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Heavy implications for the first second

15 March 2000

After a decade of running, the results from CERN’s research programme with
high-energy nuclear beams provide tantalizing glimpses of mechanisms that shaped
our universe.

About a microsecond after the Big Bang, the universe was a seething soup of quarks
and gluons. As this soup cooled, it “froze” into protons and neutrons, supplying the
raw material for the nuclei that appeared on the scene a few minutes later.

To
check if this imagined scenario is correct, since 1986 experiments at CERN have been
accelerating beams of nuclear particles to the highest possible energies and piling them
into dense nuclear targets. Recreating what happened in the first microsecond of
creation has so far taken many years of careful and painstaking work.

The goal
has been to use the energy supplied by the nuclear beams to recreate tiny pockets of
primordial quark-gluon plasma about the size of a big nucleus and watch them behave
as “Little Bangs”. Theorists using simulation tools predict that this soup/plasma should be formed at a temperature of about 170 MeV (about 1011degrees, or
100 000 times the temperature at the centre of the Sun) with an energy concentration
of about 1 GeV per cubic femtometre – seven times that of ordinary nuclear
matter.

The milestones of the early universe, separated by only fractions of a
second, nevertheless stretched over immense energy gaps as the Big Bang
temperature plummeted. The Little Bang experiments too have to contend with vast
swings of temperature/energy.

The experiments take snapshots of the particle
patterns emerging from these Little Bangs, but these patterns, although embedded in
the particle behaviour, are quickly masked by the surrounding nuclear debris. The
challenge is to peer through this debris to glimpse the signature of the Little
Bangs.

Ion beam experiments

The ion beams at CERN serve
several large experiments, codenamed NA44, NA45, NA49, NA50, NA52,
WA97/NA57 and WA98. Some of these studies use existing multipurpose detectors to
investigate the fruit of the heavy-ion collisions. Others are special dedicated
experiments to detect rare signatures.

On both the machine and the physics
sides, the programme is an excellent example of collaboration in physics research.
Scientists from institutes in more than 20 countries, including Italy, Japan, Germany,
France, Portugal, Russia, Finland, India, Poland, Greece, Switzerland, the UK and the
US, have participated in the experiments. The programme has also allowed a new
productive partnership to develop between high-energy physicists and nuclear
physicists, and it has considerably extended the number of scientists using CERN as a
research base, with new research centres, some of them from far afield, joining the
CERN research programme.

Estimates of the energy density established when
the colliding nuclei coalesce point to several giga-electron-volts per cubic femtometre
(CERN CourierNovember 1999 p8), suggesting that the theoretically expected
critical energy threshold has been crossed.

One important quark signature is the
J/psi particle, which is made of a charm quark and its antiquark. J/psis are rare
because charm quarks are heavy. However, theorists suspected that the production of
J/psis would be suppressed by the screening of the quark “colour” charge by the
surrounding quark-gluon matter. A strong reduction in the number of J/psis leaving
the fireball would suggest that hot quark-gluon plasma was initially present. This is
exactly what the NA50 experiment saw (CERN CourierMay 1999
p8).

Other particles – phi, rho and omega mesons – are composed of lighter
quarks and antiquarks bound together. These mesons can be seen through the
surrounding fog of dense matter via their decay into pairs of weakly interacting
particles – for example, electron-positron pairs – which pierce through the surrounding
strongly interacting material. In a quark-gluon plasma, the quarks and antiquarks find
it difficult to lock onto each other and therefore their signals get smeared out, as seen
in the NA45 experiment.

Another encouraging sign seen quite early in CERN’s
heavy-ion experiments was the increased production of particles containing strange
quarks. The ion projectiles only contain up and down quarks – no strange quarks.
High-energy proton-proton or electron-positron collisions provide enough energy to
synthesize strange quark-antiquark pairs, but for the nucleus-nucleus collisions the
fraction seen by the WA97 experiment was markedly higher. The greater the
strangeness content of the emerging particles, the more their production levels were
increased. For example, the yield of Omega baryons containing three strange quarks
was 15 times normal.

In principle the cleanest quark signals are the
electromagnetic ones, and WA98 has seen some preliminary signs of an increased
yield of single photons radiated by quarks.

Quark chemistry

The
particles leaving the fireball retain signatures of their past, pointing back in time. In
elastic scattering when particles “bounce” off each other, only their momentum
changes. As the fireball expands, the energy density decreases until the hadrons no
longer interact – their momenta “freeze out”. The momentum distribution of the
particles leaving the fireball gives a snapshot of when this freezeout occurred, at a
temperature of about 100 MeV.

What happened if the fireball was much hotter
and denser, when quark chemistry was operating? Once the resulting subnuclear
particles emerged, their composition reflected what happened when the quarks froze.
These particle distributions serve to reveal the chemical freeze-out temperature when
quarks became subnuclear particles – around 180 MeV, which agrees with the critical
temperature predicted by theory.

Another experimental technique, based on
interferometry, is a development of the pioneering astronomical work of Hanbury,
Brown and Twiss and adapted for particle physics by Giuseppe Cocconi at CERN in
1974. Looking at correlated pairs of particles, this technique measures sizes. The rate
of expansion of the system is known, so size information can be extrapolated
backwards to reveal the original energy density and to disentangle thermal motion
from collective flow.

Heavy ions at CERN

The CERN results
obtained with lead beams are the culmination of a long programme. A proposal in
1982 from heavy-ion enthusiasts suggested that the CERN machines could be used to
accelerate beams of oxygen ions to extend interesting heavy-ion results obtained
earlier at Darmstadt’s Unilac and Berkeley’s Bevalac.

Despite CERN’s crowded
programme (the SPS proton-antiproton collider was then in full swing) and
commitments to new projects such as LEP, development work for heavy-ion beams
began at CERN through a Berkeley/CERN/Darmstadt collaboration. An important
element was CERN’s Linac 1 injector, which had already learned how to handle
deuterons and alpha particles. This was fitted with an electron cyclotron resonance ion
source from Grenoble and a radiofrequency quadrupole from Berkeley.

In the
mid-1980s, at the same time as CERN’s big machines were learning how to handle
electrons and positrons in preparation for LEP, an experimental programme got
under way at CERN’s SPS synchrotron using 200 GeV/nucleon oxygen ions.
Complementary data came from a programme at Brookhaven’s AGS synchrotron
with beams of 14.6 GeV/nucleon.

CERN soon extended the range of its
experimental programme by supplying sulphur beams at 200 GeV/nucleon. From
1993, equipped with the new Linac 3 injector and its ion source, and in a
collaboration between CERN and institutes in the Czech Republic, France, India, Italy,
Germany, Sweden and Switzerland, the reach of the experiments was considerably
extended using the much heavier lead projectiles.

The future

These
results, announced on 10 February at CERN, resulted in a blaze of media hype.
However, they are not definitive and have to be followed up. While all of the pieces of
the puzzle seem to fit a quark-gluon plasma explanation, it is essential to study further
this new form of matter to characterize its properties fully and confirm the
quark-gluon plasma interpretation. Where exactly is the energy threshold for the new
state of matter? What are the critical sizes of the produced fireballs? What is the actual
transition? In a succinct analogy from theorist Maurice Jacob, “We have seen boiling
water but we do not yet know what steam looks like, nor how the boiling
goes.”

Although the ion beam experiments at CERN continue, the focus of
heavy-ion research now shifts to the Relativistic Heavy Ion Collider at Brookhaven,
which starts experiments this year. Due to start in 2005, CERN’s Large Hadron
Collider experimental programme will include a dedicated heavy-ion experiment,
ALICE.

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