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CERN | Geneva | Switzerland

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The coolest and hottest fluid

In September, the Large Hadron Collider (LHC) operators at CERN attempted a new trick: putting in collisions protons in one beam and lead ions in the other. Usually, the LHC operates with two beams of identical particles (protons or ions) circulating in opposite directions in the accelerator. Here is what is expected from this new setup.

These ions are atoms stripped of all their electrons, leaving only the nucleus. Lead ions contain 82 protons plus 126 neutrons, all held together by the nuclear force.  Protons are also composite objects made of three quarks and bound together by “gluons”, the particles carrying the nuclear force.

So when two such heavy ions collide at nearly the speed of light, I dare anyone to describe where each quark and each gluon will end up. Already, trying to predict where fifteen billiard balls go after breaking the pack is tough enough. But when each projectile is made of hundreds of particles, it becomes impossible.

At first glance, it would seem all we could get out of this is just a mess. But this turns out to be the coolest and hottest mess one will ever see. From the most energetic collisions comes a new form of matter called the quark-gluon plasma.

There are three very well known state of matter: solid, liquid and gaseous. Lesser known is the fourth state of matter called plasma. This is what one finds inside a neon tube when the electric current applied is strong enough to strip the gas of its electrons. Positively charged ions and negatively charged electrons float around freely, having enough energy not to recombine.

The quark-gluon plasma is just one step above this. Imagine there is enough energy around that not only the atoms but the nucleons (the name given to protons and neutrons, the particles found inside the nucleus) break apart and coexist in some sort of an extremely energetic fluid. This is as hot as it got instants after the Big Bang. What is so cool about it though, is that this plasma exhibits collective behavior, meaning quarks and gluons do not float freely but have collective properties. The most spectacular of them is that this fluid has no viscosity and behaves as a perfect fluid. If you try to confine it in a container, it just flows up the container’s wall and spread all over the place.

The ALICE experiment is dedicated to the study of the quark-gluon plasma. Each year, the LHC operates for a few weeks with lead ions instead of protons. ALICE collects data both during proton-proton collisions and heavy ions collisions. Even when only protons collide, the projectiles are not solid balls like on a billiard table but composite objects. By comparing what can is obtained from heavy ion collisions with proton collisions, the ALICE physicists must first disentangle what comes from having protons in a bound state inside the nucleus as opposed to “free protons”.

So far, it appears that the quark-gluon plasma only formed during heavy-ion collisions since they provide the necessary energy density over a substantial volume (namely, the size of a nucleus). Some of the effects observed, such as the number of particles coming out of the collisions at different angles or momenta, depend in part on the final state created. When the plasma is formed, it reabsorbs many of the particles created, such that fewer particles emerged from the collision.

By colliding protons and heavy ions, scientists hope to discern what comes from the initial state of the projectile (bound or free protons) and what is caused by the final state (like the suppression of particles emitted when a quark-gluon plasma forms).

Already, with only one day of data taken in this new mode, the ALICE collaboration just released two papers. The first one presents the measurements of the charged hadrons density produced in proton-ion collisions and compares the result with the same measurement after proper normalization performed in proton-proton and ion-ion collisions. The second compares the transverse momentum distributions of charged hadrons measured in proton-ions and proton-proton collisions.

The ultimate goal is to study the so-called “structure function”, which describes how quarks and gluons are distributed inside protons, when they are free or embedded inside the nucleus.

More will be studied during the two-month running period with protons colliding on heavy ions planned for the beginning of 2013.

A “snapshot” of the debris coming out of a proton-lead ion collision captured by the ALICE detector showing a large number of various particles created from the energy released by the collision.

Pauline Gagnon

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