Physics at 13 TeV: ALICE - scratching under the surface

ALICE’s wonderland materialises where the lead-lead ultrarelativistic collisions happen in the LHC. With a jump of over one order of magnitude in collision energy from the Relativistic Heavy Ion Collider (RHIC) and using state-of-the-art detectors, the experiment studies the quark-gluon plasma, a state of matter that existed during the Universe’s infancy.


The hugely hot medium was observed to behave almost like an ideal fluid, which, although absorbing their energy, leaves single propagating quarks and gluons almost undeflected, enhances the production of strange quarks, suppresses the production of particles made of quarks and antiquarks, and seems to be emitting light in the early stages of its expansion. “The data from the first LHC run have already challenged some of the notions that had emerged from the previous RHIC programme,” says Federico Antinori, ALICE Physics Coordinator. “The abundance of hard probes, that is, high-energy particles that interact with the medium, has allowed us to get much clearer insights into the properties of this very peculiar state of matter. We have also had surprises in the study of proton-proton and proton-lead collisions, where intriguing signals of possible collective behaviour have been observed unexpectedly.”

The higher-energy collisions of Run 2 will allow ALICE to dig deeper and explore farther in this new land. “We expect a significant increase in the statistics for all types of collision systems. In addition, the increase in collision energy will translate into a further gain in the cross-sections for hard probes,” explains Federico Ronchetti, ALICE Run Coordinator. “The increased statistics will allow us to perform much more detailed measurements. We will look at the production of several types of particles as a function of the orientation with respect to the collision plane; we will be able to classify events according to the shape of the expanding medium; and we will carry out multi-particle correlation studies.”

The control of the experiment’s operating luminosity is a challenge for ALICE. “We will need to monitor the beam overlap region carefully with our online systems to allow the experiment to run safely at its design luminosity and to ensure proper feedback to the LHC operators,” says Antinori. “Sophisticated algorithms will be employed at the trigger level, to minimise the fraction of pile-up events in the sub-detectors, and offline, to deal with the residual pile-up.”

The ALICE detector is installed at Point 2 of the LHC. Not far from the interaction area is the transfer line from the SPS for the injection of the clockwise beam into the LHC. Such a setup implies additional vacuum equipments and experiment protection systems such as collimators, which are a potential source of spurious interactions and hence of unwanted background for the experiment. “These components have been refurbished during LS1 and we expect improved background conditions during our proton-proton operations,” concludes Ronchetti. “Overall, we really think that with Run 1 we have just scratched the surface, and that much more is yet to come with Run 2!”

Check out more of our Physics at 13 TeV series in "LHCb - a new data-processing strategy" and "TOTEM - a new era of collaboration with CMS". For a theory perspective on the next run, read "Life is Good at 13 TeV".

by Antonella Del Rosso