denisov lecture
TRANSCRIPT
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Detection of Muons
Dmitri Denisov
FermilabAcademic Lect ur e, Apr i l 5 2000
Outline:
Muon
Muon identificationMuon interaction with matter
Elements of muon spectrometers
uabsorbers and magnets
utracking detectors
Examples of muon detection systems
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Muon
Discovered by Anderson and Nedermeir in1936 in cosmic rays. Till 1947 (piondiscovery) was considered Yukawa particle,but very low interaction rate was a puzzle
Since that time muon parameters are welldefined
u Charge +/- 1u Mass 105.658389 MeV
u Lifetime 2.19703 sec
u Decay (100%) e
u No strong interaction
Major sources of muons
u cosmic (decays of pions p-> )
s ~102 muons/m2.sec, E>1GeV
u accelerators
s low Pt muons product of mesons decay,100m long 40GeV/c pion beam line
~2% of pions will decay muon energy is between Ebeam and
Ebeam/2
s high Pt muons product of heavy objectsdecays: b, W/Z, etc.
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Why we would like to detect muons?
Muons are easy to detect with highaccuracy and low backgrounds: nostrong interaction
Long lifetime
Lepton decay channels for many ofheavy objects are clean and have lowbackgrounds:u ->
u W-> , Z ->
u t -> bW ->
Many of new particles searchescontain muon(s) as final detectableparticles
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Particle IdentificationMethods
Detection of muons consists of twomajor steps:u identification
u muon parameters measurement
The direct way for identification is tocompare parameters of particle inquestion with known values:u mass
u charge
u lifetime
u decay modes
Typical method for a few GeV muonsis to measure momentum and velocityof a particle:
m = p(1-1)1/2
Velocity is measured by:u Time of flightu Cherenkov, TRD
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Identification (cont)
For high energy (above a few GeV)muons identification is based on lowrate of interaction of muons withmatter:
If charged particle penetrateslarge amount of absorber withminor energy losses and smallangular displacement such particle
is considered a muon
Details and definitions of largeamount of absorber, minor energylosses, etc. will be discussed in this
Lecture
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Muon Lifetime
Muon lifetime is 2.2 sec, c=0.7km
Decay path:
p, GeV/c Decay path, km
1 7
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Instrumentation ofMuon Detectors
Major parts of muon detector:
u absorber/magnetu tracking detectors
u (electronics, DAQ, trigger, software)
Absorbers:
u most common is steel: high density
(smaller size), not expensive, could bemagnetized
u concrete, etc.
Magnets:
u dipole magnets in fixed target or solenoidmagnets in colliders:
s a few m3 in volumes field ~2T
s cryo and/or high energy consumption
u magnetized iron toroids:
s hundreds m3 volume
s saturation at ~2Ts low power consumption
u air core super conducting magnets:
s field similar to iron magnets, but nomultiple scattering
s cryo and complex design
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Tracking Detectors
There are two major requirements formuon tracking detectors:
u coordinate accuracy
u large (~103 m2) area
Other considerations include:
u resolution timeu sensitivity to backgrounds
u segmentation (triggering)
u aging
u cost
Two most common types of detectors:
u scintillation counters
u gas wire detectors
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Scintillation Counters
Used before/after absorber for muonidentification/triggering, rarely formomentum measurement
Typical size 0.1m x 1m x 1cm
Muon deposits ~2MeV of energy in a
counter, which converts into 20-200photo electrons for a typicalphototube
Major parameters of scintillationcounters:
u
very fast t ~1nsu easy to make of any size
u inexpensive in operation
u due to high muon energy depositionless sensitive to backgrounds
butu expensive per m2
u coordinate resolution is x>1cm
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Alignment of TrackingDetectors
In order to measure muon tracks withhigh precision, exact location of wires(cells) is required:u temperature variations
u movement (sink) of heavy objects
u complications due to detectors sizes andlack of space (hermeticity)
Major ways of alignment:u passive - detectors location is determined
before the run by (optical) survey and
these data are used for data analysis:~0.5-1mm
u active - continuing monitoring of chamberslocations by system of sensors (lasersbeams, etc.):
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Shielding of Muon Detectors
During HEP experiments in addition touseful particles large amount ofbackground particles are created:u particles coming from accelerator beam
losses
u spectators interactions (showers) with
accelerator and detector equipmentu neutrons
Muon detectors are sensitive tobackgrounds because:u have large area
u gas detectors have low (~1keV) threshold Most common way is to install thick
shielding to absorb backgrounds:u steel to catch hadrons
u poly to absorb neutrons
u
lead to reduce gamma fluxes Even moderate shielding could provide
factor of ~102 reduction in number ofbackground hits
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D0 Muon System
Absorbers:
u calorimeter and steel toroid: 15
u punchthrough less then 10-5
Detectors:
u tracking: drift tubes with 1mmresolution, 3 layers
u trigger: scintillation triggercounters, time resolution 1ns
Momentum resolution (muon systemonly):
u multiple scattering limit is 18%
u p/p is ~50% at 200GeV/c
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uon ys emUpgrade
Forward
trigger: scintcounters
Shielding
Forwardtracker: mini-drift tubes
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Muon Detection - Summary
Muon detection is based on ability ofmuon to penetrate thick absorberswith minor energy losses
Muon spectrometer momentumresolution is limited by multiplescattering in absorber and coordinateaccuracy of tracking detectors:
p/p = (ms2 +(xp/b)2)1/2
Due to the size of muon systems(~103m2) scintillation counters and/orgas detectors are used
Backgrounds estimation is importantand shielding installation may benecessary
Precision determination of detectors
location is achieved via alignment For very high energy muons (above
0.2TeV) electromagnetic radiationbecomes an issue