I think we all agree that the currently available results from the Heavy Ion programme at the CERN SPS give clear indications that some very interesting phenomena must occur at the early stages of a Pb-Pb collision, at the present energies. The braver among us would go further and claim that this ``interesting phenomenon'' is actually the creation of deconfined QCD matter, the long sought-for grail of heavy-ion physicists: the QGP. At this particular moment I am not interested in discussing ``details'' such as whether a real (thermalized) QGP is formed or whether we have ``only'' reached a state where deconfined gluons move freely within some local bubbles. By paying too much attention to details we might fail to understand the main point.

By the time of the Quark Matter 96 conference, May 1996, I believed that the goal of the Heavy Ion community in general, and of the SPS experiments in particular, was to provide compelling evidence that the experimental results could only be explained within the QGP scenario. We had to convince the sceptics (the normal way to make progress in science) that the conventional models failed to explain the existing results, even assuming absurdly high densities of hadronic matter.

I was aware that a few people were already claiming that we had reached the production of the QGP state of matter in sulphur collisions, but I considered those people to be simply a small group of ``revolutionaries'': a provocative element that should always be encouraged and cherished in any scientific field, to push for faster progress. Shortly before (and during) Quark Matter 97, I realized (to my surprise) that such a scenario of QGP production in S-S collisions, at 200 GeV, was actually considered to be ``a very reasonable possibility'' by a large fraction of our community. Indeed, as I was told by some people, this was the basic reason why some SPS heavy-ion experiments were interested in running at a lower energy (40 GeV).

While still recovering from this change of reference frame, I found myself involved in the process of providing CERN's Director General with some ``consensual'' statements from the SPS heavy-ion community, to be used in his report to the CERN Council during its December session. Despite the effort of Peter Sonderegger to reach some list of agreed statements, which would also have provided the basis of the requested `Press Release', it became clear that our community was not yet ready to overcome its ``internal differences'', even with such a top-level external motivation.

I had practice in coping with the group of ``sceptics'' but not in facing divergences among two groups of ``believers''. I consider that these two situations are very different. The first is quite common in science, when a team makes a more or less fundamental discovery and has to convince the rest of the scientific community that the experimental results they obtained cannot be explained within the framework of the models so far considered ``conventional''. Some scientific progress is made when the observed phenomena find a natural explanation (or are sometimes even predicted) in models incorporating ``new ideas''. This is a perfectly normal situation and, in fact, in a healthy scientific field we must praise those who try to find ``conventional solutions'' just as much as (or maybe even more than) those who have the illuminating ideas leading to revolutionary advances.

The second situation is of a rather different nature. We agree that a very high-density state of (eventually deconfined) matter is produced at SPS energies, but we are unable to decide whether this happens in S-S just as well as in Pb-Pb collisions, or whether it only happens in head-on Pb-Pb collisions. I feel that this discussion is not entirely confined within purely scientific grounds. I have heard people from both groups saying that the others would only accept that the production of the QGP state of matter in the SPS has been established if their ``own'' signatures were implicated in the discovery. The ``others'' don't seem to be driven by a pure and objective scientific motivation. I have also heard some people say that we should not announce that a QGP has already been produced at the SPS, otherwise we would lose one of the best reasons to build the ALICE experiment. I cannot understand such an idea. LEP was not build to discover the Z0 but rather to precisely study its properties. The construction of the LHC will not stop in case the Higgs boson is found next year at LEP. Anyway, if we cannot make up our mind regarding the results already achieved at the SPS, how can we effectively push for future experiments?

We are all passionately involved in this field and we should not accept this unpleasant situation in a passive and unconcerned manner. Are we going to wait for 1998's December Council meeting before we re-attempt a broad consensus?

I would be perfectly happy if we concluded that there is no QGP to be found in the SPS data. That would still be a discovery in itself. However, I would be extremely disappointed if the currently available SPS data contained clear indications of QGP production, but we only found this out retroactively, two or three years from now, after having been told so by our RHIC colleagues.

I hope that my feeling is shared by some of you and that we will find some constructive way of cross-correlating the different observables to which each of our experiments is sensitive. I am sure that we will be able to extract more (and better) information from the data of each experiment after having learnt what the neighbouring experiments have to tell us. I ask you to actively participate in the construction of a common ``SPS heavy-ion spirit'', by helping to create a forum where our experiments can discuss and exchange information in an informal and constructive environment.

Among the many advantages of having this multi-experiment working group, we can count the preparation of some figures merging several points from different experiments, in a carefully coordinated way. We could also define a standard procedure to compare the different centrality classes of each experiment, usually defined from the charged particle multiplicity or the transverse energy (measured in several rapidity windows) or from the forward energy collected in a ZCD. We should be able to use the same model in all experiments to extract the corresponding impact parameter ranges or the number of participants, for instance.

Besides these obvious gains, this forum could also be the appropriate place to prepare a document reviewing the present understanding of the data collected over the years in the SPS heavy-ion programme. Such a report, reflecting a collective view rather than stressing individual accomplishments, besides providing an objective review of the field (and thereby a good basis to support future goals), would also be an excellent motivation to promote a pleasant atmosphere within our community, improving our chances of eventually reaching some conclusions.

Carlos Lourenço
January 1998