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

ALICE at the LHC 2

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

Inner Tracking System (II) 4 Design goals  Optimal resolution for primary vertex and track impact parameter Minimize distance of innermost layer from beam axis ( ≈ 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/  2 region LayerDet. Type Radius (cm) Length (cm) Resolution (  m)PbPb dN/dy=6000 rr Z Part./cm 2 Occupancy (%) 1SPD SPD SDD SDD SSD SSD

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 p T cut-off –p T resolution <≈6% for a pion in p T range MeV 5 < 60  m (r  ) for p t > 1 GeV/c Central Pb–Pb Track impact parameter resolution [  m] Poster by E. Biolcati MC simulations: p-p

ITS role in ALICE physics (II) Vertexing  Reconstruction of primary (interaction) vertex From tracks: ITS crucial to obtain resolution better than 100  m 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 6 Primary vertex resolution vs. multiplicity in p-p D +  K -     decay vertex resolution Resolution [  m] Vertex from tracks Interaction diamond:  x,y =50  m

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/d  from tracks): Larger  and p T 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 7 dN/d  10 TeV (Pythia) Poster by M. Nicassio

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 2 layer barrel Total surface: ~0.24m 2 Power consumption ~1.5kW Evaporative cooling C 4 F 10 Operating at room temperature Material budget per layer ~1% X 0SPD Half-stave Inner surface: 40 half-staves Outer surface: 80 half-staves Beam pipe Outer layer Inner layer Minimum distance inner layer-beam pipe  5 mm MCM 5 Al layer bus + extender Ladder 1Ladder 2 MCM + extender + 3 fiber link Grounding foil 13.5 mm 15.8 mm ~1 200 wire-bonds ALICELHCb1 readout chip mixed signals 8192 cells 50x425  m 2

SDD 9 Central Cathode at -HV E drift Voltage divider v d (e - ) Anodes Front-end electronics (4 pairs of ASICs) -> Amplifier, shaper, 10-bit ADC, 40 MHz sampling -> Four-buffer analog memory  HV supply  LV supply  Commands  Trigger Data  Modules mounted on ladders Carbon fiber support Cables to power supplies and DAQ SDD layers into SSD Cooling (H 2 O) tubes 70.2 mm Layer# laddersMod./ladder# modules

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

ITS Commissioning data taking 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 11 TIME

Example from PVSS online detector control and monitoring 12 Temperature (°C) Leakage current (µA) Longitudinal tracks along one half-stave (  14 cm) SPD operation and calibration SPD online event display – Cosmic run 106/120 modules stably running  Dead+noisy pixels < 0.15%  Typical threshold ≈ 2800e-  Operating temperature ≈ design value  Average leakage ≤50V ≈ 5.8 µA  Average Bus current (≈ 4.4 A)  Detector readout time: ≈ 320  s  Detector dead time: 0 up to ≈ 3kHz (multi-event buffering) ≈ 320  s 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 Side viewView along z

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

SSD operation and calibration 14 Charge ‘ratio’ Cluster charge N-side Cluster charge P-side 11 % 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

AND Trigger: SPD FastOR  Coincidence between top outer SPD layer and bottom outer SPD layer  rate: 0.18 Hz Triggering and tracking the cosmics 15 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 Layer 1 (SPD) Statistics collected ≈ 10 5 good events Goals:  Alignment of each of the 2198 ITS sensors with a precision < than its resolution (≈ 8  m 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 Layer 4 (SDD) Cosmic data sample Layer 1 (SPD) Layer 4 (SDD) Layer 1 (SPD) Layer 5 (SSD) Layer 4 (SDD) Talk by A. Dainese

Alignment 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  2 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 17 Poster by A. Rossi

Alignment: Survey for SSD 18 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

Alignment: Millepede on SPD 19 x y Track-to-track  xy at y=0 Realigned   xy =52  m   spatial =14  m (MC ideal geometry:  spatial =11  m)  xy [cm] Clusters attached to same track in acceptance overlaps realigned not realigned   xy =18  m    xy =  spatial  2   spatial =14  m (MC ideal geometry:  spatial =11  m) ALICE Preliminary

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 20 Geometry only Geometry + calibration

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 21 SSD  Cosmics with field (0.5 T)  Tracks reco in TPC+ITS  Muon Most Probable Value of dE/dx for 300  m of silicon from AD&NDT 78 (2001) 183 ALICE Preliminary

Conclusions 22 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  8  m. ≈ 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. 11 th 2008

Backup 23

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. dividerDopant inhomogeneties

Alignment: SSD survey 26 Extra Clusters - With survey   xy =48  m  point =48/√2=34  m  misal =27  m   xy =25  m  point =25/√2=18  m  misal =0  m Extra clusters - No survey  xy [cm] 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

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

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

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 29

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 30 Muons from test setup 3 scintillator trigger No Zero Supp With Zero Supp

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 ~5 μ m square. Measured Resolutions are: Δ x and Δ y~25 μ m Δ z~235 μ m Δθ x and Δθ y ~0.30e-3 ° Δθ z ~1.75e-3 ° 31