FIRST RESULTS OF THE SILICON STRIP DETECTOR at STAR Jörg Reinnarth, Jonathan Bouchet, Lilian Martin, Jerome Baudot and the SSD teams in Nantes and Strasbourg (France) and the STAR collaboration 1. The new detector The SSD has been recently installed in the central part of the STAR experiment to enhance the tracking capabilities in this region It provides information on the positions of hits and on the ionization energy loss of charged particles The SSD improves the extrapolation of tracks in the Time Projection Chamber to the hits found in the Silicon Vertex Tracker The improvement of the extrapolation will help to better characterize the tracks especially for secondary particles Thus the reconstruction efficiency for short-lived particles such as K 0, Λ and Ω improve accordingly 2. Technical Information The SSD consists of 20 ladders x 16 wafers = 320 wafers Each wafer is made up of double sided silicon sensor; 768 strips per side The resolution is 20  m in r  and 700  m in z The SSD covers 2  in r , and -1<  <+1; the surface is 1 m², 500 k channels The A128 front-end chips shows an extended input range of ±13MIPs and extra-low power consumption <350µW per channel Connections between the strips on the wafer and the A128 input channels are done via the Tape Automated Bonding The produced heat on the ladder is removed via an air cooling system On board control chips (COSTAR) monitor temperature, leakage current, etc. 3. SSD performance in Run 2005 In August 2004 the last 10 (of 20) ladders were installed at STAR Since January the SSD is included in the new beamtime The left picture shows the mean module occupancy for all wafers In Run ´05 74 million CuCu200 events with the TPC were taken 91% of all TPC events (for CuCu200) include the SSD 93% of all CuCu62 events include the SSD The picture in the middle shows the events for the different runs 4. Hit reconstruction Each module consists of a p(ositive) and n(egative) side with each 768 strips Charged particles deposit charge on one or more strips, depending on charge, angle and momentum of the particle A cluster finding algorithm is used to build clusters from the strips and to merge p- side and n-side clusters to form hits. In case of closed hits, charge matching is used to solve cluster ambiguities (left picture). The usual cluster size is 1.5 strips (middle) The right picture shows the distribution of the total cluster charge 5. Upcoming tasks For cluster charge matching p- and n-side will be calibrated (left pictures) Calibration database will be created for fast offline processing Improvement of cluster building due to the calibration will be determined The SSD will be aligned by software to determine the exact wafer positions (right pictures) After alignment the tracking parameters (errors, resolution) have to be tuned The data have to be reprocessed for tracking with the SSD Monte Carlo data with new geometry will be used in the embedding Improvement of reconstruction efficiency for real data will be determined Upgrade of some SSD parts (e.g. cooling system, noise improvement) for upcoming runs will be installed this year 6. Summary and outlook The SSD has made a very nice job so far ~90% of all TPC data include the SSD The offline software development is finished and Monte Carlo data exists The detector has to be calibrated and aligned Than the tracking can be done The SSD will definitely enhance the tracking capabilities (left picture) The fine tuning of the tracking in the SSD will be started shortly and then the precise impact of the SSD on the STAR capabilities will be determined The complete SSD+SVT setup on its way into the STAR central region. The SVT is inside the SSD. The black tubes are the air cooling system. Schematic drawing of a SSD module showing the silicon wafer, the two hybrid circuits with their electronic chips and the TAB ribbon. The silion wafer. Top: p-side; bottom: n-side with readout electronic. The black points are the COSTARs. The brown and gold horizontal strips are the TAB ribbons (Kapton+Cupper) Schematic drawing of the ladder. Each ladder consists of 16 wafers attached to a cabon-fiber structure. At the end of each side the control and ADC boards are placed. One full assembled ladder. On the left side the ADC board is placed. The p-side of the wafers are visible and are facing the beam axis once installed in STAR. Setup of SVT, SSD and TPC. Inside (yellow) the three layers of the SVT are visible. They are surrounded by the SSD (red). The blue lines are tracks detected in the TPC. The mean module occupancy for module vs. real ladder. The mean module occupancy is distributed uniformly over all the ladders. Left: In case of only one particle, crossing a wafer, there is no ambiguity to find the crossing point. Right: In case of two particles, close together, there is ambiguity, which can be solved due to charge matching of the cluster. Cluster finder Relative population of the number of strips per cluster. 60% of the particles induce some signal on one or two strips. The cluster charge distribution can be nicely reproduced by a Landau distribution. The ADC pulse of the n-side against the ADC value for the p-side for one wafer. The wafer is not calibrated well, because both sides should measure the same ADC value. Residual in x against phi. In case of a perfect aligned SSD the residual should be distributed around zero. The wave distribution is due to a shift of all the whole SSD barrel in x and y. After a global alignment the residual in x is flat. Still each ladder has to be aligned seperatly. The z position of the first point of the helix. The red curve describes only TPC tracks. Using the SSD (green) clearly constraints the z-position of the track. Number of SSD hits versus number of primary tracks. The deviation from the diagonal is due to both the secondary particles producing hits in the SSD and fake hits induced by the electronic noise. The p/n ratio of pulser ADC values for all A128 chips. The value is constant for different runs and will be calibrated. Hit reconstruction: The colored lines represent the tracks, measured by the TPC and extrapolated by the primary vertex. The straight lines are the SSD and SVT ladders. SSD hits are visible and can be associated to the tracks. STAR SSD SVT TPC Pedestal and noise values for one ladder along the beamtime. Pedestal and noise were rather constant. Number of events taken with TPC, TPC+SSD and TPC+SSD+SVT for different runs. Cu+Cu 200GeV Cu+Cu 62GeV pp