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Highlights From PHENIX I

By Achim Franz

More than 20 years ago, at Quark Matter 1987 in Nordkirchen, Germany, the first data from the CERN SPS heavy ion program were presented [1]. At that time the emphasis was on global observables like overall multiplicity, transverse energy flow, or even collision size and shape via HBT interferometry.

Now that RHIC has entered its eighth year of running and billions of events from 4 collision systems and 6 energies have been collected, interest shifts to determining the properties of the new matter with precision measurements of the distributions and systematic study of their dependence on colliding system, centrality, rapidity or even the reaction plane. RHIC also increased its luminosity by a better understanding of the machine and new techniques like stochastic cooling. The d+Au run which ended a few weeks back yielded 30 times more data for PHENIX than the previous run in 2003.

The different collisions systems varying from simple p+p, to d+Au, where cold nuclear effects should be visible, to Cu+Cu and Au+Au collisions have to be measured in the same experiment to give a baseline and to enhance, via the ratios of scaled p+p to A+A distributions, the additional collective effects which lead to the description of the perfect liquid.

Figure 1 shows on example of such a baseline measurement which is not just a great example of mT scaling but also shows the versatility of the PHENIX detector. Shown are pT spectra for several mesons measured at PHENIX in 200GeV/c p-p collisions. Only the π0 spectra were fitted with a modified Hagedorn function which fixed the shape for all other spectra via mT scaling with only the overall scale os a free parameter. These spectra are used to define the background contributions to low mass lepton pair measurements.

Figure 1 Transverse momentum spectra for different mesons from 200 GeV/c p+p collisions. See text for details.

PHENIX has added several new detectors which aid in particle identification to higher pT, reaction plane determination and can also be used as a trigger in low energy runs which are planed for the coming years. For example the Time-of-Flight wall - West (TOF-W) is made from multi-gap resistive plate chambers (MRPC). With a time resolution of 65 ps it widens the pT window for particle ID. Preliminary data from the TOF-W show that the anisotropic flow, v2, which had shown a scaling behavior with the number of constituent quarks as a function of the transverse energy KET [2], breaks from that scaling at KET/quark > 1GeV.

More v2 measurements were shown by PHENIX including the first data on J/ψ -> ee. The preliminary values are consistent with 0, but the still large systematic error bars cannot rule out any of the existing models, and data from J/ψ -> &mu&mu are still being analyzed.

Probing the medium by multiple particle correlations and disappearing and reappearing jet like structures have gotten a lot of attention in the past years [3]. Many presentations during QM08 were discussing head and shoulder structures and "ridgeology." PHENIX has measured the properties of the ridge and shown that these particles behave more like the overall bulk of particles and show a response of the medium rather than jet fragmentation.

QM08 was packed with new data and very detailed analyses, but in my opinion we can only extract the maximum information from these riches of experimental data if the experimenters and theorists agree on a set of common variables, like how to present the centrality of the collision, how to calculate the number of participating nucleons, and describe the nuclear geometry, and so on.

References

[1] Zeitschrift für Physics C, Volume 38, Numbers 1-2, March, 1988

[2] Phys. Rev. Lett. 98, 162301 (2007)

[3] arXiv:0801.4545, Phys.Lett B 649 (2007) 359 Phys. Rev. C 77 (2008) 011901 Phys. Rev. Lett. 98 (2007) 162301