Nikhef is member of the ALICE experiment, one of the detectors at CERN’s Large Hadron Collider (LHC) in Geneva. Here, over a thousand scientists work to find answers to fundamental questions about the building blocks of the Universe. We know that matter is built up from atoms, which, in turn, consist of protons, neutrons and electrons. ALICE focuses on quarks and gluons, the elementary particles locked inside protons and neutrons.
ALICE is short for ‘A Large Ion Collider Experiment’. It is one of the detectors at CERN’s LHC in Geneva. The LHC is a particle accelerator: it produces collisions between protons at extremely high energies, which disintegrate into fragments. By studying these fragments, researchers hope to discover yet unknown forms of matter.
ALICE mostly studies collisions of heavy atomic nuclei (such as lead ions). When such heavy atomic nuclei collide at speeds close the speed of light, the local temperature surges to 2000 billion degrees, 100.000 times higher than that of the Sun’s core. In this way, the researchers actually reproduce the situation that existed a few microseconds after the Big Bang, when the Universe was formed. This temperature is essential to create a so-called quark-gluon plasma, which can be used to unravel the secrets of these elementary particles.
The Universe (including ourselves) consists of matter that is made up of atoms. Each atom consists of a nucleus of protons and neutrons, around which electrons ‘circle’. Electrons are elementary particles: for as far as we know now, they can’t be split any further. Protons and neutrons can be split; they are composed of quarks, which are held together by gluons.
The quarks and gluons are locked up inside protons and neutrons, and have never been observed elsewhere. Why is this? Can we devise a way to release quarks from their proton or neutron prison? Questions also exist about the mass of quarks. For example, one would expect that the mass of a proton equals that of the three quarks from which the proton is built up. Nothing could be further from the truth. It turns out that the quarks constitute only 1% of the mass of a proton. Where does the rest of the mass come from? Can the strong interaction (which ‘imprisons’ the quarks inside the proton) provide an explanation?
To solve the mysteries surrounding quarks and gluons, the ALICE experiment attempts to bring quarks and gluons into a new state of matter (the quark-gluon plasma) at extremely high temperature, and to study it before the quarks cool down and regroup into ‘regular’ matter. The Standard Model of Particle Physics predicts the existence of the quark-gluon plasma whenever the pressure and temperature are sufficiently high, as was the case right after the Big Bang and as could still be the case inside neutron stars.
Measurements by ALICE reveal that the hot state of matter that is produced has remarkable properties. It is so dense that it absorbs even the most pervasive type of radiation. Moreover, the matter behaves as a ‘perfect fluid’ with extremely small viscosity. Further measurements at ALICE are aimed to shed light on how this can be explained in terms of changes in the strong interaction.
Research into the quark-gluon plasma will provide unique insights into the origin of the Universe – insights that can’t be gained through observations by telescopes or satellites.
Within an international collaboration, Nikhef and Utrecht University have contributed strongly to the design and construction of the Silicon Tracker at the heart of the ALICE experiment. This detector is a vital part of the ‘tracking’ system which precisely reconstructs all traces of the thousands of charged particles produced when two lead nuclei collide. By means of advanced statistical techniques these traces allow to determine the collective flow pattern of the particles that were produced. This subsequently allows to determine various properties of the quark-gluon plasma, using hydrodynamic models.
Nikhef plays a leading role in these analyses, which have already resulted in the observation that the quark-gluon plasma behaves as an ideal fluid.
Nikhef also participates in various other research projects aimed at studying the interaction of quarks at extremely high pressure and temperature.