For researchers interested in the working of the subatomic and astrophysical realms, soup is on the menu. This is no ordinary soup, however, but a seething mass of elementary particles called quarks and gluons. The whole universe is thought to have consisted of just such a soup of few millionths of second after the Big Bang that began everything. And today the soup is being sought by two groups of physicists, one of lot of which is attempting to recreate it on earth using particle colliders, while the other lot searches for it In the sky, buried in the cores of dead stars.
Both groups have reported progress, though neither has yet produced conclusive results. But definite identification of “quark matter” would have profound implications. For particle physicists, the search for such a soup provides a way of testing one of their most fundamental theories: that of quantum chromodnamics (QCD). Among astrophysicists, meanwhile, some theorists have suggested that in certain circumstances quark matter could form a stable sort of super-dense material which if its existence could be proven would literally be the strangest stuff in the universe.
The recipe for making your own quark soup from scratch goes something like this. First, take some atoms of lead. Unreel them and discard the electrons, leaving just the nuclei. Put the nuclei into a particle accelerator, spin up to almost the speed of light and heat by slamming into a target made of more lead atoms, so that they reach a temperature 100,000 times hotter than the centre of the sun. At this temperature, something rather unusual is thought to happen
Atom nuclei are bundles of protons and neutrons which, along with electrons, are the basis of normal, modern matter. But protons and neutrons are themselves made up of more fundamental particles called quarks, bound together by ‘glue” particles called (for obvious reasons) gluons. Quarks come in several varieties: a proton, for instance, consists of two “up” quarks and a “down” quark, while a neutron consists of two downs and an up. The theory of QCD makes predictions about the way quarks and gluons should behave.
Normally, the laws of subatomic physics dictate that individual quarks are never seen in the wild; they always travel around in twos or threes. At sufficiently high temperatures, however — such as those reached in a high-energy particle collider protons and neutrons are thought to disintegrate into a soup, or plasma, of individual quarks and gluons, before cooling and recombining into ordinary matter.
That is what QCD predicts. So, since 1994, an international team of researchers at CERN, the European laboratory for particle physics in Geneva, has been smashing lead nuclei together and then combing through the hail of subatomic particles that results from these collisions to look for evidence of quark-gluon plasma.
This is hard, because physicists can directly detect only particles that escape from the fireball and reach their instruments — and these particles are likely to have undergone several transmutations in their short lives. But by working backwards it is possible to discern the processes that led to the formation of the observed particles. For example, interactions between free quarks and gluons, as opposed to ordinary matter, should cause more of some types of exotic particle to be produced, and fewer of other kinds. Careful analysis of the rations of particles churned out in thousands of collisions should thus determine whether a quarkgluon plasma was made or not.
CERN researchers have announced that analysis of the results of seven separate types of collision collectively provided evidence of the creation, for the first time, of just such a soup. For a fraction of a second they had, in other words, recreated the conditions that prevailed just after the Big Bang. Admittedly, this declaration of victory came with several provisos. Ulrich Heinz, a theoretical physicist at CERN, says that more experiments at higher energies will be needed to verify the result. But, having cranked up their accelerators to achieve the most energetic collisions possible the CERN team can go no further. So the announcement also signalled a passing of the torch to the new Relativistic Heavy Ion Collider at the Brookhaven National Laboratory on Long Island, New York, which will start an experimental programme at higher energies later.
The tone of the passage is:
- Light and informative
- amusing but trite
- Perceptive and well researched