Particle Identification in High Energy Physics Experiments by MikeJenny

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									Particle Identification in High
Energy Physics Experiments
         Time-of-Flight techniques

             Matthew Jones

              June 22, 2004
             Introduction




• Subatomic particles observed as “tracks”
  using a Wilson Cloud Chamber
• Easy to distinguish two types of tracks
               Introduction
• Thick tracks:  particles (He nucleus)
  – Example: 222Rn (radon)  218Po + 
• Thin tracks:  particles
  – Example: 14C  14N + - (+ e)



         Particle Identification
     Particle Physics Experiments
  • Massive particles need energy to produce:
                  E = m c2
  • Typical experiments involve collisions:




Beam particle
Beam particle     Target particle
                   Interaction           Beam particle
                                       Decay products

                 FIXED TARGET
                COLLIDING BEAMS
       Particle Accelerators

                           2000




1931
Particle Detectors
          Particle Detectors
• Most read out electronic signals
• Event data is interpreted and visualized
  using computers:
                                Tracks
                               What kinds of
                               particles make
                               tracks in a
                               detector?
          Particle Structure

• Examples:
  – Protons and neutrons (make atomic nuclei)
  – Electrons (nucleus + electrons = atom)
  – Muons (found in cosmic rays)
• The Standard Model of Particle Physics
  describes these particles and more…
            Particle Structure
• The most fundamental “building blocks” of
  nature are quarks and leptons.      Electric charge
• Six types of quarks:
       up          charm       top            Q = +2/3
       down        strange     bottom         Q = -1/3

• Six types of leptons:
       electron     muon                      Q = -1
       e                                  Q=0

                                     Increasing mass
           Particle Structure
• Quarks have fractional charge:
  – up, charm, top have Q = +2/3 |e|
  – down, strange, bottom have Q = -1/3 |e|
• We can’t observe quarks as free particles
• We do observe particles that are made out
  of specific combinations of quarks:
  – Proton = (uud) has charge +1
  – Neutron = (udd) is electrically neutral
             Particle Structure
• Types of hadrons:
  – Baryons contain three quarks
  – Mesons contain quark anti-quark pairs
  – Glueballs and hybrids (maybe)
• Examples:
  – Pions:
  – Kaons:
• More examples:
  – D-mesons:
  – B-mesons:
                  Particle Masses
                               Light… easy to produce
Particle          Mass
proton, neutron   » 1 GeV
pions             0.14 GeV
                               Heavy… hard to produce
kaons             0.5 GeV
D mesons          1.8 GeV
B mesons          5 GeV
electron          0.0005 GeV
muon              0.11 GeV
tau               1.8 GeV

neutrinos         » 0 (!)
           Particle Decays
• Most particles are unstable and decay to
  lighter particles:
                     100%
                      64%
                      21%


                      9%



                                0.02%
                Lifetimes
• How long do these particles live?
• How far would they travel in a detector?
                        Non-relativistic, v << c

                        True for all  = v/c

• We also measure momentum in GeV:
           Typical Lifetimes
Particle   Lifetime   ct      d when p=1 GeV
Proton     1          1       1         Long
Electron   1          1       1         Long
 +   -
 ,       2.2 s     659 m   6.2 km    Long
+, -     26 ns      7.8 m   56 m      Long
K+, K-     12 ns      3.7 m   7.5 m     Long

D+, D-     1.1 ps     315 m 169 m     Short
B+, B-     1.7 ps     496  m 94 m     Short
+, -     0.29 ps    87 m   49 m     Short
       Tracks in the detector
• Charged tracks are almost always:
  – Protons
  – Electrons
  – Muons
  – Kaons
  – Pions
• All other particles decay to these before
  they can be detected directly
• How can we tell them apart?
• Why would we want to?
         Electrons and Muons
• Electrons produce Bremsstrahlung:
     Heavy nucleus                 




• High energy gamma rays produce e+e-:

                            e+

                              e-

• Electrons lose energy quickly in dense material.
• Muons don’t interact much at all!
               The CDF Detector
Muon chambers
 Calorimeter
Tracking chamber
     Pions, kaons and protons
• They all look alike:
  – A charged track in the tracking chamber
  – Energy spread out in the calorimeter
  – Nothing in the muon chambers
• How can we tell them apart?
               but first…
• Why?
    Why perform particle ID?
• Example: studying heavy mesons




• Start with a muon…
Particle reconstruction
                This is the muon…

                    These are
                   the tracks…

                  Which tracks
                  make the D+s?
      Particle reconstruction
• The tracking chamber measures momentum
• We guess at the mass
• Relativistic kinematics:
    Particle reconstruction
                         “combinatorial background”
                                (mostly pions)
       Real     K+K-




• There are a lot more pions than kaons.
     Pions, kaons and protons
• How can we tell them apart?           Their mass!
• How do we measure mass?
  – The tracking chamber measures momentum:



                                Radius of curvature (cm)
                                Magnetic field (Tesla)
                                Transverse momentum (GeV)
• Relations between mass and momentum:
                            (when v << c)

• Measure p and v… calculate m.
               Measuring  t
  Production point          Detector




• How well we can measure  t determines
  how well we can distinguish between the
  two particles.
Separation power
          Particle ID for CDF-II
• Requirements:
  – Good K- separation below »1.5 GeV/c
  – Perform measurements every 132 ns
  – Must fit in the available space
     5 cm space between
    tracking chamber and
   superconducting magnet
      Time-of-Flight Detectors
• Components:
  – Charged particle detector
    • Plastic scintillator
  – Photomultiplier tubes
    • Change light pulse into pulse of electric current
  – Electronics
    • To measure arrival time of electric pulses
• Requirements:
  – Precise enough for good K- separation
  – Do it fast. (every 132 ns!)
        Scintillation Detectors
• Plastic scintillator:
  – Emits blue light when hit by charged particles
  – Light should be emitted fast (few ns)
• Light propagates by internal reflection off
  the smooth surfaces
  – The attenuation length should be long (few m)
• Many people have used Bicron 408 for
  TOF detectors.
Bicron-408 data sheet
                  Optics
• Light propagates by internal reflection
• Light with large angles comes out later:



• We can select which light to accept:
                Photomultiplier Tubes
  • Convert light to electric pulse (106 gain)
         – Photocathode emits photoelectrons
         – Dynodes produce secondary electrons
         – Charge collected at the anode

   0 volts      200 V           400 V           600 V

Photon                                                           Current
                                                                700 voltspulse

                        300 V           500 V



 Photocathode                     Dynodes               Anode
           Photomultiplier tubes
• What about the magnetic field?


                   Photoelectron curles up in B field
  Photon




• Possible solutions:
  – Guide the light out of the B field
  – Fine mesh photomultiplier tubes
Example: The CLEO-II Detector


                      Light guides




                      Photomultiplier tubes
 Fine Mesh Photomultiplier Tubes
• Secondary electrons accelerated parallel to the
  B-field.
• Gain with no field: 106
• With B=1.4 Tesla: 104
• We also added a
  preamplifier (gain of »15).
• Attach directly to ends
  of scintillator bars.
• Manufactured by
  Hamamatsu Photonics
               Electronics
• Typical configuration:


                             ADC




• Time between START and STOP
  converted to voltage
• Voltage digitized with ADC
TOF Electronics in CDF
     How is it done in 132 ns?
• Parallel processing: pipeline architecture.
Electronics
               Offline Analysis
• Several corrections need to be applied to the
  measured time:
  – Propagation time in the scintillator
  – PMT pulse height corrections
         Pions

  – Measuring the     collision time
          Kaons




          Protons
          Offline Analysis
• Reconstructing low momentum ! K+K-:
           Remaining Issues
• Achieved goal of “100 ps” timing resolution
• Small fraction of events are biased:




• Currently a subject of investigation.
  Other Particle ID Techniques
• Ionization energy loss in gas
• Emission of Cerenkov light
  – RICH detectors
  – Threshold detectors
• Transition radiation detectors
  – Especially for e- separation
                   Summary
• A rich variety of particles exist in nature
  – But only 5 quarks (+ the top quark)
• Only observe 5 directly:
  – e§, §, §, K§, p
• Particle ID an important part of many HEP
  experiments.

								
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