Integration and alignment of ATLAS SCT Roland Härtel IMPRS Seminar May 12, 2006

Content Introduction (LHC, ATLAS, InnerDetector) SCT (layout, principle of operation) Beam tests (1995 - 2002) Alignment Combined Test Beam (2004) IMPRS Seminar 12.05.2006 Roland Härtel Integration and alignment of ATLAS SCT

Large Hadron Collider 27 km long accelerator tunnel bending radius of 2804 m pp center-of-mass energy: 14 TeV 25 ns between collisions (40 MHz) high radiation environment at interaction points HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

ATLAS InnerDetector Inner Detector used for particle track reconstruction HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

SCT Barrel and Endcap SCT = SemiConductorTracker SCT is the silicon strip detector of ATLAS and consist of 4088 modules HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

SCT Barrel and Endcap modules each SCT module has 2 x 768 readout strips the EndCap modules are wedge shaped due to the radial arrangement on a disk stereo angle of 40 mrad between readout sides HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

SCT operation principle pn diode, operated in reverse biased mode (150 V) charge collection in p+-strips readout via mirror charge in aluminum readout strips after amplification, shaping and discrimination only a binary signal is retained discrimination threshold  1 fC strip pitch is 80 m HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

SCT coordinate system Coordinate system of SCT module is a right-handed orthogonal three-dimensional frame that is aligned with one readout side (r--side) The origin of the coordinate system is at the center-of-gravity of the r--side HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Beam tests of SCT modules - Setup Beam tests with CERN SPS + beam (180 GeV) with and without 1.56 T magnetic field Beam telescope detectors were used for tracking and spatial resolution measurements (uncertainty  5 m) Sets of 12 SCT modules were tested HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Beam test - residuals Barrel Endcap Binary readout creates top hat shaped residual distributions rms = 23 m = pitch / 12 space point resolution: x = 16 m y = 850 m HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Beam test – discrimination threshold hit efficiencies > 99% and noise occupancy < 5*10-4 are required for non-irradiated modules this can be easily achieved irradiated SCT modules have only a very narrow discrimination threshold region where these requirements are met HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Beam test – charge collection Theta Phi The incident angle of a track has a great influence on charge sharing between neighboring readout strips The 10° tilt angle of SCT barrel modules results in a 20 % multi-strip cluster probability HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Beam test – Lorentz angle In magnetic field the holes that drift to p+-strips are subject to Lorentz force. This offset can be expressed in terms of an angle magnetic field of beam test: 1.56 T L (150 V) = 3.3° L (350 V) = 2.1° HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Alignment Process of deducing position of each SCT module Misalignments can be corrected for in track reconstruction software Modules are not moved back into nominal position Alignment accuracy must match intrinsic spatial resolution to minimize systematic errors HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Track based alignment The residual, i.e. the distance between fitted particle trajectory and the readout strip is used to measure misalignments distorted and not centered residual distributions can be the effect of misalignments HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Iterative approach For each module residuals and derivatives are determined, the complete geometry information is encoded there With a linear least squares method the most probable values for the alignment parameters are determined Updated alignment parameters are then used in the next iteration cycle to calculate new residuals and derivatives. HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Alignment parameters – SCT barrel HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Alignment accuracy SCT Achievable alignment accuracy after 10 iterations with 150k tracks from a single pion sample. Only SCT detector is aligned. Pixel detector frozen. HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Alignment accuracy vs number of hits HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Combined TestBeam Combined test beam in 2004 with all ATLAS subdetectors Setup was like a slice from the ATLAS barrel No beam telescope modules Other tracking detectors provide a reference frame for SCT HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Combined TestBeam - SCT residuals Iteration 1 Iteration 30 Residuals of the 8 SCT modules in CTB without alignment corrections and with alignment corrections after 30 iterations HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Combined TestBeam – alignment parameters After a few iterations the alignment parameters of most degrees of freedom converge on stable values HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT

Conclusions - Outlook Extensive beam tests with SCT modules were done to improve detector simulation and tune operating parameters of SCT Performance and behavior of SCT modules understood well Alignment algorithm for SCT is in place Validation, testing and improvement of algorithm with simulated and test beam data is ongoing SCT barrel is integrated with ATLAS drift tube detector (TRT) and takes data with cosmic rays Important step towards final integration and commissioning First alignment studies with cosmic data are being done HLL Seminar 26.04.2006 Roland Härtel Integration and alignment of ATLAS SCT