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Introduction to Particle Detection
Winter School on AstroParticle Physics@ Ooty
21st - 29th December, 2014
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Particle Physics
Ultimate deconstruction : Establish working of the universe
starting at the most microscopic level with proofs
Standard model : An extremely successful paradigm of most all
observed phenomena proved by experiments
-decay
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Fundamental building blocks
(matter & fields)
Anti-particles
 All particles have their corresponding anti-particles
 All matter particles has spin ½, called fermions
 All exchange force field particles (quanta) have spin 1, called bosons (except gravity)
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Standard Model at a glance
Weak coupling strength increases as
the interaction energy increases,
inspired the idea of unification of EM
and Weak forces at a high enough
energy; Electroweak unification
verified (exp): Standard Model
vindicated
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Particle discoveries
Particles discovered 1898 - 1964
Particles discovered 1964 - present
Higgs
B-factory era starts here
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Dr. Prafulla Kumar Behera, IIT Madras
LHC era starts here
22rd Dec. 2014
Why do we accelerate particles ?
• To take existing objects apart
• 1803 J. Dalton’s indivisible atom
atoms of one element can combine with atoms of
other element to make compounds, e.g. water is
made of oxygen and hydrogen (OH)
• 1896 M. & P. Curie find atoms decay
• 1897 J. J. Thomson discovers electron
• 1906 E. Rutherford: gold foil experiment
• Physicists break particles by shooting other
particles on them
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Why do we accelerate particles ?
 (2)
To create new particles
 1905 A. Einstein: energy is matter
E=mc2
 1930 P. Dirac: math problem predicts antimatter
 1930 C. Anderson: discovers positron
 1935 H.Yukawa: nuclear forces (forces between protons
and neutrons in nuclei) require pion
 1936 C. Anderson: discovers pion muon
 First experiments used cosmic rays that are accelerated for
us by the Universe
 are still of interest as a source of extremely energetic
particles not available in laboratories
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Generating particles
 Before accelerating particles, one has to create them
 electrons: cathode ray tube
(think your TV)
 protons: cathode ray tube
filled with hydrogen
 It’s more complicated for other particles (e.g.
antiprotons), but the main principle remains the same
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Basic accelerator physics
 Lorentz Force:
F = qE + q(vB)
 magnetic force: perpendicular to velocity, no acceleration
(changes direction)
 electric force: acceleration
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Surfing the electromagnetic wave
 Charged particles ride the EM wave
 create standing wave
 use a radio frequency cavity
 make particles arrive on time
 Self-regulating:
 slow particle  larger push
 fast particle  small push
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Surfing the electromagnetic wave
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Cyclotron
 1929 E.O. Lawrence
 The physics: centripetal force
mv2/r = Bqv
 Particles follow a spiral in a constant magnetic field
 A high frequency alternating voltage applied between D-electrodes
causes acceleration as particles cross the gap
 Advantages: compact design (compared to linear accelerators),
continuous stream of particles
 Limitations: synchronization lost as particle velocity approaches
the speed of light
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Hadron vs electron colliders
electron proton
Point-like particle
yes
no
Uses full beam energy
yes
no
Transverse energy sum
zero
zero
Longitudinal energy sum
zero
Synchrotron radiation
large
nonzero
small
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Large Electron-Positron collider
 Location: CERN (Geneva, Switzerland)
 accelerated particles: electrons and positrons
 beam energy: 45104 GeV, beam current: 8 mA
 the ring radius: 4.5 km
 years of operation: 19892000
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Tevatron
 Location: Fermilab (Batavia, IL)
 accelerated particles: protons and anti-protons
 beam energy: 1 TeV, beam current: 1 mA
 the ring radius: 1 km
 in operation since 1983
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
LHC Accelerator
accelerated particles: protons
beam energy: 7 TeV, beam current: 0.5 A
•30,000 tons of 8.4T dipole magnets
(1232 magnets)
•Energy 80 million times •Cooled to 1.9K with 96 tons of
larger than 5’’ cyclotron liquid helium
•More then $8 billion
•Energy of beam = 362 MJ
•More than 15 years
• 15 kg of Swiss chocolate
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Future of accelerators
 International Linear Collider: 0.53 TeV
 awaiting directions from LHC findings
 political decision of location
 Very Large Hadron Collider (magnet development ?):
40200 TeV
 Muon Collider (source ?) 0.54 TeV
 lepton collider without synchrotron radiation
 capable of producing many more Higgs particles
compared to an e+e collider
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Conclusions
 Motivation for particle acceleration
 understand matter around us
 create new particles
 Particle accelerator types
 electrostatic: limited energy
 AC driven: linear or circular
 Modern accelerators
 TeVatron, LHC
 accelerators to come: ILC, VLHC, muon collider…
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Event in BELLE Detector
Lead to measure CP violation and
Nobel Prize in Physics 2008
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
p-p collisions at the LHC
Protons are not simply
u, u, d quarks at high
energies, but a
complex mix of gluons,
quarks, virtual quarkantiquark pairs:
Proton Structure
Functions
p
p
Z  μμ
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Detectors and particle physics
 Detectors allow one to detect particles 
experimentalists study their behavior
new particles are found by direct observation
or by analyzing their decay products
theorists predict behavior of (new) particles
experimentalists design the particle detectors
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Overview of particle detectors
 What do particle detectors measure ?
 spatial location
 trajectory in an EM field  momentum
 distance between production and decay point 
lifetime
 energy
 momentum + energy  mass
 flight times
 momentum/energy + flight time  mass
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Natural particle detectors
 A very common particle detector: the eye
 detected particles: photons
 sensitivity: high (single photons)
 spatial resolution: decent
 dynamic range: excellent (11014)
 energy range: limited (visible light)
 energy discrimination: good
 speed: modest (~10 Hz, including processing)
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Modern detector types
 Tracking detectors
 detect charged particles
 principle of operation: ionization
 two basic types: gas and solid
 Scintillators
 sensitive to single particles
 very fast, useful for online applications
 Calorimeters
 measure particle energy
 usually measure energy of a bunch of particles (“jet”)
 modest spatial resolution
 Particle identification systems
 recognize electrons, charged pions, charged kaons, protons
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Tracking detectors
 A charged track ionizes the gas
 10—40 primary ion-electron paris
 multiplication 3—4 due to secondary ionization
 typical amplifier noise 1000 e—
 the initial signal is too weak to be effectively detected !
 as electrons travel towards cathode, their velocity increases
 electrons cause an avalanche of ionization (exponential increase)
 The same principle (ionization + avalanche) works for solid state
tracking detectors
 dense medium  large ionization
 more compact  put closer to the interaction point
 very good spatial resolution
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Calorimetry
 The idea: measure energy by total absorption
 also measure location
 the method is destructive: particle is stopped
 detector response proportional to particle energy
 As particles traverse material, they interact
producing a bunch of secondary particles
(“shower”)
 the shower particles undergo ionization (same
principle as for tracking detectors)
 It works for all particles: charged and neutral
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Electromagnetic calorimeters
 Electromagnetic showers occur due to
 Bremsstrahlung: similar to synchrotron radiation,
particles deflected by atomic EM fields
 pair production: in the presence of atomic field, a photon
can produce an electron-positron pair
 excitation of electrons in atoms
 Typical materials for EM calorimeters: large charge
atoms, organic materials
 important parameter: radiation length
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Hadronic calorimeters
 In addition to EM showers, hadrons (pions, protons,
kaons) produce hadronic showers due to strong
interaction with nuclei
 Typical materials: dense, large atomic weight (uranium,
lead)
 important parameter: nuclear interaction length
 In hadron shower, also creating non detectable particles
(neutrinos, soft photons)
 large fluctuation and limited energy resolution
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Muon detection
 Muons are charged particles, so using tracking detectors
to detect them
 Calorimetry does not work – muons only leave small
energy in the calorimeter (said to be “minimum
ionization particles”)
 Muons are detected outside calorimeters and additional
shielding, where all other particles (except neutrinos)
have already been stopped
 As this is far away from the interaction point, use gas
detectors
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Detection of neutrinos
 In dedicated neutrino experiments, rely on their
interaction with material
 interaction probability extremely low  need huge
volumes of working medium
 In accelerator experiments, detecting neutrinos is
impractical – rely on momentum conservation
 electron colliders: all three momentum components
are conserved
 hadron colliders: the initial momentum component
along the (anti)proton beam direction is unknown
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Multipurpose detectors
 Today people usually combine several types of various detectors in a
single apparatus
 goal: provide measurement of a variety of particle characteristics
(energy, momentum, flight time) for a variety of particle types
(electrons, photons, pions, protons) in (almost) all possible
directions
 also include “triggering system” (fast recognition of interesting
events) and “data acquisition” (collection and recording of
selected measurements)
 Confusingly enough, these setups are also called detectors (and
groups of individual detecting elements of the same type are called
“detector subsystems”)
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Generic HEP detector
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014
Conclusions
 Particle detectors follow simple principles
 detectors interact with particles
 most interactions are electromagnetic
 imperfect by definition but have gotten pretty good
 crucial to figure out which detector goes where
 Three main ideas
 track charged particles and then stop them
 stop neutral particles
 finally find the muons which are left
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Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014