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The Heavy Ion Physics
When time began ...
... scientists believe there was a Big Bang from
which everything in the Universe emerged. Fifteen thousand million years later,
the Universe is so huge that it would take light millions of years cross. Yet in
the beginning everything was squeezed into a tiny volume no bigger then a flea.
All the particles which make up everyday matter, from which we and everything
around us are made, had yet to form. The quarks and gluons, which in today's
cold Universe are locked up inside protons and neutrons, would have been too hot
to stick together. Matter in this state is called Quark Gluon Plasma, QGP, and
experiments at CERN are trying to recreate it.
Scientists think that QGP
might still exist today in the hearts of neutron stars which are so dense that a
piece the size of a pinhead would weigh as much as a thousand jumbo jets. But
even if QGP does exist there, it can't be studied, so to understand the first
moments of the Universe's life scientists must create QGP in the laboratory. To
do so, they smash ions, atoms stripped of electrons, into each other at very
high energy, squeezing the protons and neutrons together to try and make them
melt. Experiments at CERN through the 1980s and 1990s have smashed ions of
oxygen, sulphur and lead into stationary targets. The results have given
tantalizing hints that QGP might have been created for fleeting moments before
cooling down into ordinary matter again.
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NA45 detector |
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WA97
detector | |
The lead beam programme started in 1994, after the CERN accelerators
has been upgraded by a collaboration between CERN and institutes in the
Czech Republic, France, India, Italy, Germany, Sweden and Switzerland. A
new lead ion source was linked to pre-existing, interconnected
accelerators, at CERN, the Proton Synchrotron (PS) and the SPS. The seven
large experiments involved measured different aspects of lead-lead and
lead-gold collisions. They were named NA44, NA45,
NA49, NA50, NA52, WA97/NA57 and WA98.
Some of these experiments use multipurpose detectors to measure and
correlate several of the more abundant observable phenomena. Others are
dedicated experiments to detect rare signatures with high statistics. This
co-ordinated effort using several complementing experiments has proven
very successful. |
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WA98 Exp'ts Setup |
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NA50
detector | |
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At a special seminar on 10 February 2000,
spokespersons from the experiments on CERN's Heavy Ion programme presented
compelling evidence for the existence of a
new
state of matter
in which
quarks, instead of being bound up into more complex particles such as
protons and neutrons, are liberated to roam freely.
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At CERN's next accelerator, the LHC, lead ions
will collide head-on at energies 300 times higher than at older CERN
experiments. Physicists believe that at these energies making QGP will be a
matter of routine allowing physicists to study its properties in detail.
What actually happens when a QGP forms?
Let's take a look!
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