High energy cosmic ray events of ultra-relativistic nucleus-nucleus collisions

Nuclear physics A447 (1985) 189~196~ North-Holland. Amsterdam

189~

HIGH ENERGY COSMIC RAY EVENTS OF ULTRA-RELATIVISTIC

NUCLEUS-NUCLEUS

COLLISIONS

The JACEE Collaboration+ h b T.H. BURNETT , S. DAKE , M. FUKIC, J.C. GREGORYg, T. HAYASHI', R. HOLYNSKI=, h J. IWAI , W.V. JONESa, A. JURAKi, J.J. LORDh , 0. MIYAMURAd, H. ODAb, h T. OGATAa, T.A. PARNELLf, T. SAITOa, S. STRAUSZ , M. SZARSKAi, T. TABUKIa, f d Y. TAKAHASHI , T. TOMINAGA , J.W. WATTS', J.P. WEFELe, B. WILCZYNSKA=, h R.J. WILKES , W. WOLTER' and B. WOSIEK' a. b. c. d. e. f. g. h. 1.

Institute for Cosmic Ray Research, University of Tokyo Department of Physics, Kobe University Okayama Science University Department of Applied Mathematics, Osaka University Department of Physics and Astronomy, Louisiana State University Space Science Laboratory, NASA Marshall Space Flight Center Department of Chemistry, University of Alabama in Huntsville Visual Techniques Laboratory, University of Washington Institute for Nuclear Physics, Krakow

Cosmic ray ultra-relativistic nucleus-nucleus collision events in the TeV/A region are analyzed. The average transverse momentum of non-peripheral events is examined and the breakdown of the limiting behaviour of this quantity is observed in events with high energy density. Close pairing or clustering of tracks is observed in many events. Possible explanations of the phenomena are discussed. 1. INTRODUCTION It is commonly believed

that ultra-relativistic

nucleus-nucleus

collisions

are possible

test grounds of the formation of new phases of hadronic matter 11 under extreme conditions . However, in contrast to the idealized situation of a thermodynamical In the present development

system, actual collision processes

stage we are far from a reliable description

of the formation and

of the expected new phases, Quark Gluon Plasma and the Chiral Sym-

metric Phase, in nucleus-nucleus oration has been observing

collisions.

In this respect, the JACEE collab-

cosmic ray nucleus-nucleus

1979 and is looking for the clues of the problem Recently

would be highly violent.

several prominent

collision

events since

from the experimental 2)

side.

features have been reported.

*This work is supported by JSPS, ICR of Tokyo University and the Kashima Foundation in Japan and DOE, NASA and NSF in the USA. It is also supported by the Yamada Science Foundation in this conference. +Address for correspondence on this paper is 0. Miyamura, Department of Applied Mathematics, Faculty of Engineering Science, Osaka University, Toyonaka 560, Japan

0375-9474/86/$03.50 OElsevierScience Publishers (North-Holland Physics Publishing Division)

B.V.

TH. Burnett et al, f High energy cosmic ray events

19oc

extremely

For example,

high multiplicity

shown that their multiplicity model calculations

is comparable

hadron-hadron

collisions

momentum

momentum

for the presence

collisions

in the ultra-relativistic

relevance

on

momentum

expected

new

further effort and re-examination

observations, nucleus-nucleus

collision

in a re-examined

short range structure

2. HIGH MULTIPLICITY

events.

analysis

in the pseudo-rapidity-azimuthal

EVENTS AND AVERAGE TRANSVERSE

events is increasing gradually.

with previous

sample and additional measured

In

events

MOMENTUM

JC-0,1,2,4

are

The

events

Ca(1.5

with a calculated

value of the Multi-Chain

In the table two

different

values

shower)

average

for photons

an

head-on

Fe that

comparison collision.

momentum

of the

energy

in the central

cone) is obtained

in the very 2orward region by comparison

for the shower development.

are of

region from the with

Detail of this method and

a an

(Castagnoli method) have already been given in

fact is that values of the average events tend to be larger than the

GeV/c, of that in hadron-hadron

a

The 1050

noted

in

transverse

from a measurement

-(*)(shower events. pT

of the incident energy

these high multiplicity

of

1.

producing

reported as

multiplicity

Model assuming

the

is applicable

except for very high multiplicity showers

of

is obtained

a measurement

ref. 3. One remarkable

In

event

number

in Table

TeV/A)

large

estimation

the

stage.

two light ion induced events show relatively

analysis of overlapping

number of

presented.

total

(Nch > 150) is presented

in a lead target which has been previously

Monte Carlo simulation

very

re-examined

than 400'). It is also

each photon. &ch

The

results

event. Seven events have larger multiplicity

-(')(single pT

of

IN THE TeV/A REGION

report,

analysis.

table includes 23 events and the highest one is

presented.

induced

transverse

EVENTS

A list of high multiplicity

charged particles

they these

ray

performed.

in the TeV/A region is about 110 at the present

2.1 HIGH MULTIPLICITY

and

features

reports, new results are based on a

new events from the X-4

of

aspects

angle plane is reported.

this

in events from four balloon experiments,

comparison

new

Following

Furthermore,

An

density

of

is made for cosmic

is presented.

in

found.

In this report, results on average

analysis

of

Events

of the limiting

energy region phases.

Since 1979, six balloon flights have been successfully of analyzed

been

that breakdown

in nucleus-nucleus

the

maximum

starts with the energy

These features are suggestive

some

been

has

than the limiting value, 0.4 GeV/c

might

have

+1030ch

it

collisions.

in the FNAL and ISR region, have also

of the average transverse

several GeV/fm3.

and

of nucleon-nucleon

in terms of energy density has suggested

behaviour

TeV/A)+Em

rare events

with or larger than the

based on super-position

having larger average transverse

analysis

events such as Si(4

They are not exceptionally

have been observed').

collisions

transverse momentum limiting

in the TeV region.

value,

of 0.4

T.H. Burnett et al. / High energy cosmic ray events

Table 1. High multiplicity Projectile

Nch

Target

191c

events

E(TeV/A)

zE y

(MCM)*

28" (GeV) 2$" (singrx (shokshower) cone)

Ca

Pb

1050

1.5

Si

Em

1030

4.1

Si

Pb

780

Ca

CHO

760

100

Ca

Pb

680

0.5

Ca

Pb

457

1.8

2.1

Ar

Pb

416

1.0

1.2

Fe

Pb

400#

5#

Fe

CHO

3C0#

4#

Mg

Em

300#

50#

C

Em

287

C

Pb

280

11

C

CHO

240

28

0.55

4# 0.55

Cl

Em

216

CHO

217(154)*

3.2

5.8

0.65 0.76

0

Fe

200#

lO#

CHO

200#

20#

C

Fe

200#

5#

He

Pb

200#

20#

He

Pb

170#

34#

C

Pb

151

5.1

He

Pb

150#

50#

He

CHO

156 (122)*

32

by the Multi-Chain

Preliminary

2.2 AVERAGE TRANSVERSE

0.43

28

Mg

# Under analysis.

0.6#

l#

Li

*(MCM) is a calculation

0.55

0.95

0.52 0.62

Model assuming b = 0.

result.

MOMENTUM

AND ENERGY DENSITY OF EVENTS

IN THE TeV/A

REGION The energy density at a collision

is estimated

time

following

scale

E = (3/Z)/ p; + nl: (dNch/d'iln where

V

is

thermodynamical

chosen notion

to

be

of

Bjorken*)

(2

soft

QCD

interactions

after

the

as:

=0 / V

f:',lrA2/3.

Although

the

in the early stage of the collision

applicability

of

is not obvious,

the

quantity

seems to be useful as a working hypothesis.

additional

events

rapidity density transverse plot All

this

formula

In

For the re-examined the estimation,

we have used $,i) as much as possible,

events of which the average

ambiguity

applied.

and new

the pseudo-

is averaged over the central region ~.m=0 to 2. For the average

momentum,

the average transverse

in

is

a.re plotted.

In Fig.1, a scatter

momentum and energy density plane is presented. transverse

momentum

is measured

Black dots represent nucleus-nucleus

without

serious

events. The black

square

is an old event called the Texas Lone Star. White squares are proton CNO

events

in the JACEE experiment

density = 1. 5

of 5-p collisions

for comparison.

The dashed line shows the energy

at /s = 540 GeV by the use of the formula for E with A

. l JACEE “JACEE * pp&=540

AtA p*C(lO-100TeV)

.

GeV

.

.

l

0

_L._.~l_-_I_ energy density Fig.

Although

-

0.1

1

the new plot shows a somewhat dispersive

feature in comparison

previous one,') the growth of the average transverse

momentum

contribution explain

from several conservative

and a multiple

scattering

the drastic growth observed.

such

data is suggestive transverse

momentum

increase

a

mini-jet

it seems difficult

From a non-conservative

of the breaking of the

an

points of view such as

effect.') However,

limiting

point of

behaviour

in high energy density collisions. 6)

with the

is evident for the

events with high energy density above GeV/fm'. The origin of has been discussed

-10

Z (GeVifn&

of

the

view

to the

average

TH. Burnett et al. /High energy cosmic ray events

3. VERY SHORT RANGE STRUCTURE

IN THE TRACK DISTRIBUTION

193c

IN NUCLEUS-NUCLEUS

EVENTS very close pairing or clustering an

of tracks is frequently observed.7)

In Fig.2

event, He + Em + 117ch (preliminary energy estimation gives lTeV/A) is

in the pseudo-rapidity short distance, distribution

and azimuthal

6t~< 0.2, 60 < 20'

angle plane. Pairing of clustering

1s evident in this example. In

of pairs of tracks is given as AN~~~~/A(~~@)

N ch < 300. The excess of

very

short

range

pairs

inclusive plot. It is noted that the background of photons by matter

is one order of magnitude

for

appears

20 even

shown

at

Fig.

very

3

the

events in

with

such

an

from Dalitz pairs and conversion smaller compared to the

observed

very short range pairing. Most likely the explanation particle

for

the

observed

effect. A Monte Carlo simulation

phenomena

incorporating

effect shows that similar close pairing appears

slightly different the identical when

we

contribution geometrical

a

of in

the Fig.

Monte 2

form ANpair /AC where 5 = cosh(dQ)-cos(b@).

particle

use

event

effect is noticable

Gaussian

is negligible. information

parametrization, Although

identical particle

car10

are In

eXp(R2(pl-p2)'/2. is

already

events

shown this

for the source radius less

the effect

might be useful

the

identical

if the radius of the pion source

is not too large. In Fig. 4a the pair distribution (averaged over 20 events) and that of the

is

the

in

than

Otherwise, well

known,

in the analysis of energy density.

A+A (E > lTeV/A) _ 20 events(Nch < 300) 1996 tracks

Fig.2

Track distribution He+&

-t 117ch.

of went

B22

Fig.3

a

example,

Pair distribution of twenty nucleus events

5fm the such

194c

T.H. Burnett et al. /High energy cosmic ray events

It

would

possibilities

worthwhile

be

to

for the phenomena.

with very small Q-values.

mention

two other possible

but extra-ordinary

One is close pairing due to decay of resonances

Such a situation

is realized

if

the

resonance

mass

shifts due to a medium effect.') 20 Monte Carlo event

30

k

20 Monte Carlo event (@/lT=O.2)

30

‘IPE (R=l. 4i‘m) -

0

.1

5

Fig.bb Pairing by resonance mass shift

GeV

.l

.

5

including decays of @-like resonances

if the mass of the resonance

two kaons such as ZMK+ZMeV. The possibility small invariant mass is not excluded, massive photon production.

=1.0:

0

-2

In Fig. 4b Monte Carlo results Pairing appears

‘or

Fig.4a Pairing by identical particle effect.

:event B22

is very close to the of

although

electron

pair

are

shown.

threshold

it requires large enhancement

In fact, in a few events, a portion of pairing

of with

production

of

tracks

made typical cascade showers downstreams. These options of the explanation

should be selected

in future investigations.

4. Summary Nucleus-nucleus

events

in the TeV/A range

are

analyzed.

limiting behaviour of the average transverse momentum density the

is suggested. Very close pairing

most

likely

extraordinary present. 9)

interpretation

explanations

is

is observed

the

identical

such as resonance mass

Breakdown

the

of

in events with high energy in

many

particle

shift

are

Although

events. effect, not

other

excluded

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(1985)

JACEE collaboration, (1985) p. 187.

the proceedings

of the Quark Matter 84, Springer

JACEE collaboration,

T.H. Burnett et al., Phys. Rev. Lett. 50 (1983) 2062

at

TH.

4) J. D. Bjorken, 5) G. Arnison

Burnett

et al. /High

energy cosmic

ray events

Phys. Rev. 27 (1983) 140.

et al., Phys. Lett. 1188 (1982) 167.

6) L. Van Hove, Phys. Lett. 118B (1982) 138. 7) P. S. Freier and C.J. Waddington, 8) R. Pisarski 9) Y. Takahashi

and F. Wilczek,

AIP 49 (1979) 87.

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1952