2002 LHC days in Split

Sandra Horvat

08 – 12 October, 2002.

Ruđer Bošković Institute, ZagrebMax-Planck-Institute for Physics, Munich

Potential is here...Potential is here...

(particle detector)

Physics !Physics ! Physics !Physics !

MDT Precision Chambers in MDT Precision Chambers in ATLAS Muon SpectrometerATLAS Muon Spectrometer

A Toroidal LHC ApparatuS

S.Horvat 1/15 LHC days in Split, October 10 th 2002.

IntroductionIntroduction

• 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) pp collisions at 14 TeV (centre of mass) search for the Higgs(es)

ATLAS @ LHC:

S.Horvat 2/15 LHC days in Split, October 10 th 2002.

PhysicsPhysics

Some selected physics channels:

processes violatingCP

decays topand B icsemilepton

Ejetsleptonsdecays supersymm.

','

4

4*

misst

WZ

lZZH

lZZH

Instrumentation:Instrumentation:

• Trigger Chambers (RPC, TGC) low and high pt 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 2002.

ATLAS Muon SpectrometerATLAS Muon Spectrometer

Goal: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 ||<2.7

stand-alone stand-alone muon momentum resolutionmuon momentum resolution

of 1-10% for pt from 6-1000 GeV

Cathode Strip Chambers (CSC)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:CO2:CF4 at 3 kV• high granularity (strip pitch 5 mm)

Precision, precision, precision! Precision, precision, precision! Single cell resolution of Single cell resolution of 80 80 mm ! !

Monitored Drift Tubes (MDT)Monitored Drift Tubes (MDT)

• 2 multilayers of aluminum 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 m (chamber: 20 m r.m.s.)

S.Horvat 4/15 LHC days in Split, October 10 th 2002.

Precision ChambersPrecision Chambers

S.Horvat 5/15 LHC days in Split, October 10 th 2002.

MDT: Principle of operationMDT: 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 2002.

Drift Tube QualityDrift 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 with7 m (rms) accuracy

(X-ray method)

wire oscillation frequencyis 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 2002.

MDT Chamber Assembly MDT Chamber Assembly

Tube layers glued successively on the spacer:1. tubes positioned into combs on a

flat granite table (5 5 mm)2. spacer positioned on the table with

5 5 mm accuracy w.r.t the combs(6 precision towers on the table)

3. spacer deformations compensated(gravitational sag of 40-80 40-80 mm)

Weight: ~ 300 kgWeight: ~ 300 kgDimensions: 2x4 mDimensions: 2x4 m22

mmicrometer positioning icrometer positioning accuracyaccuracy required for each step,

monitored by optical sensorson 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 2002.

Mechanical Accuracy Tests Mechanical Accuracy Tests

X-rayoptical monitoring

wire position reconstruction:wire position reconstruction:

„We know what we are doing...“

S.Horvat 9/15 LHC days in Split, October 10 th 2002.

Chamber Performance Chamber Performance

muonsmuons

Studies in the muon beam at CERN:

Threshold dependence: 60 mV, 77 mV, 98 mVHigh 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 2002.

Drift Time Spectra Drift Time Spectra

t0 t1physical time window, tmax=t1-t0

(hits introduced by particles)

• physical window is obtainedvia 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 2002.

r-t relation 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 2002.

Single Tube Resolution 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=(rextrapol-rmeas)- 2fit

single tube resolution:

tube=(rextrapol-rmeas)- 2fit

r-t relation

Typical resolution of 80 Typical resolution of 80 m, at nominal operating conditions!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 2002.

Threshold & HV dependence Threshold & HV dependence

S.Horvat 14/15 LHC days in Split, October 10 th 2002.

Performance under irradiation Performance under irradiation

Performance is tested in ahigh radiation environmenthigh radiation environment.

Source:• 137Cs• 662 keV photons• 740 GBq (equivalent to the ATLAS environment)

S.Horvat 15/15 LHC days in Split, October 10 th 2002.

Summary 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• an average resolution of 80 80 mm 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 provide the desired muon track curvature resolution of 4040 mm.

The achieved mechanical wire positioning accuracy and tube resolution provide the desired muon track curvature resolution of 4040 mm.

+

=

S.Horvat 16/15 LHC days in Split, October 10 th 2002.

Outlook... Outlook...

H -> ZZ -> 4A T A T -- L A S L A S TT !?!?

...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)