2002 LHC days in Split

1. MDT Precision Chambers in ATLAS Muon Spectrometer Physics ! AToroidal LHC ApparatuS Potential is here... (particle detector) Sandra Horvat Ruđer Bošković Institute, Zagreb Max-Planck-Institute for Physics, Munich 2002 LHC days in Split 08 – 12 October, 2002.

2. Introduction pp collisions at 14 TeV (centre of mass) search for the Higgs(es) S.Horvat 1/15 LHC days in Split, October 10th 2002. ATLAS @ LHC: • Muon Spectrometer • Physics & Instrumentation • Monitored Drift Tube • (MDT) Chambers • Chamber Assembly • and Mechanical Accuracy • Chamber Performance • Summary 23m 46m

3. Physics Some selected physics channels: S.Horvat 2/15 LHC days in Split, October 10th 2002.

4. ATLAS Muon Spectrometer stand-alone muon momentum resolution of 1-10% for pt from 6-1000 GeV S.Horvat 3/15 LHC days in Split, October 10th 2002. • Goal: • measurement of the muon track curvature in a • magnetic field of B=0.3-2 T (air core toroid) • 3 layers, projective tower geometry • full coverage up to |h|<2.7 • Instrumentation: • Trigger Chambers (RPC, TGC) • low and high pt functionality • Precision Chambers (MDT, CSC) • high momentum resolution • Alignment • monitoring the chamber • displacements and deformations

5. Precision Chambers Monitored Drift Tubes (MDT) Cathode Strip Chambers (CSC) • multiwire proportional chambers • in the region of high muon and • background rates • 2multilayers ofaluminum drift tubes, • on each side of a spacer frame • Ar:CO2 (93:7) gas mixture at 3 bar, at 3 kV • optical alignment monitoring system • sense wires(30 mm pitch) centered • within a tube with precision of 10 mm(chamber: 20 mm r.m.s.) • 3 wire planes • position measurement: • centre of gravity of induced • charge on the cathode strips • Ar:CO2:CF4 at 3 kV • high granularity (strip pitch 5 mm) S.Horvat 4/15 LHC days in Split, October 10th 2002. Single cell resolution of 80 mm ! Precision, precision, precision!

6. MDT: Principle of operation When a charged particle traverses the tube, the gas is ionized, the electrons drift toward the wire. r • signal is read out at one wire end, thenshapedandamplifiedpresented to a discriminator (threshold reduces noise) • TDC measurestime differencebetween the muon pulse and a trigger signal • ( drift time+ offset of the signal propagation in the electronics ) • converted intoradius(not position! ) using ther-t relation • track reconstructionis provided by a multilayer chamber geometry S.Horvat 5/15 LHC days in Split, October 10th 2002.

7. Drift Tube Quality wires positioned within the tube with 7 mm (rms) accuracy (X-ray method) wire oscillation frequency is a measure of the wire tension, i.e. it determines the gravitational sag of the wire gas leak rate tested with argon at 3 bar pressure S.Horvat 6/15 LHC days in Split, October 10th 2002. • Drift tubes: • 30 mm tube diameter • length 1-6 m • 50 mm thick W/Re anode wire • connected to the positive HV Sense wire is fed through the tube with the clean air flow, avoiding any manual contact with the wire.

8. MDT Chamber Assembly micrometer positioning accuracy required for each step,monitored by optical sensorson the spacer and on the table RASNIK system S.Horvat 7/15 LHC days in Split, October 10th 2002. • Tube layers glued successively • on the spacer: • tubes positioned into combs on a flat granite table (5 mm) • spacer positioned on the table with5 mm accuracy w.r.t the combs(6 precision towers on the table) • spacer deformations compensated(gravitational sag of 40-80 mm) Weight: ~ 300 kg Dimensions: 2x4 m2

9. Mechanical Accuracy Tests X-ray sources chamber X-ray optical monitoring wire position reconstruction: wires positioned with 10 mm accuracy X-ray Scanning Device (X-Tomograph) S.Horvat 8/15 LHC days in Split, October 10th 2002. „We know what we are doing...“

10. Chamber Performance S.Horvat 9/15 LHC days in Split, October 10th 2002. Studies in the muon beam at CERN: muons • drift time spectra • r-t relation • single tube resolution Threshold dependence: 60 mV, 77 mV, 98 mV High Voltage dependence: 2980 V, 3080 V, 3180 V

11. Drift Time Spectra • physical windowis obtained • via Fermi function fit • on both ends of the • drift time spectra • approximater-t relation • can be derived from the • drift time spectra t0 t1 S.Horvat 10/15 LHC days in Split, October 10th 2002. physical time window, tmax=t1-t0 (hits introduced by particles)

12. r-t relation • Autocalibration (no external reference): r-t relation corrected using the fit residualsback to 1. 15 slices of 1 mm size S.Horvat 11/15 LHC days in Split, October 10th 2002. • Iterative method: • starting from the approximate r-t relation,calculate radii & reconstruct the track through 6 tube layers 3. Completed when the residuals are <few microns.

13. Single Tube Resolution z y S.Horvat 12/15 LHC days in Split, October 10th 2002. single tuberesolution:s2tube=s2(rextrapol-rmeas)- s2fit r-t relation • track reconstruction through5 layersy=m*z+b • interpolation to theexcluded layer (excluding layer by layer) Typical resolution of 80 mm, at nominal operating conditions!

14. Threshold & HV dependence nominal nominal • Lowering thethresholdimproves the resolution, as expected. • limited by the noise level • Increase in thehigh voltageimproves the resolution, • especially near the wire (higher gas gain lower effective threshold) • constraint: tube ageing under high radiation S.Horvat 13/15 LHC days in Split, October 10th 2002.

15. Performance under irradiation S.Horvat 14/15 LHC days in Split, October 10th 2002. • Performance is tested in a • high radiation environment. • Source: • 137Cs • 662 keV photons • 740 GBq (equivalent to the • ATLAS environment)

16. Summary + = The achieved mechanical wire positioning accuracy and tube resolution provide the desired muon track curvature resolution of 40mm. S.Horvat 15/15 LHC days in Split, October 10th 2002. • ATLAS Muon Spectrometer plays an important role in detection • of many physics channels (among them the „gold-plated“ H->ZZ->4l) • designed to provide a high resolution stand-alone momentum measurement • puts demanding requirements on the accuracy of the precision chambers, • resulting in a series of novel techniques for the chamber assembly, • as well as in the optical alignment monitoring • single tube resolution defines the chamber performance • an average resolution of 80 mm has been demonstrated • (at nominal operating conditions) • expected dependence on the changes in threshold and HV observed

17. Outlook... Put everything together (~2000 chambers) H -> ZZ -> 4m S.Horvat 16/15 LHC days in Split, October 10th 2002. ...testing the combined performance and alignment of more chambers, together with trigger chambers, in a magnetic field, with background signals... A T - L A S T!?