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Light guides from a prototype of the ALICE Zero Degree Calorimeter

Clues to the early Universe

The Universe has changed a great deal in the 13.7 billion years since the Big Bang, but the basic building blocks of everything from microbes to galaxies were signed, sealed and delivered in the first few millionths of a second. This is when the fundamental quarks became locked up within the protons and neutrons that form atomic nuclei. And there they remain, stuck together by gluons, the carrier particles of the strong force. This force is so strong that experiments have not been able to prise individual quarks or gluons out of protons, neutrons or other composite particles.

Primordial soup

Suppose, however, you could reverse the process. The current theory of the strong interaction predicts that at very high temperatures and very high densities, quarks and gluons should no longer be confined inside composite particles. Instead they should exist freely in a new state of matter known as ‘quark-gluon plasma’.

Such a transition should occur when the temperature goes above a value around 2000 billion degrees - about 100 000 times hotter than the core of the Sun! For a few millionths of a second after the Big Bang the temperature of the Universe was indeed above this value, so the entire Universe would have been in a state of quark-gluon plasma – a hot, dense ‘soup’ of quarks and gluons. Then as the Universe cooled below the critical value, the soup condensed into composite particles, including the building blocks of atomic nuclei.

Experiments at CERN’s Super Proton Synchrotron reported tantalising evidence for quark-gluon plasma in 2000. The next big step will be with the Large Hadron Collider and the ALICE experiment in particular.