PHENIX From the Spokesperson's Perspective
By Barbara Jacak
The PHENIX Collaboration has just completed a very successful Run-8. We took over 80 nb−1 of integrated luminosity, representing 160 billion sampled minimum-bias events. This provides a factor of 30 increase over Run-3, and will provide much needed reference data for cold nuclear matter effects upon probes of the hot, dense matter produced at RHIC. In particular the conclusions from J/ψ and open charm production will become much clearer using the Run-8 data as reference. In addition, PHENIX had two Muon Piston Calorimeters in place during Run-8, allowing us to search for evidence of saturation of gluons in nuclei at small momentum fraction. The p+p collision period of Run-8 was short, but we managed to add substantially to our transversely polarized spin data.
PHENIX collaborators presented many talks at the recent Quark Matter 2008 Conference held in Jaipur, India. In the two months preceding the conference, the collaboration submitted seven new papers for publication. As space in RHIC News is limited, I will comment here about only one of these. In arXiv:0801.4555 we show the results of a first energy scan to search for the onset of jet quenching in the hot, dense medium created in heavy ion collisions. Figure 1 shows that, in Cu+Cu collisions, strong jet quenching (i.e. suppression of high momentum particle production) is observed in 200 and 62.4 GeV per nucleon pair collisions, but there is no suppression at 22.4 GeV. The presence of nuclei actually enhances particle production, such as the neutral pions used in this paper to observe the jets' fate. This enhancement, long known to exist in collisions at several tens of GeV, is called the "Cronin effect", and may mask the presence of some energy loss of quarks and gluons in 22.4 GeV heavy ion collisions. However, the observed change from strong suppression to no suppression clearly shows that the remarkable opacity of the hot, dense medium of quarks and gluons discovered at RHIC has an onset somewhere between 22.4 and 62.4 GeV per nucleon pair collision energy. Future energy scans at RHIC will be able to pinpoint just where the onset is, and elucidate other properties of the medium under the same conditions.
The other new PHENIX publications report on the fate of J/ψ's in Cu+Cu collisions (arXiv:0801.0220), show the spectra and suppression of neutral pions to 20 GeV/c transverse momentum in Au+Au (arXiv:0801.4020), illustrate how to quantify energy loss parameters in different models by careful control of the different kinds of experimental uncertainties (arXiv:0801.1665), map out the correlations of jet fragments as a function of transverse momentum to study the medium response to deposited energy (arXiv:0801.4545), provide three dimensional images of the final state of the medium in Au+Au as it breaks up (arXiv:0712.4372), and present the first measurement of di-electrons in p+p collisions at RHIC, using these to extract the heavy quark production cross sections (arXiv:0802.0050).
PHENIX is currently working on construction of our VTX and FVTX silicon vertex detector upgrades. These will allow us to tag electrons and muons from the decay of mesons containing heavy quarks, via the displacement of their decay vertex from the collision point of the beams. The new detectors will separate charm and bottom decays, allowing PHENIX to investigate the extent to which these very heavy probes lose energy and even get caught up and swept along by the opaque, flowing medium. Everyone was surprised by early measurements which showed large energy loss and participation by heavy quarks in the collective motion experienced by all the other particles. The new PHENIX vertex detectors will allow us to see whether the extremely heavy bottom quarks sail on through as expected, or will stun physicists by also interacting strongly with the medium and losing a lot of energy.
At the same time, PHENIX collaborators are working hard on construction of our muon trigger upgrades, in order to be able to trigger PHENIX on muons from W decays. The W measurement will allow PHENIX to probe the quark and anti-quark polarizations using polarized p+p collisions at 500 GeV. The new trigger system consists of large resistive plate chambers in the PHENIX muon arms, and new trigger electronics that are being added to the existing muon tracking chambers. Installation of the first of these elements will begin very soon.
The PHENIX Hadron Blind Detector (HBD) is being readied for re-installation and data taking in Run-9. The HBD will provide background rejection for dileptons, allowing for much improved measurements by increasing the signal-to-background ratio from the current level of approximately 1 to 200. We eagerly anticipate construction start for the PHENIX Nose Cone Calorimeter before the end of this fiscal year. The completed NCC will be installed into PHENIX several years from now, and used to take data in both ion and polarized proton collisions. PHENIX is looking forward to the increased luminosities that stochastic cooling will bring to RHIC. In order to improve our data taking speed, we are planning a set of upgrades to our DAQ and trigger systems. These range from new Data Collection Modules, to upgrading the Event Builder, through implementation of a level 3 trigger real time analysis farm (and a number of important steps in between).
The PHENIX Collaboration is in the midst of an exciting time. We are extracting new physics from the data sets we've taken over the last couple years, and looking forward to using the RHIC-II luminosity with our new upgrades to explore the surprises of the quark gluon plasma with new tools.