Search for strange (S = −1) dibaryons by means of the reaction pp → K+X

NuclearPhysics A479 (1988) 389~ - 398~ North-Holland,Amsterdam

SEARCH FOR STRANGE pp + u+x

(S

Robert

Ralner

SIEBERT”,

lnstitut

de Physlque

389~

= -1)

DIBARYONS

BY MEANS

OF THE

REACTION

FRASCARIA

Nuclealre,

B. P.

n-1,

91406

Orsay,

France

Recent Saturne experiment searching for strange dibaryon resonances between 2050 and 2170 MeV Is described. The reactlon pp --) K+X was studied with protons of incident energy T = 2.3 GeV on a liquid hydrogen target detecting the kaons in the SPES IV spectrometer from 1. 1 to 1.4 GeV/c. The detection system including Cerenkov counters as a online particle trlgger with an efficient pion and proton rejection is described. The results for eK = 8’ and 10’ are presented, showing a strong final state Interaction Ap close to threshold and a small structure at the EN thresholds. Preliminary calculations In the one meson exchange model wlth final state Interaction are presented.

1.

INTRODUCTION For twenty

work,

five years

both theoretical

very few remain

after

proposed

non-strange

and

would

we

resonances baryons

quark

magnetic

based

forces

such

as the Q*

of QCD6

c

lndfvidual

clusters

quark

as the stability

and

of these

c’.

Qn x Qm

spectra,

states

known are

from

Bonn

University

and

n quarks

Dover7

spontaneous states

have

‘Dibaryons

generally

are as

In particular successful

a spectrum

by considerations

In

of multi-

against

(RFA)

on

likely for strange

of the NIJmegen all colour-singlet fm+n

= 6)

studied

dissoclatlon

0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

0375-9474/88/$03.50

but

for the

be explained

are predicted

predict

extensive

what is a dlbaryon

cannot

been

more

have consldered

baryons.

* On leave

states

of

proposed,

in particular

KIsslinger

which

have

the predictions

of m and

agalnst

Such

it is well

They

Aerts

subject

have been

type.

calculattons

into two clusters

and

system

which

baryon

that six-quark

on the MIT bag model.

representations

and

dlbaryons,

the most extensive

of six-quarks

modelse,

meson

the

experiments,

Heddle

exchanges’.

Bag

been

One must first define

of

In a two baryon

via hadronic

narrow

of precise

deflnltlon

states

the observed

states3a4p5

Among

resonances.

the

has

Many candidates

dibaryon

phenomenology.

Concerning

of dlbaryons

number

or bound

reproducing

field

a larger

retain

lnteractlng

In quark

the

and experimental.

by quark

dissociation

systems.

group3n4

are

arrangements

with SUf3)

the

colour-

stability emission

colour of these as well

Into two colour-singlet

R. Siebert, R. Frascaria / Search for strange (S * -I) dibaryons

39oc

The maximum flavor

of stability

representations

S = -2

state

strong

flavour

decay.

Natlonal

upper

singlet,

narrow

I = 1 states (r

can

even

be stable

been

made

since

sectlon

experiment

reactlon

d(K-,

S = -1

system

respect

to

Srookhaven structure

was

of such a state vary from 30 at CERNg,

did not show

estimates

These

looking

evidence results

for

do not

of the production

in either the S = -1 or S = 0 six quark systems. permits

a lower dlmenslonal

energetically.

Two candidates

21+1,2s+1

Quark structure

-2

(9qqs)s - (9s)s

-2

(Ws)s (WW),-

-1

(WW), (Wqs)o

predictions structure,

LL PI

emerge

cross

L

(W)B

from table

2295 2297 2330

33 pJ

2337

2+ pI

2112

- (Wl),

L3 pJ

2152

coupling

decay

widths of these

CDT),

have been calculated

the

S = -1

to P waves

states which

and

of the

are respectively

by Aerts

and Dover7

1, where

states) or the Aerts and de

- (W),

concerning

than

MaaS(MeV)

J

'bl

13 pJ

However

representatlon

(for S = -2 by Mulders,

i3 pJ

- (qq)o

-2

fiavour

ii pI

(qqss)3 - (99&

-1

x Qr

is the

be made.

-2

QI

with

at the

performed

K+)H,

no reliable

which

No narrow

of 5 to 40 nb/sr.

TABLE 1 Strange q4 - qz dlbaryons lying below the Mn ANa (for S = -1 states) thresholds predicted Swart3.

are listed

is zero and their

case,

has

cross

at the level

of the H,

for S = 0 and Is favoured

S

optlmum

could

dibaryon

Another

in the

c 8 MeV)

is no fiavour singlet

the six quark

H,

The

of the pp -B K+K+X reactIon*.

upon mass.

rule out the existence

There

the

H

this

limits for the productlon

structures

section

when the spin of the clusters

dimensionality.

called

for

by means

to 130 nb depending for S = -2,

is reached of lower

A search

Laboratory

observed:

are

S = -2

dibaryons,

baryon-baryon spin singlet

which

system. fDs)

The

are

of

strong

and spin triplet

and found to be less than

10 MeV.

R. Siebert, R. Frascaria / Search for strange (S = -II dibavons

They

are

fMDt

= 2.15

predicted

methods

proton.

Correspondlng

statistics

apart

predicted

Just below

(MDS

= 2.11

(2. 13 GeV) .

There

are

produced

In a bubble-chamber, method

give any lndlcatlon

might

by the

Improve

reactlon

types H.

observed.

have

been

the available

In the near future,

pp -+ A&

PlEKAFtZ

performed

has

already

and

above

experimental

It Interacts

for or agalnst

are

and

of rather

the existence

of the

as at LEAR lntenslve

: a lot of experlments

some

reported

data are

wlth a

A-

now avatlable.

The TT+ + d + K+ + X and K- + d -D TT- + X reactions reactlon

GeV)

several

:

states

to the complicated

and cannot

This method

beams,

MeV

EN threshold

: After the A Is produced

dlbaryons.

-

the

to look for those

- Ap-scattering poor

to Ile 40

GeV)

391c

narrow

on them

structures

of this

have

been

In his contrlbutlon

to thls

conferencelO. -

The

pfp, K+)X

elementary

KN + rrY spin-flip

non-spin-flip fK,s)

amplltude,

the

both

(FSI) and

D,

performed poor

K+ momentum

mlsslng

2.

showed

mass

= 2.65

THE

a strong

spectra

and

2.4

can

may

be

be

A

was

statlstlcally

and

bad

absolute

since

the

than

the

lower

Is a simple

studied

observed.

which

and

be produced

through

In the output channel,

In the

system final

but suffered

momentum

state

was

from

determlnatlon.

MeV)

In

and thus effects

pp -D K+X experiment

accurate

for eK = 0’ for two different

threshold

lncldent

a

Thls

In the Ap

proton

energtes

GeV).

A new pp + K+X experlment

misslng

description

An lntermedlate

experiment

the main

performed

which tocus

plane

resolutlon

utlllzes

allows

permits

of It.

the momentum time

of flight

already

makes

of 600

ps FWHM

focal

Two multldrlft

of the trajectories

located

In both

4

Natlonal

Laboratory

of momentum beam

line.

transfer

A

detailed

system can be found In ref. The SPES

4 beam

analysis

of particles

trajectories.

and

In The

the

sclntillators

1 cm)

covering

the total

plane,

allow the

also In the flnal focal

horlzontal

A hodoscope

13 sclntlllators

of good particle between

We

up to 4 GeV/c.

over 16 m.

fthlckness

12.

line Is a 32 meter

and corresponding

Is obtained

focus and long sclntlllatlon-counters

reconstruction

SPES

a preselectlon

counters,

range

measurements

In the Intermediate

Intermediate plane.

at the Saturn

the

and the detection

characterlstlcs

of 12 sclntlllators

the final focal time

The

of the spectrometer

long spectrometer

consisting

has been

to study np and TN FSI In a large

mass.

give just brlefly

best

of magnltude

preferentially

FSI just above the Ap (2054

taken

zero,

EXPERIMENT

with high resolution and

spin

The pp --t K+X reaction

Interaction

resolutlon

are

Is an order

the two baryons

Dt dlbaryons

pions

that could

Is the Dt.

at BNL In 196611

experiment

fTp

between

and

amplltude

hyperon-nucleon

interaction

kaons

the dlbaryon

and (IT, K) reactions

which

of

: Since

reaction

vertical

planes.

In the

Uslng

the

392~

R. Siebert, R. Frascaria / Search for strange (S = -I) dibaryons

inverse

matrix

measured does

of SPES

partlcie

not come

from

one

kaon

at the focal Counters

can

calculated

posltlon. and

detectlon

Is an enormous

Cerenkov

one

the target

In fact this standard as there

4,

at the target

permlts

system

flux of protons

plan.

We have

(TRCC)

the

momentum

and

Thls allows the rejection also

missing

mass

added

as a particle

D3 1

I I

10’~

In the focal

trigger

DL

about

The

to detect

that

kaons,

and several

plane

(fig. 1).

of the

analysis.

of SPES IV Is not sufflclent and plons,

angle

of any particle

four Total prlnclpie

10%

for

Refiexlon

of operation

I F3 -1 FL

I

FIGURE 1 The principle of the Total Refiexlon Cerenkov Counters (TRCC) Is shown In the upper part. The cone and the light transmission Is indicated for protons, pions and kaons. The dashed line gives the ilmit for total reflectlon. The dasheddotted line represents the direction of the particles. The arrangement of the four TRCC in the focal plane Is illustrated below.

of the

TRCC

lndlce

n wlth a reiatlve

emitted

Is quite

along

a cone

simple.

Charged

of

angle the

of the charged

particle.

Because

to reach

the

particles

and

practice, project

the

however, electrons

is not so nicely PMs

lndice

also

amplitudes. threshold

from

the

directlonal

must

If the

efflclency

particles of the

through

It depends light

because

Cerenkov

producing

protons

trigger

an

fast

fpp ( aK)

direction

the counter on the

charged

or not.

light at the

dlstrlbution

on the

and plons

in

particles

This secondary

exponentlal

by

6 of the

light is collected

depends

of

This light Is

Is ‘seen”

light.

one so that some

partlcie

As we want to reject

,

medium

effect.

to the propagatlon

Cerenkov

Is not so slmpie,

as the prlmary

a transparent

propagate

(PM9

which in turn produce

“substhreshold’

positton.

light

photomuitipilers

situatlon

crossing

light by Cerenkov

ec with respect

n of the TRCC

(e-rays)

So the

produce

opening

total

refiectlon

partlcies

velocity a > l/n

of

discriminator

(PK

< &I,

the

R. Siebert, R. Frascaria 1 Search for strange (S = -1) dibaryons

four TRCC degrees.

are

not mounted

perpendicular

Wlth this assymetrlc

(LX side for the light collectlon. light of kaons

and

the defavoured (fig. 1).

plans

can

of the particles

rejection 92 %.

of 99 The

96.

The

onllne

kaon detectlon

see

thresholds the favoured

plon

efflclency

200 plons and 100 protons can

light attenuatlon

from

reJectIon remalns

are reglstered

that the whole

background.

coming from

It shows

conditions from

on

the

reflexion,

on the summed sides

done hlgher

240

of 2.9

Sclntillator

made

The

telescopes

to monitor

the

by actlvatlon

target

of flight

and

good

ns at 1.4

the llquld hydrogen used

tlme

(280

relative

the thick

separation

GeV/c

vlewlng

can

pure

K+-spectra

Is

about

In figure

2

wlthout

fact

are

wlth still we

have

dlscrlbed

kaons

more

restrictive

six stop before

and plons

a thln CHz fllm upstream

mg/cmz) flux.

from

sides

400 -. Channel

sclntlllators between

plus

proton

90 96. Nevertheless

spectrum (In

(‘left’

signals

and those

havlng

a TOF

be seen.

either

proton

measurement

(TOF)

Impacts

an onllne

kaon.

on

by the PM

defavoured

wlth the

320

amplltudes

whereas

slgnals

we obtaln

than

by 4

Cerenkov

In the TRCC for dlfferent

to obtaln

160

Cerenkov

hodoscope

the four TRCC).

difference

one the

produced

by the plons Is seen

FIGURE 2 Time of fllght spectrum at p = 1.4 GeV/c. Gates set on the Cerenkov amplltudes (see text) .

offline

but tllted

and a defavoured

on tape for one measured

set up allows

80

by total

light produced

we put the dlscrlmlnator

(F)

side the directly

the photocathode

for the dlfferent

, Wlth the lnformatlons

‘rlght’)

one

reach

trajectories,

we get a favoured

On the favoured

side only the Cerenkov

To correct

to the particle

arrangement

393c

from the target,

Itself and a secondary

emission

Absolute

of the monitors

lzC(p,

callbratlons

pn)“C.

monltor

or are

were

R. Siebert, R. Frascaria / Search for strange (S = -I) dibaryons

394c

The results of the present 8 and

10 degree

misslng

mass

at 2.3

spectra

2060

2080

investigation

GeV proton by 2.5

2100

MeV

21;\tl;0

Include

energy. bins.

cross sectlons

for kaons emitted

They are shown directly

The

error

2'60MM(Me")

2OLO

bars

2060

are

2080

2100

2120

2lLO

2160

EN thr.

at

3 as

only statlstlcal.

APthr.

APthr.

on flgure

The

MM(MeV)

FIGURE 3 Missing mass spectra for pp -D K+X at 8K = 8*(a) and eK = 10’(b). The solid line Is the b-body phase space for pp + K+Ap and pp + K+ffN)+.

thresholds phase addition shown

for the A and I:

space

of these in

compared

-

calculations

two spectra

flgure

4.

to the

3-body

the Ap threshold state

interactlon

- the cN threshold structure

production of the

On

which

flgure phase

mass reglon

4

where

mass

region

where

observed

data

taken

and absolute

two

(3-BPS)

calculations

the 3-BPS

is strongly

correspond

to the

pp -D K+tZN) +.

In shape

clearly

a small

structure

different

The

value

regions

is as

: dlstorted

appears

: one at 2130 MeV which corresponds to the Ap channel,

other

observes

and

by the A-p final

The

wldth

at

at threshold.

eK = 8 and 10’ with a wldth of 10 MeV seems

the

experiments13.

pp -D K+ np

are very slmllar one

space

the EN channel structure

The full lines

shown.

(FSI)

seen at both angles

two components

are

reactions

the other one at 2138

BNLl”

of the

to the strong

and

observed

at eK = 12’ Is In progress

In

other

effect and

Thls

to have

coupling

of

MeV could correspond

to

previous

bubble

Is a few

MeV.

can

light on it.

shed

The

chamber analysls

of

395c

R. Siebert, R. Frascaria / Search for strange (S = -1) dibaryons

A.U.

0

$t”;“,,

2080

2100

21;0&;;0

2160 MM(~,J~v)

FIGURE 4 Sum of the spectra of figure 3. The solid line is the The dashed line represents 3-body phase space. the prellmlnary calculation in an One-Meson Exchange model with final state Interaction Ap (see text) .

3.

ONE

MESON

EXCHANGE

The reactions been

interpreted

have

been

avallable.

MODEL

of meson

production

in terms

achieved

In

In the case

the

case

state

Ferrarl’s

interaction

cross

section

predlctlons

proton.

enhancement

In the n-p

at zero

degree

lnteractlon. of the Graph

done

such as flgure

requlred

wlthln

Our model

amplltudes.

parameterlzed

the

one

are

based

upon

5 leads to a linear Due

to

wlth recent

K+p phase

in the s-wave

are

In the slxtles

section

have

Due to

where

for the forward low energy

In our spectra

exchange

older

work

relation the

final

differential

interaction

which

mechanism

shlft analysls.

on flgure on

between K+p

dlrectlon.

by a separable

data

exhlblts

with strong

to threshold.

by the graph

antlsymmetry and backward

done

of the pp -r K+AP dlfferentlal

kaon

an

extensive

to total cross

a strong

seen

close

calculations

is best Illustrated

both In the forward

is consistent

mass

with experiments

the pp --t K+np reaction.

is not the case

can undergo

Is clearly

mlsslng

here prellmlnary

calculations

-B K+p

Thls effect

restricted

Thls

role.

where

calculations

dominates

have traditlonally

agreement

productlon

early

were

the A produced

the spectator

We present

pion

mechanism

plays a minor

since

of

colllslons

Fair

of mesons.

of kaon production

shown that the kaon exchange a lack of data

in nucleon-nucleon

of exchange

pion

cross section

with A-p 5.

final

Technlcal

production

state details

off nuclei.

the pp + K+AP and the K+p

elastic

scattering

amplitude

is

We took It to be the same which The A-p final state interaction

potential

with parameters

adjusted

was to a

396~

R. Siebert, R. Frascaria / Search for strange (S = -1) dibaryons

FIGURE 5 Graph used for the pp -B K+ap amplitude state interaction np.

scattering (a

length

= -1.80

a and effective

fm,

done although

r0 = 3.42

way

of treating

spectra

the

below

the

pp --t K+X reaction

final

at 2.3

GeV.

The region

terms

pair.

This is consistent

predictions

compared

in figure

energy resolution

An overai

s-state

normaiisation

was

for

the

4 with

with our

missing

the

data

mass on

the

of 10 MeV was convoluted

calculations.

mass spectra

at around

2095

we are improving

calibration.

of the meson

conclusions

Our

of the FSI is well reproduced.

At the moment an absolute

are

and triplet

in order to simplify the calculations

for the A-p

interaction.

threshold

for the singlet

to get an absolute

in this model.

kinematics state

rN

with the theoretical

missing

No attempt

thls is quite posslbie

we have used non-relativistic

rO identical

range

fm).

with finai-

seems

MeV which is not accounted

this caicuiatlon

Calculations

to get results

of the cross sectlons

(T or K) exchange

on the structure

A tiny structure

observed

in the

by this calculation.

for eK # 0 and to get

around

model will be performed in this

to appear

the EN threshold

in

that will allow definite

region.

ACKNOWLEDGMENTS The pp -P K+X experiment G.

EILANPIED*. J.

T.

ERNST-,

m

IPN

Orsay

(France).

Orsay

et

de

BOIVIN+. FRASCARlA*.

U.

NeuchEtei

Univ.

of South

Univ.

of Bonn.

de

R.

BONN

(USA)

BOVET***. J.F.

between

J.P.

DIDELEZ*.

GERMONE?.

PERDRISAT+++. SIEElERT*.

(RFA),

(Suisse), Carolina

E.

Ch.F.

SAGHAl-.

IPN U.

M. R.

MAYER-KLICKUKQ. B.

* ** *f* .

at LNS is a collaboration

E.

J.P.

J.Y.

8.

J.

physicists:

EGGEF?

GROSSlORD++.

PREEDOM*.

WARDE*.

the foliowlng

T.

REPOSEUR*.

YONNET+

w

CEN

Saclay

(France).

+

LNS

Saclay

(France).

++

IPN

Lyon

+++

Colleg.

(France).

William

6 Mary.

Williamsburg.

VA

(USA).

R. Siebert, R. Frascaria / Search for strange (S = -I) dibalyons

397c

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