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Preparation for ITS alignment

Preparation for ITS alignment. What? ITS detectors, target alignment precision Why? Impact of misalignments How? Strategy and methods How well? First results from simulation. A. Dainese (INFN – LNL) for the ITS alignment group (CERN, LNL, NIKHEF, PD, TS). Inner Tracking System (ITS).

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Preparation for ITS alignment

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  1. Preparation for ITS alignment • What? ITS detectors, target alignment precision • Why? Impact of misalignments • How? Strategy and methods • How well? First results from simulation A. Dainese (INFN – LNL) for the ITS alignment group (CERN, LNL, NIKHEF, PD, TS) Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  2. Inner Tracking System (ITS) • Silicon Pixel Detector (SPD): • ~10M channels • 240 sensitive vol. (60 ladders) • Silicon Drift Detector (SDD): • ~133k channels • 260 sensitive vol. (36 ladders) • Silicon Strip Detector (SSD): • ~2.6M channels • 1698 sensitive vol. (72 ladders) SSD ITS total: 2198 alignable sensitive volumes  13188 d.o.f. SPD SDD Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  3. yloc zloc xloc ITS detector resolutions & target alignment precisions • Full: initial misalignments as expected from the mechanical imprecision after installation, actually set to 20-45 mm at the sensor level, probably higher at the ladder or layer level (~100 mm), more later... • Residual: expected misalignment left after applying the realignment procedure(s). Target ~0.7resol. ~20% degradation of the resolution detector local c.s.: xloc~rfglob, yloc~rglob, zloc = zglob Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  4. rec. track e Primary Vertex B X d0 Impact of ITS misalignmenton tracking performance • Effect of misalignment on d0 (and pt) resolutions studied by reconstructing misaligned events with ideal geometry • Estimated effect on D0Kp significance null residual full full+ A.D, A.Rossi Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  5. Introduction of “realistic” misalignment • Why “realistic”? • misalignment should follow hierarchy of hardware structure; each level should be misaligned (and then realigned) • magnitude of initial misalignments should be realistic (input from hardware experts) • misalignments at the same hierarchical level should be correlated • E.g., for SPD: barrel / half-barrel / sector / half-stave / ladder # sensitive volumes: 240 / 120 / 24 / 2 / 1 magnitude of misal. (mm):<1000 / ~100 / ~100 / 10-50 / 5 (up to now only the ladder was misaligned) • Transition to realistic misalignment is in progress (requires changes to the ITS geometry) • Will provide: • better playground for preparation of realignment procedures • better estimate of effects of residual misalignments on performance Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  6. ITS alignment with tracks:general strategy • Data sets: cosmics + first pp collisions (and beam gas) • use cocktail of tracks from cosmics and pp to cover full detector surface and to maximize correlations among volumes • Start with B off, then switch on B  possibility to select high-momentum (no multiple scattering) tracks for alignment • General strategy: • start with layers easier to calibrate: SPD and SSD • good resol. in rf (12-20mm), worse in z (120-830mm) • ITS z resol. provided by SDD anode coord. (20mm)  easily calibrated  can be included from the beginning in alignment chain • global ITS alignment relative to TPC (already internally aligned) • finally, inclusion of SDD (drift coord: rf), which probably need longer calibration (interplay between alignment and calibration) • Two independent track-based alignment methods in preparation: • global: Millepede 1 (ported to ALICE for muon arm alignment) • local: iterative method based on residuals minimization Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  7. Expected muon rate through ITS inner layer (~200cm2, ~40m underground): ~0.02 Hz ( ~104m/week) Trigger with SPD layers (tracks with 10-12 points): Trigger with ACORDE (“peripheral” tracks with 4-8 points in SSD-SDD) FastOR (FO) of the 20 chips on 2 half-staves For each half-barrel (A side, C side): 20 FOs outer layer, 10 FOs inner layer Any logic combination of these 30 FOs Preparation for cosmics data (1) Cosmics Run in February 2008  D.Elia A side SPD FastOR C side • Option being considered for cosmics: 2Layers coinc. (≥2FOs inn layer & ≥2FOs out layer) purity (fraction with 1 m with 4 SPD hits): ~97%, inefficiency (fraction of lost m with 4 SPD hits): ~19% Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  8. no misal. full misal. sd0=12mm sd0=21mm Preparation for cosmics data (2) • Cosmics at ALICE: p > 10 GeV/c, <p>~20 GeV/c(Hebbeker, Timmermans, 2001) • Cosmics tracking in ITS: • modified stand-alone ITS tracking (cluster-grouping algorithm from Torino) • 98% efficiency (12 points, 6inward+6outward) for muons that leave 12 hits, with B=0 and B=0.5T • high eff (~80-90%) also for muons crossing only outer layers • robustness tested with “extreme” misalignment scenarios • tracks prolongation from TPC to ITS being optimized for cosmics • Preparing first d0 resolution meas. by cosmics two-track matching A.D., A.Jacholkowski Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  9. SPD inner SDD inner SSD inner 103 103 103 102 102 102 10 10 10 1 1 1 800 800 400 400 Cosmics in the ITS A.Rossi • 50k m’s through inner ITS layer (~5 weeks of cosmics data) • Track multiplicity per module: • Volume correlations. Number of modules correlated to a given module: SPD inner 6 pts tracks 12 pts tracks Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  10. Track-based alignment in the barrel (ITS, TPC, TRD, ...)- Framework Overview - C.Cheskov Reconstruction Reconstruction Reconstruction Reconstruction Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  11. ITS alignment with tracks:the global approach of Millepede M.Lunardon, S.Moretto • 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 (cosmics + pp) • Linear problem: the points residuals can be expressed as a linear function of the (global, di) alignment params and (local δi)tracks’ params • At the moment being tested with cosmics tracks and B=0 • Strategy for tests: • validate algorithm with “fast simulation” (no detector response) • introduce full detector response Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  12. yloc zloc xloc Millepede with fast simulation SSD SPD SDD • 5 weeks of cosmics • (50k in SPD1), B=0 • Tracks with 12 pts • 375 modules • (out of 2198) xLOC input result difference yLOC zLOC Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  13. Millepede with fast simulationRMS of (result-input) As a function of the number of minimum # of points per track • 5 weeks of cosmics • (50k in SPD1), B=0 • Also tracks • with <12 pts SPD x yz 5mm! better with “peripheral tracks”  SDD similar to SPD Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  14. Millepede with fast simulationRMS of (result-input) As a function of the number of minimum # of points per track • 5 weeks of cosmics • (50k in SPD1), B=0 • Also tracks • with <12 pts SSD x yz much better with “peripheral tracks”   Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  15. Millepede: fast vs. full simulation • Deterioration of results with full detector simulation • This triggered investigations on the different steps of the simulation, with misalignment  found problems with overlaps that caused shift of ITS rec. points  will be solved in new ITS geometry • For the moment, continue alignment studies without misalignments Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  16. Millepede: full simul., no misal. • about 5 weeks of cosmics, B=0 • 663 modules ( 135 SPD + 104 SDD + 424 SSD ) M.Lunardon, S.Moretto Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  17. z f bottom top SPD: worse on the sides  need pp tracks Millepede: full simul., no misal.SPD only, SPD+SSD • about 5 weeks of cosmics, B=0 • 612 modules ( 192 SPD + 420 SSD ) M.Lunardon, S.Moretto Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  18. ITS alignment with tracks: iterative local method C.Cheskov A.Rossi • Determines alignment params by minimizing track-to-points residuals • Local: works on a module-by-module basis • Iterations are used to take into account correlations between the alignment params of different modules Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  19. Iterative local method: full simul., no misal. • about 5 weeks of cosmics, B=0 • no iterations (not necessary without misalignment) A.Rossi Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  20. Iterative local method:full simul. with misal., iterations SPD inner: mean, RMS iterationsconvergence Dx (mm) global z (cm) global f worse on the sides  need pp tracks A.Rossi Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  21. ITS-TPC relative alignment • Relative alignment of ITS and TPC (3 shifts + 3 angles) with straight tracks (including cosmics) • Alignment requirements: given by TPC resolutions: • shifts: ~100 mm • angles: ~0.1 mrad • Method (under development): • Assume that TPC and ITS are already internally aligned and calibrated • Use independently fitted tracks in the ITS and the TPC • Alignment params are estimated by a Kalman filter algorithm • Proof-of-principle test with “toy” tracks M.Krzewicki Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  22. Summary • The ITS alignment challenge: determine 13,000 parameters with a precision of ~10 mm • Track-based alignment using cosmics and pp collisions • preparation for cosmics reconstruction in ITS • Two independent algorithms under development • Millepede (global) • local iterative method • Promising results even with cosmics only, should be much better with cosmics + pp tracks • ITS alignment relative to TPC also under study Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  23. EXTRA SLIDES Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

  24. Fast Simulation • A muon direction is generated • Intersection points with misaligned detectors are evaluated in local coordinate systems • large misal. order of 100 mm • Points smeared with given resolutions • Use tracks with 4-12 points • Advantages w.r.t. standard sim: • clean situation, without simulation/reconstruction effects • faster Terzo Convegno sulla Fisica di ALICE - LNF, 14.11.07 Andrea Dainese

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