Strange particle production in sulphur-sulphur interactions at 200 GeVc per nucleon.

NUCLEAR PHYSICS A

Nuclear Physics AS66 (1994) 499c-502c North-Holland, Amsterdam

Strange Particle Production Interactions

Presented

in Sulphur-Sulphur

at 200 GeV/c

by A.C. Bayes, The University

per nucleon.

of Birmingham,

UK.

The WA94 Collaboration

S. Abatzis’, A. Andrishetto lo, F. Antinori’, R.P. Barnes4, A.C. Bayes4, M. Benayoun’, W. Beusch’, R. Blaes’ , I.J. Bloodworth4, J. Bohm7, A. Bravarr3, J.N. Carney4, B. de la Cruzg, S. Clewer4, J.P. Davies4, D. Di Bari2, C.J. Dodenhoff4, J.P. Dufey5, D. Elia2, J. Engelfried 5a, D. Evans’, R. Fini’, B.R. French’, B. Ghidini’, H. Helstrup3, A.K. Holme3, A. Jacholkowski2, T. Kachelhofferi2, J. Kahane’, V.A. Katchanov”, J.B. Kinson4, A. Kirk’, K. Knudson 5, P. Ladron de Guevarag, J.C. Lassalle’, V. Lenti’, Ph. Leruste’, R.A. Loconsole’, G. Lovhoiden3, V. Manzari2, K. Martens5’, A. Michalon12, M. Morando”, U. Mueller”, F. Navach2, J-L. Narjoux’, M. Passaseo’, F. Pellegrini”, A. Penzo13, E. Quercigh’, R. Ricci’, L. Sandor7, K. Safaiik5 G. Segato”, H.W. Siebert5’, A.V. Singovsky”, M. Se&‘, R. Se&‘, T.F. Thorsteinsen3, G. Undheim3, J. Urban7, G. Vassiliadis’, 0. Villalobos Baillie4, A. Volte’, C. Voltolini”, M.F. Votruba4, G. Waeldersb and P. Zavada7.

Athens’-Bari2-Bergen3-Birming&am4-CERN5 -College Madrid’-Padua’ -Serpukhov”-Strasbourgr2

de France6-KoSice7-Legnaro’ -Trieste13

“Now at Fermilab. *NOW at Physikalisches Institut, Univ. Heidelberg ‘Now at Institut fuer Kernphysik, Univ. Maim.

Abstract The WA94 experiment is designed to study the production of baryons and antibaryons in sulphur-sulphur interactions at 200 Omega spectrometer, equipped with MWPCs operated in the to measure particles in the central rapidity window with pi > _ results are presented on the production of A, x, E- and ;-.

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stran e and mu’ltistrange GeV Bc per nucleon. The butterfly mode, is used 1.0 GeV/c. Preliminary

resewed.

soot

A.C. Bayes et al. I Strange particle production in sulphur-sulphur interactions

The

WA94

experiment

interactions

in the

strange

antistrange

and

ultrarelativistic Plasma that

hadronic

the case of multistrange The

WA94

sensitive out

of the

data

is expected where could

used

in the

rapidity

hundred

rates

probe

is shown

region,

in heavy

ion

production

of

the

[2] that

the

dynamics

a Quark yield

is formed,

“normal”

in figure

PT so that

kinetnatic

into

if a QGP

for which

run

The

tl re strangeness

1.

The

The

relative

to in

are low.

apparatus

is

are recorded

low multiplicity

and reduces

of

Gluon

notably

yields

only a few tracks

in a central collision.

in this

of V” and cascade

the reconstruction

1991

produced

at high

[3] that

and medium

1)~.

proposed

be enhanced

and antibaryons

produced

to high

it has been

suggested

baryons

particles

medium

to be a useful

It has been

at central

several

to be taken

at strange with

interactions

apparatus

to tracks

to look region

ion collisions

be formed.

in normal

aims

rapidity baryons

heavy

may

[l]

central

allows

the background

in

candidates.

hadron calorimeter (9 blocks)

Y concrete

=I

uu

shield

I

Figure 1: Layout of the WA94 Experiment-1991 The

incoming

sulphur

target

multiplicity which are

sample

which

the event in the

Omega

magnetic

so that

charged

particles

length). require

The

by a quartz Tracks

Proportional

MWPCs

are “butterflied”

the

the

target

if pT > 0.6 GeV/c

can

reach

placed

target

then

upstream pass

range

Chambers

[4], meaning their

sensitive

and 2.4 5 YLAB 5 3.2.

that

the

microstrips

of 2.2 < n < 3.5. (MWPCs)

of the

through

of 2 6 and the silicon

in the pseudorapidity

MultiWire from

Cerenkov

from

a hit multiplicity

multiplicity

seven

field.

field only

ions are identified

interaction

detectors

recorded

magnetic

sulphur (2%

X

run

placed

Tracks in the

they are modified

regions

through

the

AL. Bayes et al. I Strange particle production in sulphur-sulphur interactions

501c

The tracks recorded in the MWPCs are traced back towards the target and we consider any pair of oppositely charged tracks as a V” candidate if (i) the distance of closest approach between the two oppositely charged tracks is < 1.0 cm, (ii) each decay track traces through the seven MWPCs, (iii) the angle between the V’s line of flight from the target and the sum of the three-momenta for the V” decay tracks is < 0.75”, i.e. the candidate comes from the target, and (iv) the decay distance is > 135 cm. Furthermore when looking at A and x candidates we demand that the decay tracks have non-zero impact parameters at the target plane in the bend direction of the magnetic field, hereafter called Ay (2 4.0 cm for the ~-(7r’) and > 2.0 cm for the p@) from a A(@). This reduces the background caused by tracks from the target crossing and imitating V” decays. We require that the A (iz) candidates lie in the kinematic region Icy] > 0.45: cy is the Podolanski-Armenteros variable [5] given by (Y = (q,’ - qL)/(qt + q;), where qt and qL respectively represent the positive and negative decay tracks momentum parallel to the direction of motion of the V”. We then select candidates in a 50 MeV interval centred on the A mass, giving 56140 A and 18014 x candidates. In the unambiguous region (0.45 < Icy] < 0.6) we find 28787 A and 8783 ?i candidates, giving an uncorrected ratio of K to A of 0.305 f 0.004. 111 order to select E:- and z any combination considered as a cascade candidate if

of a A or ;i- with a charged track is

(i) the distance of closest approach between the line of flight of A (K) and the charged track is < 1.6 cm, (ii) each decay track traces through the first four MWPCs, (iii) the decay distance is > 90 cm, (iv) the cascade candidate comes from the target (Ay of the cascade impact with respect to the target centre < 2.0 cm), and (v) the decay pion does not come from the target (Ay of the charged track impact with respect to the target centre > 4.0 cm). The condition that the V” points back to the target is not required for cascade analysis.

A.C. Bayes et al. I Strange particle production in sulphur-sulphur interactions

502c

The effective mass distributions for the candidates, interpreted as AT- and i2sf are _ shown in figures 2 (a) and (b); clear peaks are seen at the ? aud =- positions. The = is 0.53fO.04 and the acceptances and reconstruction efficiencies uncorrected ratio z-/Zfor the cascade candidates are now being calculated.

0)

Is

b)

v) 2 : 300 l&l 200 100

50

\ 400 1

r

1

r

GeV/c

Figure

2: Effective

mass

distributions

GeV/c

for cascade

candidates

as (a) AT-

and

(b) XT+

;;We have presented results of a preliminary study of central A, K, ? and c- production in 200 A GeV/c sulphur-sulphur interactions. They have been successfully reconstructed giving clean mass peaks. We have preliminary uncorrected ratios of x/A = 0.305 f 0.004 and z-/Z= 0.53 f 0.04. Work is underway to correct this data to take account of the geometrical acceptance aud reconstruction efficiency of the apparatus for the different particle species. Furthermore the x to A ratio has to be corrected for cascade “feed-down” where the As and ks come from Z decays [6]. 200 GeV/c proton-sulphur data will be taken this autumn to compare with the above sulphur-sulphur data.

References [l] WA94 Proposal,

CERN/SPSLC/Sl-5

(1991).

[a] J. Rafelski and B. Muller, Phys. Rev. Lett. 48 (1982) [3] H. C. Eggers and J. Rafelski XXIVth

Rencontres

[4] W. Beusch et al. , Nucl. Instr. and Methods A249 [5] J. Podolanski

and R. Armenteros,

[6] S. Abatzis et al., Phys. Lett. B270

123.

(1986) 2334 (E)

de Moriond, March 1989. (1986) 391.

Phil. Mag 45 (1954) (1991)

1066;56

13.