STAR Mid-Rapidity Upgrades: The Detector Gets Even Better
by Jim Thomas for the STAR Collaboration
STAR was constructed to investigate the behavior of strongly interacting matter at high energy density and to search for signatures of the quark-gluon plasma (QGP). Key features of the nuclear environment at RHIC are a large number of produced particles (typically about 1500 into the acceptance of the detectors at mid-rapidity) and high momentum particles from hard parton scattering. STARís primary scientific mission is to understand the evolution of the collision process in ultra-relativistic heavy ion collisions and to measure as many signatures of the hypothesized strongly interacting QGP phase transition as possible.
In order to accomplish this goal, STAR was designed to make measurements of particle production over a large solid angle, featuring detector systems for high precision tracking, momentum analysis, and particle identification at the center of mass rapidity. The large acceptance of STAR makes it particularly well suited for the measurement of event-by-event correlations in heavy ion collisions and for the detection of hadron jets.
The STAR detector uses a Time Projection Chamber (TPC) as its primary tracking device. The TPC records the tracks of particles by measuring their ionization energy loss (dE/dx) over a large distance, in a magnetic field. Particle momenta can be measured this way over a range from 100 MeV/c to 30 GeV/c and they can be individually identified over a momentum range from 100 MeV/c to greater than 2 GeV/c.
The STAR TPC is shown schematically in Figure 1. The TPC measures 4 meters in diameter and is 4.2 meter long, making it the largest TPC in the world (for a few more months). It sits in a large solenoidal magnet that operates at 0.5 Tesla. The paths of primary ionizing particles passing through the TPC volume are reconstructed with high precision from the released secondary electrons which drift to the readout end caps at the ends of the chamber. The net result is a snap-shot of the event and these snap-shots can be combined to make the beautiful physics results that have appeared in over 75 publications by the STAR collaboration.
However, this isnít good enough. We would like to extend the capabilities of the suite of STAR detectors in order to take the data faster, to identify the particles at even higher momentum, identify muons as well as hadrons, and explore the heavy flavor frontier.
The STAR TPC data acquisition upgrade will be among the first of the STAR upgrades to be operational next year (Run 8). The STAR data acquisition system is fast and flexible. It receives data from multiple detectors and these detectors have a wide range of readout rates. Currently the events are processed at input rates up to 100 Hz. In order to acquire data at even higher rates, we have designed a new set of electronics for the TPC. The new system, called DAQ1000, will acquire data at rates up to 1000 Hz. The new technology that makes this upgrade possible are the front-end chips and associated readout electronics being developed for the ALICE TPC at CERN. These chips are truly the Ďnext-generationí of TPC front-end chips and were designed based on our experience with the STAR electronics and built using specifications that are nearly identical to the STAR specs. Thus, the replacement of the existing STAR electronics with a derivative of the newer CERN electronics will mean an order of magnitude increase in performance.
Particle Identification at mid-rapidity is accomplished by several techniques but one of the most powerful is the measurement of the particles energy loss (dE/dx) in the TPC gas. This technique works extremely well at momenta below 1 GeV/c but is subject to ambiguous identification of pions, kaons, and protons above ~1 GeV/c. These ambiguities can be resolved by measuring the time of flight (TOF) of the particles from the vertex to the outer radius of the TPC. Therefore, we will start the installation a Time of Flight (TOF) upgrade in run 8 and the full upgrade will be complete in run 9. The new TOF modules will cover 2 in azimuth and will be located at the outer radius of the TPC. The technology for the upgrade is relatively new; each module will use multi-plate resistive plate chambers (MRPC) and they will be fabricated by a consortium of STAR collaborators in the US and China. The TOF system will make measurements with a precision of 85 picoseconds, or said another way, it will double the momentum range over which particles can be directly identified in STAR.
At almost twice the radius of the new TOF system, we will install a new muon identification system that will allow us to track and identify the decay of heavy vector mesons at mid-rapidity. This capability will be new and unique at RHIC. The essential technology will be the placement of double-stacked MRPC modules outside of the magnet iron that surrounds the TPC. The signal from these detectors will be used to tag muons coming from the collision vertex because the muons can easily penetrate the magnet iron while the more abundant blast of pions from the collision cannot. The pions will either be stopped or create showers in the iron. Only at high transverse momentum ( > 10 GeV/c) can a significant fraction of pions penetrate the steel or create a shower that reaches the MRPC modules. The pion showers can be rejected by precise timing measurements and good position resolution determined by the MRPC strips. To achieve even greater discrimination between muons and pions, we will look at the track of the particle in the TPC. The shape of the track (perhaps including a decay 'kink') and the energy loss (dE/dx) of the track reveals information about the track. Monte Carlo simulations suggest that muons and pions can be identified with high reliability using the dE/dx information. This means that using a simple analysis procedure, we can achieve muon/ pion separation by a factor of 200. In addition, the TOF detector, mentioned above, will be able to reject all the kaons and protons. With this kind of discrimination, we can pursue a very active program of muon measurements to detect the J/y and ° at mid-rapidity.
In order to extend STARís particle identification capabilities further into the heavy flavor domain, we will install several layers of high resolution silicon trackers starting with a pixel detector at 2.5 cm radius from the collision point. The full suite of detectors will be called the Heavy Flavor Tracker (HFT). The proposed configuration starts with two tracking layers comprised of monolithic CMOS pixel arrays using 30 mm x 30 mm square pixels. The Pixel layers will lie at radii of 2.5 cm and 7.0 cm, respectively. They provide 135 million pixels of information for every event studied. In order to provide graded resolution between the TPC and the pixel layers, two additional high rate conventional silicon barrel layers are proposed at intermediate radii of 12 cm and 17 cm. These Intermediate Strip Tracker (IST) layers provide space-points with high accuracy in r-f and in the z direction between the Pixel layers and the existing Silicon Strip Detector (SSD), reducing the number of possible track combinations that can connect with hits on the outer layer of the pixel detectors. This is particularly crucial to enable accurate measurements in high multiplicity environments. The HFT provides tracking information for short lived particle decays displaced by 100 microns, or less, from the collision point. As an example, the decay D0 ģ K- + p+ can be identified directly by using the Pixel detector to select K- and p+ tracks while the fast moving D0 cannot be seen because it is a neutral particle that doesnít leave a track in the detector. When combined with the existing STAR TPC and SSD, the HFT constitutes an integrated state-of-the-art mid-rapidity inner tracking system which is unique at RHIC. This tracking system will significantly extend the reach of the STAR scientific program. It will afford efficient topological reconstruction of D and B mesons down to low transverse momenta (for Dís ≥ 500 MeV) illuminating their in-medium interactions and the properties of the strongly interacting quark-gluon plasma.
A similar conclusion can be reached regarding the overall package of STAR upgrades; in every way, the predicted STAR performance gets even better.