A microsecond after the Big Bang, when the exploding fireball of the newborn Universe was only a few kilometres across, all matter existed in a special state.
The basic building blocks of matter - quarks and electrons - floated freely in an incredibly hot, dense soup. As the Universe grew and cooled, the quarks bound together into the protons and neutrons that abound today.
This is what physicists think happened at the beginning of the Universe. To prove it, teams at particle accelerators all over the world have been racing to recreate that primordial soup - called quark-gluon plasma.
Physicists at CERN, near Geneva, claimed to have seen signs of such a plasma after smashing lead ions into each other (New Scientist print edition, 12 February 2000). But not everyone was convinced and the experiment closed before the researchers could follow up their results. Now teams at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, New York, are claiming the prize.
Their heavyweight equipment fires gold ions at one another, creating 10 times the energy thought to be needed to make the quark-gluon plasma. During the last run in 2001, all four of RHIC's detectors - STAR, PHENIX, BRAHMS and PHOBOS - saw a peculiar effect called jet quenching.
Normally, when two ions collide they scatter two jets of particles in opposite directions, rather like billiard balls. But in the gold-gold experiment, sometimes only one jet was picked up by the detectors. This matches what you would expect for a soup of free quarks - if a collision occurred near the edge of the plasma, one particle would be kicked loose while the other would be swallowed up by the plasma.
But theorists wondered if the missing jets could be to do with the high energy of the gold nuclei, rather than any new kind of matter. To prove their case, the teams ran the experiment again, this time colliding gold ions with smaller deuterium ions. Although the energy of the gold ions was the same as before, the overall energy was not high enough to make quark-gluon plasma.
The results are still being analysed, but to those involved the answer is clear. "We do not see jet suppression in the deuterium-gold collisions," says Barbara Jacek from PHENIX, a result confirmed by researchers at the other detectors.
That suggests the jet quenching must have been due to quark-gluon plasma, not the gold ions themselves. So the teams are confident that their gold-gold collisions did create quark-gluon plasma for the first time.
"It is inconceivable that what we see in the gold-gold collisions is just an extremely hot gas of ordinary matter," says Thomas Ulrich from STAR. "This is new physics."
The researchers plan more experiments to be absolutely certain of their result. They also plan to carry out collisions at lower energies in the hope of seeing the transition from ordinary matter into quark-gluon plasma.