ALICE: The best is yet to come

The ALICE wonderland is the ion-ion collisions. However, the proton run was intensely used by the collaboration to get to know its detector in detail and to produce its first results in QCD-related matters. This very successful preparatory phase will now allow ALICE to enter the uncharted territory of the quark-gluon plasma at the extreme energies provided by the LHC.


The ALICE detector is optimized to study ion-ion collisions in which quark-gluon plasma may be formed. This type of matter, which existed a few moments after the Big Bang and appears when quarks and gluons are deconfined to form a highly dense and hot soup, has been studied at CERN’s SPS in the 1990s and later, from 2000 onwards, at much higher energy at RHIC in the US. Now it’s ALICE’s turn. “Quark-gluon plasma is created at very high temperatures but starts to cool down very quickly to become normal matter again. The high energy of the LHC puts us much higher above the threshold of its formation. The plasma will therefore stay around much longer and this will allow us to study its properties in more detail”, explains Jurgen Schukraft, ALICE Spokesperson.

There are a number of very clear predictions about how things should or should not change moving from RHIC to the LHC. “At RHIC, scientists have discovered that the new state of matter behaves like an ideal fluid with no viscosity. That was a big surprise. Now we have to see whether the new state remains an ideal fluid also at the energies of the LHC. We expect so. If we observe that it behaves very differently than we will have to carefully study its properties”, says Schukraft. If the experimental conditions at the LHC are favourable, the collaboration will be able to observe these basic properties very quickly, hopefully within a few weeks into the ion run which just started.

The very effective and rapid response of the ALICE detector has also been made possible by all the work that the collaboration carried out during the proton run. “The first LHC run allowed us to understand our detector in detail and also to test the whole chain right up to the data analysis”, he says. “Indeed, in order to fully understand what happens in heavy ion collisions, we needed to carefully measure the processes that occur in proton collisions. This is what we call ‘comparison data’: it’s the first floor of the building, which is now ready to receive the second floor”.

The data collected during the proton run was also used to tune the Monte Carlo software that is used by physicists to simulate what happens in the physics processes. “We were able to correct discrepancies between the simulation parameters and the information coming from real data. In some cases, variables used in Montecarlo were wrong by a factor of four with respect to what they should have been”, says Schukraft.

Last but not least, within the first few months of data taking with proton beams, ALICE was able to produce some interesting results on open questions in QCD, such as the fate of the baryonic number and its distribution when two protons collide at very high energy. Having laid a solid foundation, ALICE can certainly be confident that its first steps in the ion wonderland will meet the collaboration’s high expectations.

by CERN Bulletin