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Dear Berndt and Peter,
Thank you for writting a draft of the summary of HIPS98
meeting. Following your request I send you my comments.
These comments are formulated after reading Uli's corrections
to your text.I agree with most of his corrections.
Thus writting my remarks, in order to avoid repeatitions,
I assume that Uli's corrections are already included in your
text.
Let me first describe a main reason for my comments.
In your carefully formulated introductory paragraph
you correctly state that the data seem to indicate
creation of QGP in A+A collisions at SPS.
On the other hand in the discussion of experimental
results you do not indicate precisely what is the
relevance of various results for your original conclusion.
In fact `low mass lepton pairs' are not related to QGP creation.
Production of rare hadrons (J/psi, Omega) is obviously related
but this relation is now not known to us
(e.g. it is not at all clear whether one expects more or less
aboundant production of J/psi and Omega for confined matter
in comaprison to QGP).
Finally hadronic freeze-out conditions may help to deduce
conditions at the early stage of the collision but relevant
results (undersaturation of strangeness in hadron gas and
possibility to extrapolate energy density to the early stage)
are not realy discussed in your summary.
In my opinion the results which are the closest related to
the creation of QGP are results on pion and strangeness
multiplicities: their collision energy dependence and the
absolute values at SPS.
This is why I suggest to include them in your report.
Suggested modifications of your draft and their justifications
are listed below.
1. Introduction, first paragraph:
'The new data provide strong support .... is created
in Pb+Pb collisions at the SPS'.
exchange 'Pb+Pb' to 'A+A (from S+S to Pb+Pb)'.
All important new features observed in the Pb+Pb collisions
were already observed in S+A collisions.
The only effect which seems to be new in central Pb+Pb collisions
is `low' production of J/psi mesons.
But as I will describe below this conclusion fully depends on the assumed
(and questionable) A-dependence of charm production.
Therefore it should not be treated as an evidence of a new
physics which occurs
only in central Pb+Pb collisions (in contrary to many others
experimental results).
2. Introduction, second paragraph:
remove 'suppression of'
See comments to J/psi paragraph.
at the end of the first sentence add:
'and energy dependence of pion and strangeness production.'
3. J/psi suppression paragraph.
exchange 'suppression' to 'production' in the whole paragraph
(three times).
Term 'J/psi suppression' implies specific and for many of us
controversial interpretation of J/psi data.
It originates from the picture in which 'pre-J/psi-meson'
is produced following A-dependence expected from pQCD and
it is further suppressed by interactions with participant
nucleons, hadronic comovers and/or QGP.
The basic assumption of this model is however questionable:
a. there are no strong theoretical arguments that charm production
has to follow 'pQCD-A-dependence'. On the contrary due to low
mass of charm quarks one usually questions applicability of
pQCD to charm production (see recent review hep-ph/9702287,
and our discussion during HIPS meeting).
b. there are no experimetal data on charm production in
A+A collisions, which allows for an experimental
confirmation of the assumption. On the contrary the analysis
of dimuon mass spectrum for Pb+Pb collisions presented
by NA50 in Chamonix indicates problems of pQCD based approach
in reproducing the data ('charm like enhancement' reported
by E. Scomparin).
c. data on open charm production in p+A interactions are too
poor to distinguish between two extreme case: 'pQCD-A-dependence'
(alpha = 1) and A-dependence characteristic for pion
production (alpha=0.9).
d. The A-dependence measured for J/psi production is the
same as the A-dependence measured for pion production both for
p+A and Pb+Pb collisions. In the 'suppression like' interpretation
this scaling behaviuor (similar to that observed for pions and
strangeness) is accidental.
The scaling behaviour seems to suggest that A-depepndence of charm
production may be similar to the A-dependence of pion production.
Thus, in this case, low production of J/psi is caused by low
production of charm and not by J/psi suppression!
In summary, only direct data on open charm production in A+A
collisions can allow for unique interpretation of J/psi results.
This is why, writing about J/psi results now, we can not limit
ourself to a single, very specific, interpretation.
4. J/psi suppression paragraph.
exchange 'smaller statistical and systematic errors' to
'smaller statistical but larger systematic errors'
Here NA50 experts should help me but, as far as I understand
the experimental procedure, the systematic errors in the case
of the normalization to minimum bias cross sections are significantly
larger. This is the reason why one usually presents the ratio
J/psi/DY.
5. J/psi suppression paragraph.
remove text which starts from the sentence:
'If the analysis .... ' to the end of the paragraph.
New data of NA50 do not show any more threshold like
behaviour previously reported at about E_T = 50 GeV.
The ratio of J/psi multiplicity to pion multiplicity is
independent of A for p+A and A+A collisions.
Thus there are no experimental indcations for a possibilty
to observe threshold like behaviour in J/psi production.
6. Low-mass lepton pairs.
I fully agree with Uli's comments.
7. Hadron thermometry.
I propose to change the title to 'Hadronic freeze-out conditions'
as this paragraph contains more than only discussion of the
temperature.
In the first sentence change 'Pb+Pb' to 'A+A'.
Statistical description of Pb+Pb data is not better
than the description of S+S data.
In the second sentence add after 'at a temperature'
'similar for all high energy collisions (from e+e- to
Pb+Pb) and'
Chemical freeze-out (hadronization) temperature is similar for all
highe energy reactions from e+e- to Pb+Pb.
Remove last sentence which starts from 'Furthermore ....'
Again note that chemical freeze-out (hadronization) temperature
is the same in e+e- and Pb+Pb collisions, thus in both cases
close to the expected critical temperature for QGP creation.
On this basis
we do not want to conclude that QGP is created in e+e- interactions!
and therefore we should also not do it for Pb+Pb collisions.
At the end add the following sentences:
'At the chemical freeze-out strangeness in the hadron gas is significantly
underpopulated. This is in fact expected when the full equilibration of
strangeness in QGP phase takes place before hadronization.
8. Omega enhancement.
I agree with Uli's comments.
9. Add paragraph:
'Energy dependence of pion and strangeness yields.
Collision energy dependence of pion and strangeness
production in A+A collisions can be studied using data
obtained at SPS and at other accelerators operating at
lower energies in BNL, JINR, LBL and GSI.
The results are consistent with the hypothesis that
transition to deconfined matter takes place between
top AGS energy (15 A GeV) and top SPS energy (158 A GeV).
In fact also absolute yields of pion and strangeness at
SPS are consistent with the statistical model of QGP
creation at SPS.'
10. Last paragraph.
I agree with Uli's comments to this paragraph.
However he did not go beyond general remarks.
Therfore I propose the following specific formulation:
'The results discussed above suggest a continuation
of heavy ion experiments at SPS. The main goal of the near-term
programme is to search for the region in collision
energy and size of the colliding nuclei in which transition
to deconfined matter takes place.
Experiments NA45, NA49 and NA57 plan to ....... such as
40 GeV/nucleon. At lower beam energies one expects lower
energy density at the early stage of the collision and therefore
one hopes to be able to identify energy region in which
transition takes place. The expected signatures are
characteristic energy behaviour (in some cases non-monotonic changes)
of pion and strangeness production, effects connected
with softening of equation of state, long fireball life
time measured by Bose-Einstein correlations and specific
pattern of particle flow, and finally changes in event-by-event
fluctuations.
The NA50 and NA57 will attempt to extent .....
It apears desirable for both experiments .....
NA45 is ready for .....
The main goal of long-term programme discussed by NA49, NA50 and NA57
is to study open charm production in A+A collisions at top SPS
energy. This results are urgently needed due to the observation of
significant anomalies in J/psi production.'
(.... - take original text)
best regards
Marek Gazdzicki