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Muon Identification in LHCb

Muon Identification in LHCb. Outline: Introduction Physics requirements Background conditions Overview of the Muon System Physics Performance Muon trigger Muon identification MWPC Detector Performance and aging studies Detector construction Triple Gem Detector

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Muon Identification in LHCb

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  1. Muon Identification in LHCb Outline: • Introduction • Physics requirements • Background conditions • Overview of the Muon System • Physics Performance • Muon trigger • Muon identification • MWPC Detector • Performance and aging studies • Detector construction • Triple Gem Detector • Performance and aging studies B.Schmidt Workshop on Muon Detection in the CBM Experiment

  2. Physics Goals: The Muon system of LHCb is primarily used to trigger on muons produced in the decay of b-hadrons: b X ; In particular:B  J/(+-) Ks ; Bs J/(+-)  ; Bs +- The muon momentum is measured precisely in the tracking system The muon system identifies muons from tracks in the tracking system Requirements: Modest momentum resolution (~20%) for a robust PT -selective trigger Good time resolution (a few ns) for reliable bunch-crossing identification Good muon identification (~90%); small pion-misidentification (~1%) Introduction 0 0 0 B.Schmidt

  3. Background sources in the LHC environment: ,K  X decays main background for L0 muon trigger Shower particles hadron punch-through including shower muons Low-energy background induced by n- processes contributes significant to chamber hit rate Machine background, in particular high energy beam-halo muons Requirements: High rate capability of chambers Good ageing properties of detector components Detector instrumentation with sufficient redundancy Introduction B.Schmidt

  4. Overview of the LHCb Muon Detector M1 M2 M3 M4 M5 -> Projectivity to interaction point • 435m2 of detector area with 1380 chambers • Hadron Absorber of 20 thickness in total • 5 Muon stations, with 2 independent layers/station -> redundancy • Layers are logically ORed -> high station efficiency B.Schmidt

  5. Detector status: snapshot from the pit In the pit To be installed Tracking system RICH1 Vertex Detector Calorimeters RICH2 Magnet Muon System B.Schmidt

  6. Muon Detector Layout • Space point measurement • 4 Regions with granularity • variable with distance from • beam axis • - 0.6cm x 3.1cm in Region 1, • 5cm x 25cm in Region4 • Particle flux varies between • - 0.5 MHz/cm2 (M1 Region 1) • - 0.2 kHz/cm2 (M5 Region 4) Chamber Detector Technology :-MWPC will be used in most regions - triple GEMs are used for M1R1 Electronics: - System has 126k FE-channels, reduced to 26k readout channels B.Schmidt

  7. Muons in LHCb B.Schmidt

  8. Muon Trigger Algorithm Level 0 Muon Trigger: • Muon track finding: • Find seed pad in station M3 • Find pads within opened search windows (FOI) in stations M2, M4 and M5 • Use pads found in M2 and M3 to extrapolate to M1 and find pad in M1 within FOI • Stations M1 and M2 are used for the PT-measurement • -> Muon Trigger exploits multiple scattering in the muon shield by • applying tight search windows B.Schmidt

  9. Background Simulation B.Schmidt

  10. Particle Rates in the LHCb Muon System B.Schmidt

  11. Level 0 Muon Trigger Trigger Performance: • Muon system includes realistic chamber geometry and detector response -> Muon System is robust B.Schmidt

  12. Level 0 Muon Trigger Beam halo muons: • Distribution of energy and radial position of halo muons 1m upstream of IP travelling in the direction of the muon system • Muons entering the experimental hall behind M5 give hits in different BX in the muon stations -> No significant effect • Halo muons are present in ~1.5% of the bunch crossings • About 0.1% of them cause a L0 muon trigger B.Schmidt

  13. Muon Identification Algorithm: • Extrapolate reconstructed tracks with p > 3GeV/c and first hits in Velo from T10 to the muon system (M2 etc.) • Define a field of interest (FOI) around extrapolation point and • Define minimum number of stations with hits in FOIs • M2+M3 for 3  p  6 GeV/c • M2+M3+(M4 or M5) for 6 p  10 GeV/c • M2+M3+M4+M5 for p  10 GeV/c B.Schmidt

  14. Muon Identification Performance: Nominal Maximal background background p>6GeV/c Sx<0.053  94.00.3 94.30.3 90.00.6 Me 0.780.09 3.50.2 0.60.1 M 1.500.03 4.000.05 1.20.05 MK 1.650.09 3.80.1 1.20.1 MP 0.360.05 2.30.1 0.30.1 Additional cuts on slope difference Sx between tracking and muon system and pare required in case of large bkg. -> M ~ 1%  ~ 90% B.Schmidt

  15. Muonic Final States B0 J/(+-) Ks: • Well established CP-violating decay from which angle  in the unitary triangle can be determined. • J/ (+-) reconstruction: - oppositely charged tracks identified as muons. - Mass of dimuon pair consistent with J/ mass -> More than 100k ev./year expected in LHCb Bs +-: • Decay involves FCNC and is strongly suppressed in the Standard Model -> BO mass resolution 18 MeV/c2 -> ~10 signal events over 3 bkg expected per year LO performance for both decays: • L0 trigger acceptance of fully reconstructed events is 98%. • L0 muon acceptance is 95% with>70% triggered by muon trigger alone. 0 B.Schmidt

  16. MWPC Readout Types Region 4: Anode wire readout Region 3: Cathode pad readout Region 1+2: (in stations M2+M3) Combined anode and cathode readout -> Different readouts and variations in input capacitance pose special requirements on FE-chip B.Schmidt

  17. HV HV HV HV MWPC Requirements and Layout Requirements: • High rate capability of chambers -> up to 0.2 MHz/cm2 • Good ageing properties of detector components: -> up to 1.6 C/cm2 on cathodes and 0.3 C/cm on wires • Detector instrumentation with good redundancy and time resolution -> each chamber has (generally) 4 layers -> 99% station efficiency in 20ns time window 2 independent ASD ORed on FE Board (similar ORing scheme also applies to cathode pads) 4 separate HV supplies to anode wires Cathode pads PCB Structural panel B.Schmidt

  18. Geometry and Fields Volt /cm Pitch: 2 mm Gap: 5 mm Wire: 30 µm HV: 2650 V Cathode field: 6.2 kV/cm Wire field: 262 kV/cm Gain: 105 Total avalanche charge: 0.74pC cm B.Schmidt

  19. Gas Properties In Ar/CO2/CF4 40/40/20 we expect for 10 GeV muonsabout: ~ 42 clusters/cm ~ 2.35 e-/cluster, -> 99 e-/cm Drift velocity of 90 µm/ns is ‘saturated’ meaning that a small change in electric field does not affect the time resolution. B.Schmidt

  20. Front-End Electronics FE-chip specifications: • Peaking time ~ 10ns • Rin: < 50  • Cdet : 40-250pF • Noise: <2fC for Cdet=250pF • Rate: up to 1MHz • Pulse width: < 50ns • Dose: up to 1Mrad 3x4mm prototype, (2.5x3mm final) FE-chip ‘CARIOCA’: • 0.25m CMOS technology • 8 channel ASD • fully differential • 10ns peaking time • 30mW/channel UFRJ Rio, CERN B.Schmidt

  21. Chamber Performance: Time resolution 20ns • Optimum amplifier peaking time ~10ns • Intrinsic time resolution is about 3ns B.Schmidt

  22. Chamber Performance: Efficiency Cathode Efficiency: Anode Efficiency: WP WP -> Plateau length is about 350 – 450 V B.Schmidt

  23. High rate behaviour 1.2 MHz/wire pad, 40kHz/cm2 • Time RMS is unchanged - no space charge effect, as expected • 4% efficiency loss at 1MHz due to signal pileup, as expected B.Schmidt

  24. Chamber Performance High rate performance: (old result obtained with 1.5mm pitch) Time resolution stable (no space charge effects) Small Efficiency drop due to pile up B.Schmidt

  25. Comparison with Simulation Double gap chamber: • 95% efficiency if the threshold • is set to 30% of the average signal. • 99% efficiency if the threshold is • set to 20% of the average signal. • In order to have a double gap • chamber well within the plateau we • want to be able to use a threshold of • ~15% of the average signal. • Full simulation: • - Primary ionization (HEED) • - Drift, Diffusion (MAGBOLTZ) • - Induced Signals (GARFIELD) -> Good agreement, we understand our detector B.Schmidt

  26. Chamber Specifications • Gas Gain variations: • Working point should not move out of the plateau • The gas gain should be for 95% of the area within 25% of G0 • For the remaining 5% of the area the gas gain should be within 50% of G0 • A gain change of a factor 1.25 (1.5) corresponds to a voltage change of • ±34 (62)V around 2650 V, corresponding to ±1.25 (2.25)%. • -> The gas gain changes by a factor 1.25 (1.5) if the wire surface field • changes by ±1.25 (2.25)%. • Resulting chamber specifications: • Gap: 95% in 90m 1% 5% in 180m 2% • Pitch: 95% in 50m 0.35% 5% in 100m 0.7% • Wire y-offset: 95% in 100m 0.1% 5% in 200m 0.4% • Wire plane y-offset: 95% in 100m 0.1% 5% in 200m 0.4% • -> Added in squares : 1.07% 2.2% B.Schmidt

  27. Chamber Construction • Production Program: • Within two years ~1500 chambers had to be build • in 6 production centres. • Quality assurances (QA) is a key issue • Example for QA: • Measurement of wire pitch • with CCD cameras using, • telecentric lenses B.Schmidt

  28. Wire Pitch Measurement 2.1 2.0 1.9 0 75 150 225 300 375 450 525 600 Small pitch fluctuations mainly due to precision of combs -> 99% of wires within (2.00 ±0.05)mm Quality check of method: Reproducibility of result within 1.5m (RMS) on average -> Method well adapted for reliable QA B.Schmidt

  29. Example for Gas Gain Uniformity 2.1 2.0 1.9 0 75 150 225 300 375 450 525 600 B.Schmidt

  30. MWPC Aging studies • Two long term aging tests have been performed: • 1st test at GIF in 2001, where charges of 0.25 C/cm have been accumulated in 6 months. • Open loop gas system has been used • Gas mixture: Ar / CO2 / CF4 (40%, 50%, 10%) • 2nd test at ENEA Cassacia in June 2003, where charges of 0.5 C/cm have been accumulated in 1 month (using the 25kCi (!) Co source). • One chamber has been in open loop, one in closed loop • Gas mixture: Ar / CO2 / CF4 (40%, 40%, 20%) • Parameters controlled: • Relative gas gain: In both tests one gap was used as reference gap to avoid complicated corrections for P and T variations. The reference gap was only for very short periods per day under HV and always flashed with the fresh gas. • Dark currents, including self-sustaining rest current following the beam off. The dark currents were measured with current monitors with a resolution of 1 nA. B.Schmidt

  31. Summary of Results from Casaccia Accumulated charges: • No signs for aging from the measured parameters: • Relative currents did not change. • Malter currents were not observed. • No variations in current ratios after/before Casaccia from measurements with GIF and with an Am-source. B.Schmidt

  32. Analysis of Surfaces 1st the wires . . . Strong FR4 etching of open surfaces CF4 is a phantastic gas ! CF4 is a terrible gas ! Bubbles under Kapton foil Detaching of the ground grid between the pads (due to FR4 etching)

  33. Conclusions Conclusions: • Although no drop in gas gain was visible after the Casaccia aging test, the materials exposed to CF4show strong surface etching. • The effects are very similar for the chambers in the two gas system used in Casaccia (vented and re-circulation gas-system). • The effect has been stronger the 2nd aging test, where the gas mixture contained 20% CF4, than in 1st test, where 10% CF4 have been used. • Plans: • Repeat the test at GIF in the coming months, where the test takes however 5 times longer than in Casaccia (source strength 25kCi). • New mixture with less/no CF4 . • Both gas loops will be used. • Control of O2 and H2O and contamination is foreseen. B.Schmidt

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