STAR meets its Data and Detector Development Goals in Run 7
By William Christie
The STAR Collaboration had a very successful FY07 physics run. In spite of the usual challenges in running machines as complex as the RHIC Collider and the STAR Detector system, the STAR Collaboration met all of its physics data set goals, as well as its ongoing detector technology development goals.
STAR used the return to top energy (root(s) = 200 GeV) Au on Au running in Run 7 to pursue a better understanding of the most compelling features that have been observed in the ultra relativistic nuclear collisions at RHIC. We did this by running trigger combinations that included twelve distinct physics triggers, as well as a number of monitoring, normalization, and background triggers. We had physics triggers that were tuned to gather data sets which will allow STAR to extract high energy photons, which will be used to get direct photon cross sections, gamma-jet correlations to study energy loss for tagged jet energies, as well as provide a high statistics sample of high Pt tagged events for studying the three particle correlations which were reported by STAR at QM06. We had a set of triggers which selected data sets to allow us to make a qualitative advance in understanding the origin of the suppression of non-photonic electrons from D and B semi-leptonic decays. We had multi level triggers for upsilons in heavy ion collisions, as well as a very selective (low rate) trigger for J/psi's produced in ultra peripheral collisions.
To take advantage of our multi-layered silicon detectors for
the study of B and D decays, we used a new Vertex Position
Detector (VPD) that allowed us to select minimum bias events
that were centered in our silicon detectors, with a narrow
distribution of collision vertices along the beam line. The
trigger level position resolution that we achieved with the VPD
is shown in Fig. 1, which shows the difference between the
collision vertex along the beam axis as determined by the online Time Projection Chamber (TPC) tracking and the VPD. This trigger level vertex resolution of ~5 cm is plotted as a function of the a TPC track multiplicity parameter (proportional to impact parameter), and shows that the position resolution was independent of impact parameter over all but the most peripheral collisions. Fig. 2 shows that, using this VPD based trigger, our minimum bias data set has a vertex distribution along the beam axis with a FWHM of just 20 cm.
Despite some difficulties with power supply failures, the RHIC Collider delivered an integrated beam luminosity for the experimental program that was very close to the maximum of their pre-run estimated range. STAR used, and needed, every bit of this good collider performance to reach our rare trigger data set goal of sampling 600 inverse microbarns of luminosity, as well as accumulating our minimum bias goal of collecting 75 Mevts centered in our Silicon detectors. Fig. 3 shows that we achieved ~ 101% of both of these goals.
STAR also managed to setup a trigger, and collect a very small data set of events, during the one day low energy test run that was carried out during run 7. There were some clock stability issues associated with the collider having to run the low energy (root(s) = 9.1 GeV) configuration with 366, as opposed to the usual 360, RF buckets per RHIC revolution. Due to some very effective, real time, expert work on this issue by Robert Michnoff and others at C-AD, the clock glitches were reduced to a level that we could run the STAR system, setup a trigger, and collect some events.
Finally, in addition to the physics program, STAR made very significant progress on developing and testing the new detector technologies that will be employed in our upgrade program. We had 1 Read out Board (RDO, 144 RDO in entire TPC) and 29 FEEs of the new TPC electronics system (aka DAQ1000). This DAQ1000 test electronics was successfully tuned and debugged early in the run, and for the last month or so was included in the regular physics program, and operated by the Shift crews. We also tested a small three layer Active Pixel Sensor (APS) silicon technology, both outside and inside the STAR magnet, and very close to the beam pipe. This device worked as expected, and as well as allowing the experts involved to work out issues involved with operating the device in the STAR environment, they were able to collect data on the hit densities they'll have to contend with due to both collisions and beam related backgrounds. We successfully, installed, debugged, designed a trigger for, and collected a data set for, a Muon Test Detector based on large readout pad MRPC chambers, placed outside the STAR magnet iron. Throughout the run the new FMS detector (large array of Pb-glass cells at forward rapidities) was checked out cell by cell, the new electronics was completely installed, and significant progress was made in commissioning these new electronics. The FMS will play a major role in the planned d-Au and polarized pp running in Run 8.
In summary, STAR had a very successful physics run in Run 7, and we are eagerly working to extract what we expect to be a rich pallet of exciting physics from the accumulated data sets.