Strange quark matter in the cosmic radiation

STRANGE QUARK MATTER IN THE COSMIC RADIATION

Takeshi SAITO

Institute for Cosmic Ray Research, University of Tokyo, Tokyo, iapan

AnomaIOL5 massive nuclei of charge of Z=14 and mass of about 370 amu were observedin high energy cosmic rays . Assuming that the observed nuclei are really SQM, anempirical mass formula for SQM is derived on the basis of a Fermi-gas model . A newballoon program to confirm the existence of SQM in the cosmic radiation is reported .

1 . INTRODUCT10NStrange quark matter (SQM) is the matter

consisting of roughly equal numbers of up,down and strange quarks . It is believed thatsuch matter is the true ground state of QCD andis absolutely stablel .2 . The SQM nuggetsmight have been created at the phase transi-tion from quark gluon plasma to nuclearmatter in the early Universe . However it isdifficult tc detect ttog at present becausethey probably would have evaporated and couldnot survive to the present timed . Possiblesources of the present day SQM would becollision

of neutron stars or the neutronstars with a superdense quark surface andquark stars with thin nucleon envelope4 .

Theresulting lumps of SQM thus produced could bedetected at the Earth . However, there

havebeen no direct search for SQM in the cosmicradiation .

We reported two novel nuclei of Z= 14 and mass of about 370 amu which werefound in high energy cosmic rays In 1981 .

The

is urgently necessary to confirm the existenceof SQM in the cosmic radiation .

Candidates of SQM found in cosmic rays in1981 are described in section 2. In section 3,

0920-5632/91/$03 .50 © 1991 - Elsevier Science Publi,hers B.V .

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Nuclear Physics B (Proc. Suppl.) 24B (1991) 184-190North-Holland

properties of SQM are discussed on the basis ofa Fermi-gas model, and an empirical mass formu-la of SQM is derived . A new balloon experi-ment to confirm the existence of SQM incosmic rays is reported in section 4 .

2 . STRANGE QUARK MATTER IN COSMIC RAYSThe novel nuclei with a high baryon-to-

charge ratio have been reported in Ref . 5 .Fig . 1 shows a schematic view

of the

instrument which was flown from SanrikuBalloon Center in 1981 to study the chemicalabundances and energy spectra of cosmic rays .A

plastic Cherenkov counter (C, refractiveindex, n=1 .5) and a scintillation counter

measured primary charges of cosmic rayswith an accuracy of 0 .4 unit of charge .

Aliquid Cherenkov counter (L, n=1 .270)measured thr particle energies aroundcutoff rigidity of 10 GV . Two pairs ofX-Y

crossed

multi-tube proportional coun-ters (MTV"C) determined particle trajectories

tion of [C1/C2]*[Sl/S2]*[L1/L2/L3/L4], whoserate was 11 cps . Out of 106 events, 1 .27X105 events satisfied the condition of[C1*C2*S1*S2*L1*L2*L3*L4] . Trajectories of the

SQM was our best explanation for the novelnucle15 . On the other hand, Price recalled asperïal event which was found in their reas-

of cosr,lc rays within an accuracy of about 0 .5cm fir a single particle as well as multiplepanicles.

s::ssment of their monopole candidates . It is During the balloon flight for 28 hours atalso possible to interpret the event as a an atmospheric depth of 9 g/cm2 , about 106nucleus with a high baryon-to-charge ratio . It events were collected under the trigger condi-

T. Saito/Strange quark matter in the cosmic radiation

FIGURE 1Schematic view of the instrument used in 1981

particles were measured with the MTPC forevents within 4 standard deviations(4 a ) froma response line of cosmic rays in the samecounter .

The counter noises

and particleshitting the photomultiplier were eliminated

totally by the tracking procedure .

Finally,3 .7X104 nuclei with Z ? 5

were accepted

for

studies of chemical abundances and spectra of

cosmic rays . Fig . 2 shows the scatter plot of

C and S, where

C and S

are mean pulseheights of Cherenkov counter, C=[C1+C2]/2and

of scintillator,

S=[SI+S2]/2 .

These

pulse heights were normalized to that

of

a relativistic singly charged particle . Thebroken

curves show the 10 a lines from a

response line of cosmic rays . The 269 events

outside the 10 a lines in Fig . 2

were

abandoned "a priori" as background events in

the cosmic ray studies, because the 269

events would never disturb our results derived

from 3 .7X104 events .

Origin of the 269 outside events were

studied event by event In 1988, because if the

nuclei with a high baryon-charge ratio exist

in the cosmic radiation, they might be found

C(A . Z)iC (,e-1 .Z=i>

185

FIGURE 2Scatter plot of Cherenkov outputs C vs scin-tillator outputs S . The broken curves snow the10a line from a response line of cosmic rays .The fragmentation events are shown by thesmall solid circles and the clipping particlesby the open circles . The solid circles showsthe anomalous events .

In the outside region . It was found that, out

of 269 events, 245 events in the C > S were

originated by projectile fragments which wereproduced

In the Cherenkov radiator or the

wall of vessel . On the other hand, out of 24events in the C < S, 22

events were

found

to

be produced by the clipping particles,

which are shown by the open circles in

Fig . 2 .

Two events shown by the solid cir-

cles were passing through the center of the

detectors .

The dependencies of S and C on p (=v/c)

are shown by the solid curves and the corre-

sponding energies are shown by the dotted lines

in Fig .

2,

where S = aZ2/,8 2

and

C

=

bZ2(1

- 1/n2 16 2 ) .

The charges and energies of

the

two events are obtained as Z=14, and E=440

and 460 MeV/nucleon, respectively .

After

reporting the anomalous nucle15 , we got many

proposals for sources making the anomalous

signals, which were not mentioned in Ref-5 .

TABLE 1 : POSSIBLE: BACKGROUND SOURCES------------------------------------------------

Backgrounds Probabilities------------------------------------------------

Table 1 shows the background sources and their

probabilities in event numbers . The effects

due to (1) secondary particle productions Inthe scintillator, (2) albedo particles, (3)

partially ionized nuclei, (4) low energy nucleigetting over the geomagnetic rigidity of 10 GV,

studied In Ref . 5 . Background sources of(6) and (7) were proposed by readers of

5 .

One tends to consider that the tail

thethe

the

isthe

were(5),

Ref .

of scintillation distribution, (5), makessignals of two events .

Fig.3

showsobserved

pulse

height distribution o .'scintillator for oxygen nuclei, which

fitted by a Landau distribution shown bysolid curves ;

F(x)=0 .295exp{-0.5(x0-85+e-x)} for x>0

F(x)=0 .302exp{-0.5(x+e-x)}

for x<0

In

this expression, the two events appeararound x=50 . Then, the probability of theanomalous signals produced by the scintillatoris estimated as < 10-10 .

Specialists in passive detector techniquestend to consider that the anomalous signalswere originated by coincidence of a pair of Arand 0 nuclei which are produced from a colli-sion of Fe nucleus with the atmospheric nucleiabove the instrument . However, probabilityof such case was estimated to be smaller than

T. SaitolStrange quark matter in the cosmic radiation

NcVch

100

io

FIGURE 3Pulse height distribution of scintillator forOxygen nuclei

10 -8 when considering the fragmentation proba-bility of Fe - Ar+0, 6X10`4 , probability ofcoincidence of two fragments, 10-3 in whichthe Ar fragment must fire the

scintillatorand the MTPC-2 "without" firing the Cherenkovcounters and MTPC-1, and at the same time the0 fragment must fire the

Cherenkovcounters and M'CPC-1 "without" firing thescintillator and MTPC-2, and the detectionefficiency for Ar fragment to the total col-lecting power, 10-2 .

Coincidence of a clipping nucleus and adelta-ray was also proposed for source of theanomalous signals . A c1IDping nucleus madesignals of scintillator and the MTPC-2,and at the same time a delta-ray produced fromthe same nuclei fires the MTPC-1 .

However,the probability of such a coincidence Isnegligible small, <10 in the presentexperiment .

All the background sources proposed so farhave been totally excluded .

It has been

(1) Secondary Particles 5109

(2) Albedo Particles 5106

(3) Partially Ionized Nuclei < 109

(4) Low Energy Nuclei 5107

(5) Tail of Scintil . Dist . < 16-10

(6) Fragments, Fe - Ar+O 5108

(7) Clip . Par . and S Rays < 109

concluded that the two events are nuclei of

Z=?4 and E=450 MeV/nucleon . The mass of nuclei

is given by A=RZ/(E2+2mE) 1/2 ,

where

R is

rigidity (momentum per charge), E is

energy

per nucleon, and m is the nucleon mass .

The

minimum value of A is given as AMin=137

by

substituting the above formula the measured

values of Z=14 and E=0 .45 GeV/nucleon,

and

the minimum rigidity in the experiment, 10 GV .

When we assume that rigidity

spectrum of

anomalous nuclei have the same exponent

as that of cosmic ray nuclei,

we cRn use the

observed mean rigidity of cosmic rays, 27.2 GV

for R . Then the mean mass of the nuclei

is

given as A=370 . We have introduced SQM in

order to understand the high baryon to charge

ratio for the nuclei . Flux of the SQM candi-

dates is shown in Fig . 4 .

i

i

N

8uH

N

âNZ


R1Sid1ty/'rotsi EnerByv/ev

FIGURE 4Flux of SQM candidates . The solid circle showsthe value at 10 GV and the open circle that atthe total energy . A slant mark shows theregion expected from a new experiment, whereenergy spectrum for SQM is assumed to be thesame exponent as that of cosmic rays, 1 .7 .

18 7

3 . PROPERTIES OF SQMAssuming that the nuclei of A=370 and Z=14

are really SQM, we derived the relationships

between parameter :, describing SAM and the nrissformula of SQM . In order to describe SQM, weused a simple Fermi gas model in which u, d

and s quarks are constrained in volute V by a

bag pressure B2 . The state of SQM is de-

scribed by the thermodynamic potentials

S2 i (i=u,d,s,e) being functions of chemicalpotentials

u i, the strange quark mass

mS ,

B

and V. The chemical equilibrium maintained by

weak interactions establishes u d= u S (= u ) . The

mass formula for SQM is essentially the same

to the Bethe-Weizsacker model for aormal nuclei

consisting of a volume term, a surface term,

and the Coulomb term ;

E=1 W ( gi+ Q i)+B1V + 4 n

a R2 +

(3/5) a Z 2/R,

where V=4 a R3/3 and a is surface tension .

This formula contains five free parameters,

u , Au , mS , B and V .

By

minimizing energies

with respect to the volume V and charge Z, we

can deduce the number of parameters from five

to

three .

We chose

e 0, ms and

A

for

the

three parameters, and all other parameters are

expressed by these three parameters . The

state of SQM is thus determined by these three

parameters and by the additional QCD coupling,

a C .

The relationships between three

parame

ters

a o , mS

aC are obtained by

normalizing

to the observed values of A=370 and Z=147 .

Fig . 5 she-,s the relation of mS and e0 for

different

values of

aC = 0 .0, 0.3,

0 .6

and

1 .0 . These three parameters are strongly

correlated each other as seen in Fig . 5 . Then

the strange quark mass will become to about 140

MeV if one accepts the

aC value of around 0 .1

which is derived from accelerator experiment .

Fig . 6 shows the derived empirical mass formula

for SQM .

It is nearly constrained for the

different values of

aC as well as

to,

188

280

250

â 220

190

160

130 E

100 ~. . . . . . . . .

1

.

.

.

.

i

.

.

.

.i

~

.

._.

î880 890 900 910 920 930 940

E,, �/A

(Me V)

FIGURÉRelation of ms on EMin for a =0, 0 .3 . 0 .6 .1 .0

FIGURE 6Empirical mass formula for SQM .

T. Salto/Strange quark matter in the cosmic radiation

ac=1 . 0

-

0. 6 i

0.0

once one point of P, and Z is giver . The A-Zrelation of normal nuclei is indicated by thedashed curve for comparison in Fig .6 .Recently, Price recalled to us a special eventwhich was found in their reassessoient of theirmonopole candidate7 .

One possible interpre-

tation for the event is as a massive particleof

Z=46 and A > 1000 amu which is shown by

the

open circle .

It is remarkable that it isconsistent with the derived mass formula . When

SQMs will be detected in future experiment,they might distribute around this curve .

4 . SEARCH FOR SQM

A new balloon experiment8has 10 timeshigher sensitivity than the previous experiment .

Strong points of the new instrumentunder construction are ; (1) Massive He nuclei

in the detection range (Z > 10

in

experiment) . (2) Mass of candi-h is determined directly byE and R, ( the magnetic rigidity

is applied for R in the previous(3) Particle identification is

done with passive detectors .Fig . 7 shows a schematic view of SQM tele-

scope, which is a hybrid system combininga counter system with photo-sensitive passivedetectors . The charges of incident particlesare measured with the scintillation counters,S1 and S2 with an accuracy within 0 .3 unitof charge . The velocities (energies pernucleon) are measured with the Cherenkovcounters, C1 and C2 . The changes of ioniza-tion energy loss, dE/dX are measured withthe scintillation counters of S3, S4, SS andS6 .

The MTPCs ( mulct-tubbed proportionalcounters) measure the tracks of singleparticles as well as multiple particles withinan accuracy of about 0 .5 cm . The passivedetector is a stack of sandwiches consistingof a CR-39 plate, a nuclear emulsionplate .

The cascade detector is a stack ofsandwiches of a

lead plate of 5 mm thickness,a nuclear emulsion plate and two X-ray films .

The magnetic rigidity of the Earth is usedas a rigidity filter to select several tensof candidate events among extensive numbers of

are included

the previousdate events,

measuring Z,

of the Earthexperiment) .

FIGURE 7Schematic view of a new instrument under con-struction for balloon experiment from Sicily

normal cosmic ray events . About 108 normalcosmic rays, including protons are expectedfrom the 100 m2*str exposure at Sicily . Asthe cutoff rigidity at Sicily is 8 GV, all thenormal cosmic rays have relativistic velocities ( p = 0 .97359, E= 3 .17 GeV/nucleon) .

Therelationships between Cherenkov outputs (ameasure of E) and scintillation outputs (ameasure of dE/dX) are the same as those inFig.2 . In the same way in the previous experi-ment, events outside 10 a from a response lineof normal cosmic rays are selected as thecandidate events .

The nlr¢ber of candidateevents

selected by this procedure Is

ex-

pected as a few to teat events per one balloonflight and 10 to 50 events from the total

exposure, 100 m2*sr*h which is planned in the

first phase of the balloon program .The magnetic rigidities R of the candi-

date events are determined by measuringthe multiple scattering angles in the emulsion plates .

An accuracy In the rigiditymeasurement is about 21 % when fifteen emul-


sion plates are applied .

Fig. 8 shows

therelationship between rigidities and the Cherenkov outputs for Helium, Carbon

and Siliconnuclei .

The photoelectron number in Fig . 8Includes the collection efficiency of thepresent experiment .

As shown b the crossesin Fig . 8, the masses of the anomalous

nucleiare determined within accuracy of 20X.

Forthe

nuclei with energies below 320

MeV/..̂,changes of dE/dX are measured with thescintillators of S1 to S6 .

Masses

of

theanomalous nuclei are determined within accura-cy of 5 % for nuclei stopping in thedetector, and 20 % for nuclei leaving

fromthe detector .

C( is . Z)/CZ-1)

NP.e.1000 .

Rigidity

189

FIGURE 8RelaiionEhips between Cherenkov outputs andrigidities for He . C and Si nuclei

It is important to know whether the massive

nuclei are really SQM or other massive parti-

cles, when the massive nuclei are detected .

For particles interacting inside the detector,

secondary particles are tracked and their

ion

emission

angles and energies are measured .

If the massive nuclei are SQM, many

strange

fragments

from the primary SQM might

be

detected as V-particles in the nuclear emul-

0ca .

If

they are Centauro

type

events9 ,

which might be caused by the explosion of a

glob of highly dense matter", no n O

produc-

tion will be observed in the cascade detec-

tor .

In the case of other charged massive

particles like a technibaryon-nucleus atomll,

a pair consisting of a charged massive

particle and a normal nucleus will be detected

after their atomic collisions . The collision

cross section for such a process would be

larger than several mb .

Schematic

picturesof possible particles in this category

are

shown in Fig . 9 .

FIGURE 9Schematic pictures of massive particles ;(1) SQM, (2) Centauro type event, (3) Techni-baryon-nucleus atoms .

The first balloon experiment, launchingfrom Milo station, Sicily and recovering atSpain, is scheduled for summer of 1993 asJapan-Italy cooperative experiment8 . Relativeabundances of anomalous events to cosmic rayswas given as about 2 .6 X 10-7 at the samerigidity and 2 .1X10-5 at theenergy from the previous experiment .anomalous events are expected from

same total

Severalthe 100

m'*hour exposure at Sicily . Thespectrum of anomalous nuclei is shown by the


expected

slant marks in Fig.4 . in the new experiment,

masses of the anomalous events will be meas-

ured directly and pf.rtlcle Identification will

be performed . Our proposed experiment has

the potential to finally confirm the existence

of SQM in the cosmic radiation . Such a con-

firmation would certainly open a new field in

nuclear physics and astrophysics .

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2 . E.Farhi and R .L.Jaffe, Phys . Rev . D30(1984) 2379 . M.S .Berger and R .L.Jaffe,Phys . Rev . C35 (1987) 213, K.Takahashi andR .N .Boyd, Ap .J . 327 (1988) 1009 .

3 . C .AIcock and E .Farhi, Phys . Rev . D32(1985) 1273 ; J .Madsen et al ., Phys . Rev .D34 (1986) 2947 ; C .Alcock and A .Olinto,Phys . Rev . D39 (1989) ?1233 .

4 . P .Haensel et al ., Astron . Astrophys . 160,(1986) 121 ; C .Alcock et al . Ap . J .310(1986) 261 .

5 . T.Salto, Y .Hatano, Y.Fukada and H.Oda,Phys . Rev . Lett . 65 (1990) 2094 .

6 . P.B.Price, E .K.Shirk, W .Z .Osborne andL.S.Pinsky, Phys . Rev . D18 (1978) 1382 .

7 . M.Kasuya, T .Saito and M .Yasue, INS-Report-876 (INS, Univ . of Tokyo) May 1991 .

8 . Japan-Italy Collaboration, M.Aglietta etal ., Nuovo Cimento, to be published .

9 . C .M .G .Lattes, Y.Fujimoto and S .Hasegawa,Phys . Rep . 65 (1980) 151 .

10 . J .D .Bjorken and L .D . McLerran, Phys . Rev .D20 (1979) 2353 .

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Strange quark matter in the cosmic radiation