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Particle physicists have long wanted to create a state of matter that
existed in the earliest moments of the big bang – a quark-gluon plasma.
They hoped to create such a plasma by smashing together heavy ions. But
now the first evidence of this state of matter has come unexpectedly from
simple proton-antiproton collisions at Fermilab in Chicago.

Quarks make up ‘hadrons’, particles which interact via the strong nuclear
force. The most familiar representives of the class are the protons and
neutrons of atomic nuclei. They are held together in threes by ‘force-carriers’
known as gluons. But during the birth of the Universe in the big bang, quarks
and gluons mingled freely in an extremely dense plasma.

Particle physicists and cosmologists believe a quark-gluon plasma existed
during the first second of the big bang, 15 billion years ago (see ‘Recreating
the birth of the Universe’, New Scientist, 17 August). Their plan was to
create the primeval state of matter by slamming together heavy ions of elements
such as gold or uranium.

Now Peter Levai and Berndt Muller of Duke University in Durham, North
Carolina, claim they have found the ‘signature’ of the quark-gluon plasma
in data obtained at Fermilab and published last year.

According to their calculations, processes that take place in the quark-gluon
fireball should eject known particles at high velocities perpendicular to
the direction of the colliding beams. They predict that large numbers of
different kinds of particles should emerge with almost the same ‘transverse’
velocity (Physical Review Letters, vol 67, p 1519).

When Levai and Muller applied their test to data from the proton-antiproton
collisions at Fermilab, they found just the kind of transverse shower of
particles expected from the quark-gluon plasma. ‘The velocities for pions,
kaons and antiprotons are equal,’ they say. ‘Our results constitute a powerful
indication for the existence and formation of (the quark-gluon plasma).’

This is good news for the theorists who predicted the transition from
normal matter to the quark-gluon phase, but slightly galling for the experimenters
who are still preparing their heavy ion experiments. Nevertheless, the result
provides valuable information for them. The heavy ion experiments promise
to be much more powerful probes of the quark-gluon state than the proton-antiproton
experiments. They will create more energetic ‘mini-fireballs’, and provide
more information about what went on before normal matter existed.

In the same issue of Physical Review Letters (p 1523), N. S. Amelin
of the University of Bergen and his colleagues present calculations which
provide more details of the kind of transverse flows expected for lead-lead
and sulphur-sulphur collisions. These show that the exact nature of the
flow depends strongly on details of the theoretical model used in the calculation.
So, by eliminating some models and hopefully supporting others, future experiments
may help theorists to develop a better understanding of the way fundamental
forces work, and bring them a step closer to a satisfactory grand unified
theory which will unite all the forces of nature in one mathematical package.

Another probe of the quark-hadron transition may be the element lithium.
Its abundance in the Universe, along with that of helium, was fixed in the
earliest moments of the big bang, and it may depend on the precise details
of the transition.