The size of the fireball

  • Posted on: 20 January 2015
A technique called Bose-Einstein or HBT Interferometry allows us to measure the size and lifetime of the fireball created in heavy-ion collisions. The fireball formed in heavy ion collisions at the LHC is hotter, lives longer and expands to a larger size at lower energies.

This method is devised by the pioneering work of Hanbury, Brown and Twiss in astronomy and looks at pairs of particles (two-pion correlation functions). In hadron and ion collisions, there is enhanced production of bosons close together in phase space, due to Bose Einstein statistics. This leads to an excess of pairs at low relative momentum. The width of the excess region is inversely proportional to the size of the system at decoupling, at the point when most particles stop interacting. The quark gluon plasma behaves as a fluid, with strong collective motions described well by hydrodynamic equations. The collective flow makes the size of the system appear smaller as the pair momentum increases. The radius of the pion source is measured in three dimensions : along the beam axis, Rlong; along the transverse momentum of the pair, Rout; and in a direction perpendicular to these two, Rside, show in the figure below. The similarity between Rout and Rside indicates a short duration of the emission – explosive emission. The time when the emission reaches its maximum, with respect to the first encounter, is found to be 10-11 fm/c, significantly longer than at RHIC. The product of the three radii – an estimate of the homogeneity volume of the system at decoupling - is twice as large as at RHIC resulting in a hotter and longer-living fireball.