1. Introduction to Particle Detection
Prafulla Kumar Behera
IIT Madras, India
Winter School on AstroParticle Ooty
21st - 29th December, 2014

2. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

3. 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)
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

4. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

5. Particle discoveries Particles discovered 1898 - 1964
Particles discovered present
Higgs
B-factory era starts here
LHC era starts here
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

6. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

7. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

8. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

9. Basic accelerator physics
Lorentz Force: F = qE + q(vB)
magnetic force: perpendicular to velocity, no acceleration (changes direction)
electric force: acceleration
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

10. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

11. Surfing the electromagnetic wave
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

12. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

13. Hadron vs electron colliders
proton
Point-like particle
yes no
Uses full beam energy
Transverse energy sum
zero
Longitudinal energy sum
non-zero
Synchrotron radiation
large
small
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

14. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

15. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

16. LHC Accelerator accelerated particles: protons
beam energy: 7 TeV, beam current: 0.5 A
30,000 tons of 8.4T dipole magnets
(1232 magnets)
Cooled to 1.9K with 96 tons of
liquid helium
Energy of beam = 362 MJ
15 kg of Swiss chocolate
Energy 80 million times
larger than 5’’ cyclotron
More then $8 billion
More than 15 years
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

17. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

18. 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…
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

19. Event in BELLE Detector
Lead to measure CP violation and
Nobel Prize in Physics 2008
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

20. 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 quark-antiquark pairs: Proton Structure Functions
Z  μμ
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

21. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

22. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

23. 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)
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

24. Modern detector types Tracking detectors Scintillators Calorimeters
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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

25. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

26. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

27. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

28. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

29. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

30. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

31. 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”)
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

32. Generic HEP detector Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014

33. 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
Dr. Prafulla Kumar Behera, IIT Madras
22rd Dec. 2014