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Dear Berndt and Peter,
thank you for sending me the draft of the status report of the SPS
programme based on our discussions at the HIPS98 meeting.
Although formulated very concisely, I think that the draft prepared by
you can still profit from a few improvements, and I would like to make
some corresponding suggestions. I will give arguments for these
suggested improvement which necessarily reflect my own personal
assessment of our situation which may not in every respect agree with
the opinion of my colleagues. Where such disagreement exists we should
try to find a formulation which most us can agree with. The present
draft, however, contains formulations which I know from various phone
calls which I got on Friday raises objections from a non-negligible part
of our community and which therefore certainly require modification.
1. In the second paragraph on ``the most notable achievements over the
past year'' the discovery and quantification of directed and
elliptic flow and the qualitative breakthrough which this has
caused and will cause in our understanding of collective flow
phenomena in relativistic heavy ion collisions and of the collision
dynamics altogether is missing. Thermalization by kinetic
equilibration without collective flow makes no sense; the picture
is only complete if both phenomena are proven to exist, and a lot
has happened in this direction in the last year or so. (In this
context I would also like to refer to E. Shuryak's open letter
after HIPS98 with which I mostly agree.) You discuss radial flow in
your section on hadron thermometry, but you completely leave out
elliptic flow. The latter is, however, very important because it is
believed to be generated in the very early stages of the collision
and thus may, once quantitatively understood, become a cornerstone
for proving EARLY thermalization (possibly before hadronization). I
therefore suggest to add to the first sentence of the second
paragraph as follows:
``...below the rho-resonance, the thermal and chemical distribution
of produced hadrons, and the collective flow pattern established
in relativistic heavy-ion collisions. We now...''
Furthermore a new section 4. discussing the flow aspects should be
inserted after section 3., the section on Omega enhancement thus
becoming section 5. (see below).
2. The discussion of J/psi suppression is fine except for the last two
sentences. My first reaction was to object to the choice of the
word ``anomalous'' in the first sentence because it reminded me of
the definition of ``anomalous'' given by L. Kluberg as ``deviating
from a straight-line extrapolation of p+p and p+A data'' in the
NA50-plot of (J/psi)/DY vs. AB. This, I think, is somewhat
arbitrary since the straight line reflects a certain theoretical
prejudice based on a certain class of theoretical models which are
still not uncontroversial. But then later in your text you define
``anomalous'' implicitly by ``strongly different E_T-dependence in
Pb+Pb compared to lighter systems'' which I can live with.
However, I cannot accept the last two sentences: ``Such a behaviour
is expected if...''. The first of these two sentences suggests
implicitly (and this is how it will be read by the community at
large who has been exposed to this implicit suggestion in the
form of a definite claim or even a proven fact) that, at fixed beam
energy of about 160 A GeV, the critical energy density for
deconfinement will be crossed by changing the centrality of the
collision or otherwise the number of participating nucleons. This
suggestion is ill-placed here because it contradicts with the last
sentence of the report where it is suggested to cross the critical
energy density by reducing significantly the beam energy, and a
careful reader will notice this. Worse, this expectation can, and
this is my firm position, NOT be reconciled with the bulk of the
``anomalous'' (in the sense of interesting and clearly of
collective nuclear nature) phenomena in light hadron production
which are already seen in lighter systems (with smaller numbers of
participants) and which do NOT show a second anomaly at the point
where J/psi suppression becomes ``anomalous''. My personal
impression of the discussions in Chamonix was that, in fact, serious
doubts were raised about the existence of a direct relation between
``anomalous J/psi suppression'' (which was, in fact, interpreted --
I think, correctly -- as mostly ``anomalous Chi suppression'') and
the deconfinement threshold.
The last sentence in this paragraph is even more misleading. Even
if I represent a minority opinion I must voice my objections to the
applicability (or relevance in the context of the present data) of
the ``prediction of two consecutive thresholds due to the
dissolution of the chi_c and J/psi states''. This prediction is
based on the static picture of charmonia immersed in a pre-existing
thermalized QGP which are dissolved by Debye screening of the
confining potential. Not only is this picture completely
unrealistic in view of the strong dynamics in heavy ion collisions,
but it also suggests the same threshold for the melting of the
chi_c and the psi'. To explain the different observed thresholds for
anomalous chi_c and psi' suppression one then invokes the AD HOC
explanation that one is due to deconfinement while the other is due
to hadronic comover scattering. This is entirely ad hoc in the
sense that I have not seen a consistent calculation of both effects
within the same dynamical model.
With the last two sentences we would perpetuate 10 year old
prejudices which rely on oversimplified model calculations which,
as we should have learnt by now, have little to do with reality. In
reality the dense initial stage is highly dynamical and has a
strong time dependence which DOES depend on the size of the
collision system resp. the number of participants. In repeating
the same old claims one closes ones eyes and ears to the obvious
contradiction of such a picture with light hadron phenomenology
(without even mentioning it!), leaving the impression that we are
unable to learn from new and more comprehensive experimental
evidence.
For these reasons I strongly plead for REMOVING these two sentences
without substitute. This would take nothing away from the strong
positive statements in the first part of this paragraph, and would
avoid preempting an ongoing discussion which may revolutionize our
thinking about the phenomenon and cause us to eat our tongues in
the years to come if we stick to the present formulation.
3. In the discussion on low-mass dileptons I have a different
recollection of the theoretical discussions than the one reflected
in the sentence ``While theorists still argue...''. I found the
arguments by J. Wambach very convincing that there is no
discernible direct connection between these data and medium
modifications of hadron properties caused by an onset of chiral
symmetry restoration in the form of a reduced scalar quark-
antiquark condensate. All HIPS98 participants to whom I talked
had the same impression, and nobody from the other camp was there
or stood up in order to argue why the theorems which J. Wambach
quoted when demonstrating the nonexistence of such a connection are
wrong or do not apply. The wording given in the report does not
reflect this significant turn of events on the theoretical side. I
think that the following sentence (replacing the one quoted above)
reflects the outcome of HIPS98 more faithfully:
``These data have stimulated significant new theoretical
developments. While it is no longer clear that a direct connection
between the data and the phenomenon of chiral symmetry restoration
(in form of induced medium modifications of mesons) can be
established, it appears certain that these data will provide...''
(etc. as in the existing draft)
4. In the section on hadron thermometry I only note three typographical
errors: a colon is missing after the subsection title, and on the
top of page 2 an inverted comma is missing in ``chemical''
temperature. Towards the end J/psi is once written with a straight J
rather than with an italic J as everywhere else.
[As an aside: I found the point you make in the second last sentence
of section 3 very interesting: it implies that a measurement of
the psi' transverse momentum spectrum can distinguish between the
explanations of its ``anomalous'' suppression by hadronic comover
rescattering and by early partonic collisions or even Debye
screening: In the former case the surviving psi' mesons should pick
up the developing flow of the hadronic comovers and show much
flatter p_t spectra than the chi_c and direct J/psi mesons. I
should check what the existing data say about this... In any case,
this can be measured, of course.]
5. As mentioned above I would suggest to include a short new section
on flow between the present sections 3 and 4, with the following or
similar words:
``4. {\em Collective flow:} In addition to the radial transverse
flow discussed above, the existence of directed and elliptic
transverse flow has now been clearly established in semicentral and
peripheral Pb+Pb collisions by the NA49 collaboration. In contrast
to the former, which reflects the pressure history integrated over
the entire expansion phase, the latter is believed to be generated
very early in the collision. Monte Carlo simulations based on early
string dynamics with an ultrasoft equation of state, followed by
hadron formation and hadronic rescattering, fail to reproduce in
particular the elliptic flow whose strength was found to peak near
midrapidity. Elliptic flow thus points to an isotropic pressure
component (and thus to the onset of thermalization) at a very early
stage of the collision for which other observables and theoretical
simulations suggest energy and particle densities above the
deconfinement threshold.''
6. I feel very uneasy about the labelling of the section on
strangeness enhancement as ``Omega enhancement''. First, our
conviction that the observed strangeness enhancement pattern is
unusual and really signals an interesting effect with origins
before hadronization is based on the newly available systematics of
the enhancement pattern with strangeness content of the hadrons.
An enhancement (relative to p+A) which GROWS with the strangeness
content of the hadron (and thus with its mass!) is consistent with
statistical (``equilibrium'' or Maximum Entropy) hadronization of a
deconfined state with enhanced strangeness, but contradicts naive
expectations based on the rising thresholds for heavier
multistrange particles if you want to form them by hadronic
rescattering. Secondly, even though the enhancement effect is most
prominent for the Omega, it is also the Omega which suffers the
strongest canonical suppression in p+p and p+A (for the parameters
of the thermal fit to pp collisions at sqrt(s)=27 GeV performed
in the paper by Becattini and myself, the canonical suppression
factor for \Omega+\bar\Omega was calculated last weak by J. Sollfrank
as 12, while the observed enhancement factor is 17.) A factor 12
enhancement could thus be due simply to hadronization of a partonic
system with the SAME thermodynamic properties as in p+p, only in a
larger nuclear volume; only the remainder can be argued to be due
to real excess strangeness production before hadronization.
In other words, I think that the convincing power of the
strangeness signal cannot and should not be diluted by
concentrating the attention on the \Omega. Besides being unfair to
the other strangeness experiments, it is really only by taking all
the strange particle production features together (including the
simple global strangeness enhancement by a factor 2 or so beyond
what you can get by removing the canonical suppression in
elementary collisions) that the strangeness signature becomes more
than interesting, but convincing. The new overall systematics, and
not only the single observation of the Omega yield, is what
constitutes the impressive progress in the last two years.
I would therefore suggest to call this section ``Strangeness
enhancement'' even if this sounds less flashy. Also, the first
sentence should be changed into
``Earlier results of an enhanced overall production of strangeness
in nuclear collisions (carried mostly by kaons and hyperons) have
been confirmed and extended by precise measurements of....''
(etc. as in draft).
At the end of this paragraph I would append the following sentence:
``The observed increase of the relative enhancement with the
strange quark content of the produced particles contradicts
expectations from hadronic rescattering models in which the secondary
production of multistrange (anti)baryons is hindered by high mass
thresholds.''
7. The last paragraph suffers from the problem that it can be
misunderstood as saying that changing the E_T range (e.g. by
going to intermediate-size collision systems) changes the energy
density. This is not said explicitly, but the contrary is not
stated either. In view of this ambiguity in the formulation, the
present text could be misread as giving priority for a run at
full energy with smaller nuclei over a low-energy run. This could
have severe and undesired consequences. I agree with Shuryak,
Gazdzicki, Stock and many others that indeed in order to
significantly reduce the energy density in the collision and to get
out of the QGP phase or probe the onset of deconfinement one MUST
reduce the beam energy, not the size of the collision system.
Changing the size of the collision system provides access to
another interesting phenomenon, the approach to local
thermalization and the onset of flow in nuclear collisions (both
before and after hadronization). The two issues should, however, be
kept clearly apart and stated unambiguously in order to avoid
unnecessary complications in the discussion about what should be
done when and why.
[I would still think that a run with smaller systems would be
interesting for NA50, NA49 and NA57 even if the J/psi anomaly and a
possible (?) steplike suppression pattern had nothing to do with
deconfinement per se, but rather with having sufficient time to
destroy a strongly bound state by mostly subthreshold collisions
with secondary partons. This is also an intersting aspect of the
approach to thermal equilibrium. But I would strongly encourage
everybody not to confuse this with the goal of reaching subcritical
initial energy densities -- if we make QGP at the SPS, the only
safe way of getting out of it again are runs at lower beam energy.]
I apologize for the many words which I wrote to support, in the end,
rather modest changes in the wording of the report. But the difference
does matter, in my mind, and therefore I wanted to give you full
access to my motivations.
I also send this to the rest of your mailing list in order to provoke
a critical response from the other participants of HIPS98.
Thanks for the work you have done,
with warm regards,
Ulrich