Inclusive Hyperon Polarization : a Revi Ew

Polarizations from proton fragmentation are compared to determine the dependence on produced hyperon according to energy, target and kinematics. Polarization data for meson and neutrino beams are also given. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985212 C2-122 JOURNAL DE PHYSIQUE There are many reasons for studying hyperon production. Hyperons after all are the most common of the "stable" baryons comprising 3/4 of the baryon octet. In addition they offer an excellent method of probing the mechanisms of particle production by using the strange quark as a tracer. Spin properties can be measured as easily as cross sections since decay via the parity violating weak interaction automatically reveals the polarization. It has been known for almost ten years that A hyperons produced by high energy protons are strongly polarized. During the past decade many experiments have been carried out to determine the energy dependence of this polarization, its kinematic behavior, and the extent to which other hyperons are polarized. This ~ork,~g~mmarized in Fig. 1, has shown that the polarization is both large and pervasive. All of the high energy hyperon polarization data comes from inclusive reactions, A + B + H + anything, where A is the projectile (usually a proton but there is some data from II , K, V and Y beams), B is the target (most commonly Be but H2,D2, -C, Ir, Pb and W have also been used), a_nd H is the 2 x-0.5 produced hyperon (A,C*,C, CO ,= O ,= ,Cl,A ). The Oo ' to 20 ' i0 fraction of the parent p t i c momentum retained J / z in GeV by the hyperon is X = P;l/P:ax E Pg/(&/2) where n p + p t ~ o + x ' o ~ + B ~ E o + x ' g p 11 is the hyperon momentum parallel to the proA p + H/D-A~+X' * p + Be -=-+ X'* o p + ~ r A ' + x ' o p + ~ c ~ + x * jectile incident momentum in the senter of mass of m ~ + B ~ A O + X ' , ' + p + ~ e z + + x " . the mass of the interaction and p is the maximum X P + p --AO+X' V p + a t I + x S max 8 ~ . w A ~ + x ' p + p p + x tn -o.al+s value of that momentum. The magnitude of x indip + ~ c z ~ + x ~ o P + B ~ n + x " cates the degree to which the hyperon is a fragment of the projectile if X > O or the target if X < O . Figure 1: Inclusive hyperon The fraction of the parent particle momentum scatpolarization from representatered perpendicular to the projectile incident tive experiments using high momentum is XT = pT/pmax " pT/(6/2) where PT is energy proton projectiles. the transverse momentum of the hyperon. Since the Sing1e data points strong interaction respects parity, the only to PT= GeV/c, = 0.5 have allowed polarization is in the direction fi = C i n X AL. been wed so that the points where $. is the direction of the parent particle do not, in most cases, reH ~n fleet the total precision of and $H the direction of the hyperon. All inclusive each experiment. polarhyperon polarizations measured at high energy are izati.Ons are negaalong this direction except for that initiated by a tive except for c+,c-and CO v (V) beam. which are positive. The In terms of the constituent quarks, proton sign of the Qpolarization fragmentation into a hyperon can be viewed as the has not been determined. replacement of one valence quark ( A , C*, CO ), two valence quarks ( C, z0 , L) or all three valence quarks (Q -, K ) in the original projectile. After the recombination of the new quarks into the baryon, the quark spin is usually deduced from the hyperon polarization using constituent quark wave functions. These static "SU(6)" wave functions have been remarkably successful, although not perfect, in predicting baryon magnetic moments.15 Examination of the spin structuro of these wave functions leads to the simple rule that the_ospinpf the h ~ e r o n is in the same direction as that of the strange quark for A , = , E and R and in the opposite direction for C* , C and p. In all cases of inclusive high energy hyperon production by protons, the measured polarization of A, E', and zis in fact opposite that for C + , Cand 1'. '-l4 The CO and Qd a t a , t h e magnit u d e s o f which a r e shown i n F i g u r e 1, have l a r g e e r r o r b a r s and a r e s t i l l p r e l i m i n a r y . The most e x t e n s i v e i n v e s t i g a t i o n o f t h e kinematic b e h a v i o r o f hyperon p o l a r i z a t i o n h a s been c a r r i e d o u t f o r A p r o d u c t i o n by p r o t o n s . T h e s e d a t a r e a c h l a r g e r X t r a n s v e r s e momenta and h i g h e r e n e r g i e s t h a n any o t h e r p o l a r i z a t i o n measurements w h i l e s p a n n i n g a c r o s s , s e c t i o n r a n g e of 6 o r d e r s of magni tude . Most o f t h e d a t a , from 400 GeV F e r m i l a b e x p e r i m e n t s , i n v e s t i g a t e t h e k i n e m a t i c r e g i o n shown i n F i g u r e 2 , 0 .2 < x < 0 . 8 , 0 < pT <3.7 GeV/c. The A p o l a r i z a t i o n ,was found t o X~ b e a p a r t i c u l a r l y o . , , . , , , , , , 0.5 1.0 F i g u r e 2: Kinemat ic r e g i o n s i m p l e f u n c t i o c o v e r e d by i n c l u s i v e A p o l a r i b o t h X and pT. ~ 6 z f 9 z a t i o n measurements. The For a c o n s t a n t X , t h e d a t a f a l l w i t h i n t h e c r o s s polarization increases h a t c h e d r e g i o n . The dashed in magnitude with p l i n e i s t h e k i n e m a t i c l i m i t . T u n t i l a p p r o x i m a t e l y z P, in GeV/c 0 3.0 1 GeV/c. F o r pT > 1 5 N dp)kX '14~he~l I; GeV/c t h e p o l a r i E U -0.2 A pBe-AX1400GeVl'8~'q z a t i o n i s inde-I pBc-hX 14CC G ~ V I S 0 n i= 0.44 pendent o f pT. S D O ~ X -0.10 Ocpendrse T h i s b e h a v i o r i s .-0.3 0: F i g u r e 3. A t low 2 0 . 2 0 pT t h e p o l a r i z a t i o n 0 ppA X '1 pT in GeV/c is o n l y a v e r y weal: I ~ O O C X V I f u n c t i o n o f X b u t -0.45< P, '0.55CUlC A P P ~ X * ~ 120.34 ( 4 0 5 &V1 F i g u r e 4: I n c l u s i v e A p o l a r i z a t i o n a s a f u n c t i o n o f X f o r 0.10 ------4 a l l d a t a w i t h pT > l GeV/c from two e x p e r i m e n t s . Also shown is low pT d a t a . The l i n e i s drawn F i g u r e 3: I n c l u s i v e A p o l a r i z a t o g u i d e t h e e y e and w i l l be t i o n a s a f u n c t i o n o f pT f o r used i n t h e f o l l o w i n g f i g u r e s t o a p p r o x i m a t e l y c o n s t a n t X. A r e p r e s e n t t h e p o l a r i z a t i o n . sample o f t h e d a t a f rom t h r e e P + N A + X (400 GeV) e x p e r i m e n t s a l l u s i n g t h e same PT in GeV/c s p e c t r o m e t e r is shown. E r r o r s 2 3 when n o t shown a r e ' s m a l l e r t h a n I I t h e p o i n t s . The l i n e s a r e drawn X ' 0.2 t o g u i d e t h e e y e . 0.3 f o r pT > GeV/c t h e p o l a r i z a t i o n depends 0 4 l i n e a r l y on X f o l l o w i n g what a p p e a r s t o be a u n i v e r s a l c u r v e a s shown i n F i g u r e 4. o 5' F i g u r e 5 g i v e s a r e p r e s e n t a t i o n of t h i s p o l a r i z a t i o n a s a f u n c t i o n o f X and pT. 0.6 Because t h e p o l a r i z a t i o n depends on b o t h 0 7 X and pT, c a r e must b e t a k e n i n comparing 0.30d a t a from d P f f e r e n t e x p e r i m e n t s . For F i g u r e 5: Schemat ic o f t h e b e h a v i o r o f PT > 1 GeVIc7 d a t a need o n l y be i n c l u s i v e A p o l a r i z a t i o n a s a f u n c t i o n a s a f u n c t i o n o f X. o f X and pT. The d a t a used t o c o m p i l e T h e r e a c e two o t h e r f a c t o r s which c o m p l i c a t e t h e compar i son o f d i f f e r e n t t h e p i c t u r e comes from r e f e r e n c e s 5, e x p e r i m e n t s . The f i r s t is t h a t 17, 19. C2-124 JOURNAL DE PHYSIQUE experimenterstend to use different targets. The target dependence of A polarization has been 9,19 measured by three experiments, two at 400 GeV and one at 28 G ~ v ~ , and is shown in Figure 6. All 0.05 ApCCu.Pb1-A experiments agree that for targets with larger 4 4 e p e e * atomic number the magnitude of the polarization 400 GeV decreases as might be expected from rescattering -0''0 inside the nucleus. The effect is small, on the order of 3%. No statistically significant target o ~[CU.P~I-A dependence ha been observed for any other hyperon $-0.05 o p&-A polarization~ For simplicity it would be useful 2 4 ~ ) G ~ V 9 if future experiments would use the same target. 2*O.l0 Be has been used for the bulk of the existing U J data. g -0.15 The second problem occurs only for A production. The Adetected in the amaratus can A come from either "direct" production or from CO +Ay decay which occurs in the target. This A from CO decay complicates the interpretation 1 of the data since it carries the polariza3 tion of the CO for complete A acceptance. 20 The CO contamination will always cause the Figure 6: Target dependence of measured A polarization to be smaller in maginclusive A polarization. Each nitude than the direct polarization. The graph represents a single experidirect A polarization can be calculated if ment. Within each experiment two the amount of contamination and the A data points with the same pT also polarization from ,?? decay is known have the same value of X. p(measured)=k(~) N " P(A+CO) N~ N ~ + N ~