by Marco van Leeuwen . Published: 06 August 2012

In the afternoon of 12 July, there was a special colloquium session where the LHC experiments showed some of their recent results for CERN staff members and the summer students. I was honoured to present the latest results from ALICE. Most of those results have recently been presented in the 'Hard Probes' conference which took place in May of this year in Cagliari, Sardinia.

The goal of research in ALICE is to use high-energy collisions of heavy nuclei to produce a hot and dense state of strongly interacting matter, where the quarks and gluons that are normally bound inside protons and neutrons are free (the Quark Gluon Plasma, QGP) and to study its fundamental properties and dynamical evolution.

This will give us access to a more profound knowledge of the strong interaction, one of the 4 fundamental forces which binds matter together to form the atomic nucleus where matter in today’s universe is concentrated. The Quark Gluon Plasma is though to be a primordial state of matter which existed in the first few microseconds after the Big Bang from which the entire universe emerged.

There are two basic approaches in studying these collisions: one is to detect low momentum particles, which we think are produced when the QGP cools down and disintegrates to form hadrons. The temperature at which this happens is about 200 MeV, and the momenta of the produced particles are roughly equal to this temperature.


"There are two basic approaches in studying these collisions: one is to detect low momentum particles, which we think are produced when the QGP cools down and disintegrates to form hadrons. The other approach to study the QGP is to use highly energetic or heavy partons (quarks and gluons) that are produced from the incoming high-energy partons during the early stages of the collision."

The other approach to study the QGP is to use highly energetic or heavy partons (quarks and gluons) that are produced from the incoming high-energy partons during the early stages of the collision. These 'hard probes' propagate through the Quark Gluon Plasma and probe its properties by interacting with it.


During the ICHEP12 conference results from the study of particle production at low transverse momentum with ALICE

When investigating particle production at low transverse momentum (pt), it is found that the typical momenta of heavier particles, such as protons, are larger compared to those of lighter particles (pions and kaons). This is in agreement with expectations for freeze-out from an expanding fluid, indicating that the Quark Gluon Plasma behaves like a liquid. More precise information on these processes is obtained by investigating the azimuthal distributions of produced particles, which are sensitive to the anisotropic expansion of the Quark Gluon Plasma in non-central collisions. Results on such measurements were presented at the recent ICHEP conference in Melbourne, Australia and at Quark Matter in Annecy, France, last year.


"At the same conference, ALICE also presented first results from measurements with jet reconstruction. These jets are the remnants of high-momentum partons, which fragment into several hadrons before reaching the detectors"

The focus of the Hard Probes conference in May this year was on measurement of high-pt and heavy particles. Just before the conference, new results on heavy charm meson production were submitted for publication. These results show that also heavy quarks, with a mass larger than the expected temperatures in the Quark Gluon Plasma, interact with the medium and lose energy. At the same conference, ALICE also presented first results from measurements with jet reconstruction. These jets are the remnants of high-momentum partons, which fragment into several hadrons before reaching the detector. Jet reconstruction algorithms sum together the energies of the fragments to obtain a measure of the initial parton energy. A strong suppression of jets was found, indicating that jet fragments are spread over a larger area in heavy ion collisions than in proton-proton collisions, due to interactions with the Quark Gluon Plasma.


Hard Probes Conference organized earlier this year in Cagliari

The new results opened many interesting discussions during the above mentioned conferences and at CERN, both with theoretical physicists and with researchers from different experiments. Based on these discussions we are now refining and extending some of the measurements, before publishing the final results. In addition, we will see comparisons of our measurements with theoretical calculations, which will tell us to what extent we understand the Quark Gluon Plasma and the strong interaction.