Revista Mexicana de Física 40, Suplemento 1 (1994) 305-317

Strange matter - a new domain of nuclear physicsC. GREINER

Department of Physics, Duke UniversityDurham NC 27708-0305, U.s.A.

A. DIENER, J. SCHAFFNER AND H. STOCKERlnstitut für Theoretische Physik, J. W. Goethe- Universitiit

D-60054 Fmnkfurt, GermanyReceived 4 January 1994; accepled 13 April 1994

ABSTRACT.Relalivislic heavy ion collisions offer the possihilily lo produce exolic melaslable oreven absolutely slable slates of nuclear maller conlaining (roughly) equal number of strangenesscompared lo lhe contenl in baryon number: Slrangelels, small pieces of strange quark mal ter,were proposed as a signal of QGP formal ion. As their hadronic counlerpart, also small piecesof slrange hadronic matler may show up wilh ralher very similar properties. The resemblingmicroscopic slructure gives raise to lhe speculalion lhal bolh slales mighl have a slrong over-lap and correlation. The reasoning of both their stabilily and existence, lhe possible separa-tion of strangeness necessary for their formation and the chances for their detection are re-viewed.RESUMEN.Las colisiones relalivislas de iones pesados permilen la posibilidad de producir estadosde materia nuclear exótica metaestable o incluso absolutamente estables, conteniendo (aproxi.madamente) un número igual de extrañeza en comparación al contenido en número bariónico:Los strangelets, pequeñas porciones de materia de quarks extraña, fueron propuestos como unaseñal de la formación del plasma de quarks de gluones. Al igual que su homólogo hadrónico.pequeñas porciones de materia hadrónica extraña pueden tener propriedades muy similares. Lasemejanza en la estructura microscópica da origen a la especulación de que ambos estados podriantener una fuerte correlación y translape. Se revisan las razones de su estabilidad y existencialy la posible separación de extrañeza necesaria para su formación y las posibilidades de su de-tección.

PACS: 12.38.Mh; 21.80.+a; 25.75.+r

1. INTRODUCTION

Perhaps the only unambiguous way to detect the transient existence of a temporarilycreated quark gluon plasma (QGP) might be the experimental observation of exotic rem-nants, like the formation of strange quark matter (SQM) droplets [1]. First studies inthe context of the MIT-bag model predicted that sufliciently heavy strangelets might beabsolutely stable [21 or smaller ones at least metastable [1]. The reason for the possiblestability of SQM lies in introducing a third flavonr degree of freedom, the strangeness,where the mass of the strange quarks is considerably smaller than the Fermi energy of the

305

306 C. GREINERET AL.

T l"'rV J

150

IDO

/'/

//

/normal~,,::f'

.. ' .' .. . .::/<;....

.' . ,\', ,', '"... ' ...•:~'Crk:G:Jl)"1

PlaSmo

•

(~'

FIGURE 1. Phase diagram of (hot) nuclear matter including the strangeness degree of freedorn-~1E.MOsano possibly also strangelets establish as stahle lIlultistrallge configllrations.

quarks, thus lowering the total mass per unit baryon number of the system. Aeeordingto this pieture, SQM should appear as a nearly neutral and massive state beeause thenumber of strange quarks is nearly equal to the number of massless up or down quarksand so the strange quarks neutralize that hypothetieal fonn of nuclear matter.

Still, on the other side, strangeness remains also an experimentally as theoretieallylargely unexplored degree of freedom in strongly interaeting baryonic matter [3], Thislaek of investigation refleets the experimental task in produeing nuclei eontaining (weaklydeeaying) strange baryons, whieh is eonventionally limited by replaeing one neutron (orat maximum two) by a strange A-particle in seattering experiments with pions or kaons.However, central relativistie heavy ion eollisions provide also a souree for the formation ofmulti-hypernuclear (Sil!,!) objeets, eonsisting of nueleons, A's, E's and :'::'s. To be morespeeifie, the RQMD model 14] prediets on the average the oeeurenee of 20 A's, 10 E'sand 1 :'::'s per event for Au(11.7 AGeV)Au and of 60 1\'s, 40 E's and 5 :'::'s per event forPb(l60 AGeV)Pb. (The number of :'::'s are solely obtained by string fragmentation -re-seattering effeets are not taken into aeeount.) By employing a relativistie meson-baryonfield theory (RMF), whieh gives a rather exeellent deseription of normal nuelear and singleA-hypernuelear properties [5]' it was found that sueh eonfigurations may exist as smallmetastable objeets [6). From a more general point of view, based on these theoretieal oh-servations one is now tempted to ask about their principIe existen ce also as mueh largerobjeets.

STHANGE MATTER - A NEW DO~IAIN 01' NUCLEAR PIIYSICS 307

111 Fig. 1 we depict qualitatively what we want to emphasize and report in the follow-ing: Customarily the equation of state of hot and dense hadronic malter is characterizedby means of a phase diagramm (PB - T), where at some critical temperature and/ornonstrange baryon density eventually a phase trausition to a deconfined QGP-state doesoccur. However, the EOS to be passed through during a heavy ion collision incorporatesalso a new degree of freedom, the net strangeness (counting a surplus of strange OYerantis-trange quarks). Like the occurence ofbound nonstrange nuclear matter, multihypernuclearmatter, or small droplets (I\IEi\IOs) of this new state, may be revealed. In addition, alsothe phase transition to the deconfined state is affected by the possible conglomeration ofthe strangeness degree of freedom. In particular, if the strangelet does exist in principIe,it has to be regarded as a cold, stable and bound manifestation of that phase being aremnant or 'ash' of the originally hot QGP-state.

In chapter 2 we snmmarize the reasons for the existence 01 both this novel and exoticstates, particularly we describe our findings for stable multihypernuclear configurations inmore detai1. In chapter 3 the mechanism of strangeness sepa ratio n out of a QGP and thepossiblble distillation of a strangelet are reviewed. Also we give some arguments that asomewhat similar scenario might hold solely in the hadronic sector, assembling and local.izing more hyperons in phase space. In chapter 4 we more critically discuss the detectionpossibilities of these states by their properties and lifetimes, also in respect to the presentexperimental undertaking at I3rookhaven and at CERN.

2. STRANGE MATTER

2.1. Stmnge quark matter

Let us briefiy summarize how a stable or metastable strangelet might look like: Think ofbulk objects, containing a large number of quarks (u ...u, d...d, s ... s), so-called multiquarkdroplets. Multiquark states consisting only of u- and d-qnarks must have a mass largerthan ordinary nuclei, otherwise normal nuclei would be unstable. However, the situation isdifferent for droplets of SQM, which would contain approximately the same amount of U-,d- and s-quarks. 1f the mass of a strangelet is smaller than the mass of the correspondingordinary nucleus with the same baryon nnmber, the strangelet would be absolutely stableand thus be the true groundstate of nuclear matter [2]. Presently such a scenario cannotbe ruled out.

On the other hand, it is also conceivable that the mass per baryon of a strange dropletis lower than the mass of the strange A-baryon, but larger than the nucleon mass. Thedroplet is then in a metastab/e state, it cannot decay spontanously into A's [1,7]. Forphenomenological bag parameters 81/4 lower than 190 MeV strange quark droplets canonly decay via weak interactions. For larger B-values strangelets are instable. Due tothe strong finite size effects [11 and the wider range of the employed model parameters,smaller strangelets are much more likely to be mctastable than being absolutely stable.(\Vitten's idea [2] of absolute stable SQM droplets does work in this approach only forbag parameters around 145 MeV.)

308 C. GREINERET AL.

, "

251/2 251/2 lPl/2 lpl/2 lPl/2

O lPl/2 lPI/2 lP3/2- -

lP3/2 lP3/2~~

15112 151/2 151/22: -10 lP3/2 lp3/2 -~

>- A -=-0Oll...Q)e -20 - -W 151/2 151/2

-30 - P, n, , , ,

FIGURE2. Single partide energy 01 a ME~IO consisting 01 two 01 each baryon 01 the baryon octetexcept the E1s. The binding encrgy differenrc cancels the mass differcnce oí the strong rcactionchannels so that the wholc 'nucleusl is metas taLle.

2.2. Strange had1'Onic matter -MEMOs

The existence 01 metastable exotic multihypernuclear objects (t\IEMOs) has been pre-dicted just recently [61. A classificatioll scheme lor metastable combinations of nucleonsand hyperons exhibits that combinations of nllcleons, A's and 3's are favoured comparedto combinations with ¿;'s due to their Q-vallles in vacuum. The substantial energy re-lease Q "" 75-80 MeV in reactions like EN - AN or ¿;3 - A3 can not be overcomeby binding elfects. The situation is less clear for the reaction 3N - !lA with the smallerenergy release of Q = 23, 28 MeV. Here the reaction can be prohibited by Pauli-blockingelfects in bound strange hadron matter (SHM). An example is shown in Fig. 2, where thesingle particle levels of a strange nucleus consisting of two of each proton, neutron, A,30, 3- are plotted. Note that each baryon sits in the ls-state. The reaction 3N - AAcan not ir.duce a strong decay becanse the two A's sitting in the ls-level cause the pro-duced A's to escape in vacuum. But this is energetically unfavoured resulting in an overallmetastable compound system. The calculation was carried out in a rclativistic mean ficldmodel taking ca re of the nncleon-nucleon and nucleon-hyperon interaction. Extrapola-tions to heavier systems shows a minimum binding energy of EB/A = -13 MeV witha strangeness fraction of ¡, = ISI/A "" 0.6 and a charge fraction around Z/A "" 0.1nearly independently of the total mass. (Here and in the following ¡, counts the amountof net strangeness per baryon in the system.) The most important elfect for this rais-ing binding energy and stability comes from the vanishing Coulomb repulsion due tothe addition of the negatively charged 3-. In contrast to normal nuclei one can easilythink of baryon numbers of the order of A > 500 beca use the fission process will beremoved since the ordinary Coulomb repulsion generated by the protons can be compen-sated by a comparable number of 3-'8. Thus SHM is predicled lo be metastable evenin the bulk limil. Interestingly, multi-A hypernllclei are generally unstable again8t the

STRANGEMATTER- A NEWDOMAINOF NUCLEARPIIYSICS 309

••-5 J=t

O 56Ni

-10 O 132Sn¿ 'V 2:l8Pb

<: + 310G

"- -15 • Yc:J O 56Ni

W /':, 180Th

-20

2.01.50.50.0 1.0

fsFIGURE 3. The binding energy versus lhe slrangeness fraclion f. for several sequences of super-slrange hypernudei based on various nudear cores as indicaled. The calculalions for lhe upperlhree cores employed lhe model wilhoul slrange mesan exchange, whereas lhe calculalion for lhelower one indudes lhe slrong YY inleraclion. Y denoles purcly hyperonic maller.

reaclion AA - =.N for some crilical number of A's, Le. pure multi-A hypernuclei gel morebound by adding =.'s. E.g. 56Ni collapses for more than 14 A's to a multi-A=. hypernu-cleus.An extension of this model also implements the scarce information about the hyperon-

hyperon interaction. This is done by introducing two new meson ficlds into the theory, (J'

and <P, which couple to strange baryons only [81. Note that the weak AA interaction inthe usual relativistic mean fieId model stands in contrast to the experimental known val-ues. After the implementation of this two new meson exchanges one can easily get strongenough hyperon-hyperon potentials. The binding energy for MEMOs will be enhanceddue to this additional inleraclions. Indeed, binding energies of -21 MeV and more havebeen found with a strangeness fraction of J. :::::1 as is visualized in Fig. 3. Even negativelycharged strange nuclear systems are possibIe without loosing stability. The filled circlesin Fig. 3 represent stable aggregales of purely hyperonic A, =.0,=.- matter with EB/A aslarge as -9 MeV for J, :::::1.7 and Jq ::::: -0.3. The lightest stable object of this typeis likely to be (2A + 2='° + 2='-). Purely hyperonic matter, in contrast to SHM in gen-eral, is not stable in the bulk limit, beca use of the Coulomb repulsion generated by the:=;- 's.Therefore finite MEMOs share the two features highlighted as a possible "smoking gun"

for the existen ce of SQM, namely J, :::::1 and q/A < 0.1. This analogy goes even further ifone considers the baryon densities of these multihypernuclear objects, which ranges from 2to 3 times that of normal nuclear malter similar to SQM calculations. Neverlheless, SQMmight be absolute stable, while SHM will be unstable against weak interactions, particularfor /':,S = 1 nonmesonic decays.

310 C. GREIt'ER ET AL.

~300

~al 200....:J.•..ro....alC. 100E~

~ coupleaA coupledfree HG

OO 50 100 150 200 250 300 350 400 450

Nonstrange Chem. POt. [MeV]

FIGURE 4. The slrangeness separalion oul of a QGP occurs if lhe phase transilion lakes placebelow lhe curves presented here. In the case oC inleracling baryons in a RMF approach, lhe borderis shifted lo higher lemperatures, in parlicular if lhe ::O-particle is coupled to the mean field.Including also the additional interaclions among the hyperons (model 2 of seclion 2.2). lhe bordervanishcs and the separation will always occur.

3. STltAI'GENESS SEPAltATION

Mulliple collisions per hadron ensure lhat a syslem slarts lo equilibrate which might besuited to search for the most inleresting collective effects. In particular all exolic mul-tistrange objects need for lheir formation large strange partide numbers, high degree ofequilibration and large densities. These re'luiremenls should enlarge their produclion prob-abililies al least due lo simple coalescence argumenls. In order to get this kind of statesone should therefore use high energies lo produce enough strangeness and energy density,and heavy nuclei to gain as much equilibration as possible. If the degree of stopping andthermalization at high densilies is large enough, as predicled by the RQMD model, lheargumentation with more simple models like e.g. thermodynamic rate equations seems tobe reasonable. In the following we want lo skelch lwo relaled arguments why the produc-tion of SQ:Vl clusters, if lhey do exist in principIe, is very likely, if a baryon r¡ch and hotQGP is created in such collisions.The net strangeness of the QGP is zero from the onset, although an equal, however

large, number of strange and anlislrange quarks has been produced by gluon fusion [91.However, lhere is a physical mechanism which separates the strange quarks from theiranliparlicles [1). Herefore consider the phase transition of the QGP lo the hadron gas atsome critical temperalure. There is no reason why these difIerent 'luarks do hadronize inthe same manner and lime, especially if oue lhinks of a baryon rich system. It is 'simple'for the anlistrange qnarks to materialize in kaons 1«'18) because of the lots of light qnarksas compared lo lhe s-quarks which could only move inlo the suppressed antikaons (1(-) orthc hcavy hypcrons. HCllCC, during aH cquilibrium phasc transition a large antistrangcncssbuilds up in the hadron mal ter while the QGP retains a large strangeness excess.Figure 4 shows lhe physical region where the above outlined separation occurs [101. For a

free gas of hadronic parlicles and resonances lhe critical curve below which the mechanism

STRA:"GE ~IATTER - A I'EW OmlAl:" OF :"UCLEAR PIIYS1CS 311

starts working lies already at rather high temperatures or baryochemical potentials. lf oneineludes interaction among the baryons like in the RMF approach outlined in section 2.2,this 'border' moves even to higher temperatures. (For the Rl>lF-model 2 there indeed existno such border, the mechanism should work inside the whole (T •...•I,)-plane.) In any case,it is expected that the phase transition from a QGP to an (interacting) hadronic gas liesbelow these presented curves so that essentially the strangeness separation should alwaystake place.Furthermore, rapid kaon emission leads similarly to a finite net strangeness of the

expanding system IIJ. This, in turn, results in an even stronger enhancement of the s-quark abundance in the quark phase. This prompt kaon (and, of course, also pion) emissionmay cool the quark phase, which then condenses into metastable or stable droplets of SQI>I.

3.1. Stmngelet distillatian

For modeling the evolution of an initially hot fireball a two phase equilibrium descriptionbetween the hadron gas and the QGP was combined wilh the nonequilibrium radiationby incorporating the rapid freeze-out of hadrons from lhe hadron phase surrounding theQGP droplet during the phase transition IIJ.Two scenarios may describe the evolution to the final sta te: The quark droplet may re-

main unstable until the strange quarks have elustered inlo A-partieles and other slrangehadrons to carry away the strangeness and the plasma has complelely vanished into stan-dard parlieles. This scenario is customarily accepled. 1I0wever, if SQ~l exisls at lowtemperatures in configurations having a mass per baryon lower than the mass of theA-partiele, the hot SQM droplel would remain at the phase transition boundary muchlonger. As shown in 11], producing strange baryons like ,\ partieles is energetically moreexpensive and therefore less likely than produCÍng SQ~I like strange!ets. Toward the endof the evolution only baryons are allowed to escape from the droplel, since at this point allof the antiquarks 'lre gone. The baryons will be mostly nneleons, since lhe hyperons areheavier amI require more energy for formation. These nueleons remove energy but they donot carry away an)' strange quarks, so t1le ratio oC strange to nonstrange quarks incfeasesfurther, refining the dislillation of strangeness. \Vith a reasonable but small probability,the hot strange malter cools down lo cold lumps of size A ~ 5-50, depending on theoriginal baryon content of lhe plasma.Figure 5 givcs an impression how the hadronisalion proceeds for a large bag constant

(B1/4 = 235 I>leV -no strangelet in the groundstale) and a small bag constant (B1/4 =145 MeV). The initial parameters are a net strangeness content of J..(to) = 0.25 and anentropy per baryon ratio ~(to) = 25 in both cases: For the large bag constant the systemhadronizes completely in t ~ 8 fm/c, which is cuslomarily expected and lhus not toosurprising. Yet, a strong increase of lhe net strangeness of the syslem is found in bothsituations, and the plasma drop reaches a strangeness fraclion of f, - 1.5 when lhe volumebecomes smal!. Indeed, for the small bag constant, however, a cald strangelet emerg"s fromthe cxpansion and c\'aporatioll proccss with an approximat.c baryon llumbcr of AH '"'""17,a radius of R '" 2.5 Cm, ancl a lIt't strallgcllcss [ractian uf fs(t - 00) ;:::1.5, Le. a chargeto baryon ralio Z/A = (1 - /,)/2 - -0.2! It wouid comprise a nueleus of positive baryonnumber, but negative charge.

312 c. GREINER ET AL.

S/A ••t=25, fs=O.25

100 100al b)

80 80

O- 60 81/4=235 MeV 60(!)el

1II 40 40~

20 20

O O

2 2

O170

~ 165

>a;160(~

~1- 155

1506 8 10 12 14

t [ fm/c ]

20O. O 50 100 150 200 250

t [ fm/c ]FIGURE 5. a) Baryon number, strangeness content and temperature of the quark glob duringcomplete hadronisation as a function of time for a very large bag constant Bt = 235 MeV.The initial values are an initial baryon content of A8(tO) = lOO, an entropy per baryon ratio of~(Io) = 25 and an initial net strangeness fraction of ¡.(Io) = 0.25. Note the strong increase of thestrangeness content with time. b) The same situation as in a., however, for a small bag constantBt = 145 MeV, when a strangelet is distilled. One observes a strong decrease in the evolvingtemperature.

AIso it was found that a high iuitial entropy does not necessarily prohibit strangeletformation. The distillery works even for larger initial entropies SIA = 50 or 100. Abundantkaon production enriches the plasma rapidly with net strangeness at high entropies. Thisolfers to look for strangelet production at the highest bombarding energies available in thefuture for very heavy systems, e.g. at the CERN SPS (ELAB ~ 200 GeV IN) or at RHIC(ELAB ~ 20 TeV IN).

STRAKGE ~IATTER - A NEW OO~IAIN OF NUCLEAR PIIYSICS 313

3.2. Purely hadronic fireball

It may be possible to produce some of the lightest multihypemuclear objects envisagedhere in the laboratory by means of high energy heavy ion collisions. Estimates based ongrounds of simple coalescence yield very small production rates, for instance 3 x 10-9

events per central Au + Au collisions at 11.7 AGeV for the lightest bound =: system~o!.,{He [81.One is tempted to ask whether a separation could also manifest if initially there is

no QGP at all, Le. the fireball is only made out of hadronic constituents. Obviously, anyseparation could only happen by nonequilibrium effeets. For a more or less global chemicalequilibrium the strange and antistrange particles are distributed rather homogenouslyacross the fireball, equalizing locally on the average. It is the interior QGP phase whichdue to its different affinity assembles the strange quarks if both phases stay in chemicalequilibrium.Potential non equilibrium effects tum out to be interesting and important for the fol-

lowing two reasons [111: The mesons like the pions and the kaons are much lighter thanthe baryons, especially compared to the moderately heavy hyperons. Accordingly their av-erage veloeity should be much higher, so that a tremendous amount of the mesons shoulddecouple reasouably earlier from the surface of the fireball before an overall freeze-outof the hadrous iu the deep iuside of the system takes place. Secondly in a baryonrichfireball the kaons are much more abuudaut than the antikaons, howe\'er they possess amuch smaller auuihilatiou cross section compared to the antikaons, e.g. aKN « aRN' Inprincipie this means that lhe kaous decouple earlier from the fireball compared lo lheantikaons.To demonstrate lhe cousequences of lhis iuspired picture we simulale lhe evolution of

an iuitially very hol system by allowing only the pious aud kaons (case 1) and addition-ally also the autikaons (case 2) to evaporate from the outer layer. The initial para me-ters are specified by the eutropy per baryou conteut (SIA(/o) = 40; 25; 15) and a fixedbaryou density (PB equals two times normal nuclear matter density -the results are onlymiuorly seusitive to this choice). The results for the net strangeness enrichment of theremaining and cooling fireball are shown iu Fig. 6 as a functiou of the deereasiug temper-ature. Iudeed, iu case 1 we observe a reasouable to strong iuerease of the net strangenessfraetion ¡, in the rauge 0.3-1.1. depending ou the iuitial entropy eonteut. This inereaseoeeurs in the early evolution al still high temperatures ~ 150-170 1>leV. Iu the seco udcase this enriehment also takes place, but beiug strougly snppressed, reachiug only values¡, ~ 0.2-0.3. This last scenario, howcver, should be seen with some cautiou, beca use itdoes not pay respeet to the faet that the kaous decouple mueh earlier and that the an-tikaons may be aunihilated. Also, the produccd number of antikaous are he re nearly asabuudant as the uumber of kaons, which is not observed by experiment where the num-ber of antikaons are suppressed. So to speak, the 'truth' may lie betwecn both sceuariospresented.The strange quarks 01' the hyperous are more localized in phase space. Thiuking in

terms of a coalcsccnce picturc fOf prodllcing cxotic multistrange clustcrs. sllch an earlyiuerease in the uet strangeness coutent, whether a QGP has formcd or noto should affectthe productiou probability of these clusters.

STRANGENESS ENRICHMENT10

7l", I( , j?O.,

0.6.S/A = 40-

D.'

02 "e ~15~

0.0O 50 100 150 200 250

TEMPERATURE [MeV]250200

[MeV]50 100 150

TEMPERATURE

0.0O

314 C. GREINER ET AL.

STRANGENESS ENRICHMENT2.0

7r, J(

15

S/A = 40

. 10-25

05 15

FIGURE 6. a) Strangeness enrichment in apure hadronic fireball. Pions and kaons escape Cromtheouter layer (case 1). The initial configuration is specified by the entropy and a baryon density oCtwo times normal nuclear matter. b) In additioIl, to show the other lextreme' also antikaons areallowed to escape Cromthe system (case 2).

4. DETECT10N

It is important to note that these objects are a new lorm 01 matter, not a specific newparticle. The strange droplets produced in these reactions do not come in the form of asingle type of particle. Many different sizes of droplets may be produced, spanning a rangein mass, charge, and strangeness contento The experimental task of finding the new formof matter is therefore challenging. Here any detected particle having an unusual charge tomass ratio is a potential stmnge/et candidate.To identify a particle or cluster, its charge and mass need to be measured. To determine

that the particle is a new form of strange matter, its strangeness content must also berevealed. The experimental approach is first to find 'objects' having a peculiar or newcharge/mass ratio. (The strangeness might be Seen by interaction with a secondary nucleus:mulliple production of As, I;s, =:s and R:s in such a secondary reaction would signal itsexistence.) The key idea here is that the charge/mass ratio will be unlike that of anynormal nuclear isotope (the 8He with a Z/A = 0.25 would be the isotope candidatewith the smallest ratio). Strangelets or MEMOs would have a charge ~ O, being slightlypositively or negatively charged. In particular in the range -0.25 < Z/A < +0.25 thereexists no quasistable form of nuclei or antinuclei. Such a range will be covered by theE864 [12] experiment taken next year at ilrookhaven. E878, the successor of E858 [13]'using a focussing spectrometer at zero degree, is seizing a much smaller selected range,which in respect to cover still the full range of interest can be steadily adjusted. A similartechnique like in E878 will also be employed next year by the Newmass collaboration [14)(NA52 experiment) at CERN at much higher energics, and latcr possibly also with thcheavy Pb-beam.

STRA:<GEMATTEH- A :<EWDO~IAI:<OF NUCLEARPHYSICS 315

STRANGELET

2

>-.•....¡¡;eQ)

O

oO

----- ---- ---

.'.....

1Radius [fml

MEMO

2

2

~'",E:::>-~VleQ)

O

OO

---------- ...•..:'::"'::'":............ ..... .•. .•. .•. .•. .•. .•..•. .•.

1 2

Radius [fml

Totals-Quarksq-Quarks

FIGURE 7. a) The spatia! quark distribution of the light quarks and the strange quarks in ahypothetica! metastable strangelet with a baryon number of AB = 30 and f. = 1 (the bagparameter EI/4 '" 170 MeV was fixed in such a way that the bindillg energy is the same asfor the multihypernuclear object in b). b) The same for a multihypernuclear metastable object6 {pnA=:0:=;- }.

However, the global properties of a strangelet or a MEMO are likely to be identical(see Fig. 7): a similar small charge ¡ZI and nearly the same average baryon density ~ 3po.In principIe, to distinguish experimentally between both one has in addition to resolvethe mass E/A very aeeurately. A MEMO is only bound in the order of E8/A ~ 10 MeVwhereas the strangelet may be bound from 10-200 MeV (whieh is, of eourse, speeulation).In Fig. 7 the spatial quark distribution of a MEMO and a strangelet are depicted. Dueto the strong overlap of the eorrespondillg distributions, the MEMO would deeay into astrangelet, if the latter is energetieally more favourable and stable.

316 C. GREINER ET AL.

Employing TOF-teehniques to reveal the velocity and thus the eharge to mass ratio,the experimental setup sets a natural time seale ~ 10-7 see. So, an important question wefinally have to adress are the lifetimes of these objeets. The ¡¡fetime of a ME1>IO shouldbe similar to the A's lifetime, i.e. ~ 10-10 see. Thus an open geometry deteetional devieewill be needed to diseover their existenee. If a produeed strangelet is absolutely stable theonly energetieally possible deeay mode is the weak leptonie deeay (s ~ d, Q ~ Q' +e+¡;),whieh will turn the strangelet to its minimum value in energy at ¡,~0.8. The time-sealefor this weak proeess has been estimated [71to be ~ 10-4 see. lIenee, the strangelet wouldremain in its initial eondition.

The situation turns out to be more eomplieated, if the strangelet is metastable. Theweak eonversion rate 1l+ S ~ 1l + d for almost eold SQ~I was ealculated [151 to be ~ 10-6_10-7 see. This proeess ehanges an s-quark into a d-quark or vice versa, whiehever isenergetieally possible, and thus drives a SQM droplet to ¡,~0.8, wbere the Fermi energiesare equal (11, = I1d). Tbis estimated rate would be suffieiently small to establisb deteetion.However, there might be another similar important weak deeay ehannel, the weak nucleondeeay: s ~ d, Q ~ Q' + n. The eonversion of a strange quark is aeeompanied immediatelyby nucleon emission. Tbis deeay is energetieally possible in the range ofO.5 < ¡,< 1.7. For¡, > 1, aeeompanying nucleon emission drifts the strangelet to a higher net strangenesseontent (and henee to a larger ehemieal potential 1', for the strange quarks). Subsequentweak deeay proeesses, as deseribed here, 'heat' up tbe droplet (on a seale of a few ~leV).The droplet may be cooled by -y-radiation or by nucleon emission. Both proeesses maycompete in magnitude. Still, the lifetime is a matter of debate. At least for energetiealreasons it sbould be lower than tbe one of a hypernuclear objeet. An open geometry devieeis needed too if tbe strangelets deeay in times of tbe same order as the MEMOs.

REFERENCES

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