# The size of the fireball

This method is based on the same principle as the measurement of star sizes pioneered by 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, i.e. 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, R_{long}; along the transverse momentum of the pair, R_{out}; and in a direction perpendicular to these two, R_{side}. The similarity between R_{out} and R_{side} indicates a short duration of the emission – explosive emission. The time when the emission reaches its maximum, with respect to the first encounter, isdeduced from Rlong and 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, indicating a hotter and longer-living fireball.