denisov lecture

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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

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