Highlights from the 2008 Deuteron-Gold Run at PHENIX
By Mike Leitch, PHENIX Run Coordinator
The PHENIX Collaboration has reached the end of a very successful Deuteron-Gold (d+Au) run with over 80 nb-1 of integrated luminosity representing 160 billion sampled minimum-bias events at the end of the d+Au run on January 27th. This exceeds our original luminosity goal of 58 nb-1 by about 40% and is about 30 times larger than the 2.7 nb-1 obtained in the previous (2003) d+Au run. Given improvements in the detectors and in the shielding since 2003 and better control of beam backgrounds – this data promises to become a new standard for RHIC Cold Nuclear Matter (CNM) effects1,2, and as the baseline for heavy-ion collision data, for years to come.
Physics goals for this d+Au run include the study of CNM effects with various probes including the J/Ψ and light hadrons into both the central and forward regions. Physics issues such as the saturation of gluons in nuclei at small momentum fraction, energy loss of gluons in normal nuclear matter, transverse momentum broadening for particle production in nuclei, and normal dissociation of hidden charm and beauty (J/Ψ, Ψ’, Υ) are among the CNM effects of interest. These measurements also provide a critically needed baseline for the presently much more accurate heavy-ion measurements, especially those obtained in the 2007 Au+Au run. Without a solid baseline that describes the normal nuclear matter effects, it can be very difficult to recognize additional effects from hot-dense matter (QGP) in heavy-ion collisions – particularly in the case of the J/Ψ where the CNM effects are substantial.
In Figure 1 we show the development of recorded luminosity over the course of the run, with peaks in recent weeks reaching about 3 nb-1 per day - larger than the whole accumulation in the 2003 d+Au run. Figure 2 shows the development of the integrated luminosity recorded by PHENIX with a total of about 80 nb-1 on Jan 27th.
There has been some concern about the backgrounds, especially into the back of our “forward” muon detectors in this run. The forward muon identifier Iarocci tubes have had large currents in the South end of PHENIX for most of the run. This is the side where the deuteron beam enters our interaction region, and we have seen average currents of up to 20 μA at the beginning of stores. Vertical steering and collimation has helped at times. However some amount of higher multiplicity and associated currents on this side should be expected since there are higher physics multiplicities from the collisions in the Au-going direction. Earlier in the run additional backgrounds associated with Au beam on the other (North) end of PHENIX were observed, but have largely been eliminated thanks to careful vertical steering and collimation of the Au beam.
This year’s peak d+Au min-bias trigger rates from our beam-beam counters have reached as high as 300 khz at the beginning of a store, and store lengths have generally been 6 hours long with typical beam-beam rates at the end of stores as low as 30 to 40 khz. For these peak rates, the maximum rate of 6.5 khz that our data acquisition system can take, allows only taking a subset of the min-bias triggers, and so we rely on a number of rare “level-1” fast triggers to obtain most of our physics signals. The rare triggers we have used include dimuon triggers from the two muon identifiers, triggers from the RICH-EmCal that are tuned to capture most of the high mass di-leptons (including the J/Ψ’s) and high transverse momentum photons, and triggers on the energy in our forward calorimeters (MPC’s). In addition to these fast “level-1” triggers we also run “level-2” filtering algorithms to select small (~1%) samples of the full raw data that are enriched in J/Ψ’s and high-energy photons. The latter allows us to run near-real time analysis on this smaller sample of events to obtain prompt feedback on the rate of accumulation of these signals.
One of the measures that demonstrates the quality and quantity of the data accumulated in this run is the J/Ψ signals in the di-lepton channel – dimuons in the forward and backward muon arms, and dielectrons in the central arms of PHENIX. As explained above, our fast analysis of “level-2” samples allows us to see these in almost real time. The observed yields and mass resolution of these signals both echo the large luminosities obtained and verify the correct performance of the PHENIX detectors. The dimuon spectra from each of the two (north and south) muon arms and the combined spectra are shown in Figure 3, where one can see very large J/Ψ peaks with about 57,000 total observed. These yields and mass resolutions are very close to expectations based on previous measurements. The mid-rapidity (central arm) J/Ψ peak is seen in Figure 4, with about 4,400 J/Ψ’s observed. In both cases, the spectra shown have had random pairs subtracted using the like-sign pairs recorded by the same triggers; an approximate method that is not as precise as the mixed single tracks that will eventually be used in offline analysis.
These J/Ψ yields are unprecedented, and represent the largest obtained at RHIC so far. To illustrate this we plot the approximate number of J/Ψ’s for each year since 2002 in Figure 5 – note that the vertical scale is logarithmic. For this year’s d+Au measurement, the points for dimuons (closed symbols) and dielectrons (open symbols) are approximate projections for the yields we expect for the full run8 d+Au data set - 73,000 in the dimuon decay channel and 6,000 in the dielectron channel. We have come a long way since the handful of J/Ψ’s we had in 2002.
We expect to record approximately 160 Billion d+Au events with a total volume of approximately 430 Tbytes, a little smaller size than the 650 Tbyte data set recorded in the 2007 Au+Au run.
Despite the limited funding this year, RHIC expects to be able to operate until about the middle of March. This allows a short polarized proton-proton run of about 6 weeks, including setup time; approximately 4 weeks of physics running – which we are now preparing for. The present plan is to run most of this time at 200 GeV.
1A. Adare, et al, (PHENIX Collaboration), “Cold Nuclear Matter Effects on J/Ψ Production as Constrained by Deuteron-Gold Measurments at sqrt(S) = 200 GeV”, Nucl-ex 0711.3917, to be published in Phys. Rev. Lett..
2Cold Nuclear Matter Effects on J/Psi Production, by Ramona Vogt, RHIC News December 4th, 2007.