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P erformance of the MDT detectors of the ATLAS muon spectrometer

Fabrizio Petrucci Dipartimento di Fisica “E.Amaldi” Università Roma TRE and INFN Roma III. P erformance of the MDT detectors of the ATLAS muon spectrometer. The muon spectrometer of the ATLAS experiment MDT : operating principles

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P erformance of the MDT detectors of the ATLAS muon spectrometer

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  1. Fabrizio Petrucci Dipartimento di Fisica “E.Amaldi” Università Roma TRE and INFN Roma III Performance of the MDT detectors of the ATLAS muon spectrometer • The muon spectrometer of the ATLAS experiment • MDT : operating principles • MDT chambers : tracking, autocalibration, efficiency and resolution • Performance vs operating conditions variations • Conclusions 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 1

  2. LHC is a pp collider with a CM energy of 14 TeV,a luminosity of 1034cm-2s-1 and a bunch crossing period of 25 ns. • stot(pp) = 70mb → 109 events/s (~25 events each bunch crossing) • sH ~ 10 pb → 10-1 eventi/s ( … but low branching ratio) • Background is 10 order of magnitude grater than the signal Trigger selection is crucial H → WW(*) , ZZ(*) (Z → ee, mm, jet - jet ) (W → en, mn, jet - jet ) Example : ATLAS Experiment at the Large Hadron Collider (LHC) • Search for Higgs boson in the mass range 110 GeV<mH<1 TeV • Search supersymmetric particles (Msusy > 100 GeV) and in particular study the Higgs sector in the MSSM • Study CP violation in the B mesons system • Precision measurements of SM parameters • In general: • jet reconstruction and invariant mass resolution; • g, m, e energy and direction resolution; • Leptonic decays to detect signal above background The ATLAS detector has been planned to fully exploit LHC potential. Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 2

  3. ATLAS detector Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 3

  4. Air-core toroidal spectrometer • 3 measurement stations • Single point resolution • Detector segmentation (low occupancy & pattern recognition) • Low gas-gain (reduce ageing) 4) Must operate reliably for many years in an high rate and high background environment expecially in the forward regions. Muon Spectrometer • Requirements : • Good momentum resolution in the range 6Gev-1TeV 2) h coverage up to |h|~2.7 3) Trigger capability on single or double muons with programmable pt thresholds. Dedicated trigger chambers Solutions : Monitored Drift Tube (MDT) + Cathode Stip Chamber (CSC) : precision chambers Resistive Plate Chamber (RPC) + Thin Gap Chamber (TGC) : trigger chambers Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 4

  5. Resistive Plate Chamber (RPC) : ionizanition chambers built with two resistive plates and read-out in both coordinates with cathodic strips. Space resolution ~ 1 cm. Time resolution ~ 2 ns. RPC Monitored Drift Tube (MDT) : Proportional drift tubes of 3cm diameter and of variable length (1.8-5.2 m). Assembled in 2 multilayers of 3 or 4 tubes. Internal laser alignement system (Rasnik). Single point resolution ~ 80 mm. Maximum drift time ~ 700 ns. MDT Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 5

  6. start stop MDT (Monitored Drift Tube) ~ 100 ep/cm produced Aluminium tube, diameter=3cm, thickness=400 mm tungsten wire, 50 mm t dc pressurized Ar·CO2 gas mixture electrons drift time Working conditions : Gas Mixture : Argon (93%, high primary ionization density) - CO2(7%) Pressure : 3 bar (High pressure reduces diffusion effects) Gas gain : 2*104 (HV=3080V) Discriminator threshold : 20 primary e (3mV/e → 60mV) Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 6

  7. uniform tube illumination drift velocity space-time relation t0 starting point of time distribution tdrift = tmax –t0 ~ 700 ns depends on drift properties of the selected gas mixture tmax end of physical distribution Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 7

  8. MDT Chamber :test beam Muon beam at the CERN SPS Systems test and systems integration 2001 H8 test beam set-up 2002 H8 test beam set-up Relative positions are the same as a sector of the barrel spectrometer Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 8

  9. Track fit 1) Four possible tangent to the first and the last drift circle. Choose a reference line as the one having the smallest χ2 with the other points. 2) Two close hits in turn are used. Four possible tangent are computed and for each circle the tangent point closer to the reference line is the track point. To each track point is assigned an error using a realistic tube resolution function 3) The track parameters are the ones which better fit the track points. Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 9

  10. Residual’s mean value in the slice residuals(mm) Drift circle Reconstructed Track r-t relation correction residual time(ns) Autocalibration : finding r-t relation • Iterative procedure. • Straight line computed fitting drift circles. • Residuals are computed. • The mean value of residual’s distribution is computed in different drift time slices. • It is used as the correction to the r-t relation. Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 10

  11. Convergence of the procedure: • A unique and bias free solution requires an angular spread of the track sample of at least 100 mrad. • The number of iterations depends on the initial r-t relation. The one obtained from the integration of the drift-time spectrum is a reasonable choice. Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 11

  12. space-time relation from autocalibration method drift velocity Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 12

  13. Track fitted with n-1 points tube not included in the track Residuals (mm) “Good” hits (~efficient hits) Radius (mm) 1) Tracks that cross the tube under analysis are fitted excluding that tube. 2) Check the hit in the tube : - Hit not present - High contribution to the c2 Efficiency tube not efficient Hits due to d rays can “hide” track hits. Effect grows with radius. Residuals (mm) ~high C2 hits Radius (mm) Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 13

  14. Extrapolation error extr. error(mm) r(mm) s(r) residuals(mm) r(mm) Resolution • Selection of “good” events (single track, 8 hits, • good c2). • Residual for each tube and its extrapolation error • are computed with the track obtained with n-1 • points. • Residual’s distribution width is given by: s(r)=  [Resolution(r)]2+ [<extrapolation error>(r)]2 Resolution(r) =  [s(r)]2 - [<extrapolation error>(r)]2 Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 14

  15. resolution (mm) resolution (mm) Signed radius (mm) Signed radius (mm) Run 2011 - QBIL = 6 Nominal conditions The resolution on different layers 4 layers average resolution Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 15

  16. HV dependence Data @ different HV values from the nominal 3080 V up to 3400V 1) t0 weakly decreases (shorter time to collect the required charge). 2) ttot  1/vdrift  p/E decreases 3) time distribution risetime (space resolution) decreases (distance between ionization clusters and cluster size fluctuation) 4) noise frequency slowly increase Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 16

  17. Disctiminator threshold dependence Data @ different discriminator threshold values around the nominal 60 mV (20 primary electrons) 1) t0 weakly increase (larger charge to collect). 2) ttot has a small dependence on the threshold 3) time distribution risetime increases 4) noise frequency rapidly decreases Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 17

  18. Temperature variations Temperature excursions due to external daily variations. Gas pressure fixed. Temperature variations implies density variations. vdrift E/ Slope: -2.67±0.10 ns/K (-2.61 ns/K from Garfieldsimulation) Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 18

  19. Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 19

  20. Gas composition variations Water content variations Argon percentage variations 5.50±0.05 ns/(100 ppm of H20) -69.6±0.7 ns/(% Ar) (6.37 ns/(100 ppm of H20) from Garfield simulation) (-84.19 ns/(% Ar) from Garfield simulation) Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 20

  21. Conclusions • MDT chambers have been extensively tested • Good response uniformity • Iterative method for autocalibration of the r-t relation provided an angular scan • Efficiency has been measured and is mainly limited by -electrons • The study of the efficiency and resolution with respect to the electrict field and discriminator threshold shows the expected behaviour. • Variations in temperature or gas composition have to be monitored .... Fabrizio Petruci - 8th ICATPP Conference – Villa Olmo, Como 8 October 2003 21

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