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Colliding heavy nuclei

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 with gluons, the carrier particles of the strong force. This force is so strong that experiments are not able to prise individual quarks or gluons out of their particles for long before they recombine quickly to produce new 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 in a new state of matter known as ‘quark-gluon plasma’ (QGP).

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; as it grew from being the size of a grapefruit right up to the size of our solar system, 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 protons and neutrons.

Experiments at CERN’s Super Proton Synchrotron reported tantalising evidence for quark-gluon plasma in 2000 and the Relativistic heavy Ion Collider (RHIC) at Brookhaven National Laboratory has since pursued a broad and successful programme in which some fascinating and unexpected properties of the quark-gluon system where observed, giving us new insights into what to look for. The next big step will be with the Large Hadron Collider (LHC) .

For a period of time each year the LHC will, in place of colliding protons, collide heavy lead nuclei at close to the speed of light, recreating conditions similar to those just after the “Big Bang”, only on a much smaller scale. The quarks and gluons previously confined in each proton or neutron will then form part of a QGP. This will very quickly expand and cool and at a low enough temperature reassemble as ordinary matter. The reassembled particles fly out into the detector and studying them will help us understand how quarks and gluons behave in their plasma state. This in turn will help us to understand why and how these basic constituents of matter ever formed into protons and neutrons at all, and what keeps them in that state.

Read more about Heavy ions in CMS via the link below