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star physics results: Event-wise observables

Note: Click on a figure for a larger version.

Figure. The upper panel is a schematic representation (due to Scott Pratt) of the idea that produced hadron pairs of opposite charge will be closer in rapidity (have a narrower Balance Function) if their production is delayed. Below, data from STAR at two energies indicate that the Balance Function width (vertical axis) becomes progressively narrower with increasing collision centrality (horizontal axis).

Search for Evidence of Delayed Hadronization in Central Gold-Gold Collisions at RHIC

As a result of local charge conservation, when particle-antiparticle pairs are produced, they are correlated initially in position space. If hadronization occurs early in the scenario of a partonic phase followed by a hadronic phase, as depicted by the upper pair in the illustration, the two particles would be expected to separate in rapidity due to expansion and rescattering in the strongly interacting medium. Alternatively, delayed hadronization (the lower pair in the illustration) would lead to a stronger correlation between the measured pairs in the final state.

Measurement of this correlation involves subtracting uncorrelated charge-anticharge particle pairs event-by-event. Among the remaining charge-anticharge particle pairs, the correlation as a function of the rapidity difference between the members of the pair is characterized by the Balance Function. The Balance Function is expected to be narrower in the scenario of delayed hadronization, and thus has a bearing on the possible production of Quark Gluon Plasma.

The STAR data for more peripheral collisions are consistent with models based on a superposition of nucleon-nucleon scattering, while widths in central collisions are smaller, consistent with the trends predicted by models incorporating late hadronization.

Related STAR papers
Narrowing of the Balance Function with Centrality in Au+Au Collisions at sqrt(snn) = 130 GeV
Phys. Rev. Lett. 90 (2003) 172301
e-Print Archives (nucl-ex/0301014): Abstract | PS | PDF
Journal article: Phys. Rev. Lett. server

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