n 16O + Au collisions

689c

Nuclear Physics A525 (1991) 689c-69% North-Ho~and, Amsterdam

PROTON

RAPIDITY

DISTRIBUTIONS

Lawrence

FROM 60 GEV/N

S.R. Tonse Berkeley Lab., Berkeley CA 94720 and

‘sO+Au

COLLISIONS

USA

The NA35 Collaboration: J.Baechler,’ J. Bartke,3 H. Bialkowska,4 R. Bock,‘R. Brockmanqs P.Buncic,’ S.I. Chase,s I. Derado,’ V. Eckardt,’ J. Eschke,” C. Favuzzi,’ D. Ferenc,’ B. Fleischmann,‘~” M. Fuchqs,‘* M. Gazdzicki,” H.J. Gebauer,’ E. Gladysz, C. Guerra,’ 0.Hansen,i5 J.W. Harris,’ W. Heck,” ~~.Hoffman,’ T. Humaniq5 S.Kabana,” K. Kadija,‘,’ A. Karabarbounis,” R. Keidel,13 J. Kosiec,‘1p’4M. Kowalski,3s A.Kiihmichel,” M.Lahanas,” Y. Lee ” M. LeVine,2 A. Ljubicic,’ S. Margetis,” E. Nappi, G. 0dynieq6 G. Paic,g A. Panagiotou,” A. Petridis,” A. Piper,13 F. Posa,’ H.G. Pugh,‘F. Piihlhofert3 G. Rai,’ W. Rauch,’ R. Renfordt,‘i D. R6hrich,” H. Rothard,” K. Runge,’ A. Sandoval,’ E. Schmidt” N. Schmitz,8 E. Schmoetten,’ I.Schneider,l’ P. Seyboth,’ J. Seyerleiq8 E. Skrzypczak,‘* I’. Spine%,? R. Stock ii H. StrGbete,” L. Teitelbaum, S. Tonse,s G. Vassileiadis,‘* G. Vesztergombi,’ D! Vraniqg and S. Wenig” ‘Fakultit fiir Physik, Universitat Freiburg, D-7800 Freiburg, Germany ‘Brookhaven National Laboratory, Upton, NY 11973, USA 31nstitute of Nuclear Physics, PL-30055 Cracow, Poland 41nstitute of Nuclear Studies, PL-00681 Warszawa, Poland ‘Gesellschaft fiir Schwerionenforschung, D-6100 Darmstadt 11, Germany ‘Lawrence Berkeley Laboratory, University of California, Berkeley CA 94720, USA 7Dipartimento di Fisica, Universita di Bari, and INFN, I-70126 Bari Italy ‘Mu-Planck-Institut fiir Physik und Astrophysik, D-8000 Miinchen, Germany ‘Rudjer Boskovic Institute, 41001 Zagreb, Yugoslavia “Institut fiir Hochenergiephysik, Universit;it Heidelberg, D-6900 Heidelberg, Germany “Fachbereich Physik, Universitit Frankfurt, D-6000 Frankfurt, Germany “Physics Department, University of Athens, GR-157-71 Athens, Greece 13Fachbereich Physik, Universitat Marburg, D-3550 Marburg, Germany 141nstitute of Experimental Physics, University of Warsaw, PL-00681 Warszawa, Poland Abstract; An analysis of the proton rapidity distribution in central 160+Au collisions at 60 GeV/n measured in the NA35 streamer chamber detector at the CERN SPS is presented. The charge excess of positive particles over negative particles was measured. The rapidity distribution of the charge excess which can be associated with the primordial protons in the collision is studied in terms of the nuclear stopping power and is compared to the predictions of various models.

Central 160+-Au collisions at 60 GeV/n were studied using the NA35 streamer chamber detector. The emphasis of this study was to determine the degree of nuclear stopping at an incident energy intermediate between 14.6 GeV/n and 200 GeV/n since these two energies have been studied extensively [I, 2,3] and may be representative of different stopping regimes. Most 0375.9474/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

690~

S.R. Tonse et al. / Proton mpidiy distributions

fixed-target

experiments

at relativistic

due to high track densities. is used to measure the total E$’

emission

is expected

with

all initial

energy

a transverse

state

whereas

at 60 GeV/n

GeV/n,

experiments

A more results

model

than obtained

60 GeV/n

transverse

energy calorimeter

to 1% of the total nucleus

taken

inelastic

interacts

trajectories

collisions

measurement

along the beam

trigger

component

average

2-dimensionsal

of the projected

is labelled

the difference

between

true rapidity

units in the kinematic

increases

of matching

from different

(10,500

(fi

statistics

model

positive

collisions

nucleons

component

for pi,

for the since the

in magnitude

studies

the

by 2/r.

the distribution

of

was found to have a width of a=O.OS

in one view only, compared

than a factor

views is eliminated.

Therefore

of momentum

To compensate

vector is diminished

Measuring

by greater

the 160 Particle

and charge sign

three was used.

From simulation

that

(25~ + 30n).

(7r/2)pY is substituted

and

A high

corresponding

it is estimated

pv of the momentum.

calculation,

tracks

= 10.7 GeV/n).

the longitudinal

and quasi-rapidity

region of interest.

3-view processing,

In this approach

offline to yield the momentum

quasi-rapidity.

of This

since more energy is allowed

O-Au events

of 55 target

of a 3-dimensional

quantity

emission.

to be 1.0, 0.93 and 0.84 at incident

view of a possible

component

At 200 [2].

distribution

energy available.

of 60 GeV/n

quantities:

of pr in the rapidity

stopping[l],

reactions

for isotropic

method

In a geometrical

one camera

resulting

tracks

momentum

on film and measured

projection

particle

respectively.

of 89 central

with a cylinder

complete

for the rapidity

was used to select the most central

and one transverse

missing The

at a beam

In this analysis

consists

width

is estimated

and 200 GeV/n

cross-section.

were recorded

of each particle.

compares

and 0.9 in 160+W[2].

than that obtained

from the previous

consist

on the average

to determine

method

in isotropic

and 0.7 in 160+W

and hence there is less transverse

The data used in this analysis tracks)

in ‘60+Au[4]

[5] a 11ows a broader

of 14.5 GeV/n,

results

at 14.6 GeV/ n reveals

in i60+Au[3],

experimentally,

in 160+Au

7400 negative

which

This

= r/4(& - m;), where m; is the total mass of of stopping in an event is then given by Ey/EF”“.

0.5 stopping

of stopping

I&*).

sum ET,

collisions

ET,=

of mid-rapidity

energy distributions

uses calorimetry

= CEsin

projectile,

0.6 stopping

report

method

(EF’

the data exhibit

longitudinally

the amount

stopped

measure

as observed

in a smaller

to propagate

The simplest

a study of “Si+Au

sophisticated

particles,

energies

The

forward

derived from transverse

per event

a fully

energy

particles.

Using this approach

produced

of particles from

lack good acceptance

information

the degree of stopping.

transverse

to what

energies

Therefore

to conventional

of three since the J-view

The overall efficiency

practice

for the data sample

is 299%. The proton bution

quasi-rapidity

of negative

distribution

is obtained

by subtracting

the quasi-rapidity

distri-

ones, both calculated using the proton mass. This assumes a cancellation of produced X+ with ?r-, K+ with K- and protons with antiprotons, In isospin-symmetric systems equal number8 of leaving the original non-produced protons. produced assumed The

particles

7r+ and T-

from positive

are expected.

to be equal for rapidities data

system). 8 protons calculations

is displayed

The

integrated

initially

for the region contents

forward

those predicted

of the proton

forward

projectile

hydrodynamical

of mid-rapidity

an estimate in a 16-on-55

is displayed

distribution in Fig.1.

(Yc~=1.8

of 10.88f0.68

but smaller

quasi-rapidity

by various models

tion of the Landau

furnish

a+ and n-

production

are

of the center-of-mass.

of mid-rapidity,

for a fully stopped

A comparison

Since 160 is isospin-symmetric forward

than

protons,

the 16.5 protons

in the

16-on-55

larger

than

the

expected

from

collisions

with

collision. measured

in i60+Au

The dashed line represents

model [S] with complete

stopping

for the system

the predic16-on-55.

S.R Tome et al. / Proton rapiditya%tributions

691~

'"OtAu 60 GeV (NA35) ~-'-'--w'l ‘\

‘\

\

.

Fritiof Landau Marco a

l

protcms

IL 2

Figure

1: Proton

3

Rapidity

(Quasi-rapidity

rapidity

distributions

--*: T)

4

for protons)

for the data and several models

It has a maximum at mid-rapidity which is far above the data points. This excess of protons forward of mid-rapidity compared to the data is a result of isotropic emission in the model. Comparisons were also made varying the number of target participants in the Landau model but a reasonable fit could not be made to the data. The rapidity distribution of protons from 60 GeV/n 160+Au collisions in a multiple nucleonnucleon collision model, MARCO[‘I], is displayed in Fig.1. Using a linearly increasing x distribution (LY=~), where x is the fraction of energy plus longitudinal momentum retained per nucleon-nucleon collision the MARCO calculations reproduce the data. However, MARCO requires a flat x distribution (a=l) to fit transverse energy spectra from the same data set [7]. The rapidity spectrum from FRITIOF is also displayed in Fig.1. It has a broad peak centered at Y,,,=3.3 whereas the data continue to rise toward mid-rapidity. It thus appears that there is more stopping of protons in the data, despite the fact that FRITIOF was generated with zero impact parameter, i.e. maximum centrality. Our substitution of the charge-excess distributon for the proton distribution neglects any inequality between the K+ and K- production rates. This can be caused when the contribution of K+‘s from associated production is not cancelled by negative particles in the same rapidity bin. The expected excess of K+ production cannot be precisely estimated as sufficient kaon data have not been measured at 60 GeV/c. In the present analysis it is neglected, since the effect is estimated to be approximately 10% compared to the differences observed between the data and the models in Fig.1. This estimate is derived from the FRITIOF model which predicts only 0.5 K+ forward of center of mass, spread over about 2 units of rapidity. A comparison of the proton and 7r- rapidity distributions for the same data set is displayed in Fig.2. The data plotted forward of mid-rapidit,y show similar shapes for the two. The protons are lower by a factor of 0.35. A similarity of shape can be interpreted as emission from ii common source.

S.R Tonse et al. / Proton rapid9 distributions

692~

lGO I Au 60 GeV (NA35) .__I~ '~_~~'.-.'--,-.~ ""-7 I

.

protolIs TT-

.

I . f

oLILL--L

f

**

Quasi-rapidity

distributions

I

., I

,

,_I1

4

3

2

Figure 2:Rapidity

.

I

I for protons,

of protons

rapidity

5

for 6

and rr- from 160+Au

collisions

at 60 GeV/n

hydrodynamical model[6] indicates In summary, a comparison of our data with the Landau that the 160projectile does not stop completely in the Au target for central collisions at 60 GeV/n. energy

The MARCO distribution

measurements

multiple collision and the proton

of stopping

and that rapidity

derived

distributions

This work was supported

model does not simultaneously

rapidity

distribution.

from transverse

of primordial

These

energy spectra

baryons

in part by the United

[2] J. Schukraft

(1989).

et al, Nuc1.Phys.A

[3] J. W. Harris et al, Nuc1.Phys.A [4] W. Heck et al, Z.Phys.C

498,79c 498,133c

States Dept.

of Energy

(1989).

38,19 (1988).

[5] J. Stachel and P. Braun-Munzinger, [6] Von Gersdorff et al, Phys.Rev.C (71 C.Y. Wong et al, Phys.Rev.D

Nuc1.Phys.A

39,1385 (1989). 39,2606

(1989).

indicate

that

must be considered.

References (1990).

the transverse

alone may not be reliable

DE-AC03-76SF00098.

[l] J. Barrete et al, Phys.Rev.Lett.64,1219

describe

discrepancies

495,577c

(1989).

under contract