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Last Updated: Tuesday, 19 April, 2005, 16:26 GMT 17:26 UK
Early Universe was 'liquid-like'
Concept impression of dense matter (Brookhaven/RHIC)
The impression is of matter that is more strongly interacting than predicted
Physicists say they have created a new state of hot, dense matter by crashing together the nuclei of gold atoms.

The high-energy collisions prised open the nuclei to reveal their most basic particles, known as quarks and gluons.

The researchers, at the US Brookhaven National Laboratory, say these particles were seen to behave as an almost perfect "liquid".

The work is expected to help scientists explain the conditions that existed just milliseconds after the Big Bang.

People like me, who use model calculations, are already so excited about the data because we believe they have actually found the elusive state known as the quark-gluon plasma
Asst Prof Steffen Bass, Duke University
The details, presented to the American Physical Society in Florida, will be published across a number of papers in the journal Nuclear Physics A.

They summarise the work of four collaborative experiments - dubbed Brahms, Phenix, Phobos and Star - which have been running on Brookhaven's Relativistic Heavy Ion Collider (RHIC).

First moments

Already, the results have caused quite a stir in the research community.

"The experimental collaborations are still taking a cautious approach whereas people like me, who use model calculations, are already so excited about the data because we believe they have actually found the elusive state known as the quark-gluon plasma," commented theoretical nuclear physicist Steffen Bass from Duke University.

The QGP is the state postulated to be present just a few millionths of a second after the creation of the Universe - before the formation of matter as we know it today.

RHIC tunnel at Brookhaven (Brookhaven/RHIC)
Gold nuclei zip along the RHIC's 3.8km-long tunnel at nearly the speed of light
To create the ultra-hot, ultra-dense conditions seen in Brookhaven's RHIC, gold ions were fired at each other at near-light speeds.

Although these impacts only occur in tiny volumes, they deliver sufficient energy to "melt" the neutrons and protons that make up the nuclei, and allow their even smaller constituent quarks and gluons to roam free for a few, fleeting moments.

Cern, Europe's leading nuclear research lab, is thought to have had glimpses of this remarkable state back in 2000, but the Brookhaven work is said to have gone to another level.

Surprising data

However, the RHIC scientists are reluctant to make a firm declaration that a QGP has been achieved.

"We know that we've reached the temperature [up to 150,000 times hotter than the centre of the Sun] and energy density [energy per unit volume] predicted to be necessary for forming such a plasma," said Sam Aronson, Brookhaven's associate laboratory director for high energy and nuclear physics, but added also that aspects of the data had surprised everyone.

In a sense we are working backwards to the Big Bang by going to higher energy collisions, in order to be able to see this stuff
Prof Ken Peach, Rutherford Appleton Laboratory
There were unexpected patterns in the trajectories taken by the thousands of particles produced in the individual collisions.

These suggest that the matter revealed at the heart of those collisions was behaving not like the perfect "gas" of free quarks and gluons predicted for the GDP, but more like a liquid.

"The matter in the RHIC behaves very much like an ideal fluid; like a liquid," said Bass, who also holds an affiliation at Brookhaven.

"And a liquid is, of course, very deferent from a gas - it is very strongly interacting. Whereas what people like myself had assumed for many, many years was that a quark-gluon plasma would more behave like a dilute gas, where the particles would roam freely between scatterings for quite a distance."

Working back

The RHIC teams themselves intend to run more experiments and hope to make a more definitive statement about the nature of what they have seen in due course.

Already, some commentators are saying the data appears to match some aspects of string theory, an approach that attempts to explain the fundamental properties of the Universe using 10 dimensions instead of the usual three spatial dimensions plus time.

The international scientific community has put in place a programme of development over the next few decades which will see new machines come on stream that can probe matter states in new regions and at higher energies.

These experiments should put greater detail on our understanding of the materials and forces that built the Universe.

"In a sense we are working backwards to the Big Bang by going to higher energy collisions, in order to be able to see this stuff," said Ken Peach, the head of particle physics at the CCLRC Rutherford Appleton Laboratory in the UK.

"We can't get all the way back to 't = 0' but we can get back to some place that was near there and then look to see how this 'droplet' of primordial Big Bang material develops.

"And the assumption is that provided you can create a large enough droplet, it will evolve in the same way as the early Universe."

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