The NA57 experiment
Students: Paul Bacon, Stephen Bull, Rory Clarke
There are seven members of staff and two research students who are involved in studying relativistic heavy ion interactions at CERN in Geneva. We are the only UK group working in this exciting, and relatively new, area of particle physics although there are over two thousand physicists world-wide involved in it.
Part of a heavy ion collision event observed in a silicon detector telescope, as used in NA57
One of the most intriguing predictions of QCD (the theory of strong interactions) is the possibility of a new phase of matter, known as a Quark-Gluon Plasma (QGP). Quarks are usually confined in elementary particles, such as the proton, due to the nature of the strong force which binds them. It has been predicted, however, that, under extreme conditions of energy density and temperature, matter will undergo a phase transition into a soup of quarks and gluons where the quarks will no longer be confined. This state of matter is know as a Quark-Gluon Plasma. Another phase transition which is believed to occur at similar energy densities is know as Partial Chiral Symmetry Restoration where the mass of the quarks are dramatically reduced to their 'bare' mass.
By studying these phase transitions we hope to learn more about the nature of confinement in QCD and of mass itself. It will also have important implications in the fields of astrophysics and cosmology where it has been postulated that the Universe would have been a QGP up until about 10-5 seconds after the Big Bang and the core of collapsing stars could be dense enough for QGP formation.
In order to create the conditions needed for QGP formation we need to produce the largest possible volume of matter under conditions of extremely high energy density. We do this by accelerating fully stripped lead ions to 158 GeV per nucleon (32.8 TeV per ion), using the CERN SPS, and firing them into a thin lead target. This creates a small fireball (~10 times the size of a lead nucleus) with extremely high energy density and temperature (~ 200 MeV or 2x1012 K). In terms of temperature, this is about a million times hotter than the centre of the Sun.
When a QGP is formed in these fireballs, many more strange particles (elementary particles containing one or more strange quarks) will be produced than if no QGP were formed. We detect these particles using 13 planes of silicon pixel detectors which represent the very latest in detector technology. Each 5cm by 5cm plane contains about 100,000 tiny pixels giving high precision.
Analysis of our data to date has indeed shown an increase of strange particle production by about a factor of two over what would normally be expected. Particles containing two strange quarks have been observed to be enhanced by about a factor of four and, early results show that particles made of three strange quarks are enhanced by about an order of magnitude. We are the only heavy ion experiment to have successfully found omega hyperons (particles with three strange quarks).
In February 2000, CERN made the announcement that a new state of matter has indeed been discovered by CERN heavy ion experiments. The striking results from NA57 (including its predecessor WA97) were essential to this announcement. The Birmingham group has played a vital role in the analysis of these data.
Further information about NA57
- The NA57 home page at http://www.cern.ch/NA57/
Before NA57, the group played a leading role in other fixed-target heavy ion experiments using the Omega Spectrometer at CERN. Further Birmingham-led Omega experiments searched for glueballs, hadrons composed of bound states of gluons, as predicted by QCD.
For more information on the individual Omega experiments, please consult the following: