PRODUCTION AND POLARIZATION OF STRANGE QUARKS FROM NUCLEAR MATTER : POLARIZATION OF Λ HYPERONS AT BACKWARD ANGLES IN THE REACTIONS πA→ΛX AND πA→ ΛK0 AT 4 GeV/c

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PRODUCTION AND POLARIZATION OF STRANGE QUARKS FROM NUCLEAR MATTER :

POLARIZATION OF Λ HYPERONS AT BACKWARD ANGLES IN THE REACTIONS π A → ΛX AND π A →

ΛK0 AT 4 GeV/c

I. Arai, J. Chiba, N. Kato, H. Kitayama, T. Kobayashi, A. Manabe, T. Nagae, Y. Nagasaka, I. Nomura, V. Pantuev, et al.

To cite this version:

I. Arai, J. Chiba, N. Kato, H. Kitayama, T. Kobayashi, et al.. PRODUCTION AND POLARIZATION

OF STRANGE QUARKS FROM NUCLEAR MATTER : POLARIZATION OF Λ HYPERONS AT

BACKWARD ANGLES IN THE REACTIONS πA →ΛX AND πA → ΛK0 AT 4 GeV/c. Journal de

Physique Colloques, 1990, 51 (C6), pp.C6-591-C6-594. �10.1051/jphyscol:1990680�. �jpa-00231095�

COLLOQUE DE PHYSIQUE

C o l l o q u e C6, s u p p l e m e n t a u n022, Tome 51, 15 n o v e m b r e 1990

PRODUCTION AND POLARIZATION OF STRANGE QUARKS FROM NUCLEAR MATTER : POLARIZATION OF A HYPERONS AT BACKWARD ANGLES IN THE REACTIONS wA+N( AND VA-~AKOX AT 4 GeV/c

I. ARAI, J. C H I B A * , N. KATO, H. KITAYAMA, T. K O B A Y A S H I * , A. M A N A B E * , T. NAGAE", Y. NAGASAKA, I. N O M U R A * " " , V. PANTUEV".", M. S E K I M O T O * * , M. TANAKA, K. TOMIZAWA a n d K. YAGI

Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, capan

National Laboratory for High Energy Physics, Tsukuba, Ibaraki 305, Japan

" ~ n s t i t u t e for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188, 3 a P ~ ~ s t i t u f e of Physical and Chemical Research, Wako, Saitama 351-01.

<?PP

Institute for Nuclear Research of the Academy of Sciences of the U.S.S.R., Profsojuznaja 7a, Moscow 117 312, U.S.S.R.

L'hypBron lambda e s t une sonde i n t e r e s s a n t e permettant d ' e t u d i e r l a production de quarks e t r a n g e s e t l e u r p o l a r i s a t i o n dans l a m a t i e r e n u c l e a i r e . Nous avons mesure les p o l a r i s a t i o n s e t l e s s e c t i o n s e f f i c a c e s d i f f e r e n t i e l l e s d e s hyperons lambda p r o d u i t s s u r c i b l e de '*C aux a n g l e s a r r i e r e s (70e<0<145*). L e s p o l a r i s a t i o n s ne s o n t p a s r e p r o d u i t e s du t o u t p a r un c a l c u l base s u r une s u p e r p o s i t i o n de processus Bl6mentaires a l o r s que p o l a r i s a t i o n e t s e c t i o n e f f i c a c e s o n t b i e n r e p r o d u i t e s p a r un modele d e quarks e t de p a r t o n s .

A b s t r a c t

-

A A hyperon is a fascinating nuclear probe a t quark level. By using it, we can study the production of strange quarks from nuclear matter and their polarizations. Actually, we have measured the polarizations and the differential cross sections of A hyperons produced on 12C target a t backward angles(70°

<

Blab

<

145')' where the production of A hyperon is strongly forbidden in the qoasifree interaction of incident pions with nucleons i n nucleus, using large-aperture

spectrometcr. The polarizations are not re roduced a t all by calculation based on the superposition of elementary processes, while both the pofarizations and the cross sections are reproduced rvell in calculation based on the quark-parton models. As a next step, we have just finished the experiment on 6Li target, which is also mentioned a little bit.

1

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INTRODUCTION

A A hyperon has several characteristic features as an interesting nuclear probe. At first, the A hyperon consists of u,d and s quarks as shown in Fig. l. I t is the lightest baryon with s qaurk. The s qalirk does not exist in nuclear matter. Secondly, the production of A hyperon on nuclear matter is associated with a production of s

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S quarks pair because there is no s quark anywhere. Fig.:! shows the production of A hyperon in proton fragmentation, for example. T h e A hyperon production means the production of s -F quarks pair from nuclear matter followed by the recombination of them with other quarks. Thirdly, the A hyperon decays into ^T and p via weak interaction. Because of the parity non-conservation in weak Interaction, the decay angular distribution shows a characteristic asymmetry according to A hyperon's polarization as shown in Fig.3. Inversely, this fact enables us t o measure the polarization of A hyperon

easily. ^d

Fig.1-Quark structure of A hyperon. Fig.2-Production of A hyperon in proton fragmentation.

It shoud be noted that there is a tight correlation between A hyperon's polarization and its quark structure. Assuming (1)dominance of ( d i q u a r k , ~ quark) recombination in A production and (II)SU(G) quark model, it is concluded straightforward that A hyperon's polarization is just s quark's polarization itself as shown in Fig.4.

The production of A hyperon substantially incorporates the reaction dynamics at quark level; production of s - S qnarks pair and recombination of s quark with other quarks. In this context, A hyperon works as a fascinating nuclear probe at quark level and its polarization provides a critical information on the quark dynamics incorporated.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990680

COLLOQUE DE PHYSIQUE

Fig.3-Asymmet,ry in decay angular distribution of A. Fig.4Polarization of A and its quark structure.

2

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BACKWARD A PRODUCTION

The backward part,icle production-means a particle production under the extreme kinematics, which is highly selective and favours the appearance of special nuclear many body effect, e.g., the possible ex~stence of "something heavier than a nucleon" or various many-particle correlations in nuc1ea.r matter.

Fig.5 shows kinematics for the reaction hA ^-P

aX.

The particle a should be produced only inside t h e elastic circle under the quasifree hadron-nucleon interaction in nucleus. The backwa.rd pa.rt,icle product.ion is a particle product,ion in the shaded region, which is strongly forbidden in the ql~asifree hadron-nucleon interaction. Therefore, it needs the contribution of some exotic reaction mechanism, which should be a purely nuclear many body effect. We have a, chance t o pick up such a effect by measuring the backward particle product.ion.

Elastic Circle

y =

tanh-l(PLa/Ea)

- ^Y

Fig.5-Kinemat,ics for hA -P a X

Taking into account the arguments in preceding section, we recognize the specific features of A production

~tself as follows:(I)strangeness production;it provides a new information, (1I)poIarization;we can measure a substantially new observable, (1II)small final state interaction;the A hyperon is not distorted so much, (1V)quark dynamics;there should be the production of s - 5 quarks pair followed by the recombination of them with other quarks.

Now, it is a time to summarize the meaning of studying the backward A production. Based on the arguments so far, we can confirm that it is t o study nuclear many body effect by using a probe at quark level.

3

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EXPER.IMENT AT KEK

Rlot~vatttl 1)y thcsc consideration, we have carried out two experiments of backward A production;

T A -+ A X and ~ r l -+ AIP'X at 4 GeV/c. As for the former, we already have some physical results/l/, whilc the lntter 1s now at the stage of data analysis.

In t hc former, we have meast~red differential cross sections and polarizations of A hyperons produced a t (.he harkward anglrs from 6Y to 145" in lab.. The experiment was done by using a la.rge-apert,ure mttlt.iparticlc spcrt.rornct.cr, FANCY/2/. To identify t,he A hyperon, we inst.alled new detect.ors in t.he most inner part, of FANCY: thc t.argct chamber, which is 11nm spacing MWPC , and the vertex chamber,which is a c:ornpnct. cylinclrici~l drift cl~a.rnGcr wit.l~ many t.iny drift. cclls(sce Fig.6). The A Ilyperon decays int,o T-

arhcl p in its flight of fclw crrt. 'I'hwc tlctrct.ors arc 11sct1 1.0 tlct,cct A's tlccay \.crtex tlircctly.

'I'hc! A Ilypcr(m has i1.s lift o f 2.6 X 10-l0 s c ~ , wlticl~ is 7.S!)ctn in c ~ . IVr havc t,wo criteria to discrinlina,t.e t 1 1 ~ A:(l)(lirctc.t. (lr-t.c.c.t i o r t of cI(~c;~y vort.ox i ~ t t c l (Il)it~vitriitnl, nti1.ss calculi~t.cd

1,s

using 1noment.a of T- and p.

' I ' I t i . v(prtr*x c.111, is c~s..;c~~tl.i;~l for cl(ai~r (lis(:ri~tti~~i~l~iott o f A . I I I Pig.7, you can scc Lhc clear discrimination of A

hyperon.

/---' ..

Vertex

Chamber

\

^{W C ,}

^{l '}

500

1 ^{/ \}

^{(CT = 7.89 cm) X-}

d

_(b)

m

Omm All Events

,

m mmmm

m W, mm

m m o o

m m o m m

m -m

' ' ? ~ ~ q ~ 3 c ~ a ~ L r ~ a c a L

1050 1100 1150 l200 Invariant ~ a s s ( ~ e ~ l 2 )

Fig.6-Experimental set up for a- "C -+ AX at 4 GeV/c.Fig.7-Invariant mass spectra of ( p - , p ) system:

(a)with vertex cut,,(b)wit,hout vertex cut,.

Fig.8 and Fig.9 show our experimental results. As for the invariant cross sections, you can see exponent,ial-like behavior, which is common in other backward particle product.ions. As for the

polariza.t,ions, you can see definite positive pola,rizat,ions arround 100". Recognizing the poor statistics of previous bubble chamber experiment(unpublished;Shahbarzian, B. A. et al., Joint Instit,ut.e for Nuclear Research Repork No. El-11519 and No. PI-12138, 1979), it may be said that we established t.he d a t a for the first time. Indeed, our experiment is the first count,er experiment of backward A production. In t,his experiment, we established the method of counter experiment for backward A procluction a.nd achieved about ten times good statistics in comparison with t,he previous experiment.

10.0 6.0

n

IZc

-> A X at 4 GeV/c

-

_0.6 1.0

X Qhahbazien e t al.

,\10.0 6.0

U El

"a 1.0

5

^0.6

a 10.0

6.0

1 .o 0.6

0.1 o too 200 300 0 100 200 300

T~,,(A)

Emission Angle in Lab.(Deg.) Kinetic ~nergy(MeV)

Fig.8-1nva.ria.nt cross sections for X- 12C ^---t Fig.9-Polarizations for T- 12C + j\X at, 4 GeV/c;

AX a t 4 GeV/c.

Then, we want t o mension the theoretica1 caIculat~ions. Icobo et al.(privat,e commnnicat.ion) hasre

C6-594 COLLOQUE DE PHYSIQUE

calculated the pola.riza.tions based on the superposition of Regge pole amplitudes of elemenhry process X - N + ICoA incorpolating the effect of nucleon's Fermi motion. In Fig.9, the dotted curve and bhe dashed curve are the calcula.tions with the Fermi motion of 250 MeV/c and with that of of 500 bleV/c, respectively. As you see, the ca.lculations do not reproduce the experimental dat,a a t all. On the ot,her hand, we have calculat,ed based on quark-part,on models. One is the qnark-parton model of nuclear product.ion by Berlad et al.(the coherent tube model at quark level)/3/. It provides a diqnark structure function. The other is Thomas precession model of hyperon polarization by DeGrand and Miettinen/$/

It provides a, quark dynamics t o generate hyperon polarization. Our calcula,t.ions for cross sect,ions and polarizations are shown as solid curves in Fig.8 and Fig.9, respectively. Both t,he polarizations and the cross sect,ions are reproduced very well. The data a,re reproduced so well by t,hese rat,her naive calculations without any arbitrary parameter.

4

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FUTURE DIRECTION

Finally, we want to mention a little bit the future direction of our study. Because we have just established the method of experiment, the next step should be a systematic st.udy as follou,~: (1)A-dependence,(II)the correla.t,ion with ot,ller particles,for example, K'; for this st,udy, we need large apert,ure forward

spectromet,er, (1II)dependence of A's polarizations on emission angles and kinetic energies;for t.his study, we need t.en t,imes more statistics than the present., and so on.

Along t,his direct,ion, we have just finished the second experiment T- 6 L i -+ AIC0X a t '1 GeV/c(see Fig.10). In t.his experiment, we studied the A-dependence and the correlation with Iio. The analysis is now under progress. The results will appear in near future.

B ,

1

X f r n i

S1 S2

Forward Detector

Beam Counters Central Detector

^FDH

System _System

Fig.10-Experimental set u p for s 6 L i + AKOX a t 4 Ge7v'/c.

5

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CONCLUSIONS

The A hyperon has several specific features as a fascinating nuclear probe at quark level. By using the A hyperon, we can study the production of strange quarks from nuclear matter and their polarizations.

Especially, we have studied the backward A production T-''C + AX a t 4 GeV/c with at least ten times higher statistics than that of previous experiments. Our experiment is the first counter experiment which has measured the polarization of A hyperons produced in hadron-nucleus reactions over a wide range of backrvard angles. The polarizations are not reproduced a t all by calculation based on the superposition of elementary processes, while both the polarizations and the cross sections are reproduced well in calculation based on the quark-parton models. As a next step, we have just entered into a systematic study. There are many possibilities in future direction. We have already finished the backward A production

a- 6Lz ⁺ h I i O X a t 4 GeV/c. The analysis is now under progress. The results will appear in near future.

REFERENCES

/ l / Manabe, A. e t al., Phys. Rev. Lett. B(1989)490.

/3/ Ichimaru, K. e t al., Nucl. Instrum. Methods Phys. Res., Sect. m ( 1 9 8 5 ) 5 5 9 . /3/ Berlad, G. et al., Phys. Rev. rn(1980)1547.

/4/ DeGrand, T. and Miett,inen, H., Phys. Rev. D2.1(1981)2419.