Performance of the Silicon Strip Detector of the STAR Experiment

1. Performance of the Silicon Strip Detector of the STAR Experiment Jonathan Bouchet Subatech STAR Collaboration Berkeley School on Collective Dynamics

2. Outline • Physics Motivation • Strangeness, Charm. • Experiment • STAR, Inner tracking device. • Description of the SSD • Components, Hit reconstruction, Calibration. • Results • Efficiency, Distance of Closest Approach (DCA), V0 reconstruction using silicon information. • Outlook Berkeley School on Collective Dynamics

3. Strangeness • Primary designed to do strange particles physics. • Proposed and designed to enhance the STAR tracking capabilities in the central region. • Improve the momentum resolution. Simulation AuAu@200GeV Enhancement of  reconstruction by a factor of 4 TPC+SVT+SSD TPC+SVT TPC Berkeley School on Collective Dynamics

4. Why not Charm ? • Since few years, recent interest to measure open charm production in STAR. RAA • RAA of non photonic electron from B and D. • Models agree with data when only Charm is taken into account. • Need to disentangle charm and beauty contributions. • A direct measurement D-mesons is required. Nucl-ex/060712 K- How ? • Direct topological identification ex : D0-->K-+ + c = 123 m D0 Pointing accuracy is the key point. Berkeley School on Collective Dynamics

5. Solenoid Tracker at RHIC Solenoid Magnet (0.5 T) Silicon Vertex DetectorsSVT+SSD Time Projection Chamber Berkeley School on Collective Dynamics

6. Silicon Vertex Tracker • 3 layers (7,11,15 cm) of • 216 Silicon Drift Detector • Radiation length : 1.8%X0per layer Inner Tracker (1) Solenoid Magnet (0.5 T) Time Projection Chamber Berkeley School on Collective Dynamics

7. Inner Tracker (2) Silicon Strip Detector • 1 layer (23 cm from the vertex) of 320 detection modules. • Arranged in 20 ladders. • Pseudo-rapidity range : -1< η< 1, • Surface ~ 1 m2.. • Radiation length : 1%X0 Solenoid Magnet (0.5 T) Time Projection Chamber Berkeley School on Collective Dynamics

8. Silicon Strip Detector • double sided detector : • P/N junction reversely biased. • When a particle goes through the detector, electron-hole pairs are generated. • 768 strips per side with a pitch of 95 m. • Size = 42 x 73 mm2. • Intrinsic resolution : 20 m in r (transverse plan), 700 m in Z (along the beam axis). • Stereo angle : 35 mrad between strips of P and N sides for tracking constraints. • 2 hybrid circuits : • Support the FEE. Challenge to deal with 0.5 M readout channels ! Berkeley School on Collective Dynamics

9. Hit reconstructionCuCu@200GeV (2005) Cluster Finder • Cluster size distributions are good. • The tail is induced by the noise • Hits are essentially of type 1. • (hit density is small, no ambiguity) • Type 3: partial overlapping (charge matching needed) Type 1 Type 3 X : real hits X : ghosts hits X X X X X Berkeley School on Collective Dynamics 90% 10%

10. Gain calibration Mean=0.3 adc =11 adc • Since SSD are double-sided detectors, we expect the same charge deposit by MIP/particles on P and N side for each module. • Slight difference from P to N side charge (in ADC value due to electronic read-out. • Charge matching (correlation between P and N side) is used to discriminate hits from ghosts. • Gain calibration has been done for the CuCu data. Berkeley School on Collective Dynamics

11. Tracking Efficiency using SSD CuCu@200GeV • Defined as a binomial distribution between number of tracks coming from the TPC with a SSD hit used over all tracks in the SSD acceptance. • Different hit reconstruction efficiency ladder by ladder. • Repass alignement procedure. CuCu@200GeV MC AuAu@200GeV Hijing Berkeley School on Collective Dynamics

12. Observation : shift in the drift direction <Z> for SSD according to the magnet polarity. Lorentz effect : drift direction of the charge carriers affected by magnetic field. Lorentz effect <Z> : hit position -track position [m] -B +B • Scale the values from CMS 4T magnetic field to STAR 0.5 T [1] before after Improvement of the efficiency caused by picking the good hit. t Berkeley School on Collective Dynamics [1]:physics/0204078

13. Distance of Closest Approach • Resolution of DCA in the transverse plan limited by MCS. • Due to : • Mass (thickness of layer), distance to primary vertex. • Our case : (includes vertex resolution after alignment ) (Dcaxy)~140m/pT(GeV) CuCu@200GeV TPC TPC+SSD+SVT K0s invariant mass • Almost to our goal in vertex resolution. • Improvement on mass resolution when including at least 1 silicon hit. Berkeley School on Collective Dynamics

14. Outlook • Microvertexing techniques using the Silicon Detector of STAR are developed for the first time in heavy ion environment for charm/beauty identification. • SSD+SVT work well, and successfully took data 2005 (CuCu). • Reprocess CuCu data with improvements is on-going [1] • Ready to analyze the current AuAu data. • The Future : STAR upgrade HFT. [1] : Margetis et al,"Alignment Experience in STAR” to be published in CERN Yellow Report Berkeley School on Collective Dynamics

15. Berkeley School on Collective Dynamics

16. Results on the hits properties by the calibration • Hits are “cleaner” • Deviation decreases with the calibration. Calibration applied Berkeley School on Collective Dynamics

17. Efficiency vs centralitycucu@62GeV 0-10 10-20 20-30 30-40 >40 Berkeley School on Collective Dynamics

18. Density CuCu@62GeV CuCu@200GeV AuAu@200GeV • CuCu62, CuCu200 : Nhits/wafer ~ 4,5 • AuAu200 : Nhits/wafer ~6 Berkeley School on Collective Dynamics

19. TapeAutomated Bonding • It allows flexible connections. • It serves as a pitch adaptator. • Good yield of production. Berkeley School on Collective Dynamics

20. CuCu data reprocess Berkeley School on Collective Dynamics