HAL Id: jpa-00224614
https://hal.archives-ouvertes.fr/jpa-00224614
Submitted on 1 Jan 1985
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
EXPERIMENTAL POSSIBILITIES WITH UA2
G. Sauvage
To cite this version:
G. Sauvage. EXPERIMENTAL POSSIBILITIES WITH UA2. International Symposium On High
Energy Spin Physics 6, 1984, Marseille, France. pp.C2-729-C2-732, �10.1051/jphyscol:1985295�. �jpa-
00224614�
JOURNAL DE PHYSIQUE
Colloque C2, supplément au n°2, Tome 46, février 1985 page C2-729
EXPERIMENTAL POSSIBILITIES WITH UA2
G. Sauvage
Laboratoire de l'Accélérateur Linéaire, Université Paris-Sud, 91405 Orsay, France
Résumé - Nous donnons ici un bref résumé des résultats de physique marquants obtenus auprès du collisionneur SppS du cERN . L'accent est mis sur les limitations dues au fait que les faisceaux de protons et d'antiprotons ne sont pas polarisés.
Abstract - A brief summary of the most important physics results obtained with the SppS collider at C E R N is given. The emphasis is put on the limi- tations due to the fact that both proton and antiproton beams are unpola- rised,
The main results obtained in 1982-1983 by both experiments, UA1 and UA2, at the SppS collider at CERN, two-jet dominance and W-Z discovery, correspond to an inte- grated luminosity of about 120 nb~ . In the coming years, 1984 to 1986, the energy of the beams will increase from 270 GeV to 315 GeV (the c.m. energy will increase from /S = 54-0 GeV to /S = 630 GeV) and the integrated luminosity will be multiplied by a factor 5 to 10. The peak luminosity will be increased by an order of magnitude when the ACOL becomes available in 1987-1988, Despite the expected increase of the number of events in the coming years, there are limitations due to the fact that the beams are unpolarised, as we will see later.
Three topics will be considered^-' :
- jet physics and two-jet dominance - intermediate vector bosons physics
- indications of new physics beyond the standard electro-weak model.
JET PHYSICS AND TWQ-JET DOMINANCE
A striking feature of the hadronic physics at the CERN collider is the emergence of the two-jet dominance in agreement with QCD predictions. As soon as the total transverse energy observed in an event is high enough, about 100 GeV in the case of 0A2 (in a pseudorapidity interval |n| < 1 ) , the event is generally a two-jet event.
The jet production properties, as the transverse momentum of the two-jet system, the structure functions of the proton (and antiproton), the two-jet angular distri- butions, and the jet fragmentation properties are in good agreement with QCD
predictions and extrapolations from lower energy experiments. The two-jet production cross section is shown on the figure (UA2 results). The shaded area corresponds to different QCD predictions. The theoretical predictions are affected mainly by the following uncertainties : scale ambiguity (choice of Q ) in the leading log. term, and related to it, the choice of the A parameter in the strong coupling constant
ag(Q )> as well as the parametrization used for p and p structure functions. A possible substructure of quarks and leptons ' could manifest itself as a new contact interaction visible at large transverse momentum or at large mass and give deviations from pure QCD behaviour at large mass. This deviation is controlled by an energy scale 5 parameter, characterizing the strength of this new interaction.
The limit put by UA2, ? > 275 GeV (95% CL) is dominated by the theoretical uncer- tainties in pure QCD calculation (£ = M) . It has been pointed out at this conference that the theoretical uncertainties are much lower when one considers cross section differences with polarized proton beams in the CERN collider. This is a way to improve the limit on the £ parameter.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985295
JOURNAL
DE
PHYSIQUEUA2 1983
pp
-
jet + jet + X100 200 300
mjj IGeVI
INTERMEDIATE VECTOR BOSON PHYSICS
-
W, Z d i s c o v e r yThe W and Z v e c t o r bosons have been d i s c o v e r e d through t h e i r l e p t o n i c decays :
L L
w
'
-+a' v
Z O + R+a- where 9, i s e i t h e r a n e l e c t r o n o r a muon
I n t h e s e decays t h e l e p t o n L h a s a h i g h t r a n s v e r s e momentum, of t h e o r d e r of h a l f t h e mass of t h e v e c t o r boson ( % 25 t o 40 GeV/c) and t h e background c o n d i t i o n s due t o h a d r o n i c p r o c e s s e s a r e f a v o u r a b l e . The next t a b l e s u m a r i s e s t h e number of e v e n t s c o l l e c t e d i n b o t h UA1 and UA2 e x p e r i m e n t s .
(UA2 has no muon d e t e c t i o n ) .
The mass of t h e s e p a r t i c l e s have b e e n determined :
mW = 80.9
+
1 . 5 f 2.5 G ~ v / c ~ ( u A ~ )%
= 83.1 f 1 . 9 f 1 . 3 G ~ V / C ~ (uA2) mz = 95.6 f 1.4 f 2.9 G ~ V / C ~ ( U A I ) mZ = 92.7 ?I 1 . 7 f 1 . 4 c e v f c 2 (UA2) and a r e i n good agreement w i t h t h e p r e d i c t i o n s o f t h e ~ ~ ( 2 1 x ~ ( 1 ) s t a n d a r d model. The p r o d u c t i o n c r o s s s e c t i o n s a r e a l s o i n agreement w i t h t h e s t a n d a r d model.Due t o t h e V-A n a t u r e of t h e c o u p l i n g of t h e W $0 b o t h t h e qq and $y s y s t e m s , t h e a n g u l a r d i s t r i b u t i o n of t h e decay l e p t o n 8- i s asymmetric and h a s t h e form :
* *
2d N ( d ) / d c o s e = ( l + c o s e )
Both experiments have measured t h i s asymmetry. UA1 h a s determined t h e mean v a l u e of c o s 0% and found : < cos 0% > = 0 . 4 f 0 . 1 i n agreement w i t h t h e expected
v a l u e < c o s 8
*
> = 0 . 5 . UA2 has determined t h e v a l u e of t h e r a t i o A / V of t h e a x i a l v e c t o r c o u p l i n g t o t h e v e c t o r c o u p l i n g and found :+ 0 . 5
I
A i VI
= 1 . 0-
0 . 3The e r r o r b a r s w i l l , of c o u r s e , d i m i n i s h w i t h i n c r e a s e d s t a t i s t i c s i n t h e coming y e a r s . But i t i s important t o n o t e t h a t , w i t h u n p o l a r i s e d beams, t h e a n g u l a r d i s t r i b u t i o n of t h e decay l e p t o n i s n o t s e n s i t i v e t o t h e s i g n of t h e a x i a l v e c t o r t o v e c t o r r a t i o of t h e c o u p l i n g . I n o t h e r words one cannot d i s t i n g u i s h , w i t h such measurements, between a l e f t handed W ( a s p r e d i c t e d by t h e s t a n d a r d model) and a r i g h t handed W. The u s e of p o l a r i s e d p r o t o n s i n t h e SphS c o l l i d e r g i v e s r i s e t o asymmetries s e n s i t i v e t o t h e s i g n of t h e c o u p l i n g . A p o s s i b l e way o u t t o d e t e r m i n e t h i s s i g n , i s t o measure t h e p o l a r i s a t i o n of t h e f i n a l l e p t o n , e . g . , by s t u d y i n g t h e decay channel W + - r V and t h e decay products of t h e .c
.
However t h e background c o n d i t i o n s a r e more s e v e r e a s one i s d e a l i n g w i t h l e p t o n s ( p o r e ) of s m a l l e r t r a n s - v e r s e momenta.The p r e v i o u s remark about t h e W a p p l i e s a l s o t o any new W of h i g h e r mass, i f i t e x i s t s . I n such a c a s e , t h e d e t e r m i n a t i o n of t h e V-A o r V+A n a t u r e of t h e c o u p l i n g w i l l b e of primary importance. The use o p o l a r i s e d p r o t o n beam i n t h e SpFS c o l l i - d e r g i v e s r i s e a l s o t o s p i n asymmetries f 3 7 i n t h e c a s e of t h e Z0 p r o d u c t i o n w e l l p r e d i c t e d i n t h e framework of t h e s t a n d a r d e l e c t r o w e a k model.
-
t o p quark s e a r c h+
-The s i x t h q u a r k , t h e t o p quark, n o t s e e n i n e e a n n i h i l a t i o n s because of i t s high mass, can b e produced a t t h e CERN c o l l i d e r and s t u d i e d through t h e f o l l o w i n g decay c h a i n : p F + W +
... ,
W + t i ;,
t + b e VThe topology of such a n e v e n t is t h e presence of a l e p t o n w i t h a m3derate t r a n s v e r s e momentum ( % 10-15 GeV/c), of 2 j e t s c o r r e s p o n d i n g t o t h e b and b quarks and of m i s s i n g t r a n s v e r s e energy c o r r e s p o n d i n g t o t h e unseen n e u t r i n o . U A ~ ( ~ ) h a s r e c e n t l y r e p o r t e d t o have i s o l a t e d s i x such e v e n t s ( 3 e l e c t r o n s and 3 muons) compatible w i t h t h e above assumption, and d e r i v e d l i m i t s on t h e mass of t h e t q u a r k :
30 ~ e ~< /mt c< ~50 ~ e ~ / c ~ . The u s e of a p b l a r i s e d beam i n t h e CERN c o l l i d e r may a l s o h e l p t o i s o l a t e t h e s i g n a l a s i t g i v e s r i s e t o asymmetries i n t h e semi- l e p t o n i c decay o f t h e top.
-
h a d r o n i c decay of t h e W , Z : W , Z + q i ( q = u , d , s ,...I
The main d i f f i c u l t y encountered t o d e t e c t t h e s e decay c h a n n e l s , i s t h e abundant two-jet QCD p r o d u c t i o n . The h a d r o n i c W , Z decay w i l l appear a s a mass bump i n t h e two-jet mass d i s t r i b u t i o n . For example, UA2 s e e s a n e f f e c t of o n l y 1 t o 1 . 5 s t a n d a r d d e v i a t i o n : NW = 108 f 70 f o r an expected number of h a d r o n i c W decays of NW = 150.
I n t h e coming y e a r s , t h e i n c r e a s e d number of e v e n t s w i l l improve somewhat 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 of such a n e f f e c t . However, t h e r e a r e two ways of improving s u b s t a n t i a l l y t h e s i t u a t i o n . One c a n improve t h e mass r e s o l u t i o n by u s i n g b e t t e r c a l o r i m e t e r s t o measure t h e energy of t h e j e t s . One c a n a l s o u s e a p o l a r i s e d p r o t o n beam t o s e p a r a t e t h e p a r i t y non c o n s e r v i n g 2 j e t p r o d u c t i o n through t h e W channel from t h e p a r i t y c o n s e r v i n g 2 j e t QCD p r o d u c t i o n .
BEYOND THE STANDARD ELECTROWEAK MODEL
A s m a l l number o f unexpected e v e n t s have been observed by UA1 and"'UA2 which could i n d i c a t e t h e e x i s t e n c e of a new p h y s i c s beyond t h e s t a n d a r d e l e c t r o w e a k model. Here a r e summarizedthe d i f f e r e n t k i n d s of e v e n t s .
Z 0 + @Q-y
.
3 r a d i a t i v e Z0 decays have been observed, 2 i n UA1 and 1 i n UA2. These e v e n t s may b e produced by i n t e r n a l b r e m s s t r a h l u n g a l t h o u g h t h e i r proba- b i l i t y i s s m a l l . For example, t h e p r o b a b i l i t y of o b s e r v i n g such a r a d i a t i v e e v e n t i n t h e UA2 sample ( 8 ZO) i s l e s s t h a n 4 % . The c o r r e s p o n d i n g r a d i a t i v e W decay, W + Qvy has n o t been observed d e s p i t e t h e h i g h e r number of Wi.C2-732 JOURNAL
DE
PHYSIQUE-
mass bump i n t h e two-jet mass d i s t r i b u t i o n . Looking f o r a mass bump beyond t h e W, Z mass a t h r e e s t a n d a r d d e v i a t i o n e f f e c t has b e e n observed by UA2, a t a mass of a b o u t 150 ~ e v / c ' .-
e ~
j e t s e v e n t s . UA2 has i s o l a t e d t h r e e e v e n t s where a h i g h t r a n s v e r s e~ v ~ ~
momentum e l e c t r o n i s produced i n a s s o c i a t i o n w i t h a h i g h m i s s i n g t r a n s v e r s e energy (assumed t o b e due t o a n unseen n e u t r i n o ) and w i t h hard j e t s . Assuming t h a t t h e ev system h a s t h e mass of t h e W, t h e mass of t h e eV j e t s system c l u s t e r a t a mass of about 1.70 G ~ v / c ' . The s i g n i f i c a n c e of t h i s o b s e r v a t i o n i s weakened by t h e f a c t t h a t t h i s mass r e g i o n i s k i n e m a t i c a l l y favoured by t h e s e l e c t i o n c r i t e r i a .
-
s i n g l e photon and s i n g l e j e t e v e n t s . 2 s i n g l e photon and 5 s i n g l e j e t e v e n t s a s s o c i a t e d w i t h a high m i s s i n g t r a n s v e r s e energy have b e e n observed by t h e UA1 experiment. These e v e n t s , o r t h e i r r a t e s , a r e n o t e x p l a i n e d by c o n v e n t i o n a l s o u r c e s . Assuming t h a t t h e y a r e t h e r e s u l t s of t h e decay of a high mass s t a t e X decaying i n Z0 + j e t followed by a decay of a Z0 Snto two unseen n e u t r i n o s , t h e allowed window f o r t h e X mass i s around 17.0 GeV/c.
However, i f s u c h a s t a t e e x i s t s , t h e Z O decay i n t o two-jets should have been observed and t h e r e i s no s.ign of a mass bump i n t h e two-jet o r t h r e e - j e t mass d i s t r i b u t i o n i n U A 1 d a t a .For a l l above quoted r e s u l t s , t h e r e i s a c l e a r need f o r more s t a t i s t i c s . The coming y e a r s w i l l show which of t h e s e e f f e c t s were due t o s t a t i s t i c a l f l u c t u a t i o n s and which a r e genuine, For t h e i n t e r p r e t a t i o n of t h i s new phenomena, t h e u s e of a p o l a r i s e d p r o t o n beam i n t h e CERN c o l l i d e r can b e i m p o r t a n t .
A s a c o n c l u s i o n t h e i n t e r e s t of u s i n g p o l a r i s e d p r o t o n beams i n t h e CERN c o l l i d e r h a s been s e e n t o b e twofold :
-
i t may h e l p t o improve t h e background c o n d i t i o n s , when, f o r example one i s s t u d y i n g a p a r i t y non c o n s e r v i n g e f f e c t , a s QCD background i s p a r i t y c o n s e r v i n g .-
i t may b r i n g new i n f o r m a t i o n v e r y d i f f i c u l t t o o b t a i n w i t h u n p o l a r i s e d beams, even w i t h a n improved s t a t i s t i c s .However, t h e low c r o s s s e c t i o n s of i n t e r e s t i n g phenomena t o b e s t u d i e d , do n o t a l l o w t o l o s e t o o much on t h e l u m i n o s i t y of t h e c o l l i d e r when one u s e s p o l a r i s e d p r o t o n beams.
REFERENCES
( 1 )
-
Recent review of UA1 and UA2 r e s u l t s c a n b e found i n : RHOLF J . , P l e n a r y Report ( u A ~ ) r e s u l t sREPELLIN J . P . , P l e n a r y Report (UA2) R e s u l t s
a t t h e 22nd I n t e r n a t i o n a l Conference on High Energy P h y s i c s , Leipzig,DDR, 19th-25th J u l y , 1984.
( 2 )
-
R ~ C K L R . , S p i n e f f e c t s from quark and l e p t o n s u b s t r u c t u r e f o r f u t u r e machines ( t h i s proceeding p . )(3)
-
For example :KINNUNEN R.,LINDFORS J., Nucl. Phys. B189 (1981) 63-86
UUAPPETTA P . , SOFFER J . , S p i n asymmetries f o r W and Z p r o d u c t i o n i n pp c o l l i s i o n s
,
CPT-84lp.1650 ( t o b e p u b l i s h e d )( 4 )
