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LHCb Alignment 12 th April 2007 S. Viret Coseners Forum « LHC Startup » 1. Introduction 2. The alignment challenge 3. Conclusions.

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Presentation on theme: "LHCb Alignment 12 th April 2007 S. Viret Coseners Forum « LHC Startup » 1. Introduction 2. The alignment challenge 3. Conclusions."— Presentation transcript:

1 LHCb Alignment 12 th April 2007 S. Viret Coseners Forum « LHC Startup » 1. Introduction 2. The alignment challenge 3. Conclusions

2 1. Introduction 2. The alignment challenge 3. Conclusions

3 LHC Startup 1 S. Viret 1. Introduction 2. The alignment challenge 3. Conclusions New-physics signs are expected at the LHC Why LHCb ? 1. Physics justification 2. The detector 3. LHCb startup program They will be difficult to characterize Heavy-flavor physics will provide plenty of discriminating observables

4 LHC Startup 2 S. Viret Example : b s bsbs ssss b s Involves FCNC, forbidden by Standard Model B s decay b s W±W± t New contributions could arise and affect observable parameters (BR, A CP, A isospin ) b s t ~ ~ Need for loops involving heavy particles 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

5 LHC Startup 3 S. Viret If you want to study B-physics, its nice to have : A precise vertex reconstruction A very good particle ID An efficient trigger system A large b quark production in the acceptance 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

6 LHC Startup 4 S. Viret Beam line The LHCb experiment : 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

7 LHC Startup 5 S. Viret b b b b Beam b & b quark directions highly correlated. b quark production cross-section larger at high 100 b Production cross-section (Pythia) pTpT 230 b 10 5 b-hadrons per seconde at L =2x10 32 cm -2 s -1 (LHCb nominal lumi.) A large b quark production in the acceptance A forward geometry 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

8 LHC Startup 6 S. Viret Beam line A precise vertex reconstruction An efficient trigger system VELO (VErtex LOcator) 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

9 LHC Startup 7 S. Viret VELO VELO : Innermost part of LHCb. A detector very close to the beam (~8 mm). 42 detection modules in 2 boxes. VErtex LOcator ~1m 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

10 LHC Startup 8 S. Viret VELO modules 4.2 cm 8 mm R Module = 2 sensors (1R/1 ) glued together x y z (beam) 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

11 LHC Startup 9 S. Viret VELO is a moving detector ! During LHC beam injection, each box is retracted by 3cm from its nominal position. Then the boxes are moved back close to the beam, and data taking starts. VELO box (empty here) 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

12 LHC Startup 10 S. Viret Beam line A very good particle ID RICH 1&2 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

13 LHC Startup 11 S. Viret 2 complementary detectors : 1. Physics justification 2. The detector 3. LHCb startup program RICH 2 RICH 1 1. Introduction 2. The alignment challenge 3. Conclusions

14 LHC Startup 12 S. Viret RICH design : 1. Physics justification 2. The detector 3. LHCb startup program Photon collected by HPD detectors (484 in total RICH 1&2) Number of mirrors: RICH 1 : 4 sphericals / 16 planes RICH 2 : 56 sphericals / 40 planes Some advertising… 1. Introduction 2. The alignment challenge 3. Conclusions

15 LHC Startup 13 S. Viret Beam line Tracking System An efficient trigger system A precise vertex reconstruction 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

16 Tracking stations Trigger Tracker Outer Tracker mm 41.4 mm Trigger Tracker (1 station) Silicon Strips 183 m pitch sensor ladders 4 layers: X:U(5 o ):V(-5 o ):X 128 ladders to be aligned Inner Tracker 20% of the tracks Silicon Strips 198 m pitch 1-2 sensor ladders (336 ladders) 4 layers: XUVX Outer Tracker (3 stations) 5.0 mm Straws Double-layer straws 4 layers: X:U(5 o ):V(-5 o ):X Overlap regions between IT/OT to facilitate relative alignment Inner Tracker 125.6mm125.6mm LHC Startup 14 S. Viret 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

17 LHC Startup 15 S. Viret HLT ~2 kHz HLT (10ms): purely software. Fine event selection (streaming). L1 40 kHz LEVEL 1 (1ms): purely software. Look for a displaced vertex in the VELO (good detector alignment mandatory here). Rate (in Hz) L0 1 MHz LEVEL 0 (4 s): purely hardware. Select the events containing interesting info (, di-muons, e, & hadrons with high p T ). Pile-up rejection. Trigger strategy An efficient trigger system 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

18 2007: startup –Pilot run at 450 GeV per beam –Establish running procedures, align detectors in time and space –Integrated luminosity for physics ~ 0 fb –1 2008: early phase –Complete commissioning of detector and trigger at s=14 TeV –Calibrate momentum, energy and particle ID –Start first physics data taking, assume ~ 0.5 fb –1 –Establish physics analyses, understand performance 2009–20xx: stable running –Stable running, assume ~ 2 fb –1 /year –Develop full physics program –Exploit statistics, work on systematics LHC Startup 16 S. Viret 1. Physics justification 2. The detector 3. LHCb startup program 1. Introduction 2. The alignment challenge 3. Conclusions

19 1. Introduction 2. The alignment challenge 3. Conclusions

20 LHC Startup 17 S. Viret 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview The alignment problem: A particle passes trough a misaligned detector What happens if track is fitted using uncorrected geometry With no correction, one gets a bad quality track (or even no track at all) How could this affect LHCb results ? 1. Introduction 2. The alignment challenge 3. Conclusions

21 LHC Startup 18 S. Viret Example 1 : proper-time estimation = d · m B c · |p B | Proper-time Detector Primary vertex B-decay vertex Tracks d d new 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview 1. Introduction 2. The alignment challenge 3. Conclusions

22 LHC Startup 19 S. Viret Example 2 : trigger efficiency B s KK events HLT trigger efficiency Y axis With 0.5 mrad tilt of one VELO box, 30% less events selected These events are definitely lost!!! 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview 1. Introduction 2. The alignment challenge 3. Conclusions

23 LHC Startup 20 S. Viret A 3 steps procedure : Complete survey of every sub-detector and of all the structure when installed in the pit 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview Hardware alignment (position monitoring): Stepping motors information during VELO boxes closing OT larges structures positions constantly monitored (RASNIKs system) Laser alignment for RICH mirror positioning Software alignment Alignment precision 1. Introduction 2. The alignment challenge 3. Conclusions

24 LHC Startup 21 S. Viret 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview Software alignment strategy : Align all sub-detectors (VELO, IT, OT, RICHs) internally Align the sub-detectors w.r.t. the VELO (Global alignment). Start IT & OT, then TT (not alignable internally), RICH and finally Ecal, Hcal and Muon. 1. Introduction 2. The alignment challenge 3. Conclusions

25 LHC Startup 22 S. Viret 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview VELO alignment : how to proceed ? Alignment should be designed to be FAST (few minutes)and PRECISE (<5 m precision) Data taking Residuals monitoring End of run : VELO is open Start of run : VELO is closedIf necessary… Software alignment procedure 1. Introduction 2. The alignment challenge 3. Conclusions

26 LHC Startup 23 S. Viret Step 2 Aligned VELO Align the boxes using global fit again on primary vertices, overlapping tracks,... Step 1 Internally-aligned VELO Global fit applied on tracks ( classic & beam gas/halo ) in the two boxes Step 0 Misaligned VELO 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview VELO: the strategy 1. Introduction 2. The alignment challenge 3. Conclusions

27 LHC Startup 24 S. Viret Residuals are function of the detector resolution, but also of the misalignments From this… The geometry we are looking for is the one which minimizes the tracks residuals (in fact there are many of them but there are ways to solve this problem). … to that Global fit ? 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview 1. Introduction 2. The alignment challenge 3. Conclusions

28 x clus = i i + a j j LINEAR sum on misalignment constants i LINEAR sum on track parameters i (different for each track) LHC Startup 25 S. Viret GLOBAL FIT IDEA : Express the residuals as a linear function of the misalignments, and fit both track and residuals in the meantime: Taking into account the alignment constants into the fit implies a simultaneous fit of all tracks (they are now all correlated): We get the solution in only one step. The final matrix is huge (N tracks N local +N global ) x clus = x track + x LINEAR sum on track parameters i (different for each track) x clus = i i + x LOCAL PARTGLOBAL PART 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview But inversion by partitioning (implemented in V.Blobels MILLEPEDE algorithm), reduces the problem to a N global x N global matrix inversion !!! 1. Introduction 2. The alignment challenge 3. Conclusions

29 LHC Startup 26 S. Viret 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview VELO : MC results ( STEP 1 : modules alignment) x y BeforeAfter Resolution on alignment constants (with ~20000 tracks/box) are 1.2 m ( x and y) and 0.1 mrad ( ) Algorithm is fast (few minutes on a single CPU) Code integrated into LHCb software. MC tests made with different misaligned geometries. 1. Introduction 2. The alignment challenge 3. Conclusions STEP 2 results also within LHCb requirements

30 LHC Startup 27 S. Viret 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview VELO : testbeam results (Nov.06) The testbeam setup 10 modules installed 4 configurations (6 modules cabled) tested 1. Introduction 2. The alignment challenge 3. Conclusions

31 LHC Startup 28 S. Viret 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview VELO : testbeam results Track residuals Before alignment After alignment +26% vertices in target 1 +10% vertices in target 2 12 Vertexing 1. Introduction 2. The alignment challenge 3. Conclusions

32 LHC Startup 29 S. Viret 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview Tracking system alignment Use the same method as the VELO (global fit via Millepede) via a common alignment software framework (currently under development). Interface with Millepede is more complex than in the VELO, due to different track shapes (parabolas inst. of straight tracks). On the other hand detectors are not moving, alignment might be less frequently processed. 1. Introduction 2. The alignment challenge 3. Conclusions IT and OT are internally aligned separately, then w.r.t. each other using the overlap areas. Work is ongoing. Results expected soon.

33 LHC Startup 30 S. Viret 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview RICH alignment : principle 1. Introduction 2. The alignment challenge 3. Conclusions : Expected Cherenkov angle : Measured angle : Distortion due to mirror tilts Mirror tilt

34 LHC Startup 31 S. Viret 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview RICH alignment : results 1. Introduction 2. The alignment challenge 3. Conclusions Fit those distributions for all the mirrors combinations in order to get their individual orientation (tilt around X and Y axis). Alignment code implemented Minimization using MINUIT Measured - expected x 0.1 mrad resolution obtained, well within requirements RICH 2

35 LHC Startup 32 S. Viret 1. Whats the problem with alignment ? 2. LHCb alignment strategy 3. Sub-detectors overview Global alignment 1. Introduction 2. The alignment challenge 3. Conclusions Most critical step is tracking system alignment: VELO to T-stations (IT & OT) TT to VELO/IT/OT Strategy for step has been defined (match tracks fitted independently in both tracking systems) and successfully tested on MC. Has to be extended to step Work is ongoing.

36 1. Introduction 2. The alignment challenge 3. Conclusions

37 LHC Startup 33 S. Viret 1. Introduction 2. The alignment challenge 3. Conclusions LHCb has been designed to hunt New Physics sign in the heavy flavours sector. But in order to reach our objectives, a perfect understanding of the detector will be necessary. In particular, a very good alignment is required (for trigger, particle ID, tracking,…)

38 LHC Startup 34 S. Viret 1. Introduction 2. The alignment challenge 3. Conclusions LHCb alignment strategy has been presented (available in CERN-LHCb ).CERN-LHCb It has to take into account LHCb unique specificities (RICH, moving VELO) and requirements (online vertex trigger) Work is ongoing on many fronts, and some nice results have already been obtained (VELO testbeam alignment, RICH alignment,…). A common framework is taking shape. We are now waiting the first beams (as we cant play with cosmics )

39 Backup Slides

40 LHC Startup A0 S. Viret Align with what ? Alignment algorithm feeding has to be taken seriously ! First alignment will be determined using magnet OFF data (very important for tracking systems). Then this first alignment will be updated with magnet ON data. Specific trigger scheme for those events necessary after 2008 (work ongoing). Extensive use of specific types of tracks * (beam halo/ beam gas), mass resonances (e.g. J/ ),… * No cosmics in LHCb.

41 LHC Startup A1 S. Viret Methodology for the tests: 200 runs of events (5000 min.bias pseudo-halo) were passed trough LHCb software with the following misalignments scales (all 6 degrees of freedom are taken into account at each level): Misaligned events are produced via LHCb geometry framework. No momentum cut applied for track selection (try to rely on VELO information only) Translations (in m) ( x, y, z) Rotations (in mrad) (,, ) Module 302 Box 1001 Misalignment scales chosen using misalignment studies and hardware information

42 LHC Startup A2 S. Viret The huge matrix we had to invert is very sparse: Inversion by partitioning (implemented in V.Blobels MILLEPEDE algorithm), reduces the problem to a N global x N global matrix inversion !!! As N global 100 for the VELO, the problem could be solved in few sec. !!! k C k global HkHk HkTHkT k = 0 0 C k local … … …… … … k w k x k k w k k …… …… N global N local x N tracks

43 LHC Startup A3 S. Viret How to linearize the system ? Millepede is interesting, but linearity is a key point. Obviously VELO sensors R/ geometry is not the most linear thing in the world… Could consider module as a rigid object and thus transform (R/ ) into (X,Y) point. Module is then the basic detector element to align. R But R and sensors are precisely bonded together within a module (~10 m precision), and also precisely surveyed (~ few m precision).

44 LHC Startup A4 S. Viret VELO : MC results ( STEP 2 : boxes alignment) PV Overlap Results obtained with limited statistic ( ~1500 PVs and ~300 overlap tracks ) : offset (PV) = 17 m tilt (PV) = 99 rad offset (Overlap) = 13 m tilt (Overlap) = 40 rad Results could still be improved but are already well within trigger requirements.

45 Track fit: bi-directional Kalman fit Tracking efficiency (p>5GeV) ~94% (ghost rate ~16%) Proper time resolution ~ 40 fs B Mass resolution ~ 15 MeV 125.6mm125.6mm LHC Startup A5 S. Viret p/p Momentum Resolution p [GeV] = 14.8 m+30.4 m/p t (ip) Impact Parameter Resolution p T 1. Physics justification 2. The detector 3. LHCb startup program Tracking performance: 1. Introduction 2. The alignment challenge 3. Conclusions


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