by Panos Charitos. Published: 13 October 2012

A.M Are we currently standing at a critical point regarding the future of heavy ion physics? Do you think this is the case following the recent meeting of the European Strategy Group in Krakow?

P.B.M. I think that the recent Town Meeting that took place earlier on June 29th at CERN had already defined a certain scale of priorities that should be set in the field of ultra-relativistic heavy ions. This is the case especially regarding the European framework of research in the field. In the Town Meeting it was clearly concluded that the heavy ion programme at the LHC including the upgrade of the ALICE experiment and the intensity upgrade to 50 kHz interaction rate are the top priority of European heavy-ion physics. This priority was set forth once more in the Krakow meeting of last week (10th-12th of September) and clearly stated as a guideline in the future European research in particle physics. Therefore, I believe that things look better than in the past and in that sense I am quite optimistic, despite the difficult budgetary conditions in many countries.

The recent symposium on European Strategy for Particle Physics was held in Krakow, Poland

A.M. Which are the most important conclusions for the future direction in heavy ion physics that came from the recent meeting in Krakow?

P.B.M. One of the first priorities that clearly came out is the support for further exploring heavy ion physics based on the higher-luminosity technical capabilities of the LHC. In the case of ALICE there is an important discovery potential with 50 kHz interaction rate for lead-lead collisions. It was also stated that ALICE has an upgrade plan and has already submitted a letter of intent to the LHC Committee. It will be presented to the LHCC on Sep. 25, 2012 and soon we should have more firm directions on this issue. During the Krakow meeting, it was also stated that ALICE has a potential time scale for running and collecting data until approximately 2025.

A further outcome of the Town and Cracow meeting is the reaffirmation that very interesting physics remains to be explored at high baryon density (5 GeV < √s/(nucleon pair) <40 GeV). Finally, the complementarity of ALICE with the RHIC facilities and the heavy-ion groups of the other LHC experiments is a basic assumption that was already underlined in the Town Meeting and also came out in the Krakow Meeting.

A.M. Where does ALICE stand in terms of these directions? What is the importance for the future steps of the collaboration and how satisfied are you with the progress made so far?

P.B.M. This is a good period for the European Strategy for Particle Physics as there have been two major discoveries in High Energy Physics. The first is the Higgs or Higgs-like boson and the other one is the large mixing-angle in the neutrino sector and both clearly open up new programmes in experimental physics. Of course to realize those programmes one needs to allocate significant resources. The heavy-ion programme has clearly benefited from LHC and its unique capabilities in heavy-ion collisions. First of all the more than one order of magnitude larger collision energy compared to what is available at RHIC provides abundant production of hard probes for studying the medium of the QGP that is formed. The temperatures and densities produced at LHC are the highest that have ever been produced in a laboratory and that makes it unique for the physics programme of ALICE. I should add that this programme was made possible by the excellent performance of the LHC also for Pb beam running and the CERN accelerator team should be congratulated for this achievement. However, we note that significant effort by the CERN accelerator team is still needed in order to provide us with an upgraded intensity which is crucial for studying the QGP. Nevertheless, the effort required is finite and the resources needed should hopefully be available within the framework of the LHC improvement programme. Finally, LHC produces very small net-baryon density at mid-rapidity which corresponds to the conditions that we think existed in the early universe. I believe that currently the heavy-ion programme is favourably discussed because it addresses fundamental physics complementary to the Higss physics programme as part of exploring fully the capabilities of the LHC.

ALICE has already excellent tracking and particle identification capability. I am convinced that the upgrade that we have currently proposed will improve, by about two orders of magnitude, the performance of ALICE at high rate and will strengthen the complementarity of ALICE to the heavy-ion programmes of the other two LHC experiments, ATLAS and CMS.

Professor Peter Braun Munzinger Chairperson of the ALICE Collaboration Board

A.M. Which are the future plans of the GSI – ALICE group. What role do you think that GSI will play in the future steps –perhaps based also on what has been discussed in the Krakow meeting.

P.B.M. It is clear that the GSI-ALICE group will participate in the full energy run that will take place from 2015 on. This means until we get 1 nb -1 integrated luminosity which is the approved ALICE physics programme. Now, I am myself very enthusiastic for the ALICE upgrade and I hope that we will find the resources. For the ALICE upgrade, the GSI ALICE group needs investment funds of order of 6 M Euro, which are not currently available in the GSI budget. We have prepared a proposal to the Helmholtz society which is our umbrella organization. We also need a significant fraction of the current GSI-ALICE group to realize the upgrade project while preserving our capability for running and analysis of current ALICE experiments. Within current German funding scenario this might be not too easy since Germany has two big projects, XFEL and FAIR, which are much bigger than ALICE. However, I am firmly convinced that the unique and fundamental physics opportunities offered within the framework of the upgrade are compelling enough for its realization.

A.M. Do you think there is a strong interest among the German physics community in ultra-relativistic heavy ion collisions?

P.B.M. Well, as you might have realized there is a strong personal interest by myself that keeps me active in this field! Moreover, there are nearly 100 physicists engaged in ALICE work in Germany including a wonderful group of excellent young scientists who definitely would like to keep investigating physics in this field and are very committed. So I think that we clearly need to focus and try to do our very best to get the funding for actually realizing this. I would like to emphasize that this programme should be considered complementary to planned FAIR physics programmes and, consequently, should be funded independently of the FAIR activities.

View of the ALICE TPC just before inserting in the Miniframe

A.M. I would like to ask you about the history of the construction of the TPC as you have been personally involved in building it.

P,B.M. The Time Projection Chamber (TPC) project as an idea started even before I moved back from the U.S. to Germany. At that point I joined ALICE and my top priority was to build up within GSI a significant ALICE programme . At that time we were exploring various options until the priorities crystallized on the TPC and the transition radiation detector. In the fall of 1998 I took over the project leadership of the TPC. At that point it was not evident how to realize such a detector and we knew that it would require the development of many new components and procedures. Things had evolved in such a way that we had to find solutions for the electronics, for the read-out chambers including their special geometry, and on how to realize the field cage. In a sense we had to come up with solutions on all fronts to realize this detector. Therefore we assembled a group of people mostly from Germany (Darmstadt, Frankfurt, Heidelberg, Worms in addition to GSI) and CERN along with colleagues from Bergen, Bratislava, Copenhagen, Krakow, Lund and Zagreb, and performed simulations until we convinced ourselves that we could build a TPC that could be efficient at reasonably high rates. At that time we were not thinking of 50 kHz -as in the current upgrade proposal- but something like 5 kHz but anticipated high multiplicities of up to dN/dy=8000.

To build this detector we needed strong financial support from German funding agencies. Both GSI and the German Ministry BMBF contributed significantly through efforts of German University groups, particularly those of Heidelberg and Frankfurt. Between 1999 and 2006 the TPC was realized, the field cage was mostly built at CERN, the electronics were built as part of a collaboration between Germany and CERN and finally the readout chambers were built in Germany and one quarter in Bratislava. Following the construction phase, the detector was assembled and tested in a clean room at CERN. In early 2007 it was moved into the ALICE pit. The days of the move were probably among the four more stressful days of my life until the detector was safely installed. There followed commissioning tests in the cavern for nearly two years. I guess that you already know the rest as the TPC is now part of the ALICE detection system and is running beautifully. This success is based on the continued effort of the many talented people in the ALICE TPC collaboration and I would like to explicitely recognize this here. Following the first heavy-ion run that took place at the end of 2010 the project leadership for the ALICE TPC was taken over by Harald Appelshaeuser from Frankfurt.

Test mounting of inner and outer readout chamber plus the sealing frames to one endplate sector of the ALICE-TPC

A.M. What is the motivation for the future upgrade plans of the TPC?

P.B.M. The ALICE TPC was designed to cope with the highest conceivable charged particle multiplicities predicted for central Pb-Pb collisions at LHC energies. To cope with the large dynamic range the TPC needed to track particles from very low to high momenta and the electronics needed to provide low noise performance combined with efficient baseline restoration and zero-suppression. TPC uses means to suppress the space charge which may accumulate inside the nearly 100 m 3 volume when you have collisions at high rates. That is done by something that is called the gating grid. Essentially we allow charge from the detector volume to get to the readout detector and vice versa from the readout-detectors back to the gas volume only for events that we want to look at. This limits our trigger rate to approximately 3 kHz because if we were going beyond this limit it would mean that we would open the gate grid completely, then the amplified charge would get back into the drift volume and produce huge distortions. Actually inspecting signals at 50 kHz and not 5 kHz or rather 3 kHz (which we can do at the moment) means that we need to redesign and change the entire read-out plane as well as the electronics used in the TPC. This implies replacement of the current MWPC based detectors with detectors based on GEM technology and new electronics suitable for continuous read-out. On paper this looks very good but it is a very ambitious programme and R&D is needed for the next year to provide a firm basis for the upgrade.

A.M. Finally, I would like to ask what are your thoughts about the ALICE physics as part of the physics programme of LHC.

P.B.M. ALICE is focused on what we call quark-gluon plasma physics. This is the physics of matter produced in ultra-relativistic nucleus-nucleus collisions. Frank Wilczek coined the term 'condensed matter aspects of Quantum Chromodynamics' for this. As such it is not set to discover new particles but new connections and unearth issues such as the understanding of the transition from confinement to deconfinement of quarks and gluons, measuring the critical temperature at which it takes place and finally determining the degrees of freedom observed in and the transport properties of the quark-gluon plasma. In that respect ALICE has made really great progress and we had an impressive set of ALICE talks in the last Quark Matter 2012 conference in Washington. New results were presented that received a lot of attention and I think that we can be very proud of this.