INCLUSIVE HYPERON POLARIZATION : A REVI EW

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INCLUSIVE HYPERON POLARIZATION : A REVI EW

K. Heller

To cite this version:

K. Heller. INCLUSIVE HYPERON POLARIZATION : A REVI EW. Journal de Physique Colloques,

1985, 46 (C2), pp.C2-121-C2-129. �10.1051/jphyscol:1985212�. �jpa-00224523�

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INCLUSIVE HYPERON POLARIZATION : A REVIEW

K. Heller

School of Physios and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.

RESUME . On étudie la polarisation des hypérons produits dans la fragmentation des protons pour déterminer la dépendance par rapport aux différents hypérons, à l'énergie du faisceau, à la cible et à la région cinématique considérée. On donne aussi les résultats de la polarisation avec des faisceaux de mésons et de neutrinos.

ABSTRACT . 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

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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 ~ o r k , ~ g ~ m m a r i z e d 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- , ^{C O},- ^{= 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 pro-

A 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 - - A O + X ' V p + a t - I + x S max

8 ~ . w - A ~ + x ' ^p⁺^p

-

^{p + x}^tn^-o.al+svalue of that momentum. The magnitude of x indi-

p + ~ 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 scat- polarization from representa- tered 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 =

Cin

^X

A L .

been wed so that the points where $. is the direction of the parent particle do not, in most cases, re- H ~n

fleet the total precision of and $H the direction of the hyperon. All inclusive each experiment. polar- hyperon polarizations measured at high energy are izati.Ons are nega- along 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 Q- polarization 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 mag- netic 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 z- is in fact opposite that for

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e r r o r b a r s a n d a r e s t i l l p r e l i m i n a r y .

T h e m o s t 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 k i n e m a t i c b e h a v i o r o f h y p e r o n 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 a n d h i g h e r e n e r g i e s t h a n a n y o t h e r p o l a r i z a t i o n m e a s u r e m e n t s 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 o f 6 o r d e r s o f m a g n i t u d e . Most o f t h e d a t a , from 4 0 0 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. T h e 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: K i n e m a t i c 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 ^Xand pT. ~ 6 z f 9 z a t i o n m e a s u r e m e n t s . The F o r 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 . T h e d a s h e d i n 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 i n d e - -I

pBc-hX 14CC G ~ V I S ⁰n

-

i= 0 . 4 4 p e n d e n t 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 pp- A X '1

pT in GeV/c i s 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 * ~

1 2 0 . 3 4 ( 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 . A l s o shown i s l o w 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 b e t i o n a s a f u n c t i o n o f pT f o r u s e d 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 . s a m p l e o f t h e d a t a f r o m 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_inG e V / 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 '}

t o g u i d e t h e e y e . 0.2

0.3

f o r pT > - GeV/c t h e p o l a r i z a t i o n d e p e n d s

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

b e 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 o f 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

B e c a u s e t h e p o l a r i z a t i o n d e p e n d s on b o t h

0 7 X a n d pT, c a r e must b e t a k e n i n c o m p a r i n g

0.30-

d a t a from d P f f e r e n t e x p e r i m e n t s . F o r F i g u r e 5: S c h e m a t i c o f t h e b e h a v i o r o f

PT > 1 GeVIc7 d a t a n e e d o n l y b e 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 a n d pT. T h e d a t a u s e d 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 w h i c h

c o m p l i c a t e t h e c o m p a r i s o n o f d i f f e r e n t t h e p i c t u r e comes f r o m 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 i s t h a t 17, 19.

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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 of the data since it carries

- -

1 the polariza-

3

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 mag- inclusive A polarization. Each nitude than the direct polarization. The graph represents a single experi- direct 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 ~

where P is the h

polarization as measured, from direct A

0.2 or from CO + AY decay as indicated,

NA is the number of direct A's detected, 0.1

NZ is the number of h ! s from ZO decay detected, and N

+

^N is the total

A C

measured A sample. A Brookhaven ex-

periment measured the ratio Sigure 7: Ratio of CO'S to A's produced by a 28 GeV pr ton beam on a Be R =

---

Nz at 28 GeV, see Figure 7, _target.²¹

N~

+.Nc

(solid points)

N NA

+

^N

by detecting the Y as well as the A C

and reconstructing the C O mass. and R =

$

(open points) are shown.

The result was that R = 28 f 6% for 28 GeV protons incident on a Be target and

was approximately constant for .35 < X < .80, 0.6 GeV/c < pT < 1.3 GeV/c. Since the difference between the A and CO depends only on quark spins, this result should follow from any model which predicts hyperon polarzation. Of the few such niodels which exist, none give the correct result .22

Using the measured value of R,

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approximately the same magnitude as P(measured), its actual value is not important in determining the

correction. In fact this same experiment has a preliminary

determination of

?(X0)

=

+

^0.28^f^0.09

which is approximately equal to -~(measured).~ Thus all

A

^polari-

zation measurements at 28 GeV should be multiplied by approximately 1.4.

Although this correction is not known for any other energy, it is not un- reasonable to assume its validity for higher energies. However, in the following comparisons between

A

polarization at different energies and between

A

polarization and that of other hyperons, no correction has been made for

x0

contamination. In

1 1 1 , 1 1 1 1 1 , 1 1 1

A 400 GeV H;' i!

28 GeV H,/D:

T

l

( X I (0.361 (0.45) (0.521 (0.59) (0.691 ENERGY DEPENDENCE

Figure 8: Comparison of inclusive polarization produced by 28 and 400 GeV protons. The 400 GeV data has been interpolated when necessary, so that it

matches the X value of the 28 GeV data at each pT.

z

-0.4

P+N-A+X

8

-

^A⁴⁰⁰GeV1' -0.6 . 1500 GeV

0 2000 G ~ V '

Figure 9: Comparison of inclusive polarization produced by 400 protons with 26/26 and 31/31 GeV proton-proton

collisions. The 400 GeV data has been interpolated to match the estimated X

value of the collider data at each pT.

Figure 10: Inclusive

/\

polarization most cases the error bars are so large that produced by 12 GeV protons. The line a correction of 1.4 would not affect the represents the 400 GeV data from qualitative conclusions. Figure 4.

The polarization of A's produced by protons has been measured at proton energies

of from 12 GeV at KEK' to the equivalent of 2000 GeV at the ISR. 6'23 Given the problems of comparing one experiment to another there is remarkable agreement in both the magnitude of the polarization and its kinematic behavior. Comparisons are shown in Figures 8 and 9. In both cases the 400 Gev data has been interpolated when necessary using the kinematic behavior shown in Figure 5 so that the comparison is always made at approximately the same X for a given 2T. Since the X values for the ISR data are not published, I have estimated them using the acceptance of the experiment.24 In Figure 1 0 the 12 GeV data with a pT above 0.9 GeV/c is compared to a line representing the behavior of the 400 GeV data as a function of X .

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~ 2 - 1 2 6 JOURNAL DE PHYSIQUE

P o l a r i z a t i o n

Figure 13: 1 n c l u s i v e ~ - polarization produced by 400 GeV protons. The line represents the 400 GeV data from Figure 4.

Figure 11 : 1 n c l u s i v e ~ ~ Figure 12: ~ n c l u s i v e ~ ~ and A polarization pro- polarization produced by duced by 400 GeV protons. 400 GeV protons. The Open points are data from line represents the 400 the same experiment. GeV A data from Figure 4.

Solid points are from an

earlier experiment. All X

data were taken at a pro-

0.5 1.0

duction angle of 7.2 mrad. ^O ^r

-

%>1BeV/c POLARIZATION

P P

0

The polarizations of other hyperons pro-

duced by 400 GeV protons _-o.l- have been measured and

show a remarkable simi- larity. Figure 11 shows B a comparison of /\ ki

-0 N

and* polarization a U -0.2

taken in the same ex- ³ periment at the same _-0, time. Also shown is ^I the

A

polarization mea-

sured in an earlier ^-0.3 experiment with essenti-

ally the same apparatus at the same production angle using the same target. The agreement

Figure 15: Inclusive polarization from 400 GeV protons. The dashed line represents the

A

^polariza-

tion for the same kinematics.

L m pc"- -A+ I+

-

l400 GeV)

-

as a function of X

compared to the line representing

A

polarization. Figure 13 shows=- data in the same way. l0 T h e E - polarization may be slightly smaller than that of

A

^and

z0

between the

4

^and

x0

Figure 14: ~nclusive

2 +

polarization is as good polarization from 400 GeV as the agreement between protons. The polarization the two sets of data. is multiplied by -1. The Figure 12 shows the

so

line represents the 400 data for PT > O.9 GeV/c GeV

/\

data froin Figure 4.

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multiplied by -1. 12*25 Again there is good agreement. Finally Figure 15 gives the - A polarization as a function of p

T' ^5'18'19Also shown is a line representing the polarization at the same average value of X. It is clear that

K

polarization is much smaller than the A polarization is consistent with zero out to a pT of 2 GeV/c.

The agreement of all the hyperon polarization data in the limited kinematic regions measured leads one to believe that a single, simple mechanism is responsible for all hyperon production. One should remember, however, that 1- , ^'¹ ^and R- are only measured at one point and with larger errors. Also, no correction has been made to the A polarization for C0 dihtion. _Adifferent mechanism must be responsible for antihyperon production since A is not polarized.

The situation for A polarization produced by particles other than protons is not so clear. A Brookhaven experiment has published A polarization produced by a 16 GeV a- beam.26 Figure 16 shows this data together with the curve representing

A polarization from protons at the same average

pT ~n GeV/c value of X. It is clear these are different. The

experimenters state that the polarization measurement may suffer from biases which they could not eliminate.

r - P - hxa' The data for A polarization from a K beam has been

reviewed at this conference by TT H a u p t Most of this

z data is from low statistics bubble chamber exposures 0 t-

-0.15 dominated by energies below 10 G ~ v . ~ ~ At low energy

- II: large polarizations are very common due to the

4

_I domination of a few exclusive channels. Figure 17 g ^shows^Apolarization as a function of pT for x < O , in

-0 25 ?----_x;065--

-

effect protons producing A's on a K- target.28 The data have large errors and the value of X at each point has not been given so it is hard to compare Figure 16: Inclusive with the proton beam data. The sign of the polariza- polarization l6 GeV tion is negative as expected. Figure lea gives the pions. The dashed line re- polarization of A's produced by K-, X > 0. Again, presents the 400 GeV the sign of the polarization is the same as for polarization for the same

kinematics from PROTON HEM~SPAEBE KAON HEMISPHERE Figure 5.

!Cp -' K Kt?

-

^an$hinj ^K-pp-. n'+anythlng

I I I I I I

proton production but

in this case the strange ^a_A^{saa rvtc}_rro_6evlc quark is originally. un-

polarized since jt 04

comes from the K

.

The polarization as a function of p

T appears to be de-

creasing with energy - m -

which could mean -06.

that it is unrelated to the high energy ^-U-

proton produced 0.2 01 0 6 0.8 ,.o 4 1 4.4 *C

polarization. However, p, t ~ e v l c l

since the value of X

for each point is not Figure 17: Inclusive given and probably polarization from proton decreases with beam fragmentation using a K energy the situation beam. From the presentation is not clear. of T. Hau~t.

Figure 18: Inclusive polarization from K- fragmentation as a function of

(a) pT, (b) X. From the presentation of T. Haupt.

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C2-128 JOURNAL DE PHYSIQUE

Figure 18b shows the same data as a function of X including some from additional experiments.28 There is no evident energy dependence but the value of pT for each point, which dominates the behavior of proton produced polarization at low pT, is not given. A high statistics, high energy measurement of A polarization from K- production would be very helpful. This reaction can only give polarized strange quarks by a recombination as opposed to a quark production mechanism since the produced (ud) diquark carries no spin information in the A

.

Finally there is some preliminary data from BEBC using neutrinos to produce A's.~' For X < 0 , v

(5) +

^p^{+ A +}¹¹

+

anything is equivalent to the charged current reaction p

+

^W⁺ ^A

+

anything. The spin dependence of the interaction of the W with the proton is certainly better understood than the strong interaction. With only a few hundred events which have < 4 (~e~/c)', the A polarization observed in the production plane was 0.63

*

0.22 for the V beam and 0.50

+_

0.20 for the v beam. Perpendicular to the production plane the polarization was consistent with zero, 0.05 +_ 0.22 ('v ) and -0.02 & 0.20 (v ). The existence of A polarization in the production plane is expected for a charged current producing A's via Y*

resonances. Hopefully, future large neutrino experiments will have more data of this type with better statistics.

In summary, high energy inclusive polarization is a large and common phenomenon.

It occurs in almost every reaction investigated. I believe that the following picture is emerging from the data of high energy hyperons produced by protons.

Without a compelling theoretical guide, this picture-is based on the data presented in this review filtered by my own conservative prejdices and desire for simplicity.

1 . All hyperons produced by proton fragmentation are polarized perpendicular to the production plane. Antihyperons are not.

2. The magnitude of the polarization is independent of pT above pT of 1 GeV/c and depends approximately linearly on X.

3. The magnitude of the polarization depends approximately linearly on pT below pT of 0.8 GeV/c.

4. The magnitude and kinematic behavior of the polarization is remarkably energy independent for projectiles above 12 GeV.

If anything it increases with energy.

5 . The direction of polarization of the strange quarks is the

same for all hyperons. The opposite sign of the polarizations arises naturally from using the static quark wavefunctions in a straight forward way.

6. The C' to A ratio at production is approximately 40%. This ratio does not arise naturally from using the static quark wavefunctions in a straight forward way. Increasing the A polarization by a factor of 1.4 to correct for

z0

^dilution

means the E polarizations are smaller than those of the A

.

7 . Nuclear targets reduce the polarization of the hyperons which emerge. Rescattering in the nucleus probably reduces the correlation between scattering, momentum, and spin direction.

At this time there is no satisfactory model which shows how this behavior fits into the standard theory of strong interactions. Additional data could help clarify the situation. The most useful measurements would be large statistics experiments at high energy with good coverage in X and pT. The following is a list of examples.

The A polarization at higher pT to further press QCD. ³⁰

The Z?/A ratio at 400 GeV or above to see if the 28 GeV results apply to higher energy.

The A and polarization at the CERN or Tevatron collider to test the energy dependence.

A detailed X, p map-of the polarization of hyperons other than A . In the case of The R any additional information would be useful.

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The contribution to the A polarization from each exclusive channel.

Data using beams other than protons. The situation for K -+ A needs to be clarified.

All of the above experiments are long and difficult. There is always the possibility of a surprise which will change the picture of hyperon polarization. The most likely result, however, would be to fill out the existing picture which remains a puzzle for theory. From this point of view, the greatest need at this time is further theoretical work.

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