Strange Anti-Baryons from Quark-Gluon Plasma

Volume 262, number 2,3 PHYSICS LETTERS B 20 June 1991

Strange anti-baryons from quark-gluon plasma

Johann Rafelski Department of Physics, University of Arizona, Tucson, AZ 85721, USA

Received 5 April 1991

Experimental results on strange anti-baryon production in nuclear S--,W collisions at 200 A GeV are described in terms of a simple model of an explosivelydisintegrating quark-lepton plasma (QGP). The importance of the strange anti-baryon signal for the identification of the QGP state and for the diagnosis of its properties is demonstrated.

The general problem one encounters trying to di- The model we develop here is relatively simple and agnose by means of strongly interacting particles the is applicable mainly to the computation of ratios of presence and properties of the putative quark-gluon particle abundances within a narrow kinematic do- plasma (QGP) state of highly excited hadronic mat- main. In our approach we will only employ well es- ter is that most particles observed in high energy nu- tablished principles of statistical ensemble physics. cleus-nucleus collisions have to pass through the stage Such an approach is justified as we will be able to work of the hadronic gas (HG) consisting of individual around possible transparency, flow or spatial inhom- hadronic particles, in which their abundance and ogeneity of fireballs formed in individual nuclear col- spectrum is substantially altered. However, the com- lisions. To this objective we will consider in particu- position of the QGP phase ofhadronic matter differs lar only the narrow, central region of rapidity and from the HG phase in crucial detail: the density of particles of high transverse mass. While in such an strange anti-quarks is considerably greater. This oc- approach we forfeit certain most interesting aspects curs due to the rapid strangeness production by glue of the global reaction picture, our interpretations are based processes and inherently greater particle and model independent, and independent of tacit as- energy density in the plasma state. High strange (anti)- sumptions about unknown physics surrounding con- quark density facilitates the formation of multiply version of colored particles into asymptotically ob- strange baryons and anti-baryons not only during the servable hadrons. hadronization conversion from QGP to HG, but also Today, the search of QGP at CERN involves col- in the primordial period of the plasma evolution, as- lisions of 200 A GeV sulphur (S) nuclei with a target sociated with highest temperature and density con- nucleus which are considered at small impact param- ditions. A substantial enhancement of production eters. In the WA85 experiment [ 3,4 ] the target is the rates of multi-strange anti-baryons in nuclear colli- tungsten (W) nucleus. The NA35 experiment [ 5 ] sions [ 1 ] in particular at central rapidity and at high- takes advantage of the symmetry between target and est transverse masses [2 ] has therefore been pro- projectile, using sulphur as target. In small impact posed as a characteristic signature of QGP. This parameter S-S collisions all nucleons participate, suggestion was also made because it is difficult to find while in the asymmetric collision the effective target other mechanisms which could give rise to such consists of the tube of nucleons in the center of the abundance anomalies. The relative abundance of tungsten nucleus which is in the path of the projec- centrally produced anti-cascades -~ = s~(t, anti-hype- tile. Hence the effective mass of this target is 80-90 rons Y=~ClCl and anti-nucleons N=Cl~l(1, was sug- and the central rapidity region is yf= 2.5 + 0.1, while gested as a qualitative signature of (at least initially) S-S collision data is symmetric around the central re- explosively disintegrating QGP. gion yf= 3. These values are significantly different

from the rapidity of the projectile, yp= 6 and hence gests that we have a comparable number of the more the particles emanating from the central fireball are difficult to produce E- as compared to A. easily distinguished from the projectile fragments by There are further strong indications that the total virtue of the rapidity gap of 3-3.5 units. In these ex- abundance of strangeness grows faster than the gen- periments strange particle production has been mea- eral particle multiplicity, suggesting a more efficient sured and the results indicate the possible presence mechanism of strangeness production. This is re- of a new strange particle production mechanism. ported in terms of the ratio of strangeness to nega- The most interesting result reported in ref. [ 4 ] is tively charged particles by the NA35 experiment [ 5 ], which doubles as the multiplicity of negatives in- Rz := E-/E - = 0.39 + 0.07 creases. Similarly, in the WA85 experiment, which is fory~ (2.3, 3.0) andp± > 1 GeV/c. ( 1 ) only triggered for highest multiplicities, one finds an enhancement of the A abundance [ 7 ], when compar- In p-W reactions in the same (p±, y) region a smaller ing p-W to S-W interactions in the central rapidity value for the Rz ratio, 0.27 + 0.06, is found. The data y~ (2.4, 2.65) region as a function of the transverse for the A/A ratio reported in ref. [ 3 ], after appropri- mass spectrum. The A spectrum, when extrapolated ate corrections for contamination from cascade de- from m± e ( 1.5, 2.5) GeV to 1 GeV beats in its mag- cays [ 6 ], is nitude the spectrum of all negative particles, presumably pions, and there is an enhancement by a factor RA :=A/A=0.13 +0.03 1.7 of A, A abundances in comparison to the nega- for y~ (2.4, 2.8) andp± > 1 GeV/c. (2) tives, as the projectile changes from p to S. We further note that the WA85 transverse mass spectra of This result suggests that the ratio RA in S-W colli- A, ~ suggest a higher "temperature" (inverse slope) sions is smaller than in the p-W collisions in the same of TA = 240 + 20 MeV [ 6 ], which is somewhat greater kinematic range, as these ratios were nearly equal [ 3 ] than the other reported temperatures T= 195 MeV, before corrections for cascade contamination were derived from all momenta ranges of singly strange applied. We note that this effect may arise in part as hadrons [ 5 ], albeit the latter value is determined in a consequence of the production of A in rescattering presumably less thermalized and more transparent S- of kaons in spectator baryonic matter. Therefore it S collisions. would be of considerable advantage from the system- We must consider in detail if indeed a new physics atic point of view to consider symmetric collisions of phenomenon is being observed or if we can explain heavy nuclei, such as Pb-Pb. We also note the pre- these results consistently within a conventional par- liminary experimental values for ratios of different ticle production framework. We must consider which baryons, obtained in the kinematic domain of eqs. further information is required to narrow and per- (1), (2) [6]: haps even identify the eventually needed novel reac- -=-/A=0.6 +0.2, E/A=0.20+0.04. (3) tion mechanism. It would be in particular interesting to see if one can use the present results within a sim- If these results of eqs. ( 1 ), (2), (3) are confirmed ple QGP based model to predict a precise value of the with greater statistics, and preferentially in symmet- ratio A/p which becomes available soon [7], and ric collisions, this would contradict the view which perhaps also f~-/E-. Hence we must consider all holds that in nuclear collisions the rare central pro- these important observables as well. We have to show duction of (strange) anti-baryons should be sup- how to discriminate the available anti-baryon-bar- pressed as compared to p-A collisions. (Strange) anti- yon ratios Ri against the HG models. baryons are produced in individual N-N interac- Before proceeding to a detailed analysis of the ex- tions with preference at small XF where they would perimental results, we need to recall the relevant the- be subject to inelastic reactions with other baryons of oretical developments on which the model we em- the target and potentially projectile nuclei, leading to ploy rests [8,9 ]. Once a QGP is formed the gluonic a greater absorbing effect when a nuclear projectile is production rate dominates the process of strange used with greater atomic number. Also, eq. (3) sug- quark pair production and the time nee 'ed to satu-

334 Volume 262, number 2,3 PHYSICS LETTERS B 20 June 1991 rate the phase space (so called chemical relaxation will be thermal in appearance, while some minor dis- time constant) is smaller or at worst comparable in tortion may occur due to fragmentation processes magnitude to the estimates of plasma lifetime based which may also occur but have a negligible impact at on nearly free hydrodynamic expansion [ 1,10 ]. This high P_L. This is despite the fact that in the global had- result has been reconfirmed by more refined studies, ronization of the plasma a substantial fragmentation which included kinetic evolution of the plasma [ 11- of gluons and quarks is required in order to assure 13]. These calculations have further shown that that the entropy rich plasma phase finds a path to the strangeness abundance freezes out and does not an- relatively low entropy density of the HG. In the global nihilate, being thus characteristic of the densest state hadronization process a hadro-chemical equilibrium of the QGP. On the other hand, the HG phase cannot is established in which a substantial strangeness produce in the required abundance either strange- chemical potential prevails [ 14,15]. In this regard ness or (strange) anti-baryons at central rapidity [ 9 ] QGP differs significantly from the HG. Both in the unless one makes completely unrealistic assump- early QGP as well as in the fragmentation of QCD tions. Such can be a very high temperature T> 300 quanta there is always an exact symmetry when s, g MeV, which is inconsistent with the known particle pairs are produced, and hence/t~ 6P = 0. spectra, or the long lifetime of the HG fireball which We now obtain the particle abundances as func- needs to be more than 10 times longer than a reason- tions of fugacities: all baryons considered have spin able value of zHG< 15 fm/c, which value is consis- ½ and the flavor quantum number will be explicitly tent with the pion interferometry results. In the ex- considered, hence the other statistical factors are un- plosive hadronization of the QGP phase the ity. This allows us to ignore the isospin eigenstates, anomalous strange particle density is seen in the also because the abundance of A and/k (I=0) im- anomalous ratios of strange anti-baryons [9 ], but a plicitly includes the abundance of E ° and Zo (I= 1, transition, which is slow and allows for reequilibra- •3=0) arising from the decay X°~A°+7(74 MeV) tion of the abundances, loses much of this effect [ 13 ]. and similar for X°. Thus comparing spectra of parti- To avoid that the comparison of the theory with ex- cles within overlapping regions of m ± we find for their periment hinges on the details of the unknown mech- respective ratios anism ofhadronization it was suggested [2] that pri- E -- 2d 1 ~S 2 mordial particles, i.e. those with high p±, are R--- E- -- 2d2~ considered. These are emitted preceding the global hadronization, mostly in an explosive evaporative = exp(--2/td/T) exp(-4lts/T), (4) recombination process. The baryon and anti-baryon relative abundance arising from such a formation RA- A - 202u2s mechanism is controlled by two factors: the statistical multiplicity factors, describing the likelihood of = exp[-2(/td +/Iu)/T] exp(-2/tJT). (5) finding among three randomly assembled quarks the suitable spin-isospin of the emitted particle, and the Ignoring isospin differences for the moment, 2u chemical fugacities 2=exp(p/T) which define the =,,].d =:.~q, we obtain relative abundance of both quarks (2q) and anti- RA = (2J2q) 2 R~. (6) quarks (2~- l ), where the temperature Tand chemical In QGP we have As= 1,2q> 1, while in equilibrated, potential p are to be considered in general as func- baryon rich HG 2s/2q~ 1 [ 14]. Thus both in HG and tions of position and time. Since the probability to primordial baryon rich QGP we find that RA < R~ but find three independent particles with total energy for different reasons, and thus both phases lead to E >> T is proportional to exp ( -E~ T) in any thermal different values of R•, R=_. The cascade and lambda system, the transverse mass spectrum at high mo- ratios can easily be related to each other, showing ex- mentum will indeed be characterized by the temper- plicitly the respective isospin asymmetry factors and ature present at the primordial formation of a com- strangeness fugacity dependence. Eqs. (4), (5) imply posite particle. Thus the high momentum particle emission due to the quark recombination mechanism RA=R~exp[2(ltd--~u)/T] exp(6pJT) . (7)

335 Volume 262, number 2,3 PHYSICS LETTERS B 20 June 1991

We emphasize that eq. (7) is rather generally valid, up to a factor 2 reduction of the achievable ratio. Here irrespective of the state of the system (HG, QGP). again Pb-Pb collisions could improve the under- But clearly, we have now eliminated most of the ig- standing significantly, as the time available to pro- norance about the chemical composition of the state duce strangeness in the (hypothetical) QGP phase from which the particles are emitted - the result is would be significantly extended. The ),-factor does not now characterized by the value of#ff T, which is very appear in the ratios R which compare strange to anti- different for the different reaction scenarios. We re- strange quarks, eqs. (4), (5), as s and ~ are equally cord also an interesting prediction for the ratio of abundant during the approach to absolute chemical abundances of anti-protons to anti-lambdas to anti- equilibrium, any asymmetry arising in consequence cascade: of rescattering and is contained in the quark chemical potential. When the fireball consists of HG - the E- A 2;-' anti-strange quarks g are mostly found in kaons -~--- p- 2~_ j - exp(/~/T) exp(-#fiT) (8) K= gq, while s-quarks are distributed by strangeness exchange reactions in a rapid way between hyperons Y--sqq and anti-kaons K= sO. This rather rapid re- = >1. (9) distribution of strange quarks leads to relative chemical equilibrium, even if the absolute chemical equi- Taking the experimental value we find for the ratio librium, i.e. saturation of strangeness phase space, was eq. (9) the result 1.73 + 0.40. not reached. Therefore, abundances of Y and K are It should be remembered that all the above equa- descriptive of the thermodynamic conditions at the tions apply for ratios of particles as a function of (m ±, freeze out of the HG fireball and can suggest new y), in the domain in which one can use the Boltz- physics only by a possible enhancement of the mann approximation to the spectrum of an (as- strangeness abundance. In summary, the key differ- sumed) thermal emitter. If we were to compare global ences between the phases thus are: abundances, then we would have to allow for addi- (1) the difference of the value of ;ts=exp(~q/T) tional factors reflecting on both the partial origin of which enters in a very pronounced way e.g. in eq. (7); the particles (HG or QGP) and the size of the phase (2) the likely near saturation of the strangeness space ( oz exp (-m/T) ), while for the full phase space phase space, ifQGP was formed (~ 1 ), a highly un- integral the Boltzmann approximation may also not likely condition for HG; be always adequate. Nevertheless it is remarkable that (3) high "temperature" of the high p± strange anti- both ratios shown in eq. (8) are equal, and as shown baryons, which originate from the pre-hadronization in eq. (9) indeed greater than unity. This result has epoch of the fireball. been obtained under the assumption that strangeness In order to get some numbers it is convenient to production has saturated the full phase space avail- rewrite the isospin asymmetry factors appearing in able ("absolute" chemical equilibrium). However, eq. (7). It is customary to introduce the notation eqs. (8), (9), unlike eqs. (4), (5), are sensitive to ~t/d = ,/./q "~ O. 5t~,t/q , (10) the degree of correctness of the assumption of absolute chemical equilibrium. Denoting by a factor y < 1 #~ =#q -- 0.58pq, (11 ) the deviation from the absolute chemical equilib- /./q = 1 (]2d ...~ ]~u) , (12) rium abundance for strange and anti-strange quarks, i.e. 2s--,y2s, 2~-~ ~y2~-~, we find that the right-hand ~lUq ~- ]./d -- flu , (13) side ofeq. (8) is multiplied by the factor y, as it com- pares the degree of equilibration of strange-anti- fib =3~q . (14) quarks with up-anti-quarks. We recall that especially The isospin asymmetry ~q/~q is very small, and we in the early stages of the QGP formation, the strange- will determine it quantitatively further below in a ness phase space may not be fully saturated - calcu- QGP-fireball model. First, we need to have a mea- lations suggest [ 9 ] that one should expect up to a fac- sure of the quark chemical potential #q. This can be tor 2 under-saturation for S-W collisions, suggesting done by combining the ratios in such a way that the

336 Volume 262, number 2,3 PHYSICS LETTERS B 20 June 1991 strangeness fugacities (chemical potentials) cancel. (d> - (d> 2-Zf/Af While this is best done by comparing the K- abun- (16) (u)-(o) l+Zf/Af dance with the A abundance [ 9 ], we do not have this information presently available. However, we note In the tube model, in which all nucleons in the target in the path of the isospin symmetric projectile partic- K- R A 2u l),s A -- ~ - 2,2d2s - exp [ - (/td + 2#u ) / T] ipate in the fireball, this factor in the sulphur-tungsten collision is 1.086 (for Zr= 38, Af= 87). = exp(--#b/T) exp(0.58pq/T). (15) In the QGP we can use the analytical expressions for the flavor (quark) density and arrive at Taking RA=0.13_+0.03 and neglecting the isospin (d)-(d) ,uJT[l+(lta/TrT) 2] ~ #__~d asymmetry factor we find , (17) (u)-(fi)- lzJ T[ l + (l~/TtT) 2] - I~ /zq/T=0.52 +0.1 , where the last equality arises in the regime of interest irrespective of the composition of the source (HG, here - (#q/zrT)2 << 1. We find QGP ) of the strange (anti-) baryons. We can use this result with either eq. (4) (or eq. (5) with less preci- 6//q//./q = 0.09 sion ) and we obtain for QGP in S-W collisions. Setting As= l in QGP we /zs/T= - 0.02 + 0.06. further have #q/T( 1 +0.58#q//~q) =/~o/T= ½In(Z/E) While this result is compatible with zero, the error is too large to assume this very important value: if ks -- 0.47 _+ 0.08. were to vanish this would indicate exact symmetry Which implies between the produced strange and anti-strange quarks, which can only occur in either a baryon num- /~q/T=0.46+0.08, 8~q/T=0.041 _+0.007. ber free HG phase or in a QGP before the hadroni- We can use this result, together with/~s=0 and eq. zation process. As there is a clear baryon asymmetry (8), to predict the key strange anti-baryon ratios ex- as described by the substantial Ilq/T value deter- pected from primordial QGP (where as discussed mined above, only the latter case would be possible - above, 0 < y~< 1 characterizes the approach to abso- hence forthcoming increased precision of the mea- lute chemical equilibrium of strangeness): surements combined with additional crucial information about the ratios of different anti-baryons, eq. E- IA=fklf~=y 1.55+0.13, (8) will allow to determine the value of#s, which will E- / A = A/p= y O.64 _+O.05 , become one of the pillars of the argument for QGP origin of the strange anti-baryons. fl-/F,- =y 1.61 +0.13, Given the above determined model independent f~-/-- =y 0.62-+ 0.05. value of/zs associated with the reported high m ± > 1.5 GeV part of the central strange baryon and anti-bar- Comparing with the first results on these ratios eq. yon spectrum we now follow up the hypothesis that (3), we can extract a first estimate of strange phase these particles arise in the primordial, explosive space saturation: y=0.4_+0.2. Here we used the emission from a QGP fireball. Such a description re- strange anti-baryon ratio, to avoid the systematic duces the errors - but of course makes all results questions related to the origin of the A abundance. If "model dependent". We first estimate the isospin the strange baryon ratio is used, the result is factor encountered in the above equations, y = 0.31 _+ 0.10 _+ systematic. These results compare exp [ (/~d --/~, ) / T] = exp (8/~q/T) both for the case of well with earlier work on the strange anti-baryon ra- the HG and for the case of the QGP phase. We count tios, see fig. 2 of ref. [ 2 ] for the here implied value the number of up and down quarks and compare it /Zb/3 T= 0.46 (note that the values shown are by def- to the ratio brought into collision by the colliding inition twice the values considered here). Other re- nuclei: sults of ref. [ 2 ] show that the possible fragmentation

337 Volume 262, number 2,3 PHYSICS LETTERS B 20 June 1991 contribution to the primordial baryon and anti-bar- at the established value #q/T= 0.5, while the natural yon production is at about 10%. global abundance ratio would be (taking T= 200 MeV Another quite interesting aspect of this discussion for the "hard" component of the particle spectra) is that once #q is known, using the entropy content of 2 the QGP fireball we can predict the expected pion exp(3#q/T) iX{m__~} K2(mJT) =45 (21) multiplicity n, as compared to the baryon multiplic- f~ \mN/ K2(mN/T) " ity b, following the procedure of ref. [ 16 ]. Taking Thus the global ratio of the abundance of anti-pro- again the QGP perturbative equations of state as dis- tons to pions arising from QGP is just slightly sup- cussed e.g. in ref. [ 17 ] we find that the entropy per pressed comparing to HG, suggesting that at high m. baryon is given by (retaining only the dominant terms the ratio of re- to p will be up to a factor 3 larger than in leading order in T/#q) can be expected from thermal models. However, as S 3n 2 T(14(1-5OaJZlzt) 32(1-15aJ4rO~ this discussion shows, at given mi >/1.5 GeV there b- 2 #q\ ]-ff(l~ -t 45(1-2oq/n)J may be a remarkable abundance of anti-protons: the ratio of anti-protons to protons at high mi is = 17 (T/#q) for oq =0.6. (18) f~/p= exp(-6#q/T+~#q/T) = 0.,,,,,,_oo24na~+°'°4°. (22) For the value of T/pq determined here ( = 2.2 + 0.4) we have thus 37 + 6 units of entropy per baryon in the The relatively large error of this result indicates that primordial QGP phase at the time of multi-strange it is very sensitive to the chemical potentials and thus (anti-)baryon emission. Since at the end of the had- one may use this ratio as measured at high m± to de- ronization process the entropy content will be simi- termine the actual value of#q, provided that system- lar, though certainly somewhat larger, this permits us atic errors are reduced by choosing a symmetric col- to determine a lower limit on the pion to baryon ra- lision system. This confirms that the anti-baryon flow tio, which applies in particular to central particle is expected to be dominated by strange and multi- spectra, extrapolated from high m i to lower m., strange particles - but the anti-proton high m± flow which procedure avoids additional components in the is itself substantial, with one anti-proton being ex- spectra arising from processes occurring in the final pected for each 10-25 protons or for each 4-5 nega- evolution of the hadronic gas (e.g. A-decay, rescatter- tive pions. ing off the spectator matter, etc.). To find this pri- On several occasions in the discussion of the exper- mordial ratio we note that each baryon emitted will imental results additional or more precise data would take away entropy in the amount of have enhanced our understanding. Foremost, there is need for better statistics, and lesser systematic uncer- Sb 1.5+ m--3pq ~4 (19) b T tainty arising from the presence of a large number of non-participating baryons. We have aside of the leaving behind about 33 units of entropy; each pion "temperature" parameter at least three other quan- will carry away 4.05 units and hence the pion multi- tities to measure: the two chemical potentials (#q, #s) plicity per baryon will be n,Jb= 8.2 + 1.5. Neglecting and the parameter characterizing the approach to the small isospin asymmetry, this suggests 7t-/ equilibrium y. Hence at least four independent p = 5.4 + 1. For an experiment it is important to know (strange) anti-baryon ratios need to be measured in hov' many (negatively) charged particles occur at the order to determine the values of these parameters, same m±. To obtain this information we note that verify their consistency and show e.g. that #s = 0. This given our estimate of the high mi-protons we find requirement indeed means, adding as additional pa- that the total primordial abundance ratio is n-/ rameter the absolute normalization of the particle f~e (133,51). However, the thermal ratio of abun- spectra, that we need to determine five different par- dance at high mi is ticle (strange baryon and anti-baryon) spectra sepa- rately, with a precision of a few percent. Thus in ad- rt - =exp(3#q/T) =4.5, (20) dition to the four species (A, A, E -, E- ) we need ~'- high m±, central y further data for e.g. 1O and/or fl- or/and f~-. In ad-

338 Volume 262, number 2,3 PHYSICS LETTERS B 20 June 1991 dition, we need to compare the results of A-A colli- dances, but could hardly alter the HG based strange sions with N-A reactions. The present data only fixes anti-baryon to baryon ratios. In particular, the melt- the parameters of the QGP model we used; to con- ing of strange particle thresholds does not turn offthe firm it we need some other particle ratio as just de- particle-anti-particle annihilation, indeed it ought to scribed, and we have made clear predictions what we stimulate it - making strange anti-baryons again a key expect. Even if such experiments were to be success- signature of QGP formation. ful, it will be necessary to seek where a change in the Our conclusion is that better and somewhat ex- strange particle flow (the relative abundance of anti- panded data on (strange) anti-baryon flow, will per- cascades to anti-hyperons to anti-nucleons) occurs as mit us to identify the source of the high m± centrally a function of the CM collision energy in the likely produced particles as the primordial or/and explo- range of 2-30 A GeVcu. This is practically an impos- sive QGP state of matter. The present results are al- sible task without a variable energy heavy ion collider. ready very suggestive of this interpretation. How- When and if our predictions about further ever, the given error bars, combined with the (strange) anti-baryon abundances are experimen- systematic uncertainties accompanying asymmetric tally established, this will conflict with predictions collisions lead to a too large uncertainty. We have obtained in terms of individual hadron cascade presented here a method and provided a wealth of models which require irrespective of m± that E- are detailed predictions, which may be employed to study less than 10% of the anti-nucleon abundance, rather the evidence for the QGP origin of high P_L strange than being up to 1.6 times more abundant as sug- baryons and anti-baryons. gested here. Should individual microscopic processes driven by anti-di-quarks fragments of the projectile I would like to thank Emanuele Quercigh for dis- be the source of abundant strange anti-baryons [ 5 ], cussions of the WA85 Collaboration's experimental then the abundance of anti-nucleons will be more than results. three times greater than the abundance of anti-hyperons, due to their preference in fragmentation into light quarks. These direct reaction mechanisms are References thus in a substantial disagreement with the results expected as based on our primordial QGP model, and [ 1 ] J. Rafelski, Phys. Rep. 88 (1982) 331. in particular with the expectations presented here [2] J. Rafelski and M. Danos, Phys. Lett. B 192 (1987) 432. about the high m ± region of the spectrum. The only [3] WA85 CoUab., S. Abatzis et al., Phys. Len. B 244 (1990) 130; viable "conventional" alternative is a long lived HG, J.L. Narjoux, presentation at Quark Matter '90 Meeting which requires to be established by a quantitative and (Menton, May 1990). precise comparison with the experiment including in [4l WA85 Collab., S. Abatzis et al., Phys. Lett. B 259 ( 1991 ) particular measurement of the lifetime by e.g. a pre- 508; cision HBT pion or perhaps kaon interferometry. D. Evans, presentation at Quark Matter '90 Meeting (Menton, May 1990). Normal lived HG which is off-equilibrium in its anti- [5] NA35 Collab., J. Bartke et al., Neutral strange particle baryon contents [ 9 ] is already inconsistent with cur- production in sulphur-sulphur collisions at 200 GeV/ rent abundances of anti-cascades and anti-hyperons, nucleon, Z. Phys. C, to be published; since reactions in HG living = 30 fm/c cannot lead R. Stock, presentation at Quark Matter '90 Meeting to these ratios. Other, less conventional mechanisms (Menton, May 1990). [6] D. Evans (WA85 Collab. ), presented XXVI Rencontre de may be introduced to describe the abundant strange- Moriond 1991 (Les Arcs, Savoie, France, March 1991 ). ness production in dense and highly excited nuclear [7] WA85 Collab., S. Abatzis et al., Study of baryon and matter. Such could be the process of melting of chiral antibaryon spectra in sulphur-sulphur interactions at 200 symmetry breaking [ 18,19 ], which leads to strange- GeV/c per nucleon, CERN report CERN/SPSLC 91-5, ness enhancement as a consequence of a possible re- proposal SPSLC/P257 ( 11 January 1991 ). [ 8 ] H.C. Eggers and J. Rafelski, Intern. J. Mod. Phys. A 6 ( 1991 ) duction of strangeness thresholds and/or enhance- 1067. ment of production cross sections. Such phenomena [9] P. Koch, B. Miiller and J. Rafelski, Phys. Rep. 142 (1986) could help establish the HG equilibrium abun- 167.

339 Volume 262, number 2,3 PHYSICS LETTERS B 20 June 1991

[ 10] J. Rafelski and B. MOiler, Phys. Rev. Lett. 48 (1982) 1066; [ 15 ] K.S. Lee, M. Rhoades-Brown and U. Heinz, Phys. Rev. C 56 (1986) 2334(E). 37 (1988) 1452. [ 11 ] P. Koch, B. Miiller and J. Rafelski, Z. Phys. A 324 (1986) [ 16 ] N.K. Glendenning and J. Rafelski, Phys. Rev. C 31 ( 1985 ) 453. 823. [ 12] T. Matsui, B. Svetitsky and L.D. McLerran, Phys. Rev. D [ 17 ] J. Rafelski and A.L. Schnabel, Phys. Lett. B 207 ( 1988 ) 6; 34 (1986) 783, 2047. in: Intersections between particle and nuclear physics, AlP [ 13 ] H.W. Barz, B.L. Friman, J. Knoll and H. Schulz, Nucl. Phys. Proc., Vol. 176, ed. G.M. Bunce (AIP, New York, 1988) p. A484 (1988) 661; A 519 (1990) 831. 1068. [14] P. Koch, J. Rafelski and W. Greiner, Phys. Lett. B 123 [18] G.E. Brown, Phys. Rep. 163 (1988) 167. (1983) 151. [ 19 ] D. Lissauer and E.V. Shuryak, Phys. Lett. B 325 ( 1991 ) 15.

340