From CERN Courier:”ALICE collaboration measures the size of the fireball in heavy-ion collisions’

The ALICE collaboration has measured the size of the pion-emitting system in central lead–ion collisions at the LHC at a centre-of-mass energy of 2.76 TeV per nucleon pair. The radii of the pion source were deduced from the shape of the Bose-Einstein peak in the two-pion correlation functions.

In hadron and ion collisions, Bose-Einstein quantum statistics leads to enhanced production of bosons that are close together in phase space, and thus to an excess of pairs at low relative momentum. The width of the excess region is inversely proportional to the system size at decoupling, i.e. at the point when the majority of the particles stop interacting.

An important finding at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven was that the QCD matter created there behaved like a fluid, with strong collective motions that are well described by hydrodynamic equations. The collective flow makes the size of the system appear smaller with increasing momentum of the pair. This behaviour is also clearly visible for the radii measured at the LHC in the ALICE experiment.”

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