About the Author

Mike Leitch is a physicist from Los Alamos National Laboratory who is an APS Fellow, a member of the PHENIX Executive Council and is the PHENIX Run Coordinator for the 2007 and 2008 RHIC runs.

The 2007 Gold-Gold Run at PHENIX

by Mike Leitch

LeitchThe PHENIX collaboration has just finished our 2007 Gold-Gold run having recorded over 5 billion minimum-bias events (corresponding to about 810 microbarns-1 in integrated luminosity) using our 5 khz data acquisition event rate capability - approximately 3.4 times what was obtained in our previous 2004 Gold-Gold run. Along with detector improvements, including new subsystems that allow a reaction plane resolution up to four times better than in 2004, this data promises to yield many new and interesting physics results. Among the advances expected should be 1) substantial increases in the statistical and systematic precision of rare signals such as the J/psi and jet correlations, 2) increased reach to large transverse momentum, 3) elliptic flow measurements for the J/psi and photons, and 4) the first look at low-mass lepton pairs using Dalitz rejection from the new Hadron Blind detector.

Despite early plans to run 15 weeks of Gold-Gold collisions and 10 weeks of polarized proton collisions, because of the continuing resolution funding situation the start of the run was delayed until February 8th. An additional delay due to problems in the RHIC Cyrogenic refrigerator plant delayed the startup further resulting in a start of physics data taking of March 27th – yielding just 13 weeks of physics with Gold-Gold collisions.

Although the run was shorter than we hoped, one highlight of the accelerator operations which holds great promise for the future is stochastic cooling. We estimate that for the part of the 2007 run during which longitudinal stochastic cooling was on (for just the yellow beam), about a 16% increase in integrated luminosity was seen. With cooling on the blue beam as well, plus the potential to stochastic cool also in the transverse direction, much larger improvements may be realized in future runs.

Four new detector subsystems came online at PHENIX in this run. The Reaction Plane detector (see June 19th article by Wei Xie) along with the Muon Piston Calorimeter provide up to a factor of four improvement in the measurement for each event of the azimuthal orientation of the line connecting the centers of the two colliding Gold nuclei, called the reaction plane. The comparison of production along this direction with that perpendicular to it, e.g. for J/Psi production, is key to understanding the physics of the medium produced by the colliding Gold nuclei.

graph 1

Figure  1: Shown for Au+Au collisions is the cosine of twice the difference of the reaction plane phi angles between detectors at the two ends of PHENIX (south & north) versus centrality. A larger value translates to better resolution since the cosine of a small angle difference is largest. On the horizontal axis, more central collisions correspond to larger BBC charge. The new reaction plane (RXPN) and muon-piston calorimeter (MPC) detectors both provide a large improvement in the accuracy to which the reaction plane can be determined.

A new RPC based time-of-flight detector on the west side of PHENIX also increases the reach for identified particles up to 12 GeV/c in that region.

Finally a state-of-the-art hadron blind detector (HBD) based on triple-GEM technology was installed for this run. However, due to unexpected high voltage problems its first run should be considered mostly an engineering run, although some physics may be obtained from parts of the detector. In any case, the problems are now understood and with some modifications, should allow a fully capable refurbished detector to be fielded for the next run. This should allow unprecedented accuracy in the study of low-mass electron pairs.

Although over 500 Terabytes of raw data were recorded, a few fast “level-2” trigger selections were run in the tail-end of our data acquisition system in order to write out small samples of data enriched with J/psi’s and high transverse momentum particles. This sample, about 10% of the total size, was transferred to a large computing facility in France where early physics signals can already be seen, e.g. the dimuon J/psi peak shown below.

J psi mass

Figure  2: The J/psi --> mu+mu- signal obtained from analysis of a selected sample of data filtered online using fast level-2 algorithms. Based on this sample approximately 16,000 J/psi's can be expected in the dimuon channel from the full 2007 run's data.

Additional fast analysis at the RHIC computing center, at a large facility at Vanderbilt University, and on computers in the PHENIX counting house have provided verification of the integrity of the data for e.g. the HBD, and have prepared our analysis tools for an early start of the full analysis of the entire minimum bias data sample.

The 2007 run is the 1st installment on the large, 2400 microbarns-1, Gold-Gold sample we hope to accumulate by the end of the 2009 run. In addition we plan to firm up the cold nuclear matter baseline with a new deuteron-gold run in 2008. The latter, of course, must also come with a new polarized proton run that will allow us to continue to advance towards our spin physics goals.