Matter in the Universe is made up of atoms (and so are we). Atoms consist of a nucleus of protons and neutrons, and electrons that orbit around this nucleus. Electrons are elementary particles and, as far as we know now, these particles are indivisible. Protons and neutrons are divisible: they are built up of quarks, which are subatomic particles.
Quarks are curious particles for two reasons.
Quarks are confined into a proton or neutron and are never observed in isolation. Why is this? Can we find a way to release quarks from their proton or neutron prison? In addition, there are many questions about the mass of quarks. A proton, for instance, has a certain mass. You would think that this is the combined mass of the three quarks that the proton is made up of. This is, however, quite far from the truth because the quarks, taken together, make up for only 1% of the mass of a proton. Where does the remaining mass come from? Could the strong nuclear force (that holds the quarks confined inside the proton) have anything to do with this?
To solve the mysteries surrounding quarks, the ALICE experiment will study a new state in which matter can exist: the quark-gluon plasma. In this state, the characteristics of the strong nuclear force are altered. The researchers do, effectively, recreate the situation in which matter existed at 10 microseconds after the Big Bang (the birth of our Universe).
This is done with the ALICE experiment, one of the detectors of the Large Hadron Collider at CERN. This detector primarily studies the collisions of heavy atomic nuclei (lead ions, amongst others). Only when such heavy nuclei collide at nearly the speed of light, can we expect to create a temperature that is 100,000 times higher than that in the sun’s core. At this temperature of 2000 billion degrees, the scientists hope to be able to observe the quarks in isolation (the quark-gluon plasma), before the plasma cools down again and the quarks regroup into ‘normal’ matter.
In the past few years, the STAR experiment at the RHIC accelerator of the Brookhaven National Laboratory (USA), in which the ALICE group also participates, has already done measurements of collisions of nuclei. Even though this is done with low energy nuclei, and therefore at a lower temperature than at ALICE, the measurements do indicate that the produced hot matter state already has special characteristics. This state is so dense, that it absorbs even the most penetrating radiation. Moreover, the matter behaves like a perfect liquid, with extremely low viscosity. It could be that here the first signals of the quark-gluon plasma were already observed, but the ALICE measurements will most likely be able to clarify this further.
Nikhef and the University of Utrecht have, in collaboration with institutes abroad, contributed greatly to the design and construction of the silicon tracker in the heart of the ALICE experiment. This detector is a vital part of the tracking system that carefully reconstructs all tracks of the thousands of charged particles that are produced when two lead nuclei collide. From this, the collective flow pattern of the produced particles can be measured with advanced statistical techniques. Then, through hydrodynamic models, different characteristics of the quark-gluon plasma can be determined. Nikhef plays a leading role in such analyses, which has already led to the observation that not only at RHIC, but also at the LHC, the quark-gluon plasma seems to behave like an ideal fluid. Nikhef also contributes to several other research projects that aim to study the interaction of quarks in conditions of extremely high pressure and temperature.
The Standard Model of Physics predicts the existence of the quark-gluon plasma, provided that the pressure and temperature are high enough like, for instance, right after the Big Bang and perhaps even today in the interior of neutron stars. Studying this type of plasma in the laboratory will therefore provide us with unique insights into the birth of the Universe, insights that cannot be gained from observations with telescopes or satellites.