POSITIVE PARTICLE PRODUCTION AT LARGE TRANSVERSE MOMENTA IN THE FRAGMENTATION REGION (P 342)

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POSITIVE PARTICLE PRODUCTION AT LARGE TRANSVERSE MOMENTA IN THE

FRAGMENTATION REGION (P 342)

D. Barber

To cite this version:

D. Barber. POSITIVE PARTICLE PRODUCTION AT LARGE TRANSVERSE MOMENTA IN

THE FRAGMENTATION REGION (P 342). Journal de Physique Colloques, 1973, 34 (C1), pp.C1-

404-C1-406. �10.1051/jphyscol:1973155�. �jpa-00215235�

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D.P. BARBER

POSITIVE PARTICLE PRODUCTION AT LARGE TRANSVERSE MOMENTA I N THE FRAGMENTATION REGION (P 342)

D.P. BARBER DARESBURY

A s t r i k i n g f e a t u r e of p a r t i c l e p r o d u c t i o n a t t h e CERN ISR h a s been t h e o b s e r v a t i o n [1,7

1

t h a t a t h i g h pT t h e d i s t r i b u t i o n of secondary p a r t i c l e s produced i n t h e c e n t r a l r e g i o n d e v i a t e s from t h e e x p o n e n t i a l f a l l - o f f c h a r a c t e r i s t i c of low pT d a t a and begins t o f l a t t e n o u t . I t h a s been argued, i n i t i a l l y by Berman, Bjorken and Kogut [2], t h a t such behaviour may be c o n s i d e r e d a s evidence f o r p o i n t - l i k e c o n s t i - t u e n t s i n s i d e t h e p r o t o n . An a l t e r n a t i v e i n t e r p r e t a - t i o n [ 3 ] would be t o r e l a t e t h e s e c o n d a r i e s produced a t h i g h pT w i t h t h e decay o f massive " f i r e b a l l s " , a p p a r e n t l y produced w i t h s i g n i f i c a n t c r o s s - s e c t i o n a t ISR e n e r g i e s [4]. I n e i t h e r c a s e one would a l s o expect a n excess of h i g h pT p a r t i c l e s away from t h e c e n t r a l r e g i o n . I n t h i s l e t t e r we r e p o r t measurements of K', K+ and p r o t o n p r o d u c t i o n c r o s s - s e c t i o n s i n t h e f r a g m e n t a t i o n r e g i o n a t x=0.6 f o r a range of pT from 0.33 GeV t o 2.55 GeV.

The measurements have been made a t t h e CERN I n t e r - s e c t i n g S t o r a g e Rings w i t h a s i n g l e arm s p e c t r o m e t e r c o n s i s t i n g of a magnet and s p a r k chamber system. Par- t i c l e s were i d e n t i f i e d i n t h r e e gas Cere.lkov coun- t e r s . T h i s system and i t s o p e r a t i o n have been d e s c r i - bed i n more d e t a i l elsewhere [51. Data were t a k e n a t f o u r I S R o p e r a t i n g e n e r g i e s , w i t h c i r c u l a t i n g beams of momenta 15.5 GeV (s=960 GeV 2 )

,

22.5 GeV

2 2

(s=2025 GeV ) 26.6 GeV (s=2830 GeV ) and 31.3 (s=3920 GeV 2 )

.

At each energy t h e s p e c t r o m e t e r was s e t t o a c c e p t p a r t i c l e s w i t h l o n g i t u d i n a l momen-

tum pL ( i n t h e c e n t r e of mass) such t h a t x 0.6

,

where x = 2 p L / 6 i s t h e Feynman v a r i a b l e . The a n g u l a r range of t h i s experiment, 30 t o 170 mrad, was covered i n s t e p s of 5 mrd o u t t o 85 mrad and i n 10 mrad s t e p s t h e r e a f t e r . A t each s e t t i n g t h e acceptance was c a l c u - l a t e d a s a f u n c t i o n of pT and pL by a Monte Carlo program, which took i n t o account t h e p o s i t i o r s and shapes of t h e i n t e r s e c t i n g beams. T h i s program simu- l a t e d t h e e f f e c t s of m u l t i p l e s c a t t e r i n g , and c o r r e c - t i o n s were a p p l i e d f o r a b s o r p t i o n and decay. These l a t t e r two c o r r e c t i o n s a r e t o g e t h e r < 15 % f o r K

+ ,

< 40 % f o r K+ and a r e independent of pT

.

A s m a l l

background (< 5 %) due t o beam gas i n t e r a c t i o n s was determined g e o m e t r i c a l l y from t h e d i s t r i b u t i o n of t h e o r i g i n s of t h e r e c o n s t r u c t e d t r a c k s and was s u b t r a c - t e d . The c r o s s - s e c t i o n s were normalized u s i n g t h e l u - minosity monitoring method d e s c r i b e d i n r e f . 5.

I n o r d e r t o p r e s e n t t h e r e s u l t s a t f i x e d x

,

^{d a t a}

w i t h i n t h e range 0.55 < x < 0.65 were c o r r e c t e d t o x = - 0 . 6 u s i n g a f a c t o r exp(-b.&). Over a l i m i t e d x range such a dependence i s c e r t a i n l y a good appro- ximation t o t h e d a t a . For p i o n s and kaons t h e s l o p e b was determined from t h i s d a t a (extended i n x-range where p o s s i b l e ) and was found t o be independent of pT

.

For p r o t o n s t h e s p e c t r a a r e e s s e n t i a l l y independent of x n e a r x=0.6 [ 4 ] and t h i s c o r r e c t i o n was n e g l i g i b l e .

F i g s . 1, 2 and 3 show t h e r e s u l t s a s i n v a r i a n t

+ +

c r o s s - s e c t i o n s v e r s u s pT a t x=0.6 f o r n

,

^K ^{and p}

r e s p e c t i v e l y . The e r r o r s shown a r e t h e q u a d r a t i c sum of t h e s t a t i s t i c a l e r r o r s and a n e s t i m a t e d 5 % s y s t e - m a t i c e r r o r a r i s i n g from u n c e r t a i n t i e s i n t h e ISR l u - m i n o s i t y , t h e Monte C a r l o c a l c u l a t i o n and t h e v a r i o u s c o r r e c t i o n s a p p l i e d . I n t e r p o l a t e d d a t a from t h e lower energy experiment of Allaby e t a l . [6] a r e shown a s broken l i n e s . These a u t h o r s d e f i n e x a s pL/pL r a t h e r t h a n 2 p L / 6 ; t h e d i f f e r e n c e i s n o t s i g n i - f i c a n t f o r t h e ISR d a t a b u t i s l a r g e f o r pions and kaons a t low energy. Consequently we r e p r e s e n t t h e d a t a of r e f . 6 twice, w i t h both d e f i n i t i o n s .

The ISR d i s t r i b u t i o n s i n t r a n s v e r s e momentum a r e somewhat f l a t t e r than t h o s e o b t a i n e d a t t h e lower energy f o r a l l t h r e e p a r t i c l e s . For r e l a t i v e l y l a r g e v a l u e s o f pT (- 2.0 GeV) t h e c r o s s - s e c t i o n s f o r n

+

and

k

i n c r e a s e by a n o r d e r of magnitude i n t h e s-range 48 t o 2830 GeV 2

.

The p r o t o n c r o s s - s e c t i o n s a l s o i n c r e a s e , b u t by a s m a l l e r f a c t o r . The d a t a a t t h e f o u r ISR e n e r g i e s do n o t however show any s i g n i - f i c a n t s-dependence o v e r t h e range of x and pT co- v e r e d ; t h e y a r e t h e r e f o r e c o n s i s t e n t w i t h Feynman

s c a l i n g a s a n asymptotic s t a t e m e n t , t h e c r o s s - s e c t i o n s having e s s e n t i a l l y reached l i m i t i n g v a l u e s i n t h e ISR energy range.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973155

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LARGE TRANSVERSE MOMENTA I N THE FRAGMENTATION REGION C1-405

F i g . 1.- D i s t r i b u t i o n i n t r a n s v e r s e momentum f o r rrt a t x=0.6

.

The i n v a r i a n t c r o s s - s e c t i o n s measured i n t h i s experiment (CHLM) a r e shown a t f o u r e n e r g i e s . The broken l i n e s i n d i c a e a n i n t e r p o l a t i o n of t h e d a t a a t s=48 GeV

5

of Allaby e t a l . [ 6 ] w i t h pL/ pL (max) =O. 6 and 2pL/ &=0.6

.

10'

,

^I ^, ^, ^, ^, ^, ^, 8

PP

-

^{P + .}^...

X = a6

V T ( G . V l

-

62.6

-

--

^6.8A L L A W E l AL.

'@

10-~

-

lo-'

I , , , , ,

I , ,

0.4 0.6 0.8 la 1.2 1.4 1.6 1.8 2.0 2.2 2.L F i g . 3 . - The same a s f i g u r e 1 f o r p r o t o n s . I n t h i s

c a s e t h e d a t a of Allaby e t a l . a r e essen- t i a l l y independent of t h e d e f i n i t i o n of x

.

The shapes of t h e K i and p d i s t r i b u t i o n s i n p T a r e v e r y s i m i l a r , b o t h d e v i a t i n g from a simple expo- n e n t i a l i n t h e same way, w i t h a tendency t o f l a t t e n o u t a t low pT

.

F u r t h e ~ m o r e a s t h e s l o p e s of t h e two d i s t r i b u t i o n s a r e s i m i l a r t h e rr

+

/ p r a t i o does n o t change s i g n i f i c a n t l y w i t h pT ( a t x=0.6) and i s

" 0 . 1

.

On t h e o t h e r hand t h e d i s t r i b u t i o n of K

+

i s p e r f e c t l y c o n s i s t e n t w i t h b e i n g a simple exponen- t i a l , p o s s i b l y because of t h e somewhat l a r g e r e r r o r s . The mean s l o p e i s c l e a r l y l e s s t h a n t h e mean s l o p e s of t h e K

+

and p d i s t r i b u t i o n s , consequently t h e p r o p o r t i o n of K' i n c r e a s e s w i t h pT

.

^{For p}_T

between 1 and 2 GeV t h e s l o p e o f t h e ^K+ d i s t r i b u - t i o n i s v e r y s i m i l a r t o t h a t o b t a i n e d [ 7 ] f o r t h e no spectrum a t x=O

.

F i g . 2 . - The same a s f i g u r e 1 f o r K i

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D.P. BARBER

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