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

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

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

Instrumentation: Trigger Chambers (RPC, TGC) low and high p t functionality Precision Chambers (MDT, CSC) high momentum resolution Alignment monitoring the chamber displacements and deformations S.Horvat 3/15 LHC days in Split, October 10 th ATLAS Muon Spectrometer 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 |  |<2.7stand-alone muon momentum resolution of 1-10% for p t from GeV

Cathode Strip Chambers (CSC) multiwire proportional chambers in the region of high muon and background rates 3 wire planes position measurement: centre of gravity of induced charge on the cathode strips Ar:CO 2 :CF 4 at 3 kV high granularity (strip pitch 5 mm) Precision, precision, precision! Single cell resolution of 80  m ! Monitored Drift Tubes (MDT) 2 multilayers of aluminum drift tubes, on each side of a spacer frame Ar:CO 2 (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  m (chamber: 20  m r.m.s.) S.Horvat 4/15 LHC days in Split, October 10 th Precision Chambers

S.Horvat 5/15 LHC days in Split, October 10 th MDT: Principle of operation signal is read out at one wire end, then shaped and amplified presented to a discriminator (threshold reduces noise) TDC measures time difference between the muon pulse and a trigger signal ( drift time + offset of the signal propagation in the electronics ) converted into radius ( not position! ) using the r-t relation track reconstruction is provided by a multilayer chamber geometry When a charged particle traverses the tube, the gas is ionized, the electrons drift toward the wire. When a charged particle traverses the tube, the gas is ionized, the electrons drift toward the wire. r

S.Horvat 6/15 LHC days in Split, October 10 th Drift Tube Quality Sense wire is fed through the tube with the clean air flow, avoiding any manual contact with the wire. Drift tubes: 30 mm tube diameter length 1-6 m 50  m thick W/Re anode wire connected to the positive HV wires positioned within the tube with 7  m (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 7/15 LHC days in Split, October 10 th MDT Chamber Assembly Tube layers glued successively on the spacer: 5  m 1.tubes positioned into combs on a flat granite table (5  m) 5  m 2.spacer positioned on the table with 5  m accuracy w.r.t the combs (6 precision towers on the table)  m 3.spacer deformations compensated (gravitational sag of  m) Weight: ~ 300 kg Dimensions: 2x4 m 2 micrometer positioning accuracy micrometer positioning accuracy required for each step, monitored by optical sensors on the spacer and on the table RASNIK system

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

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

S.Horvat 10/15 LHC days in Split, October 10 th Drift Time Spectra t0t0 t1t1 physical time window, t max =t 1 -t 0 (hits introduced by particles) physical window is obtained via Fermi function fit on both ends of the drift time spectra approximate r-t relation can be derived from the drift time spectra

S.Horvat 11/15 LHC days in Split, October 10 th r-t relation Iterative method: 1.starting from the approximate r-t relation, calculate radii & reconstruct the track through 6 tube layers 3. Completed when the residuals are <few microns. 15 slices of 1 mm size 2.Autocalibration (no external reference): r-t relation corrected using the fit residuals back to 1.

S.Horvat 12/15 LHC days in Split, October 10 th Single Tube Resolution z y track reconstruction through 5 layers y=m*z+b interpolation to the excluded layer (excluding layer by layer) single tube resolution:   tube =   (r extrapol -r meas )-  2 fit r-t relation Typical resolution of 80  m, at nominal operating conditions!

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

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

S.Horvat 15/15 LHC days in Split, October 10 th 2002.Summary 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 80  m an average resolution of 80  m has been demonstrated (at nominal operating conditions) expected dependence on the changes in threshold and HV observed The achieved mechanical wire positioning accuracy and tube resolution 40  m provide the desired muon track curvature resolution of 40  m. The achieved mechanical wire positioning accuracy and tube resolution 40  m provide the desired muon track curvature resolution of 40  m. + =

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