Results of ALICE Inner Tracking System Commissioning with Cosmics at QM 2009

1. Results from the commissioning of the ALICE Inner Tracking System with cosmics Francesco Prino (INFN – Sezione di Torino) for the ALICE COLLABORATION QM 2009, Knoxville, 2009 April 2nd

2. ALICE at the LHC

3. Inner Tracking System (I) • Six layers of silicon detectors • Coverage: |h|<0.9 • Three technologies • Pixels (SPD) • Drift (SDD) • Double-sided Strips (SSD)

4. Inner Tracking System (II) • Design goals • Optimal resolution for primary vertex and track impact parameter • Minimize distance of innermost layer from beam axis (<r>≈ 3.9 cm) and material budget • Maximum occupancy (central PbPb) < few % • 2D devices in all the layers • dE/dx information in the 4 outermost layers for particle ID in 1/b2 region

5. ITS role in ALICE physics (I) • Tracking: • Prolong tracks reconstructed in the TPC • Improve momentum and angle resolution • Track impact parameter crucial for heavy flavours • Standalone ITS tracking • Track and identify particles missed by TPC due to dead zones between sectors, decays and pT cut-off • pT resolution <≈6% for a pion in pT range 200-800 MeV Poster by E. Biolcati Central Pb–Pb MC simulations: p-p < 60 mm (rf) for pt > 1 GeV/c Track impact parameter resolution [mm]

6. ITS role in ALICE physics (II) • Vertexing • Reconstruction of primary (interaction) vertex • From tracks: ITS crucial to obtain resolution better than 100 mm • From SPD tracklets: done before tracking and used as a starting point (seed) in the tracking phase. Allows for pileup tagging based on multiple vertices • Identification of secondary vertices from decays of hyperons and open charm and beauty hadrons Primary vertex resolution vs. multiplicity in p-p D+ K-p+p+ decay vertex resolution Resolution [mm] Vertex from tracks Interaction diamond: sx,y=50 mm

7. ITS role in ALICE physics (III) • Charged particle pseudorapidity distributions from SPD • Pairs of clusters, one per SPD layer, aligned to the main interaction vertex (“tracklets”) • Advantages (wrt dN/dh from tracks): • Larger h and pT acceptance • Less stringent calibration needs • Suitable for the very first data • First measurement that ALICE will be able to perform, both in p-p and Pb-Pb dN/dh reconstruction @ 10 TeV (Pythia) Poster by M. Nicassio

8. 13.5 mm 15.8 mm SPD 5 Al layer bus + extender • 2 layer barrel • Total surface: ~0.24m2 • Power consumption ~1.5kW • Evaporative cooling C4F10 • Operating at room temperature • Material budget per layer ~1% X0 MCM + extender + 3 fiber link Ladder 1 Ladder 2 MCM Grounding foil Half-stave ~1200 wire-bonds Outer surface: 80 half-staves • ALICELHCb1 • readout chip • mixed signals • 8192 cells • 50x425mm2 Inner layer Beam pipe Outer layer • Unique L0 trigger capability • Prompt FastOR signal in each chip • Extract and synchronize 1200 FastOR signals from the 120 half-staves • User defined programmable algorithms Minimum distance inner layer-beam pipe 5 mm Inner surface: 40 half-staves

9. Cables to power supplies and DAQ Cooling (H2O) tubes SDD Modules mounted on ladders Voltage divider Central Cathode at -HV Carbon fiber support Anodes SDD layers into SSD Edrift vd (e-) HV supply 70.2 mm vd (e-) Edrift • LV supply • Commands • Trigger • Data  Front-end electronics (4 pairs of ASICs) -> Amplifier, shaper, 10-bit ADC, 40 MHz sampling -> Four-buffer analog memory

10. End ladder electronics SSD r- overlap: L5: 34 ladders L6: 38 ladders Ladder • carbon fibre support • module pitch: 39.1 mm • Al on polyimide laddercables z - overlap: L5: 22 modules L6: 25 modules Hybrid:identical for P- and N-side Al on polyimide connections 6 front-end chips HAL25 water cooled Sensor: double sided strip: 768 strips 95 um pitch P-side orientation 7.5 mrad N-side orientation 27.5 mrad

11. ITS Commissioning data taking TIME • Detector installation completed in June 2007 • Run 1 : December 2007 • First acquisition tests on a fraction of modules • Run 2 : Feb/Mar 2008 • ≈ 1/2 of the modules in acquisition due to cooling and power supply limitations • Calibration tests + first atmospheric muons seen in ITS • Installation of services completed in May 2008 • Run 3 : June/October 2008 • Subdetector specific calibration runs • Frequent monitoring of dead channels, noise, gain, drift speed … • Cosmic runs with SPD FastOR trigger • First alignment of the ITS modules + test TPC/ITS track matching • Absolute calibration of the charge signal in SDD and SSD

12. Temperature (°C) Leakage current (µA) SPD operation and calibration Example from PVSS online detector control and monitoring • 106/120 modules stably running • Dead+noisy pixels < 0.15% • Typical threshold ≈ 2800e- • Operating temperature ≈ design value • Average leakage current @ ≤50V ≈ 5.8 µA • Average Bus current (≈ 4.4 A) • Detector readout time: ≈ 320 ms • Detector dead time: • 0 up to ≈ 3kHz (multi-event buffering) • ≈ 320 ms at 40 MHz trigger rate • Max readout rate (100% dead time ): ≈ 3.3 kHz • FastOr trigger with ≈ 800 ns latency June 15, 2008 First “signs of life” of the LHC SPD online event display – Cosmic run Side view View along z Longitudinal tracks along one half-stave (14 cm)

13. SDD operation and calibration Display of 1 injector event on 1 drift side of 1 module • 247 out of 260 modules in DAQ • Calibration quantities monitored every ≈ 24 h • Fraction of bad anodes ≈ 2% • <Noise> ≈ 2.5 ADC counts • Signal for a MIP on anodes ≈ 100 ADC • Drift speed from dedicated runs with charge injectors Drift speed on 1 anode during 3 months of data taking Drift speed on 1 drift side from fit to 3 injector points Measurement of vdrift vs. anode and vs. time crucial to reach the design resolution of 35 mm along rf vdrift = mE T-2.4 Lower e- mobility / higher temperature on the edges

14. SSD operation and calibration Charge ‘ratio’ • 1477 out of 1698 modules in DAQ • Fraction of bad strips ≈ 1.5 % • Charge matching between p and n sides • Relative calibration from 40k cosmic clusters • Important to reduce noise and ghost clusters 11 % Cluster charge N-side Cluster charge P-side

15. Triggering and tracking the cosmics • Trigger: SPD FastOR • Coincidence between top outer SPD layer and bottom outer SPD layer • rate: 0.18 Hz AND • ITS Stand-Alone tracker adapted for cosmics • “Fake” vertex = point of closest approach between two “tracklets” built in the top and bottom SPD half-barrels • Search for two back-to-back tracks starting from this vertex

16. Cosmic data sample • Statistics collected ≈ 105 good events • Goals: • Alignment of each of the 2198 ITS sensors with a precision < than its resolution (≈ 8 mm for SPD ! ) • Fundamental to reach e.g. the required resolution on track impact parameter for heavy flavour studies • dE/dx calibration in SDD and SSD Talk by A. Dainese Layer 5 (SSD) Layer 4 (SDD) Layer 1 (SPD) Layer 4 (SDD) Layer 4 (SDD) Layer 1 (SPD) Layer 1 (SPD)

17. Alignment Poster by A. Rossi • Two track-based methods to extract the alignment objects (translations and rotations) for the 2198 ITS modules: • Millepede (default method, as for all LHC experiments) • Determine alignment parameters of “all” modules in one go, by minimizing the global c2 of track-to-points residuals for a large set of tracks • Iterative approach • Align one module at a time by fitting tracks in the other modules and minimizing the residuals in the module under study • Plus and hardware (based on collimated laser beams, mirrors and CCD cameras) alignment monitoring system • Monitor physical movements of ITS with respect to TPC • Strategy for the track-based alignment: • Use geometrical survey data as a starting point • Measurements of sensor positions on ladders during SDD and SSD construction • Hierarchical approach: • Start from SPD sectors (10) , then SPD half staves (120), then SPD sensors (240) • After fixing SPD, align SSD barrel (w.r.t. SPD barrel), then SSD ladders (72) … • After fixing SPD and SSD, move to SDD (which need longer time for calibration) • Include SDD calibration parameters: • Time Zero = time after the trigger for a particle with zero drift distance • Drift speed for modules with mal-functioning injectors

18. Alignment: Survey for SSD • Three methods to validate SSD survey information • Fit track on one SSD layer (2 points)  residuals on other SSD layer • Fit one track on outer layer, one track on inner layer  distance and angles between the two tracks • Extra clusters from acceptance overlaps  distance between two clusters attached to same track on contiguous modules ALICE Preliminary

19. Alignment: Millepede on SPD sDxy=52 mm   sspatial=14 mm (MC ideal geometry: sspatial=11 mm) Realigned y Track-to-track Dxy at y=0 ALICE Preliminary x Dxy [cm] Clusters attached to same track in acceptance overlaps sDxy=18 mm  sDxy=sspatial2  sspatial=14 mm (MC ideal geometry: sspatial=11 mm) not realigned realigned

20. Alignment: Millepede on SDD • Interplay between alignment and calibration • TimeZero and DriftSpeed for modules with mal-functioning injectors included as free parameters in the Millepede • Resolution along drift direction affected by the jitter of the SPD FastOR trigger (at 10 MHz  4 SDD time bins) with respect to the time when the muon crosses the SDD sensor Geometry only Geometry + calibration

21. dE/dx calibration • SDD • Larger drift distance  larger charge diffusion  wider cluster tails cut by the zero suppression • Effect quantitatively reproduced by Monte Carlo simulations • SSD • Cosmics with field (0.5 T) • Tracks reco in TPC+ITS • Muon Most Probable Value of dE/dx for 300 mm of silicon from AD&NDT 78 (2001) 183 ALICE Preliminary

22. Conclusions • Successful commissioning run with cosmics during summer 2008 for the ALICE Inner Tracking System • Several calibration runs to check the stability of operation and performance over ≈ 3 months of data taking • Collected statistics of cosmic tracks allowed for: • Most of SPD modules realigned to  8mm. ≈ 1/2 of SSD modules (the ones close to the vertical with higher statistics) realigned. SDD on the way • dE/dx signal calibration in SDD and SSD • ITS ready for the first LHC collisions 7-track event collected with circulating LHC beam2 on Sept. 11th 2008

23. Backup

24. SDD performance vs. time

25. SDD correction maps • All the 260 SDD modules have undergone a complete characterization (map) before assembling in ladders • Charge injected with an infrared laser in > 100,000 known positions on the surface of the detector • For each laser shot, calculate residual between the reconstructed coordinate and the laser position along the drift direction • Systematic deviations due to: • Non-constant drift field due to non-linear voltage divider • Parasitic electric fields due to inhomogeneities in dopant concentration Ideal Module Non-linear volt. divider Dopant inhomogeneties

26. Alignment: SSD survey • Three methods to validate SSD survey information • Fit track on one SSD layer (2 points)  residuals on other SSD layer • Fit one track on outer layer, one track on inner layer  distance and angles between the two tracks • Extra clusters from acceptance overlaps  distance between two clusters attached to same track on contiguous modules Extra Clusters - With survey Extra clusters - No survey • xy=25 m • point=25/√2=18 m • misal=0 m • xy =48 m • point=48/√2=34 m • misal=27 m Dxy [cm] Dxy [cm]

27. Alignment: Iterative on SPD Poster by A. Rossi • Minimization track-to-track residuals at Y=0 • Result from iterative approach

28. Alignment: Millepede SPD+SSD • SSD Millepede realignment at ladder level + survey data for modules • Single track impact parameter resolution ≈ 30/2 ≈ 21 mm Talk by A. Dainese DATA, B=0 realigned (Millepede) ALICE Preliminary s ~ 30 mm

29. LHC injection tests Aug/Sep 2008 • The SPD was operational during the beam injection tests and provided relevant information on the background levels in ALICE

30. SDD dE/dx vs. drift time • Cluster charge dependence on drift distance • Larger drift distance  larger charge diffusion  wider cluster tails cut by the zero suppression • Effect quantitatively reproduced by Monte Carlo simulations • Cross checked with cosmic clusters collected with and without zero suppression • No dependence of cluster charge on drift distance observed without Zero Suppression Muons from test setup 3 scintillator trigger No Zero Supp With Zero Supp

31. ITS Alignment Monitoring System • Laser based system which uses a spherical mirror (1x magnification) to determine the movement of the laser/camera module and the mirror relative to each other. • Any 3 mirror/camera pairs yield movement measurements for all 6 degrees of freedom. • Resolution is limited only by CMOS pixel size ~5μm square. • Measured Resolutions are: Δx and Δy ~25μm Δz ~235μm Δθx and Δθy ~0.30e-3 ° Δθz ~1.75e-3 °