Download pdf - Results on the impact parameter trigger for the selection of charmed particles

Nuclear Physics B (Proc. Suppl.) 1B (1988) 303-310 303 North-Holland, Amsterdam

RESULTS ON THE IMPACT PARAMETER TRIGGER FOR THE SELECTION OF CHARMED PARTICLES

J . F . Baland 4, W. Beusch x, M. Dameri 2, G. Darbo 2, M. VavenportX~ R. V i a f e r i a s , J • P . Dufey ~, B.R. French l , J . J . G a r c i a ~ ( ) , F. Grard 4, A. J a c h o l k o w s k i ~, K. Knudson x, J . C . L a s s a l l e I , P. Legros 4, V N a r i o l i a ( ~ ) C. Meroni s F. M u l l e r ~, A. O l c e s e 2, B. O s c u l a t i 2, N. R e d a e l l i ~, L. Ross i ~, G. Tomasin i 2 and G. Vegni a.

x GERm, European O r g a n i z a t i o n f o r N u c l e a r R e s e a r c h , Geneva, S w i t z e r l a n d 2 D i p a r t i m e n t o d i F i s i c a d e l l ' U n i v e r s i t b and S e z i o n e INFN, Genoa, I t a l y s V l p a r t i m e n t o d l F i s i c a d e l l ' U n i v e r s i t b and S e z i o n e INFN, Mi l an , I t a l y 4 U n i v e r s i t ~ de l ' E t a t , Mons, Belg ium

We present the results on a new trigger scheme for the selection of s h o r t l i v e d p a r t i c l e s • These r e s u l t s were o b t a i n e d in a t e s t run o f t he WA82 e x p e r i m e n t a t t h e CERN Omega s p e c t r o m e t e r . We have measu red , f o r charmed p a r t i c l e s , an e n r i c h m e n t f a c t o r o f ~ 15 o v e r an i n t e r a c t i o n t r i g g e r .

I . INTRODUCTION

H a d r o p r o d u c t i o n i s a r i c h s o u r c e o f charmed p a r t i c l e s . The e x p e r i m e n t a l r e s u l t s o b t a i n e d up t o now a r e , however , s t i l l s u f f e r i n g from t h e i r l i m i t e d s t a t i s t i c s . Th i s i s due , m o s t l y , t o t h e absence o f an e f f i c i e n t t r i g g e r i n g scheme a b l e t o e n r i c h c o n s i d e r a b l y t h e charm semple w h i l e s t i l l s e l e c t i n g a good f r a c t i o n o f t h e charm c r o s s s e c t i o n .

A new t r i g g e r s t r a t e g y , based upon t h e s e l e c t i o n o f e v e n t s w i t h e v i d e n c e o f a s e c o n d a r y v e r t e x , has been t e s t e d [ 1 , 2 ] .

This pape r d e s c r i b e s t h e t r i g g e r s t r a t e g y ( s e c t . 2 ) , t h e c o n d i t i o n s under which t h e t e s t was pe r fo rmed ( s e c t . 3) and f i n a l l y p r e s e n t s t h e r e s u l t s on t h e c a p a b i l i t y o f t h e t r i g g e r t o s e l e c t charmed p a r t i c l e s ( s e c t . 4).

2. THE TRIGGER SCHEME

The i d e a i s t o t r i g g e r on e v e n t s t h a t show e v i d e n c e o f a s e c o n d a r y v e r t e x u s i n g t h e d a t a from t h e f r o n t - e n d m i c r o v e r t e x t r a c k i n g sys tem. In o r d e r to s i m p l i f y t he a l g o r i t h m t h a t f i n d s t he s e c o n d a r y v e r t i c e s we used a t h i n (~ 1 mm) t a r g e t and a s p e c i a l g e o m e t r i c a r r angemen t o f t h e m t c r o s t r t p d e t e c t o r s t h a t t a k e p a r t in t h e t r i g g e r .

The p r l n c l p l e o f t h e t r i g g e r can be i l l u s t r a t e d w i t h t h e h e l p o f f i g . 1. Three m i c r o s t r i p d e t e c t o r s a r e p l a c e d such t h a t t h e i r p i t c h d i v i d e d by t h e i r d i s t a n c e f rom t h e t a r g e t i s a c o n s t a n t ; t h e s i g n a l f rom each s t r i p i s d i s c r i m i n a t e d and r e a d ou t i n d l v i d u a l l y .

(~) U n i v e r s i t ~ de S a n t a n d e r , Spa in ( ~ ) P o l i t e c n i e o d l B r e s c i a , I t a l y

0920-5632/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

304 L. Rossi / Selection of charmed particles

lz ~ zx projection

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F i g . 1 S k e t c h of t h e t r i g g e r m l c r o s t r i p in t h e v e r t e x r e g i o n .

For a l l t r a c k s coming f rom 0, t h e p r i m a r y v e r t e x :

n(a) = n(b) = n(c)

where n i s t h e number o f t h e s t r i p h i t by t h e t r a c k , n o r m a l i z e d t o t h e i ncoming beam z c o o r d i n a t e m e a s u r e d by a t e l e s c o p e o f 20 ~m p i t c h m i c r o s t r i p s .

If the track does not point to 0 within the accuracy of the vertex d e t e c t o r , t h e n :

n ( a ' ) ~ n ( b ' ) ~ n ( c ' )

I t i s t h e n enough t o e r a s e a l l t h e h i t a d d r e s s e s t h a t a r e e q u a l i n a t l e a s t two o f t h e t h r e e m i c r o s t r i p s d e t e c t o r s . The s u r v i v i n g h i t s , i f any , a r e u sed t o f i n d p o t e n t i a l s t r a i g h t t r a c k s , d e f i n e d by t h r e e p o i n t s , and t h e i r i m p a c t p a r a m e t e r s w i t h r e s p e c t t o t h e v e r t e x .

The e v e n t i s a c c e p t e d i f any o f t h e i m p a c t p a r a m e t e r s l i e s w i t h i n a p r e s e t i n t e r v a l o f , s a y , 0 . 1 t o 1 . 0 mm (*) and t h e t r a c k segment found by t h e m t c r o s t r i p s i s c o n f i r m e d by one o f t h e Omega p r o p o r t i o n a l chambers p l a c e d 3 m e t e r s d o w n s t r e a m t h e t a r g e t .

Low momentum t r a c k s , t h a t may show r a t h e r l a r g e i m p a c t p a r a m e t e r s b e c a u s e o f m u l t i p l e s c a t t e r i n g , a r e e x c l u d e d s i n c e t h e h o r i z o n t a l a c c e p t a n c e o f t h e p r o p o r t i o n a l chamber c o r r e s p o n d s , i n t h e Omega m a g n e t i c

(*) The i m p a c t p a r a m e t e r o f a decay t r a c k f rom a p a r t i c l e w i t h l i f e t i m e T has an a v e r a g e v a l u e = c~; i . e . ~ 200 tna f o r D p a r t i c l e s

L. Rossi / Selection of charmed particles 305

field, to a momentum selection of p ~ 3.5 GeV/c. The fast read-out of t h e m i c r o s t r i p s and t h e s u b s e q u e n t d e c i s i o n w i l l t a k e , u s i n g t h e programmable Mice p r o c e s s o r e 3 0 0 ~sec [3 ] .

3. DESCRIPTION OF THE TEST RUN

We tested the Impact Parameter Trigger (IPT) [1] in July - August 1986 with a 1.3 mB thick active target and the focussing microstrip detector placed inside the OMEGA magnet (B = 1.8 T). A negative beam of 360 GeV/c momentum was used. Out of nine days allocated, five were devoted to the setting up and four to the data taking. 10 s interactions in the active target were recorded onto tape.

The s e t - u p used d u r i n g t h e d a t a t a k i n g i s shown in f i g . 2; 15 m i c r o s t r i p d e t e c t o r s ( s i x in t h e beam, n i n e in t h e v e r t e x r e g i o n ) were u sed , a t o t a l o f ~ 8000 c h a n n e l s was r e a d o u t . The mean e f f i c i e n c y of t he d e t e c t o r s i s 93%, t h e a c c u r a c y i s , as e x p e c t e d , 1/412 o f t h e p i t c h ( f i g . 3) and does no t change a p p r e c i a b l y w i t h t he s t r i p o r i e n t a t i o n in t h e m a g n e t i c f i e l d .

The t a r g e t was composed of 3 x 200 twa a c t i v e s i l i c o n s h e e t s i n t e r l e a v e d w i t h 200 tnn o f t u n g s t e n and 200 ~ o f m y l a r , t h e t o t a l t h i c k n e s s o f t h e t a r g e t was 1 .3 mm.

4. ANALYSIS OF THE IMPACT PARAMETER TRIGGER

4 . 1 Background e v a l u a t i o n

8 .10 s e v e n t s , c o r r e s p o n d i n g t o a s e n s i t i v i t y o f 50 e v e n t / ~ b , have been f i l t e r e d t h r o u g h t h e t r i g g e r a l g o r i t h m i n c l u d i n g t h e 3 .5 GeV/c c u t

256 c~anne|s Active Z~" Y~ ~20F m target

512 charmers 6 2Diem

2 DOUBLETS of SO ~m j pitch detectors are

prated lm upstream the target

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~ u

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F i g . 2 WAg2 v e r t e x d e t e c t o r s e t - u p used in t h e 1986 t e s t run .


N

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100

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o b t a i n e d t h r o u g h t h e c o r r e l a t i o n b e t w e e n t h e t r a c k r e c o n s t r u c t e d i n t h e m i c r o s t r i p p l a n e s and one o f t h e Omega p r o p o r t i o n a l c h a m b e r s .

A b o u t 19 000 e v e n t s ( i . e . 2 .3% o f t h e i n t e r a c t i o n s ) s u r v i v e d t h e IPT , i . e . c o n t a i n e d a t l e a s t one t r a c k w h i c h had an I m p a c t P a r a m e t e r ( I P ) w i t h r e s p e c t t o t h e p r i m a r y i n t e r a c t i o n i n t h e t a r g e t o f more t h a n 100 ~m i n t h e x - z p l a n e ( w h e r e t h e x d i r e c t i o n i s t a k e n a l o n g t h e beam and t h e z - d i r e c t i o n i s a l o n e t h e m a g n e t i c f i e l d ) . The i m p a c t p a r a m e t e r d i s t r i b u t i o n ( r e l a t i v e t o t h e beam p r o j e c t e d a t t h e t a r g e t c e n t r e ) f o r any 3 - p o i n t t r a c k i s shown f o r a s u b s a m p l e o f t h e d a t a i n f i g . 4 ( a ) . The same d i s t r i b u t i o n , b u t r e l a t i v e t o t h e r e c o n s t r u c t e d p r i m a r y v e r t e x p o s i t i o n , i s p r e s e n t e d i n f i g . 4 ( b ) . The HWHM o f t h i s d i s t r i b u t i o n (10 ~m) i n d i c a t e s t h a t we s h o u l d be i n p o s i t i o n t o m e a s u r e l i f e t i m e s as s h o r t a s 0 . 1 p s .

A f r a c t i o n (~ 15%) o f t h e t r i g g e r s i s due t o i n t e r a c t i o n s o c c u r r i n g e i t h e r i n t h e l a s t m l c r o s t r i p d o u b l e t o r i n t h e m y l a r window i n f r o n t o f t h e a c t i v e t a r g e t . T h e s e i n t e r a c t i o n s w i l l be v e t o e d i n t h e f u t u r e d a t a by e x c l u d i n g l a r g e e n e r g y r e l e a s e i n t h e l a s t m i c r o s t r i p and s u p p r e s s i n g t h e m y l a r window.

The r e s u l t a n t t r i g g e r r a t e f o r e v e n t s o r i g i n a t i n g i n t h e t a r g e t i s , t h e r e f o r e , 21t, t w i c e t h e v a l u e c a l c u l a t e d i n [ 1 ] . We h a v e f o u n d t h a t p a r t

20 pm pitch detector

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F i g . 3 D i s t r i b u t i o n o f r e s i d u e s on a t r a c k f i t f o r :

( a ) a 20 ~m p i t c h d e t e c t o r . ( b ) a 50 ~m p i t c h d e t e c t o r .

F i g . 4 ( a ) I m p a c t p a r a m e t e r d i s t r i b u t i o n r e l a t i v e t o t h e beam p r o j e c t e d a t t h e t a r g e t c e n t r e f o r any 3 - p o l n t t r a c k .

( b ) I m p a c t p a r a m e t e r d i s t r i b u t i o n r e l a t i v e t o t h e r e c o n s t r u c t e d v e r t e x p o s i t i o n ( o n l y t h o s e 3 - p o i n t t r a c k s t h a t h a v e b e e n a s s o c i a t e d t o t h e p r i m a r y v e r t e x a r e t a k e n i n t o a c c o u n t ) .


10

-30 -½0

(0.5%) o f t h i s d i f f e r e n c e i s due t o an i n c o r r e c t p o s i t i o n i n g o f t h e a c t i v e t a r g e t ( i t s c e n t r e was found t o be 0 . 3 - - . u p s t r e a m f rom t h e t h e o r e t i c a l f o c u s o f t h e t r i g g e r i n g m i c r o s t r i p s ) . The r e m a i n i n g d i f f e r e n c e (0.5%) can he due t o t h e a p p r o x i m a t i o n s i n t h e M o n t e - C a r l o s i m u l a t i o n ( e f f e c t s due t o e n e r g e t i c 6 - r a y s , t o c r o s s t a l k b e t w e e n e l e c t r o n i c c h a n n e l s , t o s h o r t c i r c u i t b e t w e e n s t r i p s , were n o t s i m u l a t e d b u t a r e p r e s e n t i n t h e d a t a ) .

I n c o n c l u s i o n t h e a n a l y s i s o f t h e IPT shows t h a t t h e b a c k g r o u n d b e h a v e s r o u g h l y as e x p e c t e d and a l l o w s us t o c o n s i d e r a 2% t r i g g e r r a t e as a s a f e w o r k i n g h y p o t h e s i s f o r ou r e x p e r i m e n t b a s e d on t h e i m p a c t p a r a m e t e r t r i g g e r [ 2 ] .

4 . 2 S e a r c h f o r charm i n t h e f i l t e r e d e v e n t s

We t h e n p u r s u e d t h e a n a l y s i s o f t h e d a t a i n o r d e r t o s ee i f t h e t r i g g e r s e l e c t s charm d e c a y s as c a l c u l a t e d [ 1 ] .

We f i r s t s e a r c h e d amongs t t h e e v e n t s w i t h an i n t e r a c t i o n i n t h e t a r g e t , f o r a 3 - p r o n g s e c o n d a r y v e r t e x i n t h e x - z p l a n e wh ich c o n s i s t e d o f t h e t r i g g e r t r a c k ( I P > 100 ~m) and two o t h e r 3 - p o i n t t r a c k s h a v i n g IP > 40 ~m. T h r e e - p r o n g s e c o n d a r y v e r t i c e s ( C 3 ' s ) were a c c e p t e d as c a n d i d a t e d e c a y s i f t h e v e r t e x was a t l e a s t 0 . 8 mm o u t s i d e t h e t a r g e t . The x - d i s t r i b u t i o n o f t h e v e r t i c e s i s shown i n f i g . 5. Assuming t h a t t h e 7 u p s t r e a m v e r t i c e s a r e g h o s t c o m b i n a t i o n s o f t h r e e t r a c k s , we can c o n c l u d e t h a t 30 o u t o f 37 d o w n s t r e a m v e r t i c e s a r e t r u e C3 v e r t i c e s (S/N = 4 ) , w h i c h , a s s u m i n g t h e y a r e due t o cha rm, c o r r e s p o n d s t o a ~ - - n u c l e o n ~ D ± c r o s s s e c t i o n o f 0 . 8 ~ b a r n u n c o r r e c t e d f o r t r i g g e r and d e t e c t i o n e f f i c i e n c y ; t h e c o r r e c t e d v a l u e would be ~ 4 . 5 ~b i n a g r e e m e n t w i t h d a t a on D ± p r o d u c t i o n [ 4 ] .

F i g . 6 ( a ) shows a d i s p l a y o f one o f t h e C3 e v e n t s , f i g . 6 ( b ) shows a zoomed v i ew of t h e same e v e n t .

i , [ I i , i

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F i g . 5

rl -10

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We t h e n c o n s i d e r o n l y t h e d o w n s t r e a m v e r t i c e s ; t h e d i s t r i b u t i o n o f i m p a c t p a r a m e t e r s o f e m e r g i n g t r a c k s i s shown i n f i g . 7 ( a ) , t h e mean i m p a c t p a r a m e t e r v e r s u s t h e minimum d i s t a n c e o f t h e s e c o n d a r y v e r t e x f rom t h e t a r g e t c e n t r e i s shown i n f i g . 7 ( b ) , i t s a s y m p t o t i c v a l u e , once c o r r e c t e d f o r t h e c u t s i n t r o d u c e d by t h e t r i g g e r , c o r r e s p o n d t o a l i f e t i m e T = 1 p s .

The C3 events measured by the LEBC-EHS Collaboration were then passed through our selection criteria (the sensitivity of LEBC-EHS is 50 ev/~b, equivalent to the s e n s i t i v i t y o f o u r t e s t r u n ) . F i g . 7 ( c ) p r e s e n t s t h e i m p a c t p a r a m e t e r d i s t r i b u t i o n f o r t h o s e d e c a y s t h a t s u r v i v e d o u r f i l t e r and i t can he d i r e c t l y compared w i t h f i g . 7 ( a ) .

D i s t r i b u t i o n o f t h e C3 v e r t i c e s p o s i t i o n s a l o n g t h e beam d i r e c t i o n . The d a s h e d a r e a c o r r e s p o n d s t o t h o s e d o w n s t r e a m v e r t i c e s t h a t may be momentum b a l a n c e d ,


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F i g . 6 D i s p l a y o f one C3 e v e n t :

( a ) Norma l v i e w ( t h e m i c r o s t r i p p l a n e s o f t h e v e r t e x d e t e c t o r a r e i n d i c a t e d ) .

( b ) Zoomed v i e w o f t h e t a r g e t r e g i o n .

We f i n a l l y s e l e c t o n l y t h o s e e v e n t s w h e r e a d o w n s t r e a m v e r t e x c o u l d be momentum b a l a n c e d ( d a s h e d a r e a i n f i g . 5 ) , r u n t h e f u l l r e c o n s t r u c t i o n

p r o g r a m ( b r i d g i n g t h e t r a c k s f o u n d i n t h e m i c r o s t r i p s w i t h t h e t r a c k s f o u n d

i n Omega) and a s s u m i n g f o r e a c h C3 t h e C a b i b b o f a v o u r e d d e c a y D ± ~ K ~ ± ~ ±, we o b t a i n e d t h e i n v a r i a n t mass d i s t r i b u t i o n o f f i g . 8 .

The D ± mass r e g i o n c o n t a i n s 4 e v e n t s ( b a c k g r o u n d N 0 . 3 e v e n t s ) w h i c h a g r e e s w i t h t h e n u m b e r e x p e c t e d f r o m t h e o r i g i n a l p r o p o s a l [ 1 ] .

A f u r t h e r v e r i f i c a t i o n o f t h e a b i l i t y o f t h e IPT t o s e l e c t s h o r t l i v i n g p a r t i c l e s h a s b e e n p e r f o r m e d l o o k i n g f o r K~ d e c a y i n g b e f o r e t h e f i r s t m i c r o s t r i p o f t h e v e r t e x d e t e c t o r . We h a v e s e l e c t e d e v e n t s w i t h 2 - p r o n 8 s e c o n d a r y v e r t i c e s w h e r e e a c h t r a c k o f t h e s e c o n d a r y v e r t e x h a s an i m p a c t p a r a m e t e r ( i n XZ p r o j e c t i o n ) o f a t l e a s t 100 t,m and t h e n p r o c e e d e d , as i n t h e c a s e o f D ±, t o c o m b i n e t h e m l c r o v e r t e x and t h e s p e c t r o m e t e r i n f o r m a t i o n and t o r e c o n s t r u c t t h e ~+w- i n v a r i a n t mass o f t h e d e c a y . The r e s u l t s , shown i n f i g . 9 , c o r r e s p o n d t o 20% o f o u r p r e s e n t s t a t i s t i c s .

5. CONCLUSIONS

The i m p a c t p a r a m e t e r t r i g g e r h a s b e e n shown t o work as a m e t h o d f o r t h e s e l e c t i o n o f s h o r t l i v e d p a r t i c l e s i n h a d r o n i c i n t e r a c t i o n s . We h a v e m e a s u r e d - w i t h i n t h e a v a i l a b l e s t a t i s t i c s - t h a t t h e i m p a c t p a r a m e t e r t r i g g e r s e l e c t s a b o u t 1 / 4 o f t h e c h a r m e d e v e n t s p r o d u c e d r e s u l t i n g i n an e n r i c h m e n t f a c t o r ~ 15 o v e r an i n t e r a c t i o n t r i g g e r .


20

10

100 500

WA 82-DATA 37 C3 DECAYS (111 Tracksl (23 at x>6mm from primary vertex: dashed area)

(b) [grn]

26C / '.

2~0

220 200

21 I I I l & 6 8 10

Fig. 7b

N

30

25

20

I I I N 1 3O EHS DATA

(filtered through WA82 algorithm) 35 C3 DECAYS (105 Tracks) (26 at x>6mm from primary vertex:

dashed area)

lO i

rEpr/zn = ,~- ~, ~ I 1000 b {pm] 100 500 1000

Fig. 7a Fig. 7c

b [t~m]

Fig . 7 (a) D i s t r i b u t i o n of the impact pa r ame te r s f o r t he downstream C3 decays found a f t e r the a n a l y s i s of the WA82 t e s t e x p e r i m e n t .

(b) Mean impact pa r ame te r v e r s u s minimum d i s t a n c e between the p r ima ry and s econda ry v e r t e x f o r the C3 decays of f i g . 7 ( a ) .

(c) D i s t r i b u t i o n of the impact pa r ame te r s f o r the C3 decays found in the charm EHS e x p e r i m e n t s . Only t hose decays t h a t s a t i s f y the WA82 t r i g g e r a l g o r i t h m are c o n s i d e r e d .

1(]

> L~

s

I UNDERFLOW

i i i I I

[3-* INVARIANT MASS OISTRIBUTION 1 bin=L,0 MeV

O'* mass

I-I , I ~ 1 n , ~ FI, n 1100 1300 1500 17'00 1900 2100

Mass [HEY]

F i g . 8 D ± i n v a r i a n t mass p l o t ( s ee t e x t f o r d e t a i l s ) .

These r e s u l t s open up the p o s s i b i l i t y of h igh s t a t i s t i c s h a d r o p r o d u c t i o n e x p e r i m e n t s [2] .

The same t r i g g e r p r i n c i p l e may a l s o be adap ted f o r the s e l e c t i o n of b e a u t y p a r t i c l e s , i n t h i s case a p p r e c i a b l y h i g h e r e n r i c h m e n t f a c t o r s shou ld be expec ted because of the l a r g e decay m u l t i p l i c i t y and the r a t h e r long b e a u t y l i f e t i m e .


c

>

o

L_

Z

~82' -

5 6 UNOFL

1 17 F'~ ['1 [-- ] [ -- ]

I I l I

I0

11 I IUNDFL11_11 i

L,120 460 500

I [ I I ~---- I I

200/0 TEST RUN STATISTICS ' V2 in XZ PLANE

rn(n ~) DISTRIBUTION

a}

I

M (~T~)

7 0 V L F

, , R I] ~j I I I ]

I#pTzl < 30MeV

<m(~n)):/+95HeV a m = 5 HeY

bl --4

10VFL]

540 580 M e V / c 2

F i g . 9 o i n v a r i a n t mass p l o t K S

(a ) any c o m b i n a t i o n (b ) o n l y t h o s e c o m b i n a t i o n s t h a t a r e t r a n s v e r s e momentum

b a l a n c e d in t h e XZ p l a n e (EpTZ < 30 M e v / c ) .

REFERENCES [1] C E R N - G e n o v a - M i l a n - M o n s - S a n t a n d e r C o l l a b o r a t i o n , P r o p o s a l f o r a t e s t

o f an i m p a c t p a r a m e t e r t r i g g e r a imed a t h i g h s t a t i s t i c s h e a v y q u a r k s t u d y , CERN/SPSC 85-62 SPSC/P 218.

[21 B o l o g n a - CERN - Genova - M i l a n o - Mons C o l l a b o r a t i o n . High s t a t i s t i c s s t u d y o f cha rm h a d r o p r o d u c t l o n u s i n g an i m p a c t p a r a m e t e r t r i g g e r (WAg2). CERN/SPSC 87-01 SPSC/P218 Add1.

[3] J . A n t h o n l o z - B l a n c e t a l . , NICE, a f a s t u s e r p r o g r a m m a b l e e m u l a t o r o f t h e PDP-11, CERN-DD/80-14.

[4] M. A g u i l a r - B e n i t e z e t a l . , Z e i t . f o r P h y s . C. P a r t i c l e s and F i e l d s 31 (1986) p . 491.