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ALICE: The heavy-ion challenge

19 September 2008

Résumé

Quand les premières idées concernant ALICE ont été lancées, la physique des ions lourds était encore balbutiante, et personne ne savait ce qu’on pouvait attendre du LHC. Les expériences ont depuis révélé les premiers signes du plasma quarks-gluons et la collaboration ALICE a adapté son détecteur pour l’exploration de ce nouvel état de la matière.

When the ideas for ALICE were first formed at the end of 1990, the heavy-ion programme was still in its infancy and very little was known about what physics to expect or what kind of detector would be required. Nevertheless, an expression of interest for a dedicated general-purpose heavy-ion detector was presented at Evian in 1992. "That’s the first appearance of ALICE," recalls Jürgen Schukraft, who has been at the helm of the experiment since its inception in 1991. "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. It was akin to planning for the International Linear Collider with a centre-of-mass energy of 1 TeV based on knowledge from Frascati’s ADONE machine, one of the first electron–positron colliders running at 3 GeV."

Sixteen years later, the field of heavy ions is in a mature state. The ALICE collaboration has the benefit of results from the heavy-ion programmes at the SPS and at Brookhaven’s RHIC, to use as guidance, allowing an infinitely better idea of what to look for, as well as the kind of detectors and the precision needed. Heavy ions will collide at the LHC with energy levels 28 times higher than at RHIC and 300 times higher than at the SPS, representing a huge jump in energy density. "The field of heavy ions has gone from the periphery into a central activity of contemporary nuclear physics," explains Schukraft. "The exciting thing about the LHC is that because of the huge jump in energy compared with RHIC, there are many open questions to be answered and lots of surprises to be expected. While we don’t know the answers yet, today at least we know some of the questions."

ALICE will study the quark–gluon plasma (QGP), the first evidence of which was discovered at RHIC and the SPS, and will continue the investigations by confirming interpretations and testing predictions at the LHC. "Back in 1992, we were imagining what the quark–gluon plasma would look like and we expected it to behave like an ideal gas, but what we found is that it behaves like a perfect fluid, so it is completely different," says Schukraft. "This was a very big surprise, because instead of being weakly interacting, or gas like, it is strongly interacting. It is the best fluid anyone has ever found in nature, much better than liquid helium, for example." He adds: "The discovery that QCD matter is more like a fluid, was made at RHIC. We now expect to see it flow at about the same strength at the LHC if our understanding is correct – because it can’t get any better than ‘ideal’ – or we will be scratching our heads if it behaves differently."

Another question on the minds of the ALICE collaboration is whether there is not only QGP, but yet another unusual state of matter called colour glass condensate (CGC), which may form at high gluon densities in heavy nuclei. While QGP is hot and dense, CGC is cold and dense, and would exist in the initial state – before the nuclei collide – and then melt away. "We hope to discover new aspects of QCD in the strongly coupled regime, where the strong interaction is actually strong," says Schukraft. "One of the central concepts of the Standard Model is phase transition and spontaneous symmetry breaking. The QCD phase transition is the only one accessible to study by experiment and ALICE will measure its properties and parameters."

As the field of heavy ions has unfolded, the ALICE collaborators have been flexible in changing or adding to their detector. Over the course of time, 50% of new detector components have been added to the original Letter of Intent submitted in the spring of 1993, as a result of the new data from the SPS and RHIC. This includes the muon spectrometer (CERN Courier December 2007 p30), a transition-radiation detector and the electromagnetic-jet calorimeter, scheduled to be completed in 2011 (CERN Courier June 2008 p27). "Now we know better what we need for this new regime," explains Schukraft. In addition, some detectors had to be invented from scratch – such as the time-of-flight detector, which was impossible to build at the time the original design was made, and silicon pixel detectors, which were not around then (CERN Courier July/August 2008 p28).

ALICE is expecting to receive 1 PB of data for the one month per year of heavy-ion operation, at a rate of more than 1.25 GB/s, which presents a huge challenge. According to Schukraft, state-of-the-art technology in data-collection infrastructure during the 1990s worked at a rate of 10 MB/s. "Most people thought 1 GB/s would be a real challenge to reach and that we would have to find a way to reduce the data volume. There were many discussions on how to handle this huge amount of data, yet today within a factor of 2–3 it is quite common. However, 15 years ago one could not dream of handling such a large amount of data at such a rapid rate," he says. He expects that the heavy-ion data taking will start by the end of 2009 and soon after begin to show the first interesting results.

Although the collaboration’s main interest is heavy-ion collisions, for most of the year ALICE will be running using proton–proton collisions, which is important for comparing measurements from the lead–lead collisions. The detectors are optimized for complete particle identification at angles close to 90°, detecting particles from extremely low to fairly high momentum. During the proton runs, ALICE collaborators will be tuning the Monte Carlo generators and evaluating the background and detector performance for QCD measurements, such as charm and beauty production at low transverse momentum.

"What we are doing at the LHC is very exciting," says Schukraft. "The LHC is really amazing in its ability to combine three different approaches in one machine: high-energy phenomena, producing new particles to be studied by ATLAS and CMS; indirect effects of virtual high-mass particles, studied in LHCb; and distributed energy that heats and melts matter, to be studied by ALICE. We look forward to studying lead–lead collisions at LHC energy scales."

About the author

Jürgen Schukraft talked to Carolyn Lee, CERN.

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