Virtual Journal on QCD Matter

The one that started it all was the first experimental paper from RHIC. (Here “it” is obviously defined as the RHIC era; there of course is a rich history of heavy ion results from the Bevalac, AGS and SPS fixed target programs.)

The first published RHIC result

Charged particle multiplicity near mid-rapidity in central Au + Au collisions at √s_NN = 56-A/GeV and 130-A/GeV

from the PHOBOS Collaboration in the summer of 2000 provided the long-anticipated first data on charged particle multiplicities in central Au+Au collisions at RHIC energies. These days, when we routinely contemplate exotic phenomena such as heavy quark flow, plasma instabilities, Mach cones, etc., it may be hard to recall how little was known about a phenomenom as basic as multiplicity. The PHOBOS data ruled out predicted dramatic increases in multiplicity from mini-jets or parton casades, and instead were consistent with models that invoked gluon saturation in the initial state.

A few months later, the PHENIX Collaboration published the first study of the centrality dependence of the charged multiplicity:

Centrality dependence of charged particle multiplicity in Au – Au collisions at √s_NN = 130-GeV

Saturation models predicted that the variations of multiplicity with centrality (i.e., the number of participants) show a characteristic variation, directly related to the running of the strong coupling constant (or, equivalently, DGLAP evolution of the gluon structure function), as described in

Hadron production in nuclear collisions at RHIC and high density QCD

The PHENIX results did indeed show a variations with the number of particpants suggestive of saturation effects (although it slighly predated the above theoretical preprint, and therefore does not reference this explanation of the results). Subsequent measurements by PHOBOS

Centrality dependence of charged particle multiplicity at mid-rapidity in Au + Au collisions at √s_NN = 130-GeV

and STAR

Production of charged pions and hadrons in Au+Au collisions at √s_NN = 130-GeV

were consistent (at a very satisfying level) with the PHENIX results on this variation. A complete analysis of the variation with centrality and beam energy for both multiplicity and transverse energy, along with a comparison to a wide variety of models is found in this later PHENIX paper

Systematic studies of the centrality and √s_NN dependence of the dE_T/dη and dN_ch/dη in heavy ion collisions at mid-rapidity ,

which establishes that models based on saturation effects in the initial state give a good overall description of the observed systematic trends. (See Larry McLerran’s Color Glass Condensate Primer for a very nice introduction to the theoretical literature on saturation phenomena.)

While the excitement of pinning down these multiplicity variations perhaps has waned, understanding the saturation effects in the initial state remains an absolutely essential input to hydrodynamic calculations. And it’s safe to assume that this will again become a hot topic as first data from heavy ion collisions at the LHC appear in the near future. Finally, it must be mentioned that there are very practical aspects to knowing the dependence of charged particle multiplicity on centrality and especially beam energy. Imagine trying to design a ~$100M detector and not knowing the required segmentation for good performance to better than a factor of 2. And costs scale as (at least!) the first power of that same factor of 2.

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