20 years ago ALICE started its amazing adventure in the wonderland of strong interactions and the study of extraordinary forms of matter like the Quark Gluon Plasma.
CERN’s ion programme has a long history and was initiated in 1986 with the acceleration of oxygen ions at 60 and 200 GeV/nucleon, and continued with sulphur ions at 200 GeV/nucleon up to 1993. The results of the light-ion programme strongly supported its continuation with heavier-ion beams. In particular, the energy densities reached during the collisions appeared to be high enough to be interesting, and many of the suggested signatures for the onset of a quark–gluon plasma phase turned out to be experimentally accessible. The experience gained was instrumental in assessing the feasibility of experiments with lead ions and for indicating the necessary detector modifications. Seven experiments participated in the lead-age adventure.
Following the previous successes of the heavy-ion physics programme at CERN the idea of a heavy-ion dedicated experiment that would study lead-lead collisions at the new energy scale of the LHC was discussed. During the previous years, the experience gained was instrumental in assessing the feasibility of experiments with lead ions and for indicating the necessary detector modifications that were needed to move with the lead-age adventure at the new scale of the LHC.
The first appearance of ALICE was in the Evian meeting back in 1992. Jurgen Schukraft recalls that: "We had to do enormous extrapolations because the LHC was a factor of 300 higher in centre-of-mass energy and a factor of 7 in beam mass compared with the light-ion programme, which started in 1986 at both the CERN SPS and the Brookhaven AGS." A Letter of Intent for a new experiment at the LHC was submitted on 1 March 1993 to the LHC Committee that was formed shortly after the Evian meeting. It marks the first official use of the name ALICE and it was signed by 230 people coming from 42 institutes around the world. It was clearly describing the proposal of the ALICE Collaboration for building a dedicated heavy-ion detector to exploit the unique physics potential of nucleus-nucleus interaction at LHC energies and where the formation of a new phase of matter, the quark gluon plasma is expected. The submission of the letter of intent was followed by a detailed technical proposal that was submitted two years later in 1995 and shortly endorsed by the LHCC and the CERN management.
ALICE studies strong interactions by using particles – created inside the hot volume of the Quark Gluon Plasma as it expands and cools down – that live long enough to reach the sensitive detector layers located around the interaction region. The physics programme at ALICE relies on being able to identify all of them – i.e. to determine if they are electrons, photons, pions, etc – and to determine their charge. This involves making the most of the different ways that particles interact with matter. Over twenty years, ALICE has developed a wide range of R&D activities, confronted many challenges in designing and building new detectors that could cope with the physical challenges at the new energy scales. One should also refer to the big data challenge as heavy-ion collisions produce petabytes of data that need to be stored and later analysed in order to get new physics results.
Following the first run, ALICE successfully reported on the formation of QGP and offered a new insight on the nature of strong interacting matter at extreme densities. The existence of such a phase and its properties are a key issue in QCD for the understanding of confinement and of chiral-symmetry restoration. Wherever you look, from the energy loss of fast quarks to quarkonia, from the details of the dynamical evolution of the system to the very first study of charmed hadrons and the loss of energy, the interplay between collective features of the medium and jet production, to name just a few, the ALICE results stand out for their quality and relevance. Following the recent proton-lead run that opens new horizons for the heavy-ion community at CERN, ALICE is now looking forward to a series of upgrades during the LS1.
Paolo Giubellino notes: “This has been the result of many years of work and dedication of all of us, and we can all be proud of now sharing this remarkable harvest. It has been an enormous effort, but we can now say it was really worth it, and all share the happiness for this wealth of results. We all contributed to this accomplishment, and we should all draw from it even more motivation to go forward for the next many years to come!"
We thought that this issue of ALICE MATTERS should be a special issue dedicated to the ALICE anniversary. Of course it is hard to fill twenty years of history in a few articles but we tried to come up with some stories from the early days of the collaboration. Of course there is a long list of names and we hope to include all of them in the ALICE timeline on which we are currently working.
I would like to finish this short review with one quote by Andree Gide which I am taking from Emanuele Querchigh “One does not discover new lands without consenting to lose sight of the shore for a very long time”*. Happy Birthday ALICE and wishing you to find many more islands in the wonderland of quarks and gluons!
March 2004: Permanent closure of back doors
now blocked by muon spectrometer.
The Images were taken from a presentation given by Jurgen Schukraft at a CERN Colloquium in 2007 (29/11/2007) and hence I would like to thank Jurgen for kindly sending me this material. The full video of the talk is available here
*According to Quercigh the same quote made by Howell Pugh thirty-one years ago, at the beginning of the SPS heavy-ion adventure. See also: H.G. Pugh, in Ref. , 1 (1981).
ALICE: The heavy-ion challenge
An ALICE Timeline
CERN Courrier July/August 1994
Emanuele Quercigh, FOUR HEAVY-ION EXPERIMENTS AT THE CERN-SPS— A TRIP DOWN MEMORY LANE, Acta Physica Polonica B.
E. Quercigha,& K. Safarik , Ion physics at the SPS: the experimental challenge, Physics Reports 403–404 (2004) 51–56
CERN Courrier, May 1992
ALICE enters new territory in heavy-ion collisions