Volume 262, number 2,3
PHYSICS LETTERSB
20 June 1991
Strange anti-baryons from quark-gluon plasma
J o h a n n Rafelski
Department of Physics, Universityof 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-baryonsignal for
the identification of the QGP state and for the diagnosis of its properties is demonstrated.
The general problem one encounters trying to diagnose by means of strongly interacting particles the
presence and properties of the putative quark-gluon
plasma ( Q G P ) state of highly excited hadronic matter is that most particles observed in high energy nucleus-nucleus collisions have to pass through the stage
of the hadronic gas ( H G ) consisting of individual
hadronic particles, in which their abundance and
spectrum is substantially altered. However, the composition of the Q G P phase ofhadronic matter differs
from the H G phase in crucial detail: the density of
strange anti-quarks is considerably greater. This occurs due to the rapid strangeness production by glue
based processes and inherently greater particle and
energy density in the plasma state. High strange (anti)quark density facilitates the formation of multiply
strange baryons and anti-baryons not only during the
hadronization conversion from QGP to HG, but also
in the primordial period of the plasma evolution, associated with highest temperature and density conditions. A substantial enhancement of production
rates of multi-strange anti-baryons in nuclear collisions [ 1 ] in particular at central rapidity and at highest transverse masses [2 ] has therefore been proposed as a characteristic signature of QGP. This
suggestion was also made because it is difficult to find
other mechanisms which could give rise to such
abundance anomalies. The relative abundance of
centrally produced anti-cascades -~= s~(t, anti-hyperons Y=~ClCl and anti-nucleons N=Cl~l(1, was suggested as a qualitative signature of (at least initially)
explosively disintegrating QGP.
The model we develop here is relatively simple and
is applicable mainly to the computation of ratios of
particle abundances within a narrow kinematic domain. In our approach we will only employ well established principles of statistical ensemble physics.
Such an approach is justified as we will be able to work
around possible transparency, flow or spatial inhomogeneity of fireballs formed in individual nuclear collisions. To this objective we will consider in particular only the narrow, central region of rapidity and
particles of high transverse mass. While in such an
approach we forfeit certain most interesting aspects
of the global reaction picture, our interpretations are
model independent, and independent of tacit assumptions about unknown physics surrounding conversion of colored particles into asymptotically observable hadrons.
Today, the search of QGP at CERN involves collisions of 200 A GeV sulphur (S) nuclei with a target
nucleus which are considered at small impact parameters. In the WA85 experiment [ 3,4 ] the target is the
tungsten (W) nucleus. The NA35 experiment [ 5 ]
takes advantage of the symmetry between target and
projectile, using sulphur as target. In small impact
parameter S-S collisions all nucleons participate,
while in the asymmetric collision the effective target
consists of the tube of nucleons in the center of the
tungsten nucleus which is in the path of the projectile. Hence the effective mass of this target is 80-90
and the central rapidity region is yf= 2.5 + 0.1, while
S-S collision data is symmetric around the central region yf= 3. These values are significantly different
0370-2693/91/$ 03.50 © 1991 - ElsevierScience Publishers B.V. ( North-Holland )
333
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from the rapidity of the projectile, yp= 6 and hence
the particles emanating from the central fireball are
easily distinguished from the projectile fragments by
virtue of the rapidity gap of 3-3.5 units. In these experiments strange particle production has been measured and the results indicate the possible presence
of a new strange particle production mechanism.
The most interesting result reported in ref. [ 4 ] is
Rz := E - / E - = 0.39 + 0.07
fory~ (2.3, 3.0) andp± > 1 G e V / c .
( 1)
In p - W reactions in the same (p±, y) region a smaller
value for the Rz ratio, 0.27 + 0.06, is found. The data
for the A / A ratio reported in ref. [ 3 ], after appropriate corrections for contamination from cascade decays [ 6 ], is
RA : = A / A = 0 . 1 3 +0.03
for y~ (2.4, 2.8) a n d p ± > 1 G e V / c .
(2)
This result suggests that the ratio RA in S-W collisions is smaller than in the p - W collisions in the same
kinematic range, as these ratios were nearly equal [ 3 ]
before corrections for cascade contamination were
applied. We note that this effect may arise in part as
a consequence of the production of A in rescattering
of kaons in spectator baryonic matter. Therefore it
would be of considerable advantage from the systematic point of view to consider symmetric collisions of
heavy nuclei, such as Pb-Pb. We also note the preliminary experimental values for ratios of different
baryons, obtained in the kinematic domain of eqs.
(1), (2) [6]:
-=-/A=0.6 +0.2,
E/A=0.20+0.04.
(3)
If these results of eqs. ( 1 ), (2), (3) are confirmed
with greater statistics, and preferentially in symmetric collisions, this would contradict the view which
holds that in nuclear collisions the rare central production of (strange) anti-baryons should be suppressed as compared to p-A collisions. (Strange) antibaryons are produced in individual N - N interactions with preference at small XF where they would
be subject to inelastic reactions with other baryons of
the target and potentially projectile nuclei, leading to
a greater absorbing effect when a nuclear projectile is
used with greater atomic number. Also, eq. (3) sug334
20 June 1991
gests that we have a comparable number of the more
difficult to produce E - as compared to A.
There are further strong indications that the total
abundance of strangeness grows faster than the general particle multiplicity, suggesting a more efficient
mechanism of strangeness production. This is reported in terms of the ratio of strangeness to negatively charged particles by the NA35 experiment [ 5 ],
which doubles as the multiplicity of negatives increases. Similarly, in the WA85 experiment, which is
only triggered for highest multiplicities, one finds an
enhancement of the A abundance [ 7 ], when comparing p - W to S-W interactions in the central rapidity
y~ (2.4, 2.65) region as a function of the transverse
mass spectrum. The A spectrum, when extrapolated
from m± e ( 1.5, 2.5) GeV to 1 GeV beats in its magnitude the spectrum of all negative particles, presumably pions, and there is an enhancement by a factor
1.7 of A, A abundances in comparison to the negatives, as the projectile changes from p to S. We further note that the WA85 transverse mass spectra of
A, ~ suggest a higher "temperature" (inverse slope)
of TA = 240 + 20 MeV [ 6 ], which is somewhat greater
than the other reported temperatures T = 195 MeV,
derived from all momenta ranges of singly strange
hadrons [ 5 ], albeit the latter value is determined in
presumably less thermalized and more transparent SS collisions.
We must consider in detail if indeed a new physics
phenomenon is being observed or if we can explain
these results consistently within a conventional particle production framework. We must consider which
further information is required to narrow and perhaps even identify the eventually needed novel reaction mechanism. It would be in particular interesting
to see if one can use the present results within a simple QGP based model to predict a precise value of the
ratio A / p which becomes available soon [7], and
perhaps also f ~ - / E - . Hence we must consider all
these important observables as well. We have to show
how to discriminate the available anti-baryon-baryon ratios Ri against the H G models.
Before proceeding to a detailed analysis of the experimental results, we need to recall the relevant theoretical developments on which the model we employ rests [8,9 ]. Once a QGP is formed the gluonic
production rate dominates the process of strange
quark pair production and the time nee 'ed to satu-
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rate the phase space (so called chemical relaxation
time constant) is smaller or at worst comparable in
magnitude to the estimates of plasma lifetime based
on nearly free hydrodynamic expansion [ 1,10 ]. This
result has been reconfirmed by more refined studies,
which included kinetic evolution of the plasma [ 1113]. These calculations have further shown that
strangeness abundance freezes out and does not annihilate, being thus characteristic of the densest state
of the QGP. On the other hand, the H G phase cannot
produce in the required abundance either strangeness or (strange) anti-baryons at central rapidity [ 9 ]
unless one makes completely unrealistic assumptions. Such can be a very high temperature T > 300
MeV, which is inconsistent with the known particle
spectra, or the long lifetime of the H G fireball which
needs to be more than 10 times longer than a reasonable value of zHG< 15 fm/c, which value is consistent with the pion interferometry results. In the explosive hadronization of the Q G P phase the
anomalous strange particle density is seen in the
anomalous ratios of strange anti-baryons [9 ], but a
transition, which is slow and allows for reequilibration of the abundances, loses much of this effect [ 13 ].
To avoid that the comparison of the theory with experiment hinges on the details of the unknown mechanism ofhadronization it was suggested [2] that primordial particles, i.e. those with high p±, are
considered. These are emitted preceding the global
hadronization, mostly in an explosive evaporative
recombination process. The baryon and anti-baryon
relative abundance arising from such a formation
mechanism is controlled by two factors: the statistical multiplicity factors, describing the likelihood of
finding among three randomly assembled quarks the
suitable spin-isospin of the emitted particle, and the
chemical fugacities 2 = e x p ( p / T ) which define the
relative abundance of both quarks (2q) and antiquarks (2~- l ), where the temperature Tand chemical
potential p are to be considered in general as functions of position and time. Since the probability to
find three independent particles with total energy
E >> T is proportional to exp ( - E ~ T) in any thermal
system, the transverse mass spectrum at high momentum will indeed be characterized by the temperature present at the primordial formation of a composite particle. Thus the high momentum particle
emission due to the quark recombination mechanism
20 June 1991
will be thermal in appearance, while some minor distortion may occur due to fragmentation processes
which may also occur but have a negligible impact at
high P_L. This is despite the fact that in the global hadronization of the plasma a substantial fragmentation
of gluons and quarks is required in order to assure
that the entropy rich plasma phase finds a path to the
relatively low entropy density of the HG. In the global
hadronization process a hadro-chemical equilibrium
is established in which a substantial strangeness
chemical potential prevails [ 14,15]. In this regard
QGP differs significantly from the HG. Both in the
early QGP as well as in the fragmentation of QCD
quanta there is always an exact symmetry when s, g
pairs are produced, and hence/t~ 6P = 0.
We now obtain the particle abundances as functions of fugacities: all baryons considered have spin
½ and the flavor quantum number will be explicitly
considered, hence the other statistical factors are unity. This allows us to ignore the isospin eigenstates,
also because the abundance of A a n d / k ( I = 0 ) implicitly includes the abundance of E ° and Zo ( I = 1,
•3=0) arising from the decay X ° ~ A ° + 7 ( 7 4 MeV)
and similar for X°. Thus comparing spectra of particles within overlapping regions of m ± we find for their
respective ratios
E --
R - - - E - --
2 d 1~ S 2
2d2~
= exp(--2/td/T) exp(-4lts/T),
RA- A -
(4)
202u2s
= exp[-2(/td +/Iu)/T] exp(-2/tJT).
(5)
Ignoring isospin differences for the moment, 2u
=,,].d =:.~q, we obtain
RA = ( 2 J 2 q ) 2 R~.
(6)
In Q G P we have As= 1,2q> 1, while in equilibrated,
baryon rich H G 2s/2q~ 1 [ 14]. Thus both in H G and
primordial baryon rich QGP we find that RA < R~ but
for different reasons, and thus both phases lead to
different values of R•, R=_.The cascade and lambda
ratios can easily be related to each other, showing explicitly the respective isospin asymmetry factors and
strangeness fugacity dependence. Eqs. (4), (5) imply
RA=R~exp[2(ltd--~u)/T] e x p ( 6 p J T ) .
(7)
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We emphasize that eq. (7) is rather generally valid,
irrespective of the state of the system (HG, QGP).
But clearly, we have now eliminated most of the ignorance about the chemical composition of the state
from which the particles are emitted - the result is
now characterized by the value of#ff T, which is very
different for the different reaction scenarios. We record also an interesting prediction for the ratio of
abundances of anti-protons to anti-lambdas to anticascade:
E-
A
-~--- p-
=
2;-'
2~_j - e x p ( / ~ / T ) e x p ( - # f i T )
(8)
>1.
(9)
Taking the experimental value we find for the ratio
eq. (9) the result 1.73 + 0.40.
It should be remembered that all the above equations apply for ratios of particles as a function of (m ±,
y), in the domain in which one can use the Boltzmann approximation to the spectrum of an (assumed) thermal emitter. If we were to compare global
abundances, then we would have to allow for additional factors reflecting on both the partial origin of
the particles ( H G or Q G P ) and the size of the phase
space ( oz exp ( - m / T ) ), while for the full phase space
integral the Boltzmann approximation may also not
be always adequate. Nevertheless it is remarkable that
both ratios shown in eq. (8) are equal, and as shown
in eq. (9) indeed greater than unity. This result has
been obtained under the assumption that strangeness
production has saturated the full phase space available ("absolute" chemical equilibrium). However,
eqs. (8), (9), unlike eqs. (4), (5), are sensitive to
the degree of correctness of the assumption of absolute chemical equilibrium. Denoting by a factor y < 1
the deviation from the absolute chemical equilibrium abundance for strange and anti-strange quarks,
i.e. 2s--,y2s, 2~-~ ~y2~-~, we find that the right-hand
side ofeq. (8) is multiplied by the factor y, as it compares the degree of equilibration of strange-antiquarks with up-anti-quarks. We recall that especially
in the early stages of the QGP formation, the strangeness phase space may not be fully saturated - calculations suggest [ 9 ] that one should expect up to a factor 2 under-saturation for S-W collisions, suggesting
336
20 J u n e 1991
up to a factor 2 reduction of the achievable ratio. Here
again P b - P b collisions could improve the understanding significantly, as the time available to produce strangeness in the (hypothetical) QGP phase
would be significantly extended. The ),-factor does not
appear in the ratios R which compare strange to antistrange quarks, eqs. (4), (5), as s and ~ are equally
abundant during the approach to absolute chemical
equilibrium, any asymmetry arising in consequence
of rescattering and is contained in the quark chemical potential. When the fireball consists of H G - the
anti-strange quarks g are mostly found in kaons
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 redistribution of strange quarks leads to relative chemical equilibrium, even if the absolute chemical equilibrium, i.e. saturation of strangeness phase space, was
not reached. Therefore, abundances of Y and K are
descriptive of the thermodynamic conditions at the
freeze out of the H G fireball and can suggest new
physics only by a possible enhancement of the
strangeness abundance. In summary, the key differences between the phases thus are:
(1) the difference of the value of ;ts=exp(~q/T)
which enters in a very pronounced way e.g. in eq. (7);
(2) the likely near saturation of the strangeness
phase space, i f Q G P was formed ( ~ 1 ), a highly unlikely condition for HG;
(3) high "temperature" of the high p± strange antibaryons, which originate from the pre-hadronization
epoch of the fireball.
In order to get some numbers it is convenient to
rewrite the isospin asymmetry factors appearing in
eq. (7). It is customary to introduce the notation
~t/d = ,/./q "~ O. 5t~,t/q ,
(10)
#~ =#q -- 0.58pq,
(11 )
/./q = 1 (]2d ...~ ]~u) ,
(12)
~lUq ~- ]./d - - flu ,
(13)
fib = 3 ~ q
(14)
.
The isospin asymmetry ~q/~q is very small, and we
will determine it quantitatively further below in a
QGP-fireball model. First, we need to have a measure of the quark chemical potential #q. This can be
done by combining the ratios in such a way that the
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PHYSICS LETTERS B
strangeness fugacities (chemical potentials) cancel.
While this is best done by comparing the K - abundance with the A abundance [ 9 ], we do not have this
information presently available. However, we note
KRA
2 u l),s
A -- ~
- 2,2d2s - exp [ - (/td + 2#u ) / T]
= exp(--#b/T) exp(0.58pq/T).
(15)
Taking RA=0.13_+0.03 and neglecting the isospin
asymmetry factor we find
20 June 1991
(d> - (d>
(u)-(o)
2-Zf/Af
l+Zf/Af
(16)
In the tube model, in which all nucleons in the target
in the path of the isospin symmetric projectile participate in the fireball, this factor in the sulphur-tungsten collision is 1.086 (for Zr= 38, Af= 87).
In the Q G P we can use the analytical expressions
for the flavor (quark) density and arrive at
(d)-(d)
(u)-(fi)-
,uJT[l+(lta/TrT) 2] ~ #__~d
,
lzJ T[ l + (l~/TtT) 2] - I~
(17)
/zq/T=0.52 +0.1 ,
irrespective of the composition of the source (HG,
Q G P ) of the strange (anti-) baryons. We can use this
result with either eq. (4) (or eq. (5) with less precision ) and we obtain
/zs/T= - 0.02 + 0.06.
While this result is compatible with zero, the error is
too large to assume this very important value: if ks
were to vanish this would indicate exact symmetry
between the produced strange and anti-strange
quarks, which can only occur in either a baryon number free H G phase or in a Q G P before the hadronization process. As there is a clear baryon asymmetry
as described by the substantial Ilq/T value determined above, only the latter case would be possible hence forthcoming increased precision of the measurements combined with additional crucial information about the ratios of different anti-baryons, eq.
(8) will allow to determine the value of#s, which will
become one of the pillars of the argument for Q G P
origin of the strange anti-baryons.
Given the above determined model independent
value of/zs associated with the reported high m ± > 1.5
GeV part of the central strange baryon and anti-baryon spectrum we now follow up the hypothesis that
these particles arise in the primordial, explosive
emission from a Q G P fireball. Such a description reduces the errors - but of course makes all results
"model dependent". We first estimate the isospin
factor encountered in the above equations,
exp [ (/~d --/~, ) / T] = exp (8/~q/T) both for the case of
the H G and for the case of the Q G P phase. We count
the number of up and down quarks and compare it
to the ratio brought into collision by the colliding
nuclei:
where the last equality arises in the regime of interest
here - (#q/zrT)2 << 1. We find
6//q//./q = 0.09
for Q G P in S-W collisions. Setting As= l in Q G P we
further have
#q/T( 1 +0.58#q//~q) =/~o/T= ½I n ( Z / E )
-- 0.47 _+0.08.
Which implies
/~q/T=0.46+0.08,
8~q/T=0.041 _+0.007.
We can use this result, together with/~s=0 and eq.
(8), to predict the key strange anti-baryon ratios expected from primordial Q G P (where as discussed
above, 0 < y~< 1 characterizes the approach to absolute chemical equilibrium of strangeness):
E - IA=fklf~=y 1.55+0.13,
E - / A = A / p = y O.64 _+O.05 ,
f l - / F , - = y 1.61 + 0 . 1 3 ,
f~-/--
= y 0.62-+ 0.05.
Comparing with the first results on these ratios eq.
(3), we can extract a first estimate of strange phase
space saturation: y=0.4_+0.2. Here we used the
strange anti-baryon ratio, to avoid the systematic
questions related to the origin of the A abundance. If
the strange baryon ratio is used, the result is
y = 0.31 _+0.10 _+systematic. These results compare
well with earlier work on the strange anti-baryon ratios, see fig. 2 of ref. [ 2 ] for the here implied value
/Zb/3 T = 0.46 (note that the values shown are by definition twice the values considered here). Other results of ref. [ 2 ] show that the possible fragmentation
337
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PHYSICS LETTERSB
contribution to the primordial baryon and anti-baryon production is at about 10%.
Another quite interesting aspect of this discussion
is that once #q is known, using the entropy content of
the QGP fireball we can predict the expected pion
multiplicity n, as compared to the baryon multiplicity b, following the procedure of ref. [ 16 ]. Taking
again the QGP perturbative equations of state as discussed e.g. in ref. [ 17 ] we find that the entropy per
baryon is given by (retaining only the dominant terms
in leading order in T/#q)
S
3n 2 T ( 1 4 ( 1 - 5 O a J Z l z t )
b-
2 #q\
= 17 ( T / # q )
]-ff(l~
32(1-15aJ4rO~
-t 4 5 ( 1 - 2 o q / n ) J
f o r oq = 0 . 6 .
(18)
For the value of T/pq determined here ( = 2.2 + 0.4)
we have thus 37 + 6 units of entropy per baryon in the
primordial QGP phase at the time of multi-strange
(anti-)baryon emission. Since at the end of the hadronization process the entropy content will be similar, though certainly somewhat larger, this permits us
to determine a lower limit on the pion to baryon ratio, which applies in particular to central particle
spectra, extrapolated from high m i to lower m . ,
which procedure avoids additional components in the
spectra arising from processes occurring in the final
evolution of the hadronic gas (e.g. A-decay, rescattering off the spectator matter, etc.). To find this primordial ratio we note that each baryon emitted will
take away entropy in the amount of
Sb
b
1.5+ m--3pq ~ 4
T
(19)
leaving behind about 33 units of entropy; each pion
will carry away 4.05 units and hence the pion multiplicity per baryon will be n,Jb= 8.2 + 1.5. Neglecting
the small isospin asymmetry, this suggests 7t-/
p = 5.4 + 1. For an experiment it is important to know
hov' many (negatively) charged particles occur at the
same m±. To obtain this information we note that
given our estimate of the high m i - p r o t o n s we find
that the total primordial abundance ratio is n - /
f~e (133,51). However, the thermal ratio of abundance at high m i is
rt ~'-
338
=exp(3#q/T) =4.5,
high m±, central y
(20)
20 June 1991
at the established value # q / T = 0.5, while the natural
global abundance ratio would be (taking T = 200 MeV
for the "hard" component of the particle spectra)
2
exp(3#q/T) iX{m__~} K 2 ( m J T ) =45
f~
\ m N / K2(mN/T)
(21)
"
Thus the global ratio of the abundance of anti-protons to pions arising from QGP is just slightly suppressed comparing to HG, suggesting that at high m .
the ratio of re- to p will be up to a factor 3 larger than
can be expected from thermal models. However, as
this discussion shows, at given m i >/1.5 GeV there
may be a remarkable abundance of anti-protons: the
ratio of anti-protons to protons at high m i is
f~/p= e x p ( - 6 # q / T + ~ # q / T ) = 0.,,,,,,_oo24na~+°'°4°. (22)
The relatively large error of this result indicates that
it is very sensitive to the chemical potentials and thus
one may use this ratio as measured at high m± to determine the actual value of#q, provided that systematic errors are reduced by choosing a symmetric collision system. This confirms that the anti-baryon flow
is expected to be dominated by strange and multistrange particles - but the anti-proton high m± flow
is itself substantial, with one anti-proton being expected for each 10-25 protons or for each 4-5 negative pions.
On several occasions in the discussion of the experimental results additional or more precise data would
have enhanced our understanding. Foremost, there is
need for better statistics, and lesser systematic uncertainty arising from the presence of a large number of
non-participating baryons. We have aside of the
"temperature" parameter at least three other quantities to measure: the two chemical potentials (#q, #s)
and the parameter characterizing the approach to
equilibrium y. Hence at least four independent
(strange) anti-baryon ratios need to be measured in
order to determine the values of these parameters,
verify their consistency and show e.g. that #s = 0. This
requirement indeed means, adding as additional parameter the absolute normalization of the particle
spectra, that we need to determine five different particle (strange baryon and anti-baryon) spectra separately, with a precision of a few percent. Thus in addition to the four species (A, A, E -, E - ) we need
further data for e.g. 1Oand/or f l - or/and f ~ - . In ad-
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PHYSICS LETTERS B
dition, we need to compare the results of A-A collisions with N-A reactions. The present data only fixes
the parameters of the Q G P model we used; to confirm it we need some other particle ratio as just described, and we have made clear predictions what we
expect. Even if such experiments were to be successful, it will be necessary to seek where a change in the
strange particle flow (the relative abundance of anticascades to anti-hyperons to anti-nucleons) occurs as
a function of the CM collision energy in the likely
range of 2-30 A GeVcu. This is practically an impossible task without a variable energy heavy ion collider.
When and if our predictions about further
(strange) anti-baryon abundances are experimentally established, this will conflict with predictions
obtained in terms of individual hadron cascade
models which require irrespective of m± that E - are
less than 10% of the anti-nucleon abundance, rather
than being up to 1.6 times more abundant as suggested here. Should individual microscopic processes
driven by anti-di-quarks fragments of the projectile
be the source of abundant strange anti-baryons [ 5 ],
then the abundance of anti-nucleons will be more than
three times greater than the abundance of anti-hyperons, due to their preference in fragmentation into
light quarks. These direct reaction mechanisms are
thus in a substantial disagreement with the results expected as based on our primordial Q G P model, and
in particular with the expectations presented here
about the high m ± region of the spectrum. The only
viable "conventional" alternative is a long lived HG,
which requires to be established by a quantitative and
precise comparison with the experiment including in
particular measurement of the lifetime by e.g. a precision HBT pion or perhaps kaon interferometry.
Normal lived H G which is off-equilibrium in its antibaryon contents [ 9 ] is already inconsistent with current abundances of anti-cascades and anti-hyperons,
since reactions in H G living = 30 fm/c cannot lead
to these ratios. Other, less conventional mechanisms
may be introduced to describe the abundant strangeness production in dense and highly excited nuclear
matter. Such could be the process of melting of chiral
symmetry breaking [ 18,19 ], which leads to strangeness enhancement as a consequence of a possible reduction of strangeness thresholds a n d / o r enhancement of production cross sections. Such phenomena
could help establish the H G equilibrium abun-
20 June 1991
dances, but could hardly alter the H G based strange
anti-baryon to baryon ratios. In particular, the melting of strange particle thresholds does not turn offthe
particle-anti-particle annihilation, indeed it ought to
stimulate it - making strange anti-baryons again a key
signature of QGP formation.
Our conclusion is that better and somewhat expanded data on (strange) anti-baryon flow, will permit us to identify the source of the high m± centrally
produced particles as the primordial or/and explosive Q G P state of matter. The present results are already very suggestive of this interpretation. However, the given error bars, combined with the
systematic uncertainties accompanying asymmetric
collisions lead to a too large uncertainty. We have
presented here a method and provided a wealth of
detailed predictions, which may be employed to study
the evidence for the QGP origin of high P_L strange
baryons and anti-baryons.
I would like to thank Emanuele Quercigh for discussions of the WA85 Collaboration's experimental
results.
References
[ 1 ] J. Rafelski, Phys. Rep. 88 (1982) 331.
[2] J. Rafelski and M. Danos, Phys. Lett. B 192 (1987) 432.
[3] WA85 CoUab., S. Abatzis et al., Phys. Len. B 244 (1990)
130;
J.L. Narjoux, presentation at Quark Matter '90 Meeting
(Menton, May 1990).
[4l WA85 Collab., S. Abatzis et al., Phys. Lett. B 259 ( 1991 )
508;
D. Evans, presentation at Quark Matter '90 Meeting
(Menton, May 1990).
[5] NA35 Collab., J. Bartke et al., Neutral strange particle
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