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Dear Peter and Berndt,
The aim of the Working Meeting on Heavy Ion Physics at SPS
(Chamonix, Sept. 25-27, 1998) was to discuss results obtained
recently by heavy ion experiments. In the first part of the meeting
we discussed the quality of the data and consistency between results
obtained by various groups.
I found this discussion onest, constructive and therefore
very usefull.
The second part of the meeting was devoted to the interpretation of the
results and discussion on future heavy ion programme at SPS.
Unfortunately due to lack of time discussion on the interpretation
could not go beyond several short scheduled presentations.
It was therefore highly incomplete.
This is probably the reason that your summary presentations
are also incomplete.
As the further planning of the heavy ion programme at SPS relies on the
correct identification of the key physics questions I decided to
'continue our Chamonix meeting' by writting this e-mail.
I would like to point out here two important results which are missing
in your oral summaries. I hope that these results and their discussion will be
included in the written report from the HIPS meeting which will be
prepeared by you.
1. J/psi Production.
It was recently found (hep-ph/9803462 and hep-ph/9809412) that the
ratio of J/psi multiplicity to pion multiplicity is the same
for p+p, p+A and Pb+Pb (from pheripheral to central) collisions.
This strongly suggests that the charm production mechanism is
similar to light quark production mechanism. This experimental
result seems to put in question the commonly used assumption that charm
production at SPS can be calculated using perturbative QCD.
On this questionable assumption the whole interpretation of the
J/psi data in terms of subsequent suppressions is based.
In fact, also new NA50 results on 'charm-like' enhancement independently
indicate that the assumption of a `perturbative nature' of charm production
at SPS is questionable.
It is therefore obvious that the understanding of J/psi results requires
data on open charm production, rather than the new data on J/psi.
2. Pion and Strangeness Production.
The energy dependence of pion and strangeness production in A+A
collisions changes between AGS and SPS energies.
The observed behaviour is consistent with one expected in the
case of transition to QGP occuring in the above energy range
(hep-ph/9809412).
This experiemntal results motivate an important part of the
future programme of the NA49
collaboration to search for the phase transition region and detect
effects connected with its crossing (characteristic increase of
pion production, non-monotonic behaviour of strangeness
production increased event-by-event fluctuations
and nonmonotonic behaviour of the expansion pattern (Shuryak's suggestion)).
It is necessary to underline that not only qualitative changes in the
energy behaviour point to creation of QGP at SPS.
Absolute multiplicities of pions and strangeness at SPS can also be
reproduced within a simple statistical model of QGP.
It is therefore clear that the priority of the heavy ion programme
at SPS for a low energy run in 1999 is essential for these questions.
Best regards
Marek Gazdzicki
p.s.
I would like also to comment on one point in Berndt's summary.
He underlined importance of the Omega measurements for the
interpretation of heavy ion results.
He concluded that the results on Kaon and Lambda yields are
not relevant. I strongly disagree with his opinion.
We are all impressed by high quality of the WA97 data on Omega, Xi and
Lambda but even these data can not help to resolve the theoretical
problems in their interpretation.
In heavy ion collisions at all energies particle ratios are strongly
different from the corresponding ratios in nucleon-nucleon interactions.
The relative changes usually increase with decreasing energy
e.g. strangeness to pion ratio is enhanced by a factor of about 3.5
at AGS and 'only' by a factor of about 2 at SPS.
For the energy which is below threshold for strangeness production in
nucleon-nucleon interactions the enhancement factor is infinite.
Thus a strong change in the particle ratio when going from nucleon-nucleon
to A+A collisions does not signal creation of QGP by itself.
What is however indicative is the absolute value which should correspond
to the value expected for QGP and the energy dependence of the yield
which should follow the energy dependence expected for phase transition.
In the case of rare hadrons (like Omega) it is impossible
to calculate their rates
expected from QGP in a reliable way. Also predictions for the
energy dependence in the transition region are difficult.
This is because their yields depend on many unknow parameters:
volume, density and temperature of the hadronizing matter and
on the properties of the hadronization process.
The situation is drasticly different when the ratio of total strangeness
to entropy is considered.
This ratio can be calculated for QGP and is almost independent of the
QGP parameters.
The only assumption needed to connect QGP value
with the value measured is the assumption that both strangeness
and entropy are approximately conserved, no other parameters are necessary.
Therefore a simple quantitative predictions are possible and they in fact
agree with the results at SPS.