POLARIZED PROTONS AND PARITY VIOLATING ASYMMETRIES

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POLARIZED PROTONS AND PARITY VIOLATING ASYMMETRIES

T. Trueman

To cite this version:

T. Trueman. POLARIZED PROTONS AND PARITY VIOLATING ASYMMETRIES. Journal de Physique Colloques, 1985, 46 (C2), pp.C2-721-C2-725. �10.1051/jphyscol:1985293�. �jpa-00224612�

JOURNAL DE PHYSIQUE

Colloque C2, supplément au n°2, Tome *6, février 1985 page C2-721

POLARIZED PROTONS AND PARITY VIOLATING ASYMMETRIES*

T.L. Trueman

Physios Department, Brookhaven National Laboratory, Upton, New York 1197 Z, U.S.A.

Résumé. - Nous dormons un résumé des possibilités d'utilisation des effets de vio- lation de parité associés à des protons polarisés, pour l'étude du mo- dèle standard, de la structure du proton et de la nouvelle physique au collisionneur du SPS.

Abstract. - The potential for utilizing parity violating effects, associated with polarized protons, to study the standard model, proton structure, and new physics at the SPS Collider is summarized.

The energy of the SPS Collider makes it ideally suited to make use of the par- ity violating asymmetry resulting from W and Z production. The cross-sections for producing these particles are known to be substantial /l/, and if they are what we believe they are, there are strong parity violating effects in their coupling to quarks. Most notable of these is that W's can only be produced by left-handed quarks. The case for Z's is not quite so striking, because the Weinberg angle reduces the size of the vector coupling from maximal, but there is nonetheless a significant difference in their coupling to left- and right-handed quarks. The issue is then to what degree do the quarks in a polarized proton inherit the polari- zation of the proton? The evidence from the SLAC-Yale experiment 111 and the pre- liminary results from the EMC /3/ experiment is that, in the region of x = m/Vs of consequence at the SPS Collider, the correlation is very strong. At the relevant value of x » .15, the measured value of the asymmetry k\ is about 1/3. Since

I e.2 (qt(x) - qT(x;) A, (x) = i i i

£ eL 2 (qt(x) + q ^ x ) )

it is clear that the quark polarization is strongly correlated with the proton polarization. Indeed, at this x the data are well described by the SU(6) relations

u (x) - u (x) = -r u (x) ,

d+(x) - d~(x) = - | dv(x).

(As x goes to 0 or to 1 the SU(6) relations have problems and theoretically there are expected to be serious differences with these simple relations,. /4/ but it doesn't seem important for the issues I wish to discuss, so my quantitative remarks today will assume these simple relations.) Of course, these are not the structure functions at the proper q for W or Z production. However, according to calcula- tions done by Hochberg /5/ and by Chiappetta and Soffer 161, the relation between the polarized and unpolarized valence structure functions are numerically well preserved by the Altarelli-parisi 111 equations.

The most striking asymmetry should be seen in the leptonic decays of the W.

Roughly 4 to 5 times as many W* should be made with left-handed as with right-handed The submitted manuscript has been authored under Contract No.

DE-AC02-76CH00016 with the U.S. Department of Energy.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985293

C2-722 JOURNAL DE PHYSIQUE

p r o t o n s , w h i l e about twice a s many W should be made with right-handed protons (yes! n o t e t h e r e v e r s a l h e r e ) a s with l e f t - h a g d e d . This i s a r e a l p a r i t y v i o l a t i n g e f f e c t , p r o p o r t i o n a l t o the p s e u d o s c a l a r o p i n c o n t r a s t t o the observed forward- backward asymmetries / 8 / - ^it is 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 product of t h e v e c t o r times a x i a l - v e c t o r coupling. One may f e e l he knows what t h e r e s u l t w i l l be and s o not be t o o e x c i t e d about doing t h i s , but c l e a r l y i f a p o l a r i z e d beam i s a v a i l a b l e it w i l l be done. I n any case it i s a d i r e c t way of measuring t h e s p i n dependent s t r u c - t u r e f u n c t i o n s and should complement HERA should p o l a r i z e d p r o t o n s u l t i m a t e l y be made a v a i l a b l e t h e r e /9/. The s t r u c t u r e of t h e proton seems t o me t o be of funda- mental importance and i t i s l i k e l y t h a t one day t h e o r y w i l l be a b l e t o sharpen t h e

i s s u e s here.

The formula f o r t h e asymmetry, n e g l e c t i n g p o s s i b l e p o l a r i z a t i o n of t h e s e a , i s g i v e n by

When P a i g e , Tudron and 1 / l o / f i r s t looked a t t h i s q u e s t i o n s e v e r a l y e a r s ago, we were looking f o r a way of p u l l i n g t h e h a d r o n i c j e t decays of t h e W ( F i g . l a ) out of t h e background ( F i g . l b ) . For t h e u s u a l l i g h t quarks t h i s background i s a couple of o r d e r s of magnitude above t h e s i g n a l and we had not been a b l e t o f i n d a s a t i s f a c t o r y way of doing t h i s . The f a c t t h a t t h e p o l a r i z a t i o n asymmetry i s s o s t r o n g makes t h i s appear t o be a good way t o a c h i e v e t h a t . The s i g n a l i n t h e asymmetry remains very s t r o n g . The main problem i s obviously t o accumulate enough events s o t h a t t h e e r r o r i n t h e background is s m a l l e r than t h e s i g n a l - t h e expected asymmetry i s a few per- c e n t of t h e background f o r a l l h a d r o n i c j e t p a i r s .

I n c i d e n t a l l y , above t h e W an asymmetry due t o W exchanges (Fig. l c ) remains a t t h e p e r c e n t l e v e l . D e v i a t i o n s from t h i s s t a n d a r d model r e s u l t due t o quark

s u b s t r u c t u r e were d i s c u s s e d a t t h e f i r s t Snowmass meeting and i n d i c a t e t h a t t h e

( 0 I

( b )

F I G . l ( a )

-

-

( c )

-

FIG. 2

-

c o l l i d e r could be s e n s i t i v e t o a s u b s t r u c t u r e s c a l e of about 1 TeV / I l l . Because of t h e f l e x i b i l i t y of t h e composite models a t t h e p r e s e n t time, it i s d i f f i c u l t t o say i f t h i s is s i g n i f i c a n t o r n o t .

I f t h e r e a r e o t h e r i n t e r e s t i n g c h a r a c t e r i s t i c s which d i s t i n g u i s h a p o t e n t i a l W-decay from t h e background, one may not need t o accumulate s o many e v e n t s i n o r d e r t o make p o t e n t use of t h e asymmetry. One immediately t h i n k s of t h e t b decays of the W. I f the r e c e n t l y r e p o r t e d UA1 c a n d i d a t e s f o r t h e t quark 1121 a r e t r u l y coming from W decays t h e r e should be a c l e a r c o r r e l a t i o n between t h e r a t e and t h e p o l a r i z a t i o n : about 40% more should be produced with l e f t - t h a n with right-handed p r o t o n s ( F i g . 2 ) . What's more t h e r e should be a s t r i k i n g d i f f e r e n c e i n t h e s i g n of t h e l e p t o n , t h e p o s i t i v e l e p t o n being a s s o c i a t e d with left-handed by a f a c t o r of 5 t o 1, t h e n e g a t i v e being a s s o c i a t e d with t h e right-handed by a f a c t o r of 2 t o 1.

Obviously any o t h e r p a r t i c l e whose source i s W-decay w i l l show t h i s same asym- metry. A c a s e i n point i s t h e s e a r c h f o r t h e super-symmetric p a r t n e r of t h e W. I n many models t h e r e i s p r e d i c t e d a wino below t h e W mass 1131 and t h e r e can be a sub- s t a n t i a l branching r a t i o f o r W decay i n t o t h i s and a photino i f t h e masses a r e r i g h t (Fig. 3 ) . This would show mainly as two j e t s ( q , q, p h o t i n o ) with missing energy 1141. I f t h i s shows up i n t h e asymmetry s i g n a l , it would determine t h e s o u r c e t o be W and g i v e a measurement of t h e branching r a t i o of t h e W t o wino + photino.

One can obviously play t h e same game with t h e Z . Although t h e asymmetry i s not expected t o be a s l a r g e as with t h e W , it is by no means i n s i g n i f i c a n t . A c a l c u l a - t i o n done by Kinnunen and L i n d f o r s 1151 shows an asymmetry of about 20%. T h e i r c a l c u l a t i o n a l s o shows an i n t e r e s t i n g e f f e c t i n t h e a n g u l a r d i s t r i b u t i o n from t h e Z decay of t h e l e p t o n p a i r : t h e tendency of t h e p o s i t r o n t o go i n t h e a n t i - p r o t o n

FIG. 3 - W decay a s s o u r c e o f winos and p h o t i n o s .

d i r e c t i o n i s enhanced with left-handed p r o t o n s r e f l e c t i n g an i n c r e a s e i n t h e l o n g i t u d i n a l p o l a r i z a t i o n of t h e 2. (This enhancement i s expected t o be very s m a l l i n t h e case of W-production because, i n t h e Drell-Yan p r o c e s s , t h e W ' s a r e produced about 99% p o l a r i z e d a l r e a d y : t h e only d e p o l a r i z a t i o n comes from W ' s produced by s e a quarks from both t h e p r o t o n and t h e a n t i - p r o t o n and t h a t i s q u i t e small a t 540 GeV.) QCD c o r r e c t i o n s t o t h i s p o l a r i z a t i o n have been c o n s i d e r e d by Chaichian, Hayashi, S o f f e r and Yamagishi i n a paper submitted t o t h i s conference 1161.

I t h i n k we would a l l f i n d t h e most i n t e r e s t i n g a p p l i c a t i o n s of t h i s t e c h n i q u e t o be the new d i s c o v e r i e s t h a t we d o n ' t have such c l e a r i d e a s about. L e t ' s t a k e t h e a p p a r e n t Z t o e+e-y e v e n t s 1171. There a r e no completely s a t i s f a c t o r y e x p l a n a t i o n s

C2-724 JOURNAL D E PHYSIQUE

of t h e s e , but t h e r e have been many d i f f e r e n t s u g g e s t i o n s . Most of them assume t h a t they o r i g i n a t e from Z decay. I f t h i s i s t h e c a s e t h e n t h e r e should be a 20%

asymmetry i n t h e p r o d u c t i o n r a t e . On t h e o t h e r hand some people', such a s Marciano /18/ and Holdom /19/, have suggested t h a t t h e s o u r c e i s a s c a l a r o r p s e u d o s c a l a r t h a t has n e a r l y t h e mass of t h e Z (Fig. 4). I n t h i s c a s e t h e asymmetry should be d i f f e r e n t , very l i k e l y z e r o .

FIG. 4

-

Haber and Kane /20/ have suggested t h a t t h e UA2 e v e n t s of a l e p t o n , a j e t , and m i s s i n g p~ /21/ could r e s u l t from d i r e c t p r o d u c t i o n of a wino. Since t h e s e would be produced only by left-handed quarks t h e r e would be a v e r y l a r g e asymmetry i n t h i s s i g n a l ( F i g . 5).

F I G . 5 - P o s s i b l e s o u r c e of UA2 l e p t o n - j e t s w e n t s .

The mono-jet e v e n t s of UA1 /22/ a r e a n o t h e r i n t e r e s t i n g c a s e . S e v e r a l a u t h o r s /23/ have suggested t h a t they r e s u l t from a s c a l a r quark. I n supersymmetry t h e r e i s a r e a l i s t i c p o s s i b i l i t y t h a t t h e l e f t - and right-handed s c a l a r quarks a r e not

d e g e n e r a t e / 2 4 / . Since one is produced from l e f t - h a n d e d quarks and t h e o t h e r from right-handed, i n p r i n c i p a l t h e r e can be an asymmetry i n t h i s s i g n a l ( ~ i ~ . 6 ) . The c r o s s - s e c t i o n can be l a r g e and s o can t h e asymmetry, but I know of no d e t a i l e d e s t i m a t e s . I f i n d t h e s e l a s t two p o s s i b i l i t i e s e s p e c i a l l y i n t e r e s t i n g because t h e y a r e t h e only ones t h a t do not r e l y on t h e b a s i c asymmetry of t h e vector-boson coupling.

FIG. 6 - P o s s i b l e s o u r c e o f UA1 mono-jet w e n t s .

A great many tantalizing things have been discovered at the Collider in a short time. Who knows what will be discovered next year whose nature could be unravelled with the aid of a polarized beam - perhaps a right-handed W, which would stand out like a sore thumb. Parity violation in the production process would be a very useful tool to have in our bag.

Acknowledgments

I would like to thank Mike Tannenbaum for very useful discussions. This work was supported under DOE Contract No. DE-AC02-76CH00016.

References

/1/ ARNISON G., et al. (UA1 Collaboration), Phys. Lett. 122B (1983) 103;

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/ 3 / GABATHULER E., reported at this cGference.

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/11/ ABOLINS M. et al., Proc. of the 1982 DPF Summer Study on Elementary Particle Physics and Future Facilities, eds. Rene Donaldson, Richard Gustafson, and Frank Paige (1982). See also the paper by RUCKL R. at this conference.

/12/ UA1 Collaboration, reported at various conferences (1984).

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/17/ ARNISON G. et al., (UA1 ~ollaboration), Phys. Lett. 126B (1983) 298;

BAGNIA P. et al., (UA2 ~ollaboration), Phys. Lett. (1983) 130.

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/24/ FAYET P., Phys. Lett. 84B (1977) 416;

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