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CERN | Geneva | Switzerland

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Partonic matter and perfect fluidity: it’s heavy ions time!

This is becoming a tradition: at the end of the year, the Large Hadron Collider (LHC) replaces the protons in the accelerator with lead nuclei. These are lead atoms stripped of their electrons, with “only” the 208 nucleons (that is protons or neutrons, the particles found inside the nucleus) left in the nucleus. So instead of having single protons colliding, they will soon bring 208 nucleons into collision. This is a bit like playing billiard not with single balls but with 208 agglomerated-balls. It’s bound to bring a few surprises!

Just like regular matter has many phases (water is found in solid, liquid and gaseous forms), nuclear matter, that is, the matter within the atomic nucleus, comes in hadronic or partonic phases.

The hadronic phase could be seen as having in fact a liquid form, when protons and neutrons are bound inside the nucleus just like water molecules, and a gaseous form, when the nuclear bounds are broken, leaving the protons and neutrons free to float around.

Then there is the partonic phase that occurs when enough energy is available to break the nucleons, leaving their constituents called partons, the quarks and gluons, in a free form. This is what is called the quark-gluon plasma, the most energetic soup ever cooked in the whole Universe, and more recently on Earth, thanks to the LHC.

It is believed that the quark-gluon plasma was the state of matter just after the Big Bang. The heat bath at the existing temperature provided enough energy for free quarks to overcome the powerful attraction created by the strong nuclear force, the strongest of all known forces.

This phase of matter is really not well known, having been studied only recently at high-energy accelerators and, of course, when the LHC brought heavy lead ions into collision around this time last year. Amazingly though, the theory describing the strong force called Quantum Chromodynamics (QCD) had predicted that nuclear matter would undergo a phase transition at a given but extremely high temperature.

Last December, under the energy released by lead-lead collisions, we could see a quark-gluon plasma had formed and that these seemingly disconnected particles exhibited a strong common behavior. But exactly how the transition from the hadronic to the partonic phase took place remains to be understood. The theory predicts this transition happens suddenly, like sublimation when ice evaporates, and not progressively like going from ice to water, where the two phases coexist at the liquefaction point. All this needs to be checked experimentally.

This year, having gained much more experience, the LHC team hopes to provide 20 to 40 times more collisions of these heavy ions, enabling the physicists from the ALICE experiment, which is dedicated to this type of research, but also from the ATLAS and CMS collaborations to study the quark-gluon plasma properties.

One spectacular behavior of this matter derived from basic principles by various theoretical methods (QCD, lattice or string theory calculations) all predict that the quark-gluon plasma is a perfect fluid, that is, its viscosity drops to near zero. Its viscosity is even smaller than superfluid helium, which is a mixture of two components, only one of which has zero viscosity. A superfluid does not stay in its container but climbs its walls and spreads out. Only two superfluids are known: liquid helium and Bose-Einstein condensates, when matter lies in the least energetic state near the absolute zero. Surprisingly, this perfect fluidity occurs at thousands of billions of degrees for the quark-gluon plasma while the two superfluids are found at the other extreme temperature near the absolute zero, namely  -273.15 degrees Celsius.

One way to study the quark-gluon plasma is to observe what happens in events where two jets of particles are created from the lead ion collisions. An event is just a snapshot taken when some heavy ions collide, showing all particles coming out of these collisions, very much like watching mini-firework. To conserve energy and momentum, the fragments must fly out in all directions. So when two jets are produced, they should proceed? back-to-back. What was observed last year is that the jet produced at the surface of the quark-gluon plasma could escape whereas the other one, the one pointing towards the hot, dense plasma, was absorbed and scattered by this very dense medium, as shown in the figure below.

Better still is to select events where one jet made of several light particles recoils against a photon. The photon can cross the quark-gluon plasma unaffected, acting as a much better indicator of its original direction than another jet that can also be affected by the plasma. Unfortunately, these events are much more rare but thanks to a new trigger, the ALICE collaboration hopes to find some of these events, even though it is not trivial to distinguish primary photons from secondary photons coming from background sources. The best probe of all is when a Z boson is created with a jet since, like the photon, it is unaffected by the plasma, but unlike the photon, it cannot be coming from a different source.

Despite all appearances, this can be regarded as applied research. The goal is to understand what happened right after the Big Bang, during the phase change from partonic to hadronic phase, all the way to getting clues to matter creation and star formation, to better understand how matter took shape.

Before moving to the ion-ion program, the LHC team will attempt to collide protons with heavy ions in preparation for a new program for next year. Endless fun ahead!

Pauline Gagnon

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Collisions of heavy ions where two energetic jets are produced as seen in the ALICE detector, represented here like a cylinder that has been unrolled. In the top figure, a event where the ions grazed each other (peripheral collision) and where both jets are clearly visible. In the bottom figure, a head-on (or central) collision. One jets is absorbed and scattered by the quark gluon plasma while traversing it, while the other, moving away from the plasma, can escape.