Relativistic Nuclear collisions

Relativistic nuclear collisions refer to experiments that use high energy collisions of heavy ions as compared to lower atomic mass atoms in particle accelerators.


The exploration of hot hadron matter and of multiparticle production has a long history initiated by theoretical work on multiparticle production by Enrico Fermi in the USA, and Lev Landau in the USSR. These efforts paved the way to the development in the early sixties of the statistical bootstrap model description of hadron production by Rolf Hagedorn.

First collisions

The first heavy ion collisions at modestly relativistic conditions were undertaken at the Lawrence Berkeley National Laboratory, LBNL, at Berkeley, USA, and at the Joint Institute for Nuclear Research, JINR, in Dubna, USSR. At the LBL, a transport line was built to carry heavy ions from the heavy ion accelerator HILAC to the Bevatron. The energy scale at the level of 1-2 GeV per nucleon attained initially yields compressed nuclear matter at few times normal nuclear density. The demonstration of the possibility of studying the properties of compressed and excited nuclear matter motivated research programs at much higher energies in accelerators available at BNL and CERN with relativist beams targeting laboratory fixed targets. The first collider experiments started in 1999 at RHIC and LHC begun colliding heavy ions at one order of magnitude higher energy in 2010.

CERN operation

The LHC collider at CERN operates one month a year in the nuclear collision mode, with Pb-nuclei colliding at 2.76 TeV per nucleon pair, about 1500 times the energy equivalent of the rest mass. Overall 1250 valance quarks collide generating a hot quark-gluon soup. Heavy atomic nuclei stripped of their electron cloud are called heavy-ions, and one speaks of (ultra)relativistic heavy-ions when the kinetic energy exceeds significantly the rest mass energy, as it is the case at LHC. The outcome of such collisions is production of very many strongly interacting particles.


There are several scientific objectives of this international research program:

  • The formation and investigation of a new state of matter made of quarks and gluons, the quark gluon plasma QGP which prevailed in early Universe in first 30 micro seconds;
  • The study of Color confinement and the transformation of color confining = quark confining vacuum state to the excited state physicists call perturbative vacuum, in which quarks and gluons can roam free which occurs at Hagedorn temperature;
  • The study the origins of hadron (proton, neutron etc.) matter mass believed to be related to the phenomenon of quark confinement and vacuum structure.

Experimental program

This experimental program follows on a decade of research at the RHIC collider at BNL and almost two decades of studies using fixed targets at SPS at CERN and AGS at BNL. This experimental program has already confirmed that the extreme conditions of matter necessary to reach QGP phase can be reached. A typical temperature range achieved in the QGP created

T = 300 \mbox{MeV/k} =3.3 \times 10^{12} \mbox{K} is 10 000 times greater than in the center of the Sun. This corresponds to an energy density

\epsilon=10 \mbox{GeV/fm}^3 = 1.8\times 10^{16} \mbox{g cm}^{-3} . The corresponding relativistic matter pressure is

P\simeq \frac{1}{3} \epsilon=0.52\times 10^{31}\, \mbox{bar}.

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