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POLARIZATION AND ELECTROMAGNETIC PROBES
J. Laget
To cite this version:
J. Laget. POLARIZATION AND ELECTROMAGNETIC PROBES. Journal de Physique Colloques,
1990, 51 (C6), pp.C6-469-C6-471. �10.1051/jphyscol:1990654�. �jpa-00230924�
COLLOQUE DE PHYSIQUE
C o l l o q u e C6, suppl6ment
au
n 0 2 2 , Tome 51, 1 5 novembre 1990POLARIZATION AND ELECTROMAGNETIC PROBES
J.M. LAGET
DPhN/SEPN, CEN-Saclay, F-91191 Gif sur Yvette Cedex, France
Rbsum6: Les p r o g r b s r e c e n t s dans l a m a i t r i s e d e s f a i s c e a u x d ' e l e c t r o n s e t d e photons p o l a r i s e s p e r m e t t e n t d ' e n v i s a g e r un programme d 1 6 t u d e s s y s t e m a t i q u e s d e s o b s e r v a b l e s d e s p i n d a n s l e s systbmes hadroniques. Les grand a x e s e n s o n t degages.
A b s t r a c t : Recent p r o g r e s s e s i n h a n d l i n g p o l a r i z e d beams of e l e c t r o n s and photons a l l o w t o u n d e r t a k e t h e s y s t e m a t i c s t u d y of t h e s p i n o b s e r v a b l e s i n h a d r o n i c m a t t e r . The main a x e s a r e given.
1. INTRODUCTION
C o n t r a r y t o h a d r o n i c p r o b e s , t h e s t u d y of s p i n o b s e r v a b l e s w i t h e l e c t r o m a g n e t i c probes i s n o t a domain of i n t e n s e a c t i v i t y . Very few r e s u l t s e x i s t !
However, t h i s s i t u a t i o n i s b e i n g e v o l v i n g q u i c k l y . Why?
F i r s t , t h e s y s t e m a t i c measurement of t h e u n p o l a r i z e d c r o s s s e c t i o n s , w i t h i n t e n s e e l e c t r o n beams d u r i n g t h e p a s t twenty y e a r s , h a s l e d t o a good u n d e r s t a n d i n g of t h e s t r u c t u r e of n u c l e a r systems and of t h e dominant r e a c t i o n mechanisms. To go f u r t h e r and t o u n d e r s t a n d i n more d e t a i l s t h e s t r u c t u r e of h a d r o n i c systems r e q u i r e t h e measurement of s p i n
o b s e r v a b l e s .
Second, two major t e c h n i c a l p r o g r e s s e s have been done d u r i n g t h e p a s t decade. O n t h e one hand, l i n e a r l y and c i r c u l a r l y p o l a r i z e d r e a l photon monochromatic beams a r e o b t a i n e d by backward s c a t t e r i n g of h i g h energy e l e c t r o n s on a l a s e r . This t e c h n i q u e i s a l r e a d y used a t F r a s c a t i and Brookhaven, and w i l l b e u s e d a t h i g h e r e n e r g i e s a t t h e European Synchrotron R a d i a t i o n F a c i l i t y (ESRF) a t Grenoble. On t h e o t h e r hand, more t h a n twenty y e a r s of work have l e d t o a g e n e r a t i o n of r e l i a b l e p o l a r i z e d e l e c t r o n s o u r c e s . They were mainly used i n p a r i t y v i o l a t i n g experiments where t h e requirement of s t a b i l i t y i s q u i t e s t r o n g . L. Cardman e x p l a i n e d u s t h a t , w h i l e p o l a r i z a t i o n s of 50% i s c u r r e n t l y o b t a i n e d f o r l a r g e peak c u r r e n t s , we may e x p e c t t o r e a c h i n t h e n e a r f u t u r e p o l a r i z a t i o n s of t h e o r d e r of 80 t o 90% f o r
c o n t i n u o u s beams i n t h e r a n g e of 10pA.
I n t h i s s e s s i o n , t h e s e p r o g r e s s e s have been reviewed by L. Cardman and G. Matone. The p o s s i b i l i t i e s opened up by t h e s e new t o o l s was reviewed by G. Matone and H. V a r t a p e t i a n f o r r e a l photons and by R. Milner f o r v i r t u a l photons. S. Cotanch and H. S h e r i f p u t a s p e c i a l emphasis r e s p e c t i v e l y on t h e r e l e v a n c e of s p i n o b s e r v a b l e s i n t h e s t u d y of hyperon
p h o t o p r o d u c t i o n r e a c t i o n s and on t h e importance of r e l a t i v i s t i c e f f e c t s i n p h o t o n u c l e a r r e a c t i o n s .
2. REAL PHOTONS
I n t h e p a s t , t h e asymmetries measured w i t h l i n e a r l y p o l a r i z e d photons were a n e s s e n t i a l i n p u t i n t h e m u l t i p o l e a n a l y s i s of t h e s i n g l e p i o n p h o t o p r o d u c t i o n r e a c t i o n s on f r e e
nucleons. Such s t u d i e s a r e s t i l l a t r a d i t i o n i n USSR, and have been e x t e n s i v e l y extended t o h e a v i e r n u c l e i . A t Erevan t h e s t u d y of t h e elementary p(;, n O ) p , d($, n O ) d and d ( i , p)n c h a n n e l s i s a c t i v e l y pursued. The f i r s t r e s u l t s of t h e s t u d y of t h e p(?, y ) p , 4 ~ e ( y , pn) and
~ i ( f , pn) r e a c t i o n s have been p r e s e n t e d a t t h e conference. While Compton s c a t t e r i n g of p o l a r i z e d photons on f r e e n u c l e o n s i s e x p e c t e d t o improve o u r knowledge of t h e i n t e r n a l s t r u c t u r e of t h e nucleons, t h e s t u d y of t h e ( f , pn) r e a c t i o n i s e x p e c t e d t o improve o u r u n d e r s t a n d i n g of t h e mechanisms which govern t h e a b s o r p t i o n of a photon by a p a i r of nucleons, and c o n s e q u e n t l y of s h o r t r a n g e c o r r e l a t i o n s .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990654
C6-470 COLLOQUE DE PHYSIQUE
I n t h e f u t u r e , t h e a b s o r p t i o n of c i r c u l a r l y p o l a r i z e d photons by p o l a r i z e d t a r g e t s ( o r t h e d e t e r m i n a t i o n of s p i n t r a n s f e r c o e f f i c i e n t s t o t h e e j e c t i l e i n r e a c t i o n s induced by p o l a r i z e d photons) w i l l open up a new e r a , and a l l o w t o u n d e r t a k e s y s t e m a t i c s t u d i e s of:
-
S p i n d e n s i t i e s of h a d r o n i c s y s t e m s ,-
I I e l i c i t y a m p l i t u d e s ,-
Many body mechanisms and f o r c e s .The a d v e n t of i n t e n s e p o l a r i z e d photon beams b a c k s c a t t e r e d on l a s e r ( t a b l e 1 ) makes p o s s i b l e t h e s e s t u d i e s . P i o n e e r i n g works have been performed a t F r a s c a t i w i t h t h e LADON f a c i l i t y : l i m i t e d t o 80 MeV, i t was u s e d mainly t o s t u d y t h e p h o t o d i s i n t e g r a t i o n of t h e deuterium. The f i r s t r e s u l t s o b t a i n e d w i t h t h e LEGS f a c i l i t y , a t Brookhaven, have been p r e s e n t e d a t t h e conference. Limited t o 300 MeV, i t w i l l be mainly used t o s t u d y meson exchange c u r r e n t s (MEC) and t h e mechanisms which i n v o l v e t h e A i n n u c l e i . The GRAAL f a c i l i t y w i l l be i n s t a l l e d on t h e ESRF, l o c a t e d a t Grenoble, and i s e x p e c t e d t o b e commisioned i n
1995. The a v a i l a b l e energy range w i l l extend up t o 1.8 GeV, and t h e p h y s i c s program w i l l f o c u s on t h e s t u d y of h a d r o n i c m a t t e r a t e n e r g i e s l a r g e r t h a n t h e A e x c i t a t i o n energy, t h e s t u d y of s t r a n g e n e s s p r o d u c t i o n , t h e s t u d y of Drell-Hearn-Gerassimov sum r u l e , and, more g e n e r a l l y , t h e s t u d y of t h e a b s o r p t i o n of h i g h energy p o l a r i z e d photons on n u c l e i .
T a b l e 1: The modern p o l a r i z e d photon f a c i l i t i e s F a c i l i t y
LADON LEGS
GRAAL
3. VIRTUAL PHOTONS
V i r t u a l photons a l l o w t o d i s e n t a n g l e t h e Coulomb p a r t of t h e c r o s s - s e c t i o n and t h e t r a n s v e r s e p a r t which r e d u c e s t o t h e r e a l photon a b s o r p t i o n c r o s s - s e c t i o n when t h e f o u r momentum Q2 of t h e v i r t u a l photon v a n i s h e s . The d e t e r m i n a t i o n of t h e Coulomb p a r t o f f e r s u s w i t h a d i r e c t way t o s t u d y t h e c o u p l i n g of t h e v i r t u a l photon w i t h t h e c h a r g e of each c o n s t i t u e n t of t h e h a d r o n i c s systems, and t o s t u d y t h e i r s t r u c t u r e .
A t low e n e r g i e s (E = 4 GeV), t h e Coulomb p a r t i s f r e e of A and p i o n p r o d u c t i o n
mechanisms (which dominate t h e t r a n s v e r s e p a r t ) , and a l l o w s t o s t u d y t h e s h o r t range b e h a v i o r of n u c l e a r systems. At h i g h e r e n e r g i e s (E
>
1 0 GeV), t h e Coulomb p a r t i s v e r y s e n s i t i v e t o h i g h e r o r d e r c o r r e c t i o n s t o P e r t u r b a t i v e QCD ( h i g h e r t w i s t s , non P e r t u r b a t i v e c o n t r i b u t i o n s , e t c . ) : f o r i n s t a n c e , t h e p u r e quark-parton model p r e d i c t s a v a n i s h i n g Coulomb c r o s s - s e c t i o n a t very h i g h momentum t r a n s f e r ( Q ~>
1 0 G ~ v ~ ) .P h y s i c s Program
D(;, p)n
.
MEC A Beyond A S t r a n g e n e s s DHG sum r u l e.
' T TEnergy MeV 8 0 300
1800
T r a d i t i o n a l l y , t h i s s e p a r a t i o n i s a c h i e v e d by means of t h e Rosenbluth method: t h e e l e c t r o n s c a t t e r i n g c r o s s - s e c t i o n must b e measured a t two v a l u e s of t h e e l e c t r o n s c a t t e r i n g a n g l e ( f o r t h e same v a l u e of t h e energy t r a n s f e r w a n d t h e few momentum Q2). But t h e p r i c e t o pay i s high. On t h e one hand, t h e measurement a t t h e most forward a n g l e c o s t s energy. On t h e o t h e r hand, s y s t e m a t i c e r r o r s have t o b e s m a l l and w e l l under c o n t r o l , t o make
meaningful1 t h e comparison of t h e c r o s s - s e c t i o n a t t h e two e l e c t r o n s c a t t e r i n g a n g l e s , e s p e c i a l l y s i n c e t h e Coulomb c r o s s s e c t i o n i s i n g e n e r a l small.
The a d v e n t of r e l i a b l e p o l a r i z e d e l e c t r o n beams and p o l a r i z e d t a r g e t s ( o r p o l a r i m e t e r s ) o f f e r s u s w i t h a n e l e g a n t way t o overcome t h o s e d i f f i c u l t i e s , by u s i n g s p i n a s a "knob" t o s e l e c t t h e p a r t of t h e c r o s s - s e c t i o n which i s d i r e c t l y p r o p o r t i o n a l t o t h e i n t e r f e r e n c e between t h e Coulomb and t h e t r a n s v e r s e p a r t of t h e t r a n s i t i o n amplitude.
I n t e n s i t y
Y I s
1 07
lo7
F i r s t Experiment
1985 1990
1995
T h i s method p r e s e n t s t h r e e a d v a n t a g e s . F i r s t , t h e t r a n s v e r s e p a r t a c t s a s a n a m p l i f i e r , which enhances t h e s m a l l c o n t r i b u t i o n o f t h e Coulomb p a r t . Second, o n l y t h e measurement o f a s p e t r i e s i s r e q u i r e d , a n d t h e s o u r c e s of s y s t e m a t i c e r r o r s a r e s t r o n g l y reduced. L a s t , t h e e l e c t r o n s c a t t e r i n g a n g l e n e e d s n o t t o b e small, a n d t h e e n e r g y of t h e beam i s s m a l l e r , by r o u g h l y a f a c t o r two, t h a n t h e energy r e q u i r e d t o p e r f o r m a R o s e n b l u t h s e p a r a t i o n . T h i s method i s t h e o n l y way t o d e t e r m i n e v a r i o u s c h a r g e from f a c t o r s and t h e s m a l l components o f t h e n u c l e a r wave-functions. A few examples a r e l i s t e d i n t a b l e 2.
T a b l e 2: Some examples of q u a n t i t i e s m e a s u r a b l e w i t h p o l a r i z e d e l e c t r o n s a n d t a r g e t s ( o r p o l a r i m e t e r s ) .
Physics
Charge form f a c t o r s
0 GE ( Q 2 ) P GE ( Q ~ )
n
...
Small Components o f t h e wave f u n c t i o n
D/S r a t i o
e A Components
...
t
. The f i r s t r e s u l t s of s u c h s t u d i e s were r e c e n t l y o b t a i n e d a t ~ ~ ~ / B a t e s , and p r e s e n t e d a t t h e c o n f e r e n c e . The asymmetry measured i n t h e i n c l u s i v e r e a c t i o n 3Re(h, e ' ) i s c o n s i s t e n t w i r h t h e v a l u e e x p e c t e d from t h e 311e s t r u c t u r e f u n c t i o n s computed i n t h e Faddeev formalism:
38e i s v e r y l i k e l y t o be a good c a n d i d a t e f o r a p o l a r i z e d n e u t r o n t a r g e t . P r e l i m i n a r y s t u d i e s of t h e D($, e ' h ) r e a c t i o n s i n d i c a t e t h a t i t w i l l o f f e r u s w i t h a good way t o d e t e r m i n e t h e e l e c t r i c form f a c t o r of t h e n e u t r o n .
Reactions
P(;, $ ) e '
...
D(;, e';); 3?ie(;, e l n ) +
+ ...
p ( e , e f p ) n O
3 + 3He(e, evn')
I n t h e f u t u r e , t h e combined u s e of r e l i a b l e p o l a r i z e d e l e c t r o n beams, of r e l i a b l e p o l a r i z e d t a r g e t s and of a 471 d e t e c t o r w i l l a l l o w t o c o v e r e x t e n s i v e l y t h i s f i e l d . The MLT/Bates p r o j e c t was p r e s e n t e d a t t h e c o n f e r e n c e . But i n t h e e n e r g y r a n g e of 1 GeV, t h e same program w i l l a l s o b e c o v e r e d a t Mainz, and p o s s i b l y a t NIKHEF and Bonn. A f t e r 1995, s p i n o b s e r v a b l e s w i l l b e a major p a r t of t h e p h y s i c s program a t CEBAF up t o 4GeV.
4. CONCLUSION
C o n t r a r y t o t h e p a s t , s p i n p h y s i c s w i l l b e a major a c t i v i t y o f t h e p r e s e n t g e n e r a t i o n o f e l e c t r o n machines, and w i l l make i t p o s s i b l e t o r e f i n e o u r dynamical d e s c r i p t i o n of n u c l e o n and n u c l e a r systems.
I n t h e f u t u r e , s p i n p h y s i c s s h o u l d b e t h e s p e c i f i c i t y of a n e l e c t r o n machine, i n t h e r a n g e of 10 GeV, aimed t o go beyond t h e P e r t u r b a t i v e QCD d e s c r i p t i o n of h a d r o n i c systems.
