and s t r u c t u r e l e s s . T h i s is c o n f i r m e d by the m o r e r e c e n t e x p e r i m e n t of B r a b s o n et al. [5 I w h i c h e x t e n d s up to 5.5 GeV c. T h e c o n c l u s i o n is that the s t r u c t u r e d e v e l o p s r a t h e r r a p i d l y b e t w e e n 7 and 10 GeV c. T h a t s o m e c h a r a c t e r i s t i c of pp s c a t t e r i n g changes rapidly around 8- 9 GeV c has already b e e n n o t e d in s o f a r a s the e n e r g y d e p e n d e n c e of the 90 ° c r o s s s e c t i o n e x h i b i t s a s h a r p b r e a k [6[ a n d that the s l o p e of the a n g u l a r d i s t r i b u t i o n s 432
LETTERS
15 March 1971
102c
PROTON-PROTON ELASTIC SCATTERING MOMENTA (GeVIc) 101
- CLYDE
(1966) &0,5.0,70,71
(1967) 14 2 (/9~) 1 71,101,121
ALLABY eta(
(1968) U 192
100
• ANKENBRANDT et al. (1968) 3.0 • ALLABY et el
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(1971) 10.0,12.0,!4.2, 240
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Fig, 1. Anguiar distributions for proton-proton elastic scqttering. The new results r e p o r t e d in this letter are those marked All'tby et hi. (1971). The data at the same incident momentum have been joined by a smooth curve. The symbol qt the end of the curves corresponding t() the low-energy data indicates 90 ° c e n t r e - o f - m a s s . The curve Inbelled G4(l) indicates the fourth power of the electromagnetic form factor of the proton. n e a r to 90 ° i n c r e a s e s a b r u p t l y [4]. It s e e m s e v i d e n t that t h e s e o b s e r v a t i o n s at l a r g e s c a t t e r i n g a n g l e s and the s t r u c t u r e at s m a l l e r s c a t t e r i n g a n g l e s a r e m a n i f e s t a t i o n s of the s a m e p h e n o m e non. S e v e r a l t h e o r e t i c a l d i s c u s s i o n s {71 h a v e s u g g e s t e d t h a t the a s y m p t o t i c pp d i f f e r e n t i a l c r o s s s e c t i o n s h o u l d be p r o p o r t i o n a l to G4(/), the f o u r t h p o w e r of the e l e c t r o m a g n e t i c f o r m f a c t o r of the p r o t o n . T h i s f u n c t i o n , n o r m a l i z e d to the o p t i c a l t h e o r e m v a l u e at 20 GeV c , is s h o w n in fig. 1, t o g e t h e r with the m e a s u r e d d i s t r i b u t i o n s F o r s i m p l i c i t y , the d i p o l e f o r m : G(I) - (1+ !/I 0-71) - 2 h a s b e e n u s e d to r e p r e s e n t G(I), s i n c e the d e v i a t i o n s of the e x p e r i m e n t a l d a t a on G(I) f r o m t h e d i p o l e f o r m m a k e no e s s e n t i a l d i f f e r e n c e f o r t h i s
l
8
Volume 34B. number 5
PHYSICS L E T T E R S
comparison. The present data show that as the e n e r g y i n c r e a s e s t h e pp a n g u l a r d i s t r i b u t i o n at m o d e r a t e m o m e n t u m t r a n s f e r (1l! ~ 1 G e V 2) d r o p s b e l o w G4(/) by a f a c t o r ~ 1 0 a s t h e s t r u c t u r e d e v e l o p s , b u t r i s e s to a b o v e G4(/) at l a r g e r momentum transfer. This and other details are b e t t e r d i s p l a y e d in fig. 2 w h i c h s h o w s a c o m p i l a t i o n of t h e a v a i l a b l e d a t a [ 1 , 3 , 4, 81 in t h e f o r m of X G4(I), w h e r e X is t h e r a t i o of t h e d i f f e r e n t i a l c r o s s s e c t i o n , dcr d/. to the c a l c u l a t e d o p t i c a l t h e o r e m v a l u e in the f o r w a r d d i r e c t i o n . Fig. 2 s h o w s , in a v e r y s t r i k i n g way, t h e d e v e l o p m e n t of t h e s t r u c t u r e a s a f u n c t i o n of t h e incident momentum. The most prominent feature is t h e s h a r p dip a t ]11 = 1.2 G e V 2, w h i c h d e v e l o p s at i n c i d e n t m o m e n t a a b o v e 7 G e V / c . T h i s dip
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I 0.011 ............ 0.1 F i g . 2. A d e t a i l e d
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.
.
.
.
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.
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differ-
entiai c r o s s sections and G4(/). The qunntitv X is the ratio of the differential c r o s s section, dff/dl, to the calculated optical theorem value in the forward direction~ The figure shows X divided by G4(I) as a function of 1 i. The lines drawn nre m e r e l y to link the points c o r r e sponding to a fixed incident momentum which is indicated at the end of each line.
15 March 1971
c o r r e s p o n d s to t h e r a p i d c h a n g e of t h e s l o p e of t h e a n g u l a r d i s t r i b u t i o n s e e n at t h e e n d of t h e d i f f r a c t i o n p e a k in fig. 1. In a d d i t i o n , fig~ 2 s h o w s at a r o u n d ill : 4 G e V 2 a f l a t t e n i n g of t h e q u a n t i t y X G 4 ( / ) s t a r t i n g at 10 G e V c a n d b e c o m ing m o r e p r o n o u n c e d with i n c r e a s i n g i n c i d e n t momentum. This additional feature corresponds to t h e p o i n t of i n f l e x i o n a t t h e e n d of t h e s h o u l d e r in t h e a n g u l a r d i s t r i b u t i o n s d i s p l a y e d in fig. 1. It a p p e a r s t h a t t h e s t r u c t u r e p r e s e n t in the pp e l a s t i c s c a t t e r i n g is q u a l i t a t i v e l y d i f f e r e n t f r o m t h a t a l r e a d y e x h i b i t e d at l o w e r i n c i d e n t m o m e n t a i n ~ i p , K - p a n d ~ p e l a s t i c s c a t t e r i n g f o r I/i " 2 G e V 2. T h e s e l a t t e r s t r u c t u r e s , w h i c h t e n d r o d e c r e a s e with e n e r g y , h a v e b e e n d i s c u s s e d in v a r ious ways and a strong connection has been made [91 with t h e o p e r a t i o n of d i r e c t c h a n n e l r e s o n a n c e s w h i c h a r e a b s e n t in t h e pp s y s t e m . An e l a b o r a t i o n of t h e i d e a s of Wu a n d Yang [7 I h a s b e e n p r o p o s e d [101 w h i c h t r e a t s t h e two p r o t o n s a s e x t e n d e d d i s t r i b u t i o n s of m a t t e r , w h o s e s h a p e i s r e l a t e d to t h e e l e c t r o m a g n e t i c f o r m f a c t o r G(/). T h i s m o d e l , w h i c h is s u p p o s e d t o h o l d a s y m p t o t i c a l l y a t i n f i n i t e e n e r g y , r e s u l t s in a n angular distribution which contains strong structure. Without further assumptions, this model c a n n o t p r e d i c t t h e o n s e t of t h e s t r u c t u r e a s a f u n c t i o n of e n e r g y . O t h e r m o d e l s h a v e b e e n s u g g e s t e d [11] w h i c h a t t e m p t to d e s c r i b e t h e e n e r g y d e p e n d e n c e of t h e e l a s t i c c r o s s s e c t i o n . Howe v e r , at t h e p r e s e n t s t a g e of d e v e l o p m e n t of t h e s e m o d e l s , no q u a n t i t a t i v e c o m p a r i s o n is p o s s i b l e . All t h e m o d e l s p r e d i c t t h a t a s e c o n d m i n i m u m at ![i ~ 5 G e V 2 s h o u l d d e v e l o p at h i g h e r e n e r g i e s . It is c o n c e i v a b l e t h a t the k n e e d e v e l o p ing a t !1! ~ 4 G e V 2 in fig. 2 will e v e n t u a l l y b e c o m e a s e c o n d d i p a s the e n e r g y i s i n c r e a s e d A c o m m o n f e a t u r e of t h e s e m o d e l s is t h a t t h e y m a y be f o r m a l l y i n t e r p r e t e d {121 in t e r m s of a multiple scattering mechanism. Qualitatively, o n e t r i e s to u n d e r s t a n d t h e pp s t r u c t u r e a s due to t r a n s i t i o n s b e t w e e n d i f f e r e n t o r d e r s of s c a t t e r ing ( s i n g l e to d o u b l e ) e t c . ) by a n a l o g y with the s c a t t e r i n g of c o m p o s i t e s y s t e m s ( n u c l e u s - n u c l e u s o r h a d r o n - n u c l e u s ) . In t h i s i n t e r p r e t a t i o n one w o u l d e x p e c t s u c h s t r u c t u r e to b e a c o m m o n p r o p e r t y of a l l h a d r o n - h a d r o n s c a t t e r i n g p r o c esses. A similar behaviour should therefore be s e e n i n ~ i p , K i p a n d ~p s c a t t e r i n g at h i g h e r e n ergies than have so far been explored. In c o n c l u s i o n , it m a y b e s a i d t h a t w h i l e t h e s t r u c t u r e in t h e pp a n g u l a r d i s t r i b u t i o n s e t s in r a t h e r s u d d e n l y b e t w e e n 7 a n d 10 G e V c i t s f u r t h e r d e v e l o p m e n t up to 30 G e V c is not v e r y r a p id t h u s i n d i c a t i n g a r a t h e r s l o w a p p r o a c h to a l i m i t i n g d i s t r i b u t i o n , if s u c h e x i s t s . F u r t h e r e x 433
Volume 34B. number 5
PHYSICS L E T T E R S
p e r i m e n t a l w o r k in t h e 7 - 10 GeV c t r a n s i t i o n r e g i o n is c l e a r l y d e s i r a b l e t o g e t h e r with e x t e n s i o n s to e n e r g i e s a b o v e 30 GeV in p a r t i c u l a r to s t u d y the p o s s i b l e d e v e l o p m e n t of a s e c o n d s t r u c t u r e a r o u n d i/i ~ 4 GeV 2. A c o m p a r i s o n with the s c a t t e r i n g of o t h e r h a d r o n i c s y s t e m s at h i g h e r e n e r g i e s will a l s o be m o s t i m p o r t a n t in d e t e r m i n i n g w h e t h e r o r not t h e r e i s a c o m m o n s c a t t e r i n g m e c h a n i s m at l a r g e m o m e n t u m t r a n s f e r s a n d v e r y high e n e r g i e s . T h e a u t h o r s w i s h to a c k n o w l e d g e the e x c e l l e n t t e c h n i c a l s u p p o r t p r o v i d e d by R. D o n n e t , M. F e r r a t and C. S. S t f i h l b r a n d t . T h e e f f i c i e n t o p e r a t i o n of the s l o w e j e c t e d p r o t o n b e a m and the i n s t a l l a t i o n of the s p e c t r o m e t e r w e r e p r o v i d e d by t h e MPS d i v i s i o n ( P r o t o n S y n c h r o t r o n ) . P. S c h a r f f - H a n s e n g a v e c o n s i d e r a b l e h e l p with the p r o g r a m m i n g of the o n - l i n e c o m p u t e r and I. J a r s t o r f f c a r r i e d out the m e a s u r e m e n t s on the activated aluminium foils. The s e c o n d a r y e m i s s i o n m o n i t o r w a s p r o v i d e d by V. A g o r i t s a s , and the s p e c i a l liquid h y d r o g e n t a r g e t w a s s u p p l i e d by G. B r a n d and L. M a z z o n e . F i n a l l y . the a u t h o r s would like to t h a n k G. S h e r i n g who p a r t i c i p a t e d in the e a r l y p h a s e of the e x p e r i m e n t .
Rc:fel.'g3tlC¢,s [1] d.V,Allaby, F, Binon. A.N. Diddens. P. Duteil, A. Klovning, R, Meunier. d . P . Peigneux. E.J.S:~chnridis. K. Sehli~pmann. M. Spighel, d . P . Stroot. A.M. Thorndikc andA:qM.Wetherell. Phys. Letters 28B (1968) 67. [2] J . B . Cumming, J. Hudis. A.M. Poskanzer andS. Kuuffmann. Phys. Rev. 128 (1962) 2392; J. B. Cumming. Ann. Rex'. Nu,el. Science 13 (1963) 261. [31 A. R. Clyde. UCRL-16275 (1966): J . V . A l l a b y . A.N. Diddens. A.Kiovning, E . L i l l e -
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thun, E.J. Sacharidis. K. Schltipmann andA.M. Wethere[|. Phys.Letters 27B (1968) 49: C.M.Ankenbrandt, A.R. Clark. BoCork. T.Elioff, L.T.Kerth and W.A.Wenzel. Phys.Rev. 170 (1968) 1223. [4] J.V.Allaby. G. Coeconi. A.N. Diddens. A.Klovning G. Matthiae. E.J.Saeharidis andA.M. Wetherell. Phys. Letters 25B (1967) 156. [51 B.B.Branson, R.R. Crittenden, R.M. Heinz. R.C. Kammerud. H.A.Neal, H.W,Paik. R.A.Sidwell and K.F.Suen. Phys.Rex,. Letters 23 (1969) 1306; B.B.Brnbson. R.R. Crittenden. R.M. Heinz. R.C. Kammerud. H.A.Neal, H.W. Paik andR.A.Sidwell. Physics Dept.. Indiana University. Bloomington, Int. Report COO-2009-18. [61 C,W. Akerlof. R.H. Hieber. A.D, Krisch. K.W. Edwards. L,G. Ratner andK, Ruddick. Ph\'s.Rev.
L e t t e r s 17 (1966) 1105: Phys. Rex'. 1 59 (1967) 1138. [7[ T.T. Wu and C.N. Yang. Phys. Rev. 137 (1965) B708, L. van Hove. Proc. Stoney-Brook Conf, on High Energy Two-body Refections (April 1966) and in: Particle interactions "It high e n e r g i e s (Scottish Universities Summer School 1966). eds. T.W. P r e i s t and L . L . V i c k (Oliver and Boyd, 1967! p. 63. sect. 4.4. R . A . C n r r i g a n , R.M. Edelstein. N.C. Hien. T . J . [8] McMahon. I. Nadelh~t. E.W. Anderson. E . J . Bleser. G.B. Collins. T. Fujii. J . M e n e s and F. Turkot. Phys. Rev. L e t t e r s 24 (1970) 683: G. Coceoni. V.T. Cocconi. A.D. Kriseh. J . O r e a r . R. Rubinstein, D.B. Sctiri. W. F. Baker. E.W. Jenkins and A . L . Read. Phys. Rev. L e t t e r s 11 (1963) 499: Phys. Rev. 138 (1965) B165. [9] H. Harari, submitted to Annuls of Physics. [10] L. Durand a n d R . Lipes. Phys. Rev. L e t t e r s 20 (1968) 637: T.T. Chou and C.N.Yang. Phys. Rex'. L e t t e r s 20 (1968) 1213. [11] S. Frautschi and 13. Margolis, Nuovo Cimento 56A (1968) 1155: C.B. Chiu a n d J , Finkelstein. Nuovo Cimento 57A (1968) 649: 59A (1969) 92. [12] J.D. Jackson. Rev, Mod. Phys. 42 (1970) 12,