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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 0370-2693/91/$ 03.50 © 1991 - Elsevier Science Publishers B.V. ( North-Holland ) 333 Volume 262, number 2,3 PHYSICS LETTERS B 20 June 1991 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, presum- ably 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 fur- ther 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.
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