From marek@hpikf1.ikf.physik.uni-frankfurt.de Tue Oct 13 14:29:39 1998 Date: Mon, 5 Oct 1998 19:06:19 +0200 (METDST) From: Marek Gazdzicki To: Carlos Lourenco Cc: Alfredo Musso , Axel Drees , Barbara Jacak , Berndt Mueller , Chris Fabjan , Claudie Gerschel , Daniel Ferenc , Dariusz Miskowiec , Dieter Rohrich , Edward Shuryak , Emanuele Quercigh , Enrico Scomparin , Federico Antinori , Grazyna Odyniec , Gunter Roland , Guy Paic , Hans Boggild , Hans Specht , Helmut Satz , Horst Stoecker , Itzhak Tserruya , Jean-Yves Ollitrault , Jochen Wambach , Johanna Stachel , Johannes Wessels , Jurgen Schukraft , Karel Safarik , Keijo Kajantie , Klaus Pretzl , Louis Kluberg , Ludovico Riccati , Marek Gazdzicki , Nicola Carrer , Nu Xu , Olivier Drapier , Orlando Villalobos Baillie , Peter Braun-Munzinger , Peter Seyboth , Peter Sonderegger , Reinhard Stock , Roman Lietava , Sonja Kabana , Steffen Bass , Terry Awes , Tom Trainor , Thomas Peitzmann , Thomas Ullrich , Ulrich Heinz , Volker Koch , Lorenzo.Foa@cern.ch, Helene Mauger Subject: Chamonix Summary 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.