IL NUOVO CIMENT0 VOL. XXXIX, N. 1 I o Settembre 1965

Search Ior B = 2, ,~'= 0 Resonances in p p Collisions a t 5.5 GeV/c .

G. ALEXANDER, N. KIDRON, B. REUTER, A. *SItAPIRA a n d G. YEKUTIELI

Nuclear Physics Department, The Weizmanu Institute el Science - Rehovoth

(ricewlto il 26 Giugno 1965)

In a study of the reaction pp -+ pp=*r:- at 4 GeV/c KIDD et al. (1) have found a significant difference in the shape of the pp=+ and pp~:- invariant mass distri- butions. In the ppT:+ mass distribution a peak was observed at (2.52• GeV with a full width of 0.12 GeV, while in the pp~- mass distr ibution this peak was absent. The 2.52 GeV peak was further enhanced by choosing only those events where the p=+ and p=- configurations were produced nonperipherally. With this selection criteria the significance of the peak was increased to about 3 standard deviations above a background estimated by using the isobaric model.

Two possible interpretations for the observed peak were proposed: a) the peak corresponds to a B = 2, S = 0 resonance; b) the peak is a result of an exchange mechanism (1). BERZI et al. (2) considered the second possibility" in more detail trying different exchange diagrams and production mechanisms. Their conclusion was that although exchange models might contribute to

(t) j . ~::IDD, L. ~J:ANDELLI, V. PELOSI, S. I{ATTI, A. SICHIROLLO, L. TALLONE, F. CONTE and G. TO~ASINI: Phys. Left., 16, 75 (1965).

(2) V. BERZI, E. REC~I and S. F, ATTI: Phys. Left., 16, 196 (I965), and references therein,

the pp~ mass distribution, they could not, however, account for the observed peak in the ppn + invariant mass dis- tribution.

The possible identification of the 2.52GeV peak with one of the Y = 2 , 8 : 0 states discussed by Dvso~ and Xuo~G (a) was pointed out by KIDD et al. (1). In the framework of the SU6 symmetlw model two baryons states decompose to

56| 5 6 = 4 6 2 0 1 0 5 0 G l 1 3 4 0 4 9 0 .

DYSON and XUON(~ proposed that all low-lying two-baryon states belong to the 49_0 multiplet. I t this case six B = 2, S = 0 ie~onanee states Do~, Dl~, D12, D~l, Dos and Dzo are predicted, where Dzj is a state having a spin J and an isotopic spin I. ~hese resonances satisfy the mass relation formula

(1) M = A d - B [ I ( I - ~ l ) d~J(Jd-1)--2] ,

where A = 1.876GeV if D01 is to be identified with the deuteron, B would be about 50MeV if the coefficients in (1) are the same order of magnitude

(s) F. J. DYSON and N. H. XvoNo: Phys. Rev. Left., 13, 815 (1964).

sE.&]~cn Fort IJ 2, ~ - 0 RESONANCES IN pp COLLISIONS AT 5.5 GeV/e 385

as in the baryon mul t ip le t or around l l 0 M e V if the 2 .52GeV peak in_ the p p . + mass d i s t r ibu t ion is identif ied wi th the De~ state.

I n this le t te r we present the resul ts of a search for B 2, S 0 resonance states lo rmed in pp collisions at 5.52 GeV/c. The following analysis is based on art unbiased sample of 1020 pp four-prong and 75 s ix-prong events using pictures t aken w i t h the Saelay 8 1 c m HBC at CERN. i n this sample the fo rmat ion of .N}* resonances was s tud ied in the react ions p p ~ p p = + = - , p p _ ~ p p . + = - . o and pp-+ p n . + . + = - and will be repor ted elsewhere. Irt the same reactions we have looked for the fo rma t ion of a pos- sible D q ( I - - 2 , J 1) in

(a) pp~l)=,t ~-w (I).n--~V,N'=) ,

(b) pp -> l)e, + =:( 1)21- :- ,,V,.~'= ) ,

and for the possible fo rma t ion of Dao (I- -3, J = 0 ) in

(c) pp~> l)ao +,=(l)ao-+,N},N~r:,) .

F o r bo th resonances only the decay mode into the smallest number of par- ticles consis tent w i th the conserva t ion laws was studied.

In Table I are shown the re la t ive p roduc t ion rates for the different D12 and Duo charge states in the reactions (a), (b) and (e). I n the same Table are also g iven the re la t ive p roduc t ion rates for the two D s ta tes in the f inal-state configurat ions de tec tab le in this ex- per iment .

h t reac t ions (a), (b) and (c) most of the f inal-state configurat ions conta in two or more neu t ra l part icles and hence could no t be identif ied in this exper iment .

TABLE I. - Branching ratios /or I)~i and Da0 rations in reactions (a), (b) and

Reac t ion

pp-> I),,, + .

p p - * D~I§ ([z:z: 1)

pp-~ D21 {-"" (I=~ = 2)

p p - ~ D a o + = =

J ! i

Charge s tates

charge states and/or ]inal-state con/igu-

D + + + _~.-- 21 D ++ 4 . 0

I D+' § =+

i D +++ +(2=)- D ++ + (2~)r

D+t 4(2 . ) +

D+++ 2, +(2=)- D~+, + ~_(2.)o

D~+I 4 ( 2 . ) + D ~ + (2r~)++

D3+o +++ (2.)=- D +++ (2=)- l)~+: (2=)0 Ds {2.)+ D~o (2.)++

Branch ing ra t io

i

6/10 3/lo 1/10

6/10 3/10

1/10

2/10 3/10

3/I0 2IlO

(c).

Detec tab le final s tates

Configurat ion

pp=+.-

p p . - . +

pp.§ pp.O . + . -

p n . + . + . _ pp=-=+.o

p p . + . - . O

pp=O.+~-

p n . + . + . - pp=-=+=o

p n = - . + . +

pp=+=+=-.-

pp .+=-=+=-

p p . - . - . + . +

F rac t i on

0.60

0.02

0.60 0.15 0.15 0.02

0.20 0.05 0.05 0.05 0.10

0.43

0.02

0.00

15/35 lO/35 6/35 3/35 1/35

-I

25 - I I N a o v o C i m c M o .

386 G. ALEXANDER, N. KIDRON, B. REUTER, A. SHAPIRA and G. Y E K U T I E L I

As seen in Table I, the D~ of reaction (a) should appear mainly in the ppu+ com- binat ion and the Dso of reaction (c) in the pp.+u+ combination. The expected formation of Dzl and D30 in the other charge configurations is smaller by one order of magnitude or more. In reae- %ion (b) the relative production rate o~ the different D~ charge states cannot be predicted uniquely since the two-pion configuration may occur in the I = 1 as well as in the I = 2 state. However, it is reasonable to expect that a pos- sible D~ resonance will be formed appre- ciably in the pp~:+ configuration unless there is a strong destructive interference between the amplitudes that lead to the I : 1 and I = 2 state of the two- pion systems.

events which could hinder the detection of a possible 2.52GeV peak T o this end we have used the same criteria as used by KIDD e$ al., tha t is we have excluded events with

o r

A2(Pi, PI= +) < 0.7 GeV 2

A2(P~, Pl~- ) < 0.7 GeV ~ ,

where A2(a, b) is the square of the four- momentum transfer from a to b, P~ is the incoming proton momentum and P1 is the momentum of the final-state nucleon which goes forward in the c.m. system. The ppr:+ and ppu- invar iant mass plots for events with the cut-off in the momentum transfer described

3 0 1

20;

loi i o I

~o~ !

L lO! b)

o LL___L 2.4 2.8 3.2

M IG e V ' ur,~ ' '

Fig. 1. - I n v a r i a n t mass distr ibution of a) p p ~ and b) ppr:-, in the react ion pp-->pp~+n-. The shaded a r ea represents the nonper iphera l

events .

In Fig. 1 the pp=+ and ppT:- inva- r iant mass distributions in the reaction pp-+ppT:+7: - are shown. Both distri- butions do not show any" enhancement at the 2.52 GeV region. We have next excluded from the sample the peripheral

5o[ [ i

' a )

30F

2o~ rj]

~_ 0 o

8O c)

A 2O

0 ~ -~ 2.4 3.2 2.4 3.2

Fig. 2. - J~)Aog invar i an t mass distr ibut ion in the reactions (1) pp __~pp~+g-uo (2) lap --~ pn~+g§ a) ppg+ of the react ion (1); 5) p p n - of react ion (1) and pnT:- of react ion (2); e) ppn* of react ion (1) and pun § of react ion (2); d) su m of all the com- binat ions in a), 5) and e). The shaded a rea repre-

sents the nonperipheral events .

SEARCH ] q - ) R B = 0, S ~ 0 RI,iSONANCES IN p p COLLISIONS AT 5 .5 G e V / c 387

before (Fig. 1, s h a d e d area) do n o t show, as well, a n y p e a k a t t he 2.52 GeV region. One e v e n t on Fig. 1 co r responds to 8.5 v.b. However , due to t h e uncer- t a i n t i e s in t h e b a c k g r o u n d i t is difficult to g ive a~t u p p e r l im i t to t h e pro- due~ion c ross - sec t ion of t h e 1)2+~ ++ in p p ~, pp,-r+=-.

I a Fig. 2 t he i n v a r i a n t mass d is t r i - b u t i o n of some ~N%~= c o m b i n a t i o n s in t he r eac t ion p p ~ ' ~ ' ~ = . are g iven . The s h a d e d a rea r e p r e s e n t s e v e n t s whe re t h e A '~ b e t w e e n t h e i n i t i a l p r o t o n s to a n y ~\ '= f ina l - s t a t e con f igu ra t i on ex- ceeded 0.7 GeV 2. T h e d i s t r i b u t i o n s do n o t show a n y e n h a c e m e n t in t he 2.52 GeV region a n d no s ign i f ican t di f ference b e t w e e n t he ppT:+ a n d t h e o t h e r d is t r i - b u t i o n s is obse rved .

F ina l ly , in Fig. 3 t h e pp~:+u+ a n d p p = + r : - § i n v a r i a n t mass d is t r i - b u t i o n s in t h e r e a c t i o n pp ~ pp~,~ r~+r:.-r~

are s h o w n t o g e t h e r w i t h ~N'~N~7:T: phase- space curves. N'o s igni f icant resonance- l ike d e v i a t i o n f rom t h e phase space are seen in a n y of t he d i s t r i bu t i ons , This is also t h e case for t h e ppr:+=+ dis t r i - b u t i o n whe re t h e poss ib le ~ormat ion of D3o is expec t ed to be mos t a b u n d a n t .

I n conc lus ion no ev idence is f o u n d for t he f o r m a t i o n of B 2, S - 0 resonances i n p p ~ , \ ~ , ~ ! n:: {n : :2, 3,4) a t 5.5 GeV/c

&

3 0 -

1 0 ' a

~. 0 I ~ ~- cu

A

~ 60 ,- _ H - ' - I

4o

2o [

0 [ ~

2.4 2.8 3.2 M ...... ': G e V )

Fig. 3. - ppg= Invar lan t mass distribution In pp--~pp~§162 a) ppn+g + mass distr ibution; b) pp~+:~- and p p g " ~ - mass distributions. The smooth curves roprescnb the phase-space distri- bu t ion for pp~r: conf igurat ion in p p - - + p p g ~ : ~ .

w h i c h ha s a c.m.s, ene rgy of a b o u t 0.42 GeV h ighe r t h a n t h e c.m.s, ene rgy of K i d d e t al.

W e would l ike to express our gra t i - t u d e to C E R N a n d to t he hydroge,1 b u b b l e c h a m b e r crew for enab l ing us to h a v e t he p -p exposure .