1L N['()V(} CIMENT~) V(>I_ I A I I A , N. 2 21 t',e.maio 196S

Neutral S trange-Part i c l e P r o d u c t i o n

in p p Col l i s ions at 6 .92 G e V / c .

Cv. ALEXANDER~ _2k. SHAPIRA

E. SI•OP017LOU (*) and G. YEK-UTIELI

Nuclear Physics Department, Weizmann Iq~stitute o] Science - Rehovoth

(ricevuto il 30 Agosto 1967)

Summary. - - Neutral strange particle production in pp collisions at 6.92 GeV/c has been studied in the 80in. HBC at BNL. Partial cross- sections for the different channels are given. No evidence for hyperon- nucleon resonances has been observed. Strong production of ~('~(1236) and Y*(1385) has been seen in the four- and five-body final states, which has been analysed in terms of a one-pion-exchange mechanism.

1. - I n recent years an increasing interest has been shown in the s t u d y of strange particle product ion in pp collisions at various energies. The main

reasons for this interest are: a) the hope to be able to s tudy the relat ive rare decay modes of the high-mass 2~* isobars into s t range particles such as

�9 N'* --> Y + K ,

Y * + K ,

Y + K * ,

since the d~'* isobars are known to be copiously produced in pp collisions;

b) the oppor tun i ty to search for the possible existence of hyperon-nueleon reso- nances. I n the present work we present results on the s tudy of neut ra l s t range

particle product ion in pp collisions at 6.92 GeV/c.

(*) International Atomic Energy Fellow on leave from the Nuclear Research Center ~ Demoeritos >), Athens.

456 G. AL:EXANDI~R~ A. S E A P I R A ~ ]~. S I M O P O U L O U and G. Y ~ K U T I E L I

2. - E x p e r i m e n t a l procedure .

The expe r imen t was carr ied out in 64 00O pictures t aken with the 80 in. I-IBC a t B r o o k h a v e n exposed to a secondary 6.92 GeV/c p ro ton b e a m of

the AGS. The con tamina t ion of the b e a m was found to be less t han 1 .2% using the

~6 [ ~ c) ~-ray count ing method . The film was scan- ned twice independen t ly for pp- in te rac t ion

8 , events associated wi th a visible V o decay.

L [1[~ Obvious e+e - pai rs have been e l iminated 0 . . . . . n I, u t ] [q I'] a l ready a t the scanning stage. The over-al l

0 0.4 08 1.2 1.6 scanning efficiency of the in terac t ion events

32 V1 b) associa ted with visible V o decay used finally I [ for analysis was e s t ima ted to be be t t e r t han

~24 [ I 9 9 % . An unbiased sample of some 700 events lying inside an inscribed fiducial vol-

I I ume was measured for identification v ia 16 ~ t h e geometr ica l and k inemat ica l T H R E S H

�9 ~ 0 and G R I b ; D programs demanding a (( good )) ; _ ~ _ ~ L mu l t i ve r t ex fit of bo th the product ion and

decay points to one or more of the react ions o | 0 0.2 - , l is ted in Table I. The k inemat ica l identifica-

16~-- ~ ~ ~') t ion was fu r the r supp lemented b y ionizat ion es t ima t ion of the outgoing t racks. Even t s

s fai l ing to yield any fit to the product ion poin t b u t hav ing a good fit of the V o decay to

0 ,n ~ [ ' ] , o.1 0.5 0.9 1.3 e i ther K o or A-hyperon , were assumed to

MM 2 (GeV 2) have more t han one unseen neu t ra l part icle

Fig. 1 . - Missing-mass-square dis- in the final s ta te unless an obvious fai lure tribution of pp-~AAOKT: uniquely cause was observed. The rel iabil i ty of the identified events, a) Missing neu- identif icat ion is i l lustrated in Fig. 1 where tron i n the reaction pp-~AnK%r +. the measured miss ing-mass square ( M M ~)

b) Missing 7: o in the reaction pp--> of events f i t t ing to the react ions pp-*.A2VK= -~ApK+r: ~ c) Missing-mass dis-

are shown. Figures l a ) and lb) show the tribution of the reaction pp--> -~ ApK% + with one or two visible M M 2 dis t r ibut ion of the missing neu t ron

V ~ decay events, and missing n~ in the react ions pp--~

-->AnK+~ + and pp-->ApK+7: ~ Figure lc) shows the M M 2 dis t r ibut ion of events which yielded a fit to the react ion p p - ~ A p K ~ ~- where bo th the K ~ and A ~ decay are visible (concentra t ion a round M M 2 = 0) and events hav ing one visible V ~ decay {concentration a round M~ and = .MA.~:).

XI.HrTRAL STIL\N(;E-PARTI('I,I,: PII(H)I'C'I'It)N IN p p coL], s o x s AT 6.92 (~('V/e 4;57

TAm,]~ I. - Partial cross-sectio,~ values /or .neutral-stra~tge-!aartich' p.rod,teetion i~ p p col- lisio~ at 6.92 G('V/c of this experiment and o/ other t l B C experiments at nearby ~nomenta.

I n i t i a l m o m c n - t u n : (GeV) 6.92

7 8 9

10 11 12 13 14

Re fe r ence P r e s e n t e x p e r i m e n t . . . . . . . . . . . . . . . . . . . . . .

No. of W e i g h t e d R e a c t i o n events l no. of a (y.b)

I e v e n t s

7.87 5.52 . . . . . . .

A S C O L I I ALEXANDER et at. (3) e t at. (1)

a (tzb) a (tzb)

A p K + EOpK + E + p K o

p p K ~ 1 7 6 npKOK + EOpKOn + E+nKOr=+ Z+pKOn ~ A n K + n + ApKO~ + A p K + n o np~OKO~ + ppF~OK+rc - p p K O K - n + E+pKOn+n- E-pKOrc+~+ EOpK+=+n - ApKOn+r: ~ AnKOr:+~ + ApK+rr+r: - pp:KOKO~=+rr - npg~OK+r:+:: - ppKOK-rr+r: npKOK-n+r : + AOpKO=+r:+rc- AOpK+~+Tr-n o AOnK+=+=+=- E+nKOrc+~+~- EOpKO~+~+n-

29.6 19.1

7.4 5.2

11.0 20.2

2.1 4.2

54.2 78.3 50.7

2.1 6.3

4.2 3.1

10.2 9.5 7.4

19.2

2.1 1.1

5.6 10.9

8.4 1.1 0.5

52.7 35.3 24.6 10.0 34.6 66.3

6.4 13.1 95.4 09.7 90.3 19.1 19.5

12.8 10.2 16.7 48.3 36.2 34.3

6.6 3.2

7.5 17.3 13.6

3.2 1.6

43.3 2= 8.0 28.9 4- 6.6 20.2 5= 7.4

8.2 5= 3.6 28.3 4- 8.5 54.3 5= 12.1

5.3 4- 3.6 10.8 4- 5.3 78.2 4- 10.6

8 9 . 8 4- 10.1 74.1 =~ 10.4 15.7 4- 10.8 16 .04- 6.4

10.5 5= 5.1 8.3 5= 4.7

13.7 4- 4.3 39.6 4- 12.8 29.6 4- 10.9 28.1 4- 6.4

5.4 • 3.8 2.6 + 2.5

6.2 4- 2.6 14.2 5= 4.3 10.1 4- 3.5

2.6 4- 2.5 1.3 4- 1.8

54.4 • 7.3 25.2 4- 5.0 14.3 4- 5.1 10.4 4- 3.3 25.0 4- 6.7 29.5 4- 7.O 21.4 4- 6.2 17.9 • 5.7

101.0 4- 10.3 72.4 5= 7.4 77.5 + 8.7 24.8 4- 12.4

9.8 4- 4.4 13.8 5= 5.2 14.7 4- 7.4 24.7 4- 5.6 21.4 4- 4.8 67.4 ~ 15.0 20.2 4- 8.3 49.0 4- 7.2

6.3 5= 2.8 7.8 4- 3.9 5.9 ::[: 3.4 3.9 -4- 2.8

23.1 4- 4.2 39.5 5= 6.3 20.3 5= 4.7

35.8 4- 9.9 16.0 • 7.5 4.5 • 4.5 6.4 • 4.0

19.4 • 10.4 28.85= 11.8

4.1 4- 4.1 9.4 5= 6.6

75.4 4- 15.7 78.4 • 12.6 62.3 4- 12.3

4.1 • 4.1

2.0 4- 2.0 17.4 4- 12.3

21.3 G- 8.1

0.95= 1

1.14- 1

! 4.95

31EmmA: ;t al. (:

a (tzb)

4 8 4 - 4 25__+ 3 1 7 1 3

3 5 = 1 1 2 4 - 3 205= 3

7 5 = 2 7 4 - 2

415=5 42-4-5 2~ 3

7 ~ 2

(:) G. ALEXANDER, 0. RENARY, N. KIDRON, A. SIIAPIRA, R. YAARI and G. Y~KU- TIELI: Phys. Rev. Lett., 13, 3 5 5 a (1964); G. ALEXANDER, O. BENARY, G. CZAP:EK, B. HABER, N. KIDRON, B. REUTER, A. SHAPIRA, E. SIMOPOULOU a n d G. YEKUTI~..LI: Phys. Rev., 154, 1284 (1967).

(2) E. BInRMAN, A. P. COLL~RAISE a n d U. NAtmNB~RG: Phys. l~ev., 147, 922 (1966). (a) G. ASCOLI, M. FmEBAUGH, E. L . GOLDWASS~R, U. E . KRUSE a n d R. D . SARD:

p r e p r i n t s u b m i t t e d to the XI[J~ International Con]erence on High Energy Physics (1966); 1V[. W. FIREBAUGIt: Ph. D. Thesis, U n i v e r s i t y of I l l inois , 1966 ( u n p u b l i s h e d ) .

3 0 - I I Nuovo Cimento A .

4.~8 G. A L ] ~ X A N D E R ~ A. SFfAPIRA~ ~ . S [ ] ~ f O P O U L O U a[l(~ G, Y / ~ K U T I ~ L [

On the identif ied sample of events a lower cut-off of 3 m m in space was in troduced on all v is ible-neutral -s trange-part ic le path lengths and correspond- ingly to each event a we ight was ass igned tak ing in account this cut-off, the po tent ia l path l ength of the V ~ in the fiducial v o l u m e and the k n o w n decay branching ratio. Fo l lowing the ident i f icat ion and acceptance procedures a to ta l of 229 event s y ie lded a un ique fit to one of the react ions l isted in Table I, 126 event s f i t ted to more than one hypothes i s and 145 events did not yie ld a n y fit to the product ion point . The rest of the events turned out to be e+e - pairs. A poss ible m e t h o d of reducing the degree of ambigu i ty is to take ad- v a n t a g e of the fact that the strange part ic le product ion is main ly going through the peripheral product ion mechan i sm. This feature is well i l lustrated in Fig. 2

/

/

2,4

~o r

w

f

x/ /

/

/

/ /

X /

/ /

/ d

\

/

. , %

/

/

J

i i i l [ ~ ' ~ ' 1 I I

/ / ,

/ /

/ , / / / X / / / /

/ / / / / / r ~ / ' / / " / J / I I " e I I I I I

o COS O N

Fig. 2. - Angular correlation of the two final-state baryons in[the uniquely identified pp-> AJ~Kn reaction events.

N E I T I U ~ I , STI{ANGE-P,~,IUI'I( 'IA", PI~OI)UCTI()N IN p] ) C() l , l , l~lON,~ AT 6..(} 0 ( ~ ( ' V / c 4 5 ! )

where l he anR'ular correlation between the lxvo outvoin~' ba ryons in the reac- ti(ms pp ~ A,N'K~'~ is showm As ~'an be seen, there is a s tron~ forward-ba(.kwar(1 pe~king of the two final-state ba ryons whi(,h ]ms been used in this w o r k to select in the ambiguous events only those hypotheses hav ing the two b a r y o n s goint~" in opposi te directions. The remain ing ambiguous events we have d iv ided to the different hypotheses in p ropor t ion to the cross-section of each of t h e m a,s de te rmined f rom the uniquely identified events .

3. - R e s u l t s and d i s c u s s i o n s .

The par t ia l cross-section values for the different identified pp collisions hav ing neu t ra l s t range par t ic les are g iven in Table I toge the r wi th values ob ta ined b y similar exper iments a t n e a r b y incident p ro ton m o m e n t s using t IBC techniques (1-3). While there has been found l i t t le difficulty in distin- guishing be tween the react ions pp--~ A p K + and pp ~ E~ +, one has to bea r in mind the inherent difficulty in differenting be tween A and Eo par t ic les in those final s ta tes hav ing an addi t ional nondecaying neu t ra l part icle . I n some cases such as in the react ion p p - ~ A p K ~ '+ i t is possible to e s t ima te the con- t amina t ion of the final s ta te b y compar ing the n u m b e r of events hav ing b o t h the A and the K ~ decay visible and the n u m b e r of events where one u has a visible decay. Using this me thod a con tamina t ion of not more t han 15 ~o E~ has been es t imated in the identified pp--~ApK~ + react ion.

The three-, four- and f ive-body f inal-s tate configurations h a v e been ana lysed in t e rms of resonances product ion. I n Fig. 3 are shown the two-body inva r i an t -mass dis- t r ibut ions of the react ions p p - - ~ A p K + and pp--~EOpK + which are compared wi th the phase-space dis tr ibut ion (solid lines). I n the inva r i an t mass dis t r ibut ion M(AK) there seems to be a concentra t ion of events in the lower-muss values which however is less p ronounced t h a n those repor ted i n o ther ex- pe r imen t s a t ne a rby incident p ro ton mo- men ta . This concentra t ion of events m a y be associated wi th the react ion

pp ~ J~+(1688)p --~ A K + p ,

or a l t e rna t ive ly be due to the per iphera l in terac t ion mechan i sm of one-pion exchange in which the A and K ~ emerge f rom the same

b) 4

4 0 "r~ 1.6 2.2 2.8 1.5 2.1 2.7

U,L 1.6 2.2 2.8 1.6 2.2 2.8

L1)

23 2.7 3.3 2.2 2.8 3.4 GeV

Fig. 3. - Two-body inveriant-mass distributions of the final states ApK + and E~pK +. Dashed lines represent phase-space distributions a) and b) M{pK+); c) M(AK+); d) M(E~ e) M(Ap); ]) 21s176

4 6 0 G. ALEXANDER, A. SI~APIRA, E. SIMOPOULOU ~ I l d G. Y E K U T I E L I

ver t ex . The fac t t h a t the expe r imen ta l peak posi t ion in M(AK) appa ren t ly shif ts f r o m ~ 1 . 7 GeV a t a round 5 GeV/c incident m o m e n t u m to , - -1 .8GeV at

8i

~ ot ~, , , ~ 0.7 1.1 1.5

1.6 2.0 2.A

I ., b)

//~-.,~['LL I1

2.2 2.6 3.0

1.a 1.8 2.2

t 1.2 1.6 210

OeV

Fig. 4. - Two-body invariant-mass dis- tributions of the finite state ApK~ +. Solid line represents the best fit to line- ar combination of phase space and ~ ( 1 2 3 6 ) , (40%) and Y~(1385), (10%) resonances. Dashed line represents phase space distribution only. a) M(A~+); b) M(Ap); c) M(K~ d) M(pK~

e) M(AK~ 1) M(pr~+).

8 GeV/c incident m o m e n t u m favours somewha t the per iphera l in te rac t ion ex- p lana t ion . I n the invar ian t -mass dis- t r i bu t ion of M(E~ +) i t is in teres t ing to note t h a t there seems to be a con- cen t ra t ion of events in the region 1.9 to 2.0 GeV which m a y possibly be due to the following react ion chain

pp --~ ,N'~+(1920)p -+ ZOK+p :

the da ta however are too meager to d raw any definite conclusions. In the M(Ap) and M(Y,0p) there does not seem to be any evidence for possible hyperon- nucleon resonances.

The react ions p p - + A J ~ K ~ are the mos t a b u n d a n t f rom the identified chan- nels l isted i n Table I . These react ions h a v e been studied in t e rms of r e s o - nance p roduc t ion b y fi t t ing tile vari- ous inva r i an t -mass dis t r ibut ions to a l inear combina t ion of phase-space dis- t r i bu t ion and the var ious possible reso- nance dis t r ibut ions described in ref. (1). I n the reac t ion p p - + A p K ~ + there is a s t rong 3~(1236) produc t ion esti- m a t e d to be ~ 4 0 % of the whole cross-

sect ion (Fig. 4). p roduc t ion of -~ 10 % and possibly also K~(890) p roduc t ion . The M(Ap) d is t r ibut ion ~which is c o m p a r e d in Fig. 4 only wi th the phase-space d i s t r ibu t ion has access to events in the higher- mass region which mos t p r o b a b l y is .due to the pe r iphe ra l na tu r e of the react ion. The per ipera l p r o d u c t i o n m e c h a n i s m is fu r the r i l lus t ra ted b y the M ( A K ~ dis t r ibut ion shown in Fig. 4 where the solid line is the expec ted d is t r ibut ion of a one-p ion-exchange react ion drawn schemat ica l ly in Fig. 5. This exchange d iag ram in which

I n the same reac t ion there is also evidence for Y*(1385)

Fig. 5 . - One-pion-exehange diagram for the reaction pp-~ -+ J ~ + + (1236) AK ~ ~ p=+ K~

N ] ' I I / F R A L ~T IL INGI~ - ] 'ART I ( ' LE I ' I r 1N [ ) | ) ( 'O] , I , IS]ON~ AT 6.92 (;(,V/c 461

we h:~w, ~q~l)roxinmt,('d the re~(,tion p p - - l)rc%'\K" by t)1)-*.~ ~(123(1)AK" , 'aa

bc expressed as folh)ws (~):

2 Llma ~

d( r _ f w P ~ [ / " 2 ( / I =) l=a((o2)f(zl 2) doJ ~ j =~/~= d(A~) ,

where o~ is the c.m. ene rgy of the upper ver tex = - p - - * A K ~ a(eo 2) is the cross-

section of the reaction rc-p-->AK ~ as funct ion of energy, and

1

F * is the flux normalization~ F~(A *) is the 7:p3~* ver tex funct ion and ](A S) is the form factor which is approxi-

2O

~2

O L - - 1.2 1.4 1.6 1 .B 2.0 2.2 2.4

GeV

Fig. 6. - ,N'rr invariant-mass distributions of the final states AnK+rr + and ApK+r: ~ Solid line represents the best fit to linear combin- ation of phase-space (60%), 2~~ (10%)

an({ Y*(1385), (30%) distributions.

ma ted b y the expression

/ ( A s ) = 1 + ~Tn~ ] '

with ~ "~ 60.

The solid curve shown in the M(AK ~

~ 8

distr ibution is calculated by the above

expression using the existing da ta on a(r:-p --~ A K ~ at different energies.

There seems to be a good agreement between the experiment and the calculated curve which is normalized to the

total number of events in the plot.

1.2 1.Z,. 1.6 1.8 2.0 2.2 GeV

Fig. 7. - h_~ invariant-mass dist~ribution of the final states AnK+r: + and ApK+rc ~ r Solid line represents the best fit to linear combination of phase-space {10%) and Y*(1385), (30%) and J~(1236), (10%) distributions. [] M(Ar:+); ~ M(A=~

(4) See for example E. F~RRARI and F. SELLERI: SUp~. ~'~OVO Cimento: 24, 453 (1962).

46 :~ G. A L E X A N D E R , A. S I ~ A P I R A , ~. S I M O P O U L O U a i l d G. Y]~KUTI ]ELI

I n the f inal-s ta te configurat ion A p K + ~ ~ and AnK+~ + the product ion of 3 ~ ( ] 2 3 6 ) is much less p ronounced (Fig. 6) in accordance wi th wha t is expected f rom the isospin considerat ion which predic ts the A~(1236) product ion cross- sect ion ra t io to be 9 :2 :1 for the react ions (10), (11) and (9) of Table I . On the o ther h a n d in the last two react ions there is seen a s t rong Y~(1385) produc- t ion in the M(A~) dis t r ibut ion (Fig. 7) e s t ima ted to be ,--30 ~o in bo th chan- nels. W e h a v e examined the possibi l i ty t h a t the Y~(1385) fo rmat ion is ma in ly a decay p r o d u c t of a h igh-mass 3~* isobar : A ~ * - + Y * ( 1 3 8 5 ) + K b y s tudying the t h r e e - b o d y inva r i an t -mass dis t r ibut ions A K ~ where the M(A~) was t aken in the Y*(1385) mass region. These dis t r ibut ions do not show any significant dev ia t ion f rom the phase-space curves. F u r t h e r m o r e assuming tha t the u is p r e d o m i n a n t l y a decay p roduc t of A~ or J~?~(2420), we expected to observe the Y~(1385) in a ra t io 9:2 :] in the f inal-s tate configurations A n n + K +, A p K + = 0 and ApK~ + respect ively , while exper imenta l ly it was found to h a v e the cross-section ra t ios ~-~23:22:9. The inva r i an t mass dis t r ibut ions of M(3~K) , M ( K ~ ) , M(A.E') and M(AK) of the final s ta tes A p K + ~ ~ and AnK+~ +

6 1.5 1.9 2.3 0.7 1.1 1.5 c:

0 [ , 1 2.1 2.5 2.9 1.6 2.0 2.4

GeV

Fig. 8 . - Two-body invariant-mass dis- tributions of the final-state ApK+r: ~ Solid lines represent the best fit to line- ar combination of phase-space (60~o), Y*(1385), (30%) and 2~'~(1236), (10%) distributions. Dashed line represents phase- space distribution only. a) M(p K +);

b) M(K+::~ c) M(Ap); d) M(AK+).

[1 ol

6 1.4 1.8 2.2

~ 12 t c)

2.1 2.5 2.9

[ - -

0.7 1.1 1.5

1.7 2.1 2.5 GeV

Fig. 9. - Two-body invariant-mass dis- tributions of the final-state AnK+:: +. Solid lines represent the best fit to line- ar combination of phase-space (60~o), Y*(1385), (30%) and 3~'~(1236), (10%) distributions. Dashed line represents phase-space distribution only. a) M(nK+); b) M(K+=+); c) M(An); d) M(AK+).

are shown in Fig. 8 and 9. There seems to be in some of the dis t r ibut ions a con- cen t r a t ion of events in the lower-mass values above the calculated phase- space d is t r ibut ions (dashed lines) or above the best fit d is t r ibut ions of phase-

Nl.;I TI{AI, ,<TI{AN(IE-I 'AIUI ' ICI ,E I ' I U ~ I I U C ' r I O N I N l i p COLI , I ,~ ION A T 6.92 (IeV/c 4(i3

Sp}|('t ~ }[lid ~ l ( ] 3 S ; . ) ) dis t r ib . t ions (solid lines). These phemmmm~, a r~ most l~robably asso(.iated with the peripllera.l na ture of the rea(q:ions similar to the effect seen in the M(AK ~ distr ibution of the ApKO,-: ~ final sta.te.

Final ly we lmve examined the

five-body final state ApK=r: con- figurations. The combined M(Ar 0

and M(oV=) distributions are shown in Fig. 10. A strong product ion of

A~(1236) and Y~(1385) i s ob- served. Other two- and three-

particle mass distributions of these reactions do not show any sig- nificant structure.

In conclusion, the main fea- tures of the neutral strange-par-

ticle product ion in pp collisions

tO

C:

~4o a)~ 40

2O

0 1.3 1.5 1.7 1.9 1.1 1.3 1.5 1.7 1.9

GeV

Fig. I0. - An and ~r~ distributions of the fi- nal states A2~'Kr:g configurations, a) M(Arr

b) ~/(2~'~).

a t 6.92 GeV/c is the strong product ion of Ar~(1236) and u resonances. Very little evidence exists for the product ion of K~(890) or o ther meson reso-

nances. No evidence has been found for the product ion of hyperon-nucleon resonances. The simple (~ quasi - two-body )) reaction feature observed in pp col- lisions leading to final state of nucleons and pions seem to be absent in the product ion of neutral strange particles.

We would like to thank Brookhaven Nat ional Labora tory , the AGS and the 80 in. HBC crews for enabling us to obtain this p ro ton exposure. Our thanks are also due to Dr. U. MAOR, Z. CARMEL~ N. KIDRON and S. TOAF for m a n y helpful discussions and remarks.

R I A S S U N T O (*)

Si ~ studiata al CERN la produzione di particelle strane neutre negli urti pp a 6.92 GeV/e nella camera a bolle ad idrogeno da 80 in. Si riportano le sezioni d'urto parziali per i differenti canali. Non si sono osservate prove a favore delle risonanze iperone-nucleone. Negli stati finali a quattro e cinque corpi ~ stata osservata una notevole produzione di ~ (1236) c Y*(1385), produzione che ~ stata analizzata in ter- mini di un meccanisrao a seambio di un pione.

( ' ) T r a d u z i o n e et cura del la R e d a z i o n e .

4~/~ G. AL]~XAND]~R, A. 8HAPIRA~ ~ . SIMOPOULOU and G. ](]~KUTI]~LI

Poac~etme HefiTpa.qlMiblX CTpallHblX qaCTHll B pp coy~apemmx ups 692 F3B/r

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