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What’s the quark matter?

29 April 1999

Careful analysis of data collected by the NA50 experiment studying high-energy heavy-ion collisions at CERN shows clear signs of new behaviour,
suggesting that under these conditions the colliding nuclear particles briefly fuse together to form a new kind of matter.

In ordinary matter,
quarks and gluons are confined inside nucleons,
the component particles of nuclei. However,
this has not always been the case. In the first split second after the Big Bang,
when the temperature exceeded 1013°,
quarks and gluons roamed around in a uniform “soup”. When the temperature dipped,
the free-ranging quarks and gluons suddenly “froze” into strongly interacting particles (hadrons),
where they have remained ever since. The only known way for them to leave this confinement is via high-energy nuclear collisions ­ “Little Bangs” ­ when small pockets of hot and dense nuclear matter simulate post-Big Bang conditions.

Over the past 20 years,
lab physics experiments have gradually increased the energy of their nuclear beams in the search for this “quark­gluon plasma”. As well as providing sufficient input energy to create Little Bangs,
another challenge is to recognize clearly the deconfined state once it has been recreated.

One suggestion,
which was made in 1986 by Tetsuo Matsui and Helmut Satz,
was to look among the emerging particles for states like the J/psi – a meson composed of a charmed quark and antiquark bound together.

Approaching plasma conditions,
the attractive force between the quark and the antiquark will be screened by gluons and lighter quarks,
and less charmed quark­ antiquark pairs will bind into J/psi states.

However,
an absorption effect also results from interactions of the produced J/psis with the nucleons while traversing the surrounding nuclear matter. Fortunately,
this conventional absorption mechanism can be understood from the study of lighter collision systems,
as has been done at CERN’s SPS synchrotron with proton,
oxygen and sulphur beams.

A sudden drop in the rate of J/psi formation,
after accounting for the normal nuclear absorption,
is considered to be a clear signature of quark­gluon plasma formation.

In 1996,
colliding 158 GeV/nucleon lead beams on a solid lead target and using an improved experimental set-up,
NA50 saw 190 000 J/psis via their decay into muon pairs,
four times the data collected in 1995. For peripheral lead­lead collisions,
where the density of nuclear matter is least,
NA50 sees the expected nuclear absorption effects,
extrapolated from studies with lighter nuclei.

However,
for more violent lead­lead collisions,
more energy is transferred and there is a maximum density of hot nuclear matter. Under these conditions,
quarks and antiquarks find it more difficult to stick together and the J/psi production rate dramatically decreases.

Under these conditions the quarks and gluons in the colliding lead nuclei briefly “forget” about their 15 billion year nuclear heritage and revert to their primeval state.

As well as the clear signs of J/psi suppression seen by NA50,
other encouraging signs that collective quark­gluon behaviour is not far away come from other experiments at CERN using heavy-ion beams,
notably NA45; seeing an excess of light electron­positron pairs; the increased yield of multiply strange particles by WA97/NA57; and several intriguing observations from the big NA49 study.

This bodes well for the
experiments that are preparing to take their first data at the
end of the
year at the
higher energies of Brookhaven’s RHIC heavy ion collider.
Their measurements should confirm beyond reasonable doubt the
current indications that high-energy nuclear collisions lead to a transition from confined to deconfined matter,
where quarks and
gluons are no longer bound inside hadrons.

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