EXPERIMENTS WITH PHOLARIZED ELECTRON BEAMS AT HERA

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EXPERIMENTS WITH PHOLARIZED ELECTRON BEAMS AT HERA

P. Schmüser

To cite this version:

P. Schmüser. EXPERIMENTS WITH PHOLARIZED ELECTRON BEAMS AT HERA. Journal de

Physique Colloques, 1985, 46 (C2), pp.C2-583-C2-592. �10.1051/jphyscol:1985272�. �jpa-00224590�

JOURNAL DE PHYSIQUE

Colloque C2, supplkrnent a u n02, Tome 46, f k r i e r 1985 page C2-583

EXPERIMENTS WITH POLARIZED ELECTRON BEAMS AT HERA P. Schmiiser

I I . I n s t i t u t ftir Experirnentalphysik, Univ. Hamburg, Luruper Chaussee 149, 2000 Hamburg 50, F.R.G.

R6sum6 - Les anneaux de stockage des e l e c t r o n s e t des p r o t o n s "HERA" q u i e s t en c o n s t r u c t i o n i DESY p e r m e t t r o n t de p r o d u i r e des e n e r g i e s du c e n t r e de masse

de p l u s de 300 GeV. Ce r a p p o r t t r a i t e d e I ' u t i l i s a t i o n d 1 6 1 e c t r o n s p o l a r i s h s l o n g i t u d i n a l e m e n t pour 1 '6 t u d e des processus des i n t e r a c t i o n s f a i b l e s e t pour l a r e c h e r c h e de nouveaux c o u r a n t s e t de n o u v e l l e s p a r t i c u l e s .

A b s t r a c t : The e l e c t r o n p r o t o n s t o r a g e r i n g HERA under c o n s t r u c t i o n a t DESY w i l l p r o v i d e c e n t e r o f mass e n e r g i e s o f more t h a n 300 GeV. The use o f l o n - g i t u d i n a l l y p o l a r i z e d e l e c t r o n s i n t h e s t u d y o f e l e c t r o w e a k processes and t h e search f o r new c u r r e n t s and new p a r t i c l e s w i l l be d i s c u s s e d .

I .

-

INTRODUCTION

Deep i n e l a s t i c l e p t o n nucleon s c a t t e r i n g has p l a y e d a fundamental r o l e i n ex- p l o r i n g t h e s m a l l e s t c o n s t i t u e n t s o f m a t t e r and t h e f o r c e s between them. The e a r l y experiments w i t h e l e c t r o n beams r e v e a l e d t h e e x i s t e n c e o f p a r t o n s as p o i n t l i k e and a l m o s t f r e e s c a t t e r i n g c e n t e r s i n t h e n u c l e o n and showed a t t h e sane t i m e t h a t about h a l f o f t h e n u c l e o n momentum was c a r r i e d by e l e c t r i c a l l y n e u t r a l o b j e c t s c a l l e d " g l u o n s " . From a comparison w i t h n e u t r i n o d a t a i t became c l e a r t h a t t h e p a r t o n s c o u l d be i d e n t i f i e d w i t h t h e q u a r k s o f t h e SU(3) symmetry o f hadrons. These e x p e r i m e n t a l o b s e r v a t i o n s were t h e b a s i c i n g r e d i e n t s o f Quantum Chromodynamics.

N e u t r i n o r e a c t i o n s p r o v i d e d much f u r t h e r i n s i g h t , i n p a r t i c u l a r t h e d i s c o v e r y o f weak n e u t r a l c u r r e n t s which was one o f t h e most i m p o r t a n t s t e p s towards a u n i f i - c a t i o n o f e l e c t r o m a g n e t i c and weak i n t e r a c t i o n s .

The e l e c t r o n p r o t o n c o l l i d e r HERA now under c o n s t r u c t i o n a t DESY enables t o ex- t e n d t h e i n e l a s t i c l e p t o n nucleon s c a t t e r i n g i n t o an e n t i r e l y new regime. The c e n t e r o f mass energy o f more than 300 GeV i s i n f a c t e q u i v a l e n t t o t h a t o f a 50 TeV l e p t o n beam i m p i n g i n g on a s t a t i o n a r y t a r g e t . A t HERA, e l e c t r o m a g n e t i c and weak r e a c t i o n s can be s t u d i e d s i m u l t a n e o u s l y and a r e o f comparable magnitude. Momentum t r a n s f e r s w e l l above t h e weak boson masses a r e a c c e s s i b l e and s u b s t r u c t u r e s i n t h e p r o t o n can be i n v e s t i g a t e d down t o a s c a l e o f 1 0 - l 7 cm.

One o f t h e u n i q u e f e a t u r e s o f t h e HERA c o l l i d e r i s t h e p o s s i b i l i t y t o o b t a i n l o n - g i t u d i n a l l y p o l a r i z e d e l e c t r o n o r p o s i t r o n beams. The e m i s s i o n o f s y n c h r o t r o n r a d i a t i o n l e a d s t o a n a t u r a l t r a n s v e r s e p o l a r i z a t i o n o f up t o 92% w i t h a

b u i l d u p t i m e o f 20 m i n u t e s a t 30 GeV. Magnet arrangements c a l l e d ::spin r o t a t o r s "

serve t o r o t a t e t h e s p i n i n t o t h e beam d i r e c t i o n i n f r o n t o f an i n t e r a c t i o n

r e g i o n and back i n t o t h e t r a n s v e r s e o r i e n t a t i o n b e h i n d i t . A degree o f l o n g i t u d i n a l p o l a r i z a t i o n o f 60 - 80% seems f e a s i b l e . W i t h such l e f t - o r r i g h t - h a n d e d l e p t o n beans t h e Standard Model o f electroweak i n t e r a c t i o n s can be t e s t e d i n g r e a t d e t a i l . I f new p a r t i c l e s s h o u l d be found a t HERA l i k e heavy l e p t o n s o r quarks, l e p t o q u a r k s o r supersymmetric p a r t i c l e s t h e p o l a r i z a t i o n w i l l be e s s e n t i a l t o d e t e r m i n e t h e h e l i c i t y n a t u r e o f t h e i r c o u p l i n g s t o known o r new c u r r e n t s .

An i m p o r t a n t q u a n t i t y t o c h a r a c t e r i z e t h e p h y s i c s p o t e n t i a l o f a new a c c e l e r a t o r i s t h e number o f e v e n t s which can be c o l l e c t e d i n a r e a s o n a b l e amount o f r u n n i n g t i n e . The d e s i g n l u m i n o s i t y o f HERA i s 6 x 1031 ~ m - ~ s - l b u t f o r a c o n s e r v a t i v e

-2 -1

e s t i m a t e I s h a l l use a v a l u e o f 1 x 1031 cm s l e a d i n g t o an i n t e g r a t e d lumino- Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985272

~ 2 - 5 84 JOURNAL DE PHYSIQUE

s i t y o f 1 pb-l p e r day. F i g . 1 shows t h e r a t e o f n e u t r a l and charged c u r r e n t e v e n t s

2 2

w i t h m o n e n t u m t r a n s f e r s Q l a r g e r t h a n a g i v e n Qo. About t e n e v e n t s of each t y p e a r e expected p e r day w i t h momentum t r a n s f e r s above t h e W mass.

F i g . 1

Counting r a t e s f o r charged and neu- t r a l c u r r e n t r e a c t i o n s w i t h

Q2 above a g i v e n Q:. The curves have been computed by P . M a t t i g (DESY) u s i n g t h e Gluck, Reya, Hoffmann p a r a m e t r i z a t i o n o f t h e p r o t o n

s t r u c t u r e f u n c t i o n s and AQCD = 100 MeV.

I n t h e q u a r k - p a r t 0 8 p i c t y r e t h e r e a c t i o n s ep -. ex and ep + vX a r e mediated by gauge bosons (y, Z o r W-) w h i c h a r e exchanged between t h e l e p t a n and one o f t h e quarks o r a n t i q u a r k s i n t h e nucleon ( F i g . 2 a ) .

current jet

XP proton

proton

e

^'

k

-

F i g . 2a

-

Diagram f o r deep i n e l a s t i c l e p t o n nucleon s c a t t e r i n g . The four-momenta o f t h e incoming and o u t g o i n g l e p t o n , t h e p r o t o n and t h e exchanged f i e l d quantum a r e denoted by k, k ' , p and q, r e s p e c t i v e l y ; xp i s t h e four-momentum o f t h e s t r u c k quark. The v a r i a b l e s used a r e

2 2

q = k - k g , Q = -q2, x = Q /(2p.q), y = ( p e q ) / ( k . p ) . F o r p r o t o n s a t r e s t : p. q = Mv = M(E-E'), x = Q 2 /2Mv, y = V I E .

The s c a t t e r e d l e p t o n appears on one s i d e o f t h e p r o t o n beam, t h e s t r u c k quark t r a n s f o r m s i n t o a narrow b u n d l e o f hadrons c a l l e d " c u r r e n t j e t " on t h e o p p o s i t e s i d e . The remainders o f t h e p r o t o n produce a j e t o f hadrons moving a l o n g t h e p r o t o n beam d i r e c t i o n . Monte C a r l o s i m u l a t i o n s o f such events ( F i g . 2b) show t h a t t h e o u t g o i n g l e p t o n and t h e c u r r e n t j e t a r e v e r y w e l l s e p a r a t e d f r o m each o t h e r and f r o m t h e p r o t o n j e t . HERA i s t h u s i d e a l l y s u i t e d t o s t u d y l e p t o n - q u a r k c o l l i s i o n s w i t h o u t "background" f r o m s p e c t a t o r quarks. The e v e n t s have a v e r y d i s t i n c t t o p o l o g y and can be e a s i l y r e c o g n i z e d . I t appears r a t h e r u n l i k e l y t o confuse them w i t h p r o t o n r e a c t i o n s on r e s t gas m o l e c u l e s i n t h e vacuum chamber o r o t h e r background.

I n o r d e r t o compute t h e p r o t o n s t r u c t u r e f u n c t i o n s , t h e k i n e m a t i c a l q u a n t i t i e s Q 2 and x, i . e . t h e squared f o u r momentum t r a n s f e r and t h e f r a c t i o n a l momentum o f t h e s t r u c k quark, have t o be determined. S i n c e t h e i n c i d e n t e l e c t r o n energy i s w e l l known i t i s s u f f i c i e n t t o measure t h e energy and a n g l e o f t h e c u r r e n t j e t . A hadron c a l o r i m e t e r w i t h good energy and a n g u l a r r e s o l u t i o n w i l l be an e s s e n t i a l f e a t u r e o f a HERA d e t e c t o r .

v direction e + q v + q + gluon

GI2 =

25000 G ~ V *

A

F i g . 2b

-

Monte C a r l o s i m u l a t i o n o f an e v e n t e l e c t r o n

+

p r o t o n

-

n e u t r i n o

+

quark

+

g l u o n

+

X . The o u t g o i n g quark and g l u o n fragment i n t o w e l l s e p a r a t e d hadron j e t s . The b l a c k d o t s i n d i c a t e t h e e n d p o i n t s o f t h e momentum

2 3 2

v e c t o r s o f t h e hadrons. F o r m o m e n t u m t r a n s f e r s Q

>

10 GeV t h e l e p t o n i s s c a t t e r e d by more than 90". (From D.H.Perkins, R e f . 1 ) .

x

=

0.5

-

current jet

vector

-

*,

gluon jet

*-. .\ ^-

quark jet

.'-- ^{*. .'}

^{\ \}

-\ \

20

u

\ >

10

$

n"

\ \

proton

-

jet vector

---

C-- 0 - - - -0-0- -+-a 0

/

I I

I

^I I I I I I ^I

4 / -5

C2-586 JOURNAL DE PHYSIQUE

11.

-

TESTS OF THE STANDARD MODEL AND SEARCH FOR NEW WEAK BOSONS

A l l wgak processes observed so f a r and i n p a r t i c u l a r t h e d i s c o v e r y o f t h e W?

and Z bosons a t t h e CERN pp c o l l i d e r a r e i n e x c e l l e n t agreement w i t h t h e s t a n d a r d model o f e l e c t r o w e a k i n t e r a c t i o n s .

W i t h HERA i t w i l l be p o s s i b l e t o t e s t t h i s t h e o r y i n t h e r e g i o n o f v e r y l a r g e s p a c e l i k e momentum t r a n s f e r s where e l e c t r o m a g n e t i c and weak processes s h o u l d be o f equal s t r e n g t h . The s t a n d a r d model p r e d i c t s q u i t e c h a r a c t e r i s t i c f e a t u r e s f o r n e u t r a l and charged c u r r e n t i n t e r a c t i o n s . The c r o s s s e c t i o n o f t h e r e a c t i o n e p ⁺ e X, n o r m a l i z e d t o t h e one-photon exchange c r o s s s e c t i o n , i s shown i n F i g . 3.

2 4

F o r Q above 10 G ~ v ' , t h e photon-z0 i n t e r f e r e n c e t e r m i s v e r y l a r g e and l e a d s t o remarkable d i f f e r e n c e s between l e f t - h a n d e d and r i g h t - h a n d e d e l e c t r o n s and a l s o between e l e c t r o n s and p o s i t r o n s .

F i g . 3

-

I n t e r f e r e n c e between photon and Z? exchange i n ep + ex w i t h l o n g i t u d i n a l l y p o l a r i z e d e l e c t r o n s o r p o s i t r o n s . P l o t t e d i s t h e r a t i o o f t h e s t a n d a r d model c r o s s s e c t i o n t o t h e one-photon exchange c r o s s s e c t i o n a t x = 0.25. (From Ref. 2 ) .

A q u e s t i o n o f c r u c i a l importance i s o f c o u r s e whether a d d i t i o n a l ZO bosons e x i s t . P o l a r i z e d l e p t o n beams a l l o w a more s e n s i t i v e search t h a n c r o s s s e c t i o n measurements.

The p o l a r i z a t i o n asymmetry between l e f t - h a n d e d e l e c t r o n s and r i g h t - h a n d e d p o s i t r o n s i s shown i n F i g . 4. W i t h a t o t a l i n t e g r a t e d l u m i n o s i t y o f 400 pb-l, Z2 masses up t o 450 GeV seem a c c e s s i b l e .

Standard model x = 0.25

M z2 = 300 GeV

-

Standard model x = 0.25

F i g . 4

-

Asymmetry between l e f t - h a n d e d e l e c t r o n s and r i g h t - h a g d e d p o s i t r o n s i n t h e s t a n d a r d model and i n a model w i t h a second Z

.

The q u a n t i t y x2 i s t h e m i x i n g parameter f o r t h e Z2. A l s o shown a r e " d a t a p o i n t s "

which m i g h t be o b t a i n e d i n a measurement w i t h a t o t a l i n t e g r a t e d l u m i n o s i t y o f 400 pb-' and a degree o f p o l a r i z a t i o n P = 0.80+0.02.

(Curves f r o m R.J.Cashmore, R e f . 3 ) .

The charged c u r r e n t r e a c t i o n ep ⁺ vX i s mediated by W' exchange w i t h p u r e l y l e f t - h a n d e d c o u p l i n g s . A d d i t i o n a l l e f t - h a n d e d W bosons can be d e t e c t e d a t HERA even if t h e i r mass s h o u l d be beyond t h e center-of-mass energy o f 300 GeV. The reason i s t h a t t h e i n t e r f e r e n c e w i t h t h e s t a n d a r d W exchange l e a d s t o a s u b s t a n t i a l i n c r e a s e i n c o u n t i n g r a t e a t l a r g e v a l u e s o f Q L ( s e e F i g . 5 ) . W, masses up t o

L

500 GeV can be determined. The s i t u a t i o n i s q u i t e d i f f e r e n t i f t h e second W couples o n l y t o r i g h t - h a n d e d e l e c t r o n s . I n t h a t case t h e r e i s no i n t e r f e r e n c e and t h e r a t e f o r eRp+vX may be q u i t e low i f t h e mass o f t h e WR i s h i g h . A l o w e r l i m i t o f 400 GeV f o r t h e mass o f a W boson w i t h r i g h t - h a n d e d c o u p l i n g s can be d e r i v e d from t h e decay o f p o l a r i z e d muons 4 ) . While i t w i l l be d i f f i c u l t t o improve such a

C2-588 JOURNAL DE PHYSIQUE

l i m i t a t HERA one has t o keep i n mind t h a t t h e experiments a t low QL a r e o n l y s e n s i t i v e t o r i g h t - h a n d e d c u r r e n t s i f t h e n e u t r a l p a r t n e r v R o f t h e r i g h t - handed charged l e p t o n i s massless o r v e r y l i g h t , m ( v R )

<

10 MeV. Since t h e r e i s no compel 1 in g t h e o r e t i c a l reason why r i g h t - h a n d e d n e u t r i n o s s h o u l d be 1 ig h t ,

"

measurements a t l a r g e QL a r e u n a v o i d a b l e t o e i t h e r e s t a b l i s h o r r u l e o u t r i g h t - handed c u r r e n t s .

F i g . 5

-

The c r o s s s e c t i o n f o r t h e charged c u r r e n t r e a c t i o n e

+

p ⁺ v

+

X i n a model w i t h two l e f t - h a n d e d W bosons, n o r m a l i z e d t o t h e s t a n d a r d model c r o s s s e c t i o n . The c o u p l i n g c o n s t a n t s o f t h e

W.,

and

W2

a r e

n

assumed t o be t h e same and a r e a d j u s t e d such t h a t a t qL = 0 t h e

2 4 2

Fermi c o u p l i n g i s o b t a i n e d . The c o u n t i n g r a t e a t Q

2

3 x 10 GeV -2 -1 i s about 100 p e r y e a r a t a l u m i n o s i t y o f 1 x 1031 cm s , so W2 masses w e l l above t h e HERA center-of-mass energy a r e a c c e s s i b l e .

To g e t an i d e a on t h e s e n s i t i v i t y which m i g h t be achieved a t HERA we c o n s i d e r t h e r a t i o R = oR/oL o f t h e r i g h t - and l e f t - h a n d e d c r o s s s e c t i o n s . Assuming t h a t t h e WR c o u p l e s t o t h e normal quarks and l e p t o n s w i t h t h e same s t r ' e n g t h as t h e s t a n d a r d

WL,

t h i s r a t i o i s g i v e n by

2 2

A t Q = 5000 GeV a r a t i o R = 0.05 corresponds t o a WR mass o f 214 GeV. F o r a degree o f p o l a r i z a t i o n o f P = 0.80+0.01 and an i n t e g r a t e d l u m i n o s i t y o f 300 pb-l, shared e q u a l l y between l e f t - h a n d e d and r i g h t - h a n d e d e l e c t r o n beams, one o b t a i n s a s t a t i s t i c a l s i g n i f i c a n c e f o r t h e c r o s s s e c t i o n r a t i o o f

R/a(R) = 5 . 8

.

The s t a t i s t i c a l s i g n i f i c a n c e depends b o t h on t h e degree o f p o l a r i z a t i o n P and on t h e u n c e r t a i n t y o ( P ) . F o r P = 0.60

t

0.02, t h e s t a t i s t i c a l s i g n i f i c a n c e ob- t a i n e d w i t h t h e same t o t a l l u m i n o s i t y i s o n l y 3.0.

111.

-

SUBSTRUCTURE I N QUARKS AND LEPTONS

One o f t h e b a s i c t a s k s o f t h e HERA experiments w i l l be t o measure t h e p r o t o n

0

s t r u c t u r e f u n c t i o n s a t l a r g e Q'. S y s t e m a t i c t e s t s o f QCD, t h e n a t u r e o f s c a l i n g v i o l a t i o n s , an a c c u r a t e measurement o f a S ( Q 2 ) and a d e t e r m i n a t i o n o f t h e quark d e n s i t i e s a r e problems o f g r e a t i n t e r e s t . I do n o t want t o d i s c u s s these q u e s t i o n s b u t r a t h e r c o n c e n t r a t e on t h e i m p l i c a t i o n s o f a s u b s t r u c t u r e . I f quarks o r l e p t o n s a r e comoosite a new s t r o n q f o r c e must b i n d t h e c o n s t i t u e n t s . The a s s o c i a t e d f i e l d quanta a r e p r o b a b l y v e r y massive because f e r m i o n s a r e p o i n t l i k e down t o 1 0 - l 6 cm.

One cannot e x p e c t t o observe them d i r e c t l y b u t a r e s i d u a l i n t e r a c t i o n , analogous t o t h e f o u r - f e r m i o n i n t e r a c t i o n i n B-decay, may be d e t e c t e d due t o i t s i n t e r f e r e n c e w i t h t h e s t a n d a r d y, ZO and W' exchanges. The new i n t e r a c t i o n can be c h a r a c t e r i z e d by a compositeness s c a l e AH w h i c h i s presumably above 1 TeV. RYckl 5, has computed

Q

t h e change i n t h e p r o t o n s t r u c t u r e f u n c t i o n F ~ ( X , Q ' ) as a f u n c t i o n o f t h e para- meter AH ( F i g . 6 ) .

F i g . 6

The p r o t o n s t r u c t u r e f u n c t i o n F (x,Q 2 2 ) a t x = 0.5 f o r v a r i o u s v a l u e s o f t h e s c a l e parameter AH.

The dashed and d o t t e d curves a r e t h e s t a n d a r d model p r e d i c t i o n and t h e one-photon exchange r e s u l t , r e s p e c t i v e l y .

C2-590 JOURNAL DE PHYSIQUE

4 2

I t appears t h a t a t Q ~ = 3 x 10 GeV v a l u e s o f A H as h i g h as 2-3 TeV may be de- t e c t e d . One has t o keep i n mind, however, t h a t t h e quark d i s t r i b u t i o n s a t such

2 2

l a r g e Q a r e unknown and t h a t t h e s t a n d a r d model c a l c u l a t i o n o f F2(x,Q ) de- pends on l a r g e e x t r a p o l a t i o n s o f p r e s e n t d a t a . P o l a r i z a t i o n asymmetries a r e i n - dependent o f such e x t r a p o l a t i o n s . As an example I show i n F i g . .7a t h e asymmetry

Depending on t h e h e l i c i t y n a t u r e o f t h e l e p t o n and quark c o u p l i n g s a t t h e f o u r - f e r m i o n v e r t e x , t h e r e s i d u a l i n t e r a c t i o n may be v i s i b l e f o r A H v a l u e s up t o 5 TeV.

Another asymmetry w h i c h does n o t depend on t h e t e c h n i c a l l y d i f f i c u l t l o n g i t u d i n a l beam p o l a r i z a t i o n i s t h a t between e l e c t r o n s and p o s i t r o n s ( F i g . 7b). Here t h e sen- s i t i v i t y i s o n l y l a r g e i f t h e l e p t o n s and quarks c o u p l e w i t h o p p o s i t e h e l i c i t i e s ( l e f t - r i g h t o r r i g h t - l e f t ) a t t h e f o u r - f e r m i o n v e r t e x .

F i g . 7a - The p o l a r i z a t i o n asymmetry A = ( u ( e i )

-

u ( e L ) ) / ( u ( e L )

+

u(e;)) i n t h e s t a n d a r d model ( d o t t e d c u r v e ) and f o r v a r i o u s v a l u e s o f t h e s u b s t r u c t u r e s c a l e A H . The l a b e l "LL" (RR) denotes l e f t - h a n d e d ( r i g h t - h a n d e d ) c o u p l i n g s f o r b o t h t h e l e p t o n s and t h e quarks i n t h e r e s i d u a l f o u r - f e r m i o n i n t e r a c t i o n .

F i g . 7b - The charge asymmetry ^BO = ( u ( e - )

-

u ( e + ) ) / ( o ( e - )

+

u ( e + ) ) i n t h e s t a n d a r d model ( d o t t e d c u r v e ) and f o r v a r i o u s v a l u e s o f AH.

I V - SEARCH FOR NEW PARTICLES

E l e c t r o n p r o t o n c o l l i s i o n s a r e i d e a l l y s u i t e d t o produce new l e p t o n s which c o u p l e t o t h e e l e c t r o n as w e l l as new quarks w h i c h c o u p l e t o t h e u and d quarks i n t h e p r o t o n . A n e u t r a l heavy l e p t o n Lo and a heavy quark Q c o u l d be produced i n t h e r e a c t i o n e q + Lo Q . P r o v i d e d new charged c u r r e n t s e x i s t which do n o t suppress t h i s process t h r o u g h s m a l l m i x i n g angles t h e r a t e s a r e f a i r l y h i g h even f o r l a r g e masses of t h e l e p t o n and quark (e.g. 1 e v e n t p e r day f o r ML = M - 50 GeV and

Q

-

a mass o f t h e exchanged W boson o f 200 GeV). The decay o f t h e heavy l e p t o n l e a d s t o pronounced e v e n t s i g n a t u r e s : l e p t o n s and h a d r o n i c j e t s w i l l emerge f r o m

t h e l e p t o n v e r t e x i n s t e a d o f t h e s i n g l e o u t g o i n g n e u t r i n o i n normal charged c u r r e n t s e v e n t s . If such heavy l e p t o n s should be d e t e c t e d , measurements w i t h l o n g i t u d i n a l l y p o l a r i z e d e l e c t r o n s would be e x t r e m e l y v a l u a b l e . Suppose t h e L o was found t o c o u p l e t o r i g h t - h a n d e d e l e c t r o n s o n l y then i t would be n a t u r a l t o assume t h a t r i g h t - handed e l e c t r o n s belong i n t o weak i s o s p i n d o u b l e t s r a t h e r t h a n i n t o s i n g l e t s as i n t h e s t a n d a r d model.

Another c l a s s o f p a r t i c l e s w h i c h c o u p l e d i r e c t l y t o e i t h e r t h e e l e c t r o n o r t h e n o r - mal quarks i n t h e p r o t o n a r e t h e i r supersymmetric p a r t n e r s . The a s s o c i a t e d p r o - d u c t i o n o f s c a l a r l e p t o n s and quarks

e + q + Z + i j o r e + q + G + q

proceeds t h r o u g h t h e exchange o f a s p i n 112 f i e l d quantum

( 7 , 2

o r A ) . The s c a l a r e l e c t r o n decays i n t o an e l e c t r o n and a p h o t i n o , t h e s c a l a r quark i n t o a quark and a g l u i n o o r p h o t i n o .

Since b o t h l a n d " g r e i n v i s i b l e t h e observed f i n a l s t a t e i n t h e r e a c t i o n

e

+

q ⁺ e

+

q i s t h e same as i n normal n e u t r a l c u r r e n t events: a s i n g l e e l e c t r o n t r a c k and a j e t o f hadrons. Due t o t h e momentum and energy c a r r i e d away by t h e p h o t i n o and g l u i n o t h e s e supersymmetric e v e n t s w i l l have a l a r g e imbalance i n t r a n s v e r s e momentum and t h e e l e c t r o n and t h e hadron j e t w i l l n o t appear b a c k - t o - back when viewed a l o n g t h e beam d i r e c t i o n . By s u i t a b l e c u t s i n c o p l a n a r i t y and t r a n s v e r s e momentum t h e s e e x o t i c events can be v e r y w e l l s e p a r a t e d f r o m c o n v e n t i o n a l n e u t r a l c u r r e n t i n t e r a c t i o n s .

The r e a c t i o n e

+

q + G

+

^ij i s much more d i f f i c u l t t o i d e n t i f y e x p e r i m e n t a l l y s i n c e t h e n u i n o w i l l i n g e n e r a l decay i n t o i n v i s i b l e p a r t i c l e s . The f i n a l s t a t e t h e r e - f o r e c o n t a i n s j u s t a s i n g l e j e t o f hadrons and cannot be d i s t i n g u i s h e d from t h e much more numerous c h a r g e d - c u r r e n t events e

+

^{q + v}

+

q ' a t l o w e r .Q' There i s , however, t h e p o s s i b i l i t y o f more c o m p l i c a t e d decay modes o f t h e n u i n o l e a d i n g t o m u l t i l e p t o n _or mixed l e p t o n and hadron f i n a l s t a t e s . I n t h a t case t h e e v e n t s e

+

q ⁺ v

+

q w i l l be unmistakable.

The r a t e f o r a s s o c i a t e d p r o d u c t i o n o f s c a l a r e l e c t r o n s and quarks has been com- 7 ) p u t e d by Jones and L l e w e l l y n - S m i t h 6 , and by A l t a r e l l i , Mele and Riickl

.

F i g . 8 shows t h e c r o s s s e c t i o n a t & = 314 GeV as a f u n c t i o n o f t h e and ij masses.

Almost 10 e v e n t s p e r day a r e expected f o r

m;;;

⁼ m- = 40 GeV. The c r o s s s e c t i o n s q

f o r t h e e l e c t r o p r o d u c t i o n o f supersymmetric p a r t i c l e s depend s t r o n g l y on t h e lepton-charge and p o l a r i z a t i o n (see R e f . 6 ) . F o r t h e charged c u r r e n t process e q ⁺ v q t h e p r e d i c t e d r a t e w i t h l e f t - h a n d e d e l e c t r o n s i s s i g n i f i c a n t l y h i g h e r t h a n t h a t w i t h r i g h t - h a n d e d p o s i t r o n s ; f o r n e u t r a l c u r r e n t SUSY e v e n t s , on t h e o t h e r hand, p o s i t r o n s g i v e a h i g h e r y i e l d t h a n e l e c t r o n s . Measurements w i t h p o l a r i z e d beams and w i t h b o t h e l e c t r o n s and p o s i t r o n s a r e h i g h l y d e s i r a b l e t o check these p r e d i c t i o n s .

JOURNAL DE PHYSIQUE

F i g . 8

The c r o s s s e c t i o n f o r a s s o c i a t e d p r o d u c t i o n o f s c a l a r e l e c t r o n s and quarks a t = 314 GeV as a f u n c t i o n o f t h e

-

e and ij masses.

(From R. J .Cashmore, Ref .3)

m6 GeV

V

-

CONCLUSION

The e l e c t r o n p r o t o n c o l l i d e r HERA opens a r i c h f i e l d o f p h y s i c s which i s complemen- t a r y t o t h e p h y s i c s p o t e n t i a l o f e'e- and p i ( p p ) c o l l i d e r s . A l a r g e f r a c t i o n o f t h e p h y s i c s programme can be covered w i t h u n p o l a r i z e d beams l i k e t h e measurement o f t h e p r o t o n s t r u c t u r e f u n c t i o n s , QCD t e s t s and search f o r new p a r t i c l e s . The l o n g i t u d i n a l e l e c t r o n p o l a r i z a t i o n i s v a l u a b l e i n t h e i n v e s t i g a t i o n o f s u b s t r u c t u r e e f f e c t s ; i t i s i n d i s p e n s a b l e f o r d e t a i l e d s t u d i e s o f t h e s t a n d a r d model, i n t h e search f o r r i g h t handed charged c u r r e n t s and f o r d e t e r m i n i n g t h e h e l i c i t y s t r u c t u r e o f t h e c o u p l i n g s o f new p a r t i c l e s .

I want t o thank R.Cashmore, W.Hollik, P.Mattig, R.Ruckl and B.H.Wiik f o r u s e f u l d i s c u s s i o n s .

V I

-

References

1 ) D i s c u s s i o n M e e t i n g "Physics w i t h ep c o l l i d e r s i n view o f HERA", Wuppertal October 2-3, 1981

2) Study on t h e P r o t o n - E l e c t r o n Storage R i n g P r o j e c t HERA

-

ECFA 80142 (1980) 3) Proceedings o f t h e Workshop E x p e r i m e n t a t i o n a t HERA, Amsterdam,

June 9-11, 1983

-

^DESY r e p o r t HERA 83/20 (1983)

4) H.Steiner, X X I I I n t e r n a t i o n a l Conference on High Energy Physics, L e i p z i g , J u l y 19-25 (1984)

5 ) R.Ruck1, P h y s . L e t t . i 2 9 B (1983), 363, Nucl.Phys. B 234 (1984), 91, CERN r e p o r t TH 3897 (1984)

6) S.K.Jones and C.H.Llewellyn Smith, Nucl.Phys. B 217 (1983), 145 7) G . A l t a r e l l i , B.Mele and R.Ruck1, CERN r e p o r t TH 3932 (1984).