1. LHCb Vertex Locator (VELO) Lars Eklund École Polytechnique Fédérale de Lausanne Liverpool University Vrije Universiteit Amsterdam

2. Vertex 05, November 7, 2005Lars Eklund, CERN2 Outline Introduction to LHCb and VELO Design of the VELO Performance plots Towards a beam test in 2006 A more specific example: –Digital filtering for x-talk corrections

3. Vertex 05, November 7, 2005Lars Eklund, CERN3 LHCb Tracker ECAL HCAL Muon chambers Magnet Angular acceptance 15 - 300mrad Proton interaction region RICH 2 RICH 1 Trigger tracker Dedicated b-physics experiment at LHC 10 12 bb-pairs per year CP violation and rare b-decays Correlated boost of the bb-pairs Single forward spectrometer The Vertex Locator

4. Vertex 05, November 7, 2005Lars Eklund, CERN4 VELO – the experimental challenge 1.Trigger on the B decay of interest (Velo part of software trigger) 2.Suppress multiple interactions (Pile-up veto part of hardware trigger) 3.Reconstruct decay as a function of time btbt BsBs KK KK  ,K   DsDs Primary vertex p p Since the oscillations are fast, it requires excellent vertex resolution Measurement of B S oscillations

5. Vertex 05, November 7, 2005Lars Eklund, CERN5 The Vertex Locator (VELO) ~1m Interaction region Downstream 21 Tracking stations Four ½ disk sensors per station Silicon micro-strip technology R-Φ geometry Minimalist view of the VELO Minimise material No conventional beam-pipe Sensors are operated in vacuum 250 μm Al foil to separate VELO from the beam vacuum Minimise extrapolation distance First active element at R = 8.2 mm Retractable detector halves 30 mm at LHC filling Pile-up veto 2 backward stations in the H/W trigger

6. Vertex 05, November 7, 2005Lars Eklund, CERN6 Velo Sensors  –measuring sensor (radial strips with an stereo angle) Non uniform radiation environment 1.3 * 10 14 n eq /cm 2 /year at R = 8 mm 5 * 10 12 n eq /cm 2 /year at R = 42 mm n + in n-bulk sensors second metal layer for signal routing 2048 micro strips per sensor 40 – 100 μm pitch R-measuring sensor (concentric strips) 42 mm 8 mm FE chip

7. Vertex 05, November 7, 2005Lars Eklund, CERN7 Detector module Cooling cookies Pitch adaptors Silicon sensor TPG/carbon fibre substrate with laminated kapton circuit Carbon fibre paddle Front-end chip (Beetle1.5) PRR in December

8. Vertex 05, November 7, 2005Lars Eklund, CERN8 Detector modules - performance Operational window: Sufficient cluster efficiency (> 99 %) Acceptable noise occupancy (< 0.1 %) Low fake cluster rate in next time bin (< 25 %) Cluster efficiency vs. S/N cut Results test beam November 2004 – 300 μm thick Φ measuring sensor Noise occupancy vs. S/N cutOverspill vs. S/N cut

9. Vertex 05, November 7, 2005Lars Eklund, CERN9 Cooling, vacuum and positioning Beam pipe Detector halves retractable by 30 mm CO2 cooling manifold Primary (beam) vacuum Secondary (detector) vacuum RF foil (250 μm Al) Vacuum bellows (to allow the retraction)

10. Vertex 05, November 7, 2005Lars Eklund, CERN10 Mechanics – status Vacuum and positioning assembly at NIKHEF Vacuum bellow

11. Vertex 05, November 7, 2005Lars Eklund, CERN11 DAQ chain front-end ASIC 2 m low mass cable 60 m twisted pair ADC and pre- processing FPGA Radiation dose: ~ MRad pre- compensating cable driver Radiation dose: ~100 kRadRadiation free area readout network PC farm Radiation free area Beetle 1.5 analogue readout 128 channels 4 serial links 900 ns readout time Kapton cables low mass flexible vacuum compatible Analogue repeaters compensates cable response COTS components 5632 links Cat 6 cableGigabit Ethernet commercial components TELL1 ADC (10 bit, 40 MHz) digital filter pedestal and common mode noise subtraction strip re-ordering clustering PC farm Linux cluster software trigger permanent storage

12. Vertex 05, November 7, 2005Lars Eklund, CERN12 Performance  IP = 14  m+35  m/p T Impact parameter resolution B s vtx resolution (mm)

13. Vertex 05, November 7, 2005Lars Eklund, CERN13 Towards the final system… Components are or will soon be in production Module production will start in December 2005 Assembly will start January 2006 –At CERN System test April 2006 –Whole detector half powered and configured –10 modules read out Beam test 2006 –Whole detector half in the beam test –Fully system verification: Detector modules, control system, DAQ, reconstruction software, alignment …

14. Vertex 05, November 7, 2005Lars Eklund, CERN14 Two systems issues 1.Interaction vertices in the silicon sensors and halo tracks 2.Digital filtering of the analogue data link

15. Vertex 05, November 7, 2005Lars Eklund, CERN15 Test beam set-up scintilator trigger proton beam interaction in the sensor Detector half operated in vacuum Use silicon sensors as targets Look for interactions in the silicon Reconstruct tracks and align Partially commissioned at installation

16. Vertex 05, November 7, 2005Lars Eklund, CERN16 Need stand-alone tracking Tracks from interaction point (R=0) are linear in the R-z plane Pattern recognition assume tracks from close to R = 0 Need for a stand-alone reconstruction package Handles vertices anywhere Feeds tracks to the alignment algorithm z [mm] R [mm] Event display of an interaction in the silicon sensor. Tracks highly non-linear in the R-z plane

17. Vertex 05, November 7, 2005Lars Eklund, CERN17 Spin-off: halo tracks in LHCb Velo Left Velo Right cartoon event display of the VELO Solution: Use beam halo tracks and interactions in the silicon sensors for alignment in LHCb Re-use algorithms from the beam test 2006 Velo divided into four weakly coupled parts in terms of alignment: Backward-Left, Backward-Right, Forward-Left and Forward-Right very few tracks traverse more than one part L-B R-B L-F R-F

18. Vertex 05, November 7, 2005Lars Eklund, CERN18 Two systems issues 1.Halo tracks and interaction vertices in the silicon sensors 2.Digital filtering of the analogue data link

19. Vertex 05, November 7, 2005Lars Eklund, CERN19 Digital filtering – the problem 32 channels serial data (800 ns) signalnoise Data is transferred on four serial links per front-end ASIC: header 25 ns Signal travels through: front-end hybrid kapton cables (2 types) vacuum feed-through Amplifier (+ PCB) 60 m TP cable Blue: no signal (pedestals) Red: signal in two channels

20. Vertex 05, November 7, 2005Lars Eklund, CERN20 Modelling the problem Consider each output as a number series, where n corresponds to the channel number x[n] :“True” data at the input (from sensor or test pulse) w[n] :Raw ADC values y[n] :Corrected values H :Transfer function of the ASIC and the analogue link G :Transfer function of the digital filter Sensor, ASIC and the analogue link Digital filter x[n]w[n]y[n] H(z) G(z) Method: (Assuming LTI: Linear Time Invariant System) 1.Determine H from the data (from beam particles or calibration pulses) 2.Find ‘G’ such that G*H = 1, implying that y[n] = x[n]

21. Vertex 05, November 7, 2005Lars Eklund, CERN21 The impulse response The transfer function H is characterised by the series h[n]: h[n] can be determined by injecting a single pulse: Obtaining a Delta function δ: 1. Internal calibration pulses Feature of the Beetle chip 2. From track data Point a track on the sensor Select tracks centered on strips Assume no charge sharing 3. look at the signal in adjacent channels 4 ASICs, 16 serial links: 32 different impulse responses h[+1] h[-1] x-talk measurements from TB November 2004

22. Vertex 05, November 7, 2005Lars Eklund, CERN22 Determining the filter algorithm (1) Sensor, ASIC and the analogue link Digital filter x[n]w[n]y[n] H(z) G(z) Postulate: The impulse response h[n] = 0, except for N time-bins g[n] = 0, except for M time-bins (Finite Impulse Response filter = FIR). requires thatand where The infinite sum is truncated to M + N – 1 conditions:

23. Vertex 05, November 7, 2005Lars Eklund, CERN23 Determining the filter algorithm (2) The truncated sum gives: N + M – 1 constraints M unknowns (the g[n]) Solve these over constrained equations with the least square method corrected x-talk from TB November 2004 4 ASICs, 16 serial links: 32 different impulse responses Boundary problems: 32 channels per link exceptions for channels 0 and 31 missing information The data header specific corrections

24. Vertex 05, November 7, 2005Lars Eklund, CERN24 Effects on the resolution Track residuals: reversed readout order for small radii Asymmetric (forward/backward) x-talk step in residuals Resolution: Smearing due to x-talk Odd/even channel dependence ~1 μm improvement Results test beam November 2004 – 200 μm thick R measuring sensor (7 degree track angle)

25. Vertex 05, November 7, 2005Lars Eklund, CERN25 Summary The Vertex Locator of LHCb uses silicon micro-strip sensors with R-Φ geometry Operated in vacuum with retractable detector halves Shows good efficiency and noise occupancy performance in the beam test Is in production – assembly will start soon System issues –System test and beam test 2006 Specific example –Corrections of the data link