Abstract

1. Performance of the STAR Silicon Vertex Tracker in Au-Au Collisions at RHICMarcelo G. Munhoz* and Jun Takahashi* for the STAR Collaboration *Universidade de São Paulo - Brazil Abstract The STAR Silicon Vertex Tracker (SVT) is constructed of 3 concentric cylinders of of silicon drift detectors. As the innermost tracking detector in STAR, the SVT plays an important role in determining the primary vertex and in identifying particles from secondary decays. Due to its high position resolution, it significantly enhances the reconstruction efficiency for short lived strange and multiply-strange particles. It also improves the two track resolution which is important for studies of Hanbury-Brown Twiss (HBT) interferometry. The SVT detector was fully installed in STAR for the first time for the 2001 RHIC run. Analysis of the data from the SVT during this period indicates the detector is providing essential data on V0 reconstruction and particle identification by ionization energy loss (dE/dx) in the silicon drift detectors. Preliminary results for the s = 200 GeV running will be presented, and a perspective on the extended physics reach provided by the SVT for future RHIC running will be discussed. Detector Description The STAR experiment at RHIC consists of several different sub-detectors of which the Silicon Vertex Tracker (SVT) is the inner most tracking detector. It is formed by 3 concentric barrels around the beam line with a total of 216 Silicon Drift Detectors (SDD). It has full coverage in the azimuth direction and a pseudorapidity coverage of -1 to 1. Technical Performance The left photo shows the complete SVT detector during it´s installation into STAR. Each SDD has an area of 6.3×6.3 cm2 and are mounted on long ladder structures. The outer barrel has 16 ladders of 7 detectors each and it has a mean radius of 14.9 cm from the beam line. The middle barrel has 12 ladders with 6 detectors at a radius of 10.6 cm and the inner barrel has 8 ladders with 4 detectors each at a radius of 6.0 cm. The SVT was successfully installed into STAR in February 2001. After a few weeks of adjustments, 91.2% of all 103,680 channels were working. The noise level stayed around 1800e- rms for most of the Au+Au run, improving considerably to 1200e- rms for the p+p run after eliminating noise introduced into the system ground by the High Voltage power supplies. A position resolution of 30 m was determined by shining a laser spot through the detector. The figure on the left shows the laser spot position during a period of 30 min. The slow rising slope is due to the detector warm-up which stabilizes after one hour. The short time oscillation corresponds to the detector temperature variation due to the water cooling system. This laser measurement will be used in the final analysis to correct for the temperature effect on the drift velocity thus improving the overall position resolution. 0.04 Position (cm) 0.03 cathodes 0.02 anodes To readout electronics A charged particle passing through the SDD leaves a cloud of ionized electron-hole pairs. Implanted cathodes on both surfaces of the detector generate an electric field in the depleted silicon that forces the electrons to drift towards the edge of the detector where they are collected by readout anodes. 0.01 120 microns 250 microns 300 microns 0.0 0 10 20 30 Time (min) Focusing anodes Electron Cloud TPC-SVT Integration: Track Matching Primary Vertex Finding The SVT is a powerful tool to enhance the STAR tracking capability. It can provide a better reconstruction of primary tracks and a better identification of secondary tracks. The figure on the left shows an example of a reconstructed event from a p+p collision. The TPC reconstructed tracks were extrapolated to the SVT planes and hits were associated to these tracks. The proximity of the SVT to the beam pipe and its enhanced position resolution should result in this detector having the capacity to find the primary vertex position with better efficiency and resolution than the TPC alone. The figure on the right shows the Z position of the primary vertex measured by the SVT compared to the TPC measurement for low multiplicity p+p events. The non-correlated points correspond to very low multiplicity events (2 tracks) where one of the detectors fail to determine the primary vertex position. The consistency of these two measurements is a clear indication of the good performance of the detector. TPC measured z vertex position (cm) SVT measured z vertex position (cm) The figure on the right and above shows the probability of matching a TPC reconstructed track to a SVT hit as a function of the beam direction (z) position of the primary vertex for p+p collisions. Once the TPC reconstructed track is matched to a SVT hit, one can measure the position residual of the hit on this track. The figure on the right shows these residuals as a function of the transverse momentum of the track. These values are mostly dominated by the TPC resolution. Strange Particles Topology Decay Reconstruction The measurement of strange hadron production is one of the main topics of Relativistic Heavy Ion studies. The reconstruction of their topology decay is the main tool to obtain their measurement. The diagram on the right shows the most important parameters that can be used to reconstruct particles as K0s short and L´s. The SVT should provide a better measurement of these parameters enhancing the STAR capability to reconstruct such topology decay. Primary Tracks Reconstruction Adding hits closer to the primary vertex to the TPC tracks should result in better resolution in the impact parameter measurement of primary tracks. The Figure on the left shows the impact parameter distribution of primary tracks for the same sample of events from Au+Au collisions, but using only the TPC for tracking (black curve) and using both, TPC and SVT for tracking (red curve). It is evident that there is considerable improvement provided by the addition of the SVT in the primary particles tracking. The figure on the left shows the invariant mass distribution of reconstructed decays of neutral particles (V0) from p+p collisions, assuming a K0s decay hypothesis. The black line shows the result obtained using TPC only reconstructed tracks. The red line shows the result obtained using TPC+SVT reconstructed tracks. The signal enhancement is clear from this figure. The top-left corner shows the ratio between these two invariant mass distributions, pointing the enhancement of almost 40% in the K0s mass region. Counts (a.u.) Primary track impact parameter (cm) Particle Identification (Energy Loss) The figure in the right shows the uncorrected yields (1/ptdNdpt) for the K0s identified in the previous figure. One can see that the SVT improves the efficiency for V0 reconstruction of low transverse momentum particles. These are preliminary results of the data processed with tight cuts made for TPC data only and thus are not optimized for TPC+SVT reconstructed tracks. Greater improvement in the efficiency of detecting low momentum tracks is expected for SVT+TPC specific cuts. TPC only STAR possesses the capability to perform particle identification with the TPC via dE/dx measurement. However, low momentum particles that do not reach the TPC or that have poor resolution due to short track length in the TPC cannot be identified with the TPC only dE/dx. For these particles, one could use the dEdx information from the SVT. The SVT has only 3 layers yielding 3 measurements for the dE/dx calculation, however, the higher energy resolution of silicon detectors yields a resolution equivalent to that of the TPC. The plot on the right shows an uncalibrated plot of dE/dx measured in the SVT as a function of particle momentum. TPC+SVT Summary The SVT dE/dx measurement can be combined with the equivalent TPC measurement to perform a cut in two dimensional space that can be more effective in separating different particle species. The left panel demonstrate this technique by performing particle identification for tracks with momentum between 0.2 GeV/c and 0.3 GeV/c where the pions are better separated from the other particles than for the conventional dE/dx method. The SVT adds important capabilities to the STAR experiment, as low pt tracking and better track resolution. This preliminary analysis shows various improvements to the performance of the overall STAR detector, as better resolution for primary tracks impact parameter measurement and higher efficiency for low pt K0s detection. The SVT energy loss measurement should provide an extra tool for particle identification. Many aspects of the analysis can still be improved, for example, a more detailed drift velocity calibration using the laser measurements and a better alignment of the detector. Gain correction calibration is yet to be applied to improve the dE/dx measurements. All these foreseen improvements will definitely enhance even more the SVT capabilities in the STAR experiment.