Transcript Particle Identification in High Energy Physics Experiments

Particle Acceleration for High
Energy Physics Experiments
Matthew Jones
June, 2006
Disclaimer
• This is not meant to be a comprehensive
review...
• I might not have included someone’s
favorite accelerator...
• Some resources I found:
– The Particle Adventure
– Particle Physics Education Sites
– Encyclopedia Britanica
Classical Mechanics
1. Specify initial conditions
2. Laws of physics predict the state of
the system as a function of time
x
Quantum Mechanics
1. Specify the initial state of a system
2. Laws of physics predict the probabilities
of various outcomes
You are not allowed to ask about what
happened in between!
Doesn’t this look like a histogram?
Quantum Mechanics
• What are these so-called laws of physics?
• How can we learn about them?
– Propose a model for the system
– Compare predictions with experiment
• Good models:
– Can be tested
– Predict lots of things
– Consistent with previous experiments
– Small number of adjustable parameters
– Simple?
Examples of Models
• Quantum Electrodynamics
– specifies the rules for calculating probabilities
– can be represented diagramatically:
Remember, we
Initial + don’t
observe
Final
+
photon...
state e e e ethestate
it’s virtual.
space
time
Other Models
• The Electroweak model:
– Similar to quantum electrodynamics, except
with extra heavy photons: W§, Z0
– Includes QED
– Also explains nuclear β-decay: npe- ν
μ
e+
+
Z0
e-
μ
-
Testing the Electroweak Model
“Probability”
of producing
W+W-
Energy of e+e- collisions
High Energy Physics
• We need high energies to look for or study
massive particles: E = mc2
– Example: e+e-  Z0, pp  H0 (Higgs boson)
• We need high intensities to do precision
studies, or look for rare events
– Example: K0  π0 ν ν (KOPIO experiment)
– Probability might be about 2x10-11
– Better odds playing the lottery (once)
– Make 1012 K0 particles... you might seen 20.
How Much Energy?
Higgs? Supersymmetry?
top quark
W§/Z0 bosons
charm and bottom quarks
anti-proton production threshold
kaon production threshold
pion production threshold
positron production threshold
x-rays: Roentgen, 1895
Particle Accelerators
• Classical kinetic energy:
• To get high energies, make
large:
acceleation
force = (mass) x (acceleration)
• Almost always use electromagnetic forces to
accelerate particles.
• Prefer to work with stable particles: electrons
and protons, but also heavy ions
Particle Acceleration
• Like charges repel:
+q
+Q
• Electric field:
E
+q
• E can be static or change with time
First Particle Accelerators
Electric field
e-
+
V
That’s why we measure energy in electron volts
First Particle Accelerators
Van Der Graaf Accelerators
Van Der Graaf Accelerators
Electrostatic Accelerators
Fermilab proton source
(Cockcroft-Walton)
Fermilab Pelletron
Electrostatic Accelerators
• Advantages:
– Simple
– Relatively inexpensive
– Good for studying nuclear physics
• Disadvantages:
– High voltage breakdown (sparks!)
– Either the voltages get very large or the
sizes get very big
– Can’t get to really high energies
Circular Accelerators
• Don’t provide all the acceleration at once
• Just give a particle a little push each
time it comes around in a circle
• Various configurations:
– Cyclotron
– Betatron (only of historic interest now)
– Synchrotron
Cyclotrons and Synchrotrons
• Magnetic fields bend charged particles:
Radius in centimeters
Momentum in MeV/c
(E2 = m2c4 + p2c2)
Magnetic field in Gauss
(104 Gauss = 1 Tesla)
Example: Fermilab Tevatron ring: p≈2 TeV/c = 106 MeV/c, superconducting
magnets produce B=4.2 Tesla = 42000 Gauss  r = 79,365 cm = 0.794 km
Divide r by 2 if the particle has charge 2e...
Cyclotrons
• Classic description:
Lawrence’s Cyclotron (c. 1930)
Cyclotrons: the start of Big Science
“...discoveries of
unexpected character
and of tremendous
importance.”
Berkeley 184” diameter 100 MeV cyclotron (ca. 1942)
Cyclotrons Today
• Still used today for small accelerators:
– Radiation therapy
– Production of medical isotopes
• But also for high intensity proton sources
• Example: 600 MeV cyclotron at TRIUMF
–
–
–
–
Pion and muon beams
Low energy high precision experiments
Radiation therapy and biophysics
Nuclear physics
• Maximum possible energy is about 600 MeV
– Can’t make anything heavier than a pion
Linear Accelerators
Rolf Widreröe (1928)
L.W. Alvarez (1946)
Linear Accelerators
Fermilab 400 MeV proton linac
Linear Accelerators
2 mile long Stanford Linear Accelerator: can
accelerates electrons to about 50 GeV
Synchrotrons
• Magnets bend the beam in a circle
• Accelerated in RF cavities
• Magnetic field has to change to keep
radius constant
Accelerating RF cavity
Bending magnets
Synchrotrons
•
•
•
•
•
•
•
•
First synchrotron: 70 MeV (1947)
Brookhaven Cosmotron: 3 GeV (1952)
Berkeley Bevatron: 6 GeV (1954)
AGS, PS, ISR, SPS, DORIS, PETRA, ...
Fermilab: 400 GeV (1972)
LEP: 100-200 GeV e+e- collider
Fermilab Tevatron: 2 TeV p-pbar collider
LHC: 14 TeV p-p collider in LEP tunnel
Technical Aspects
• The circulating beams come in “bunches”
• More intense beams pack more particles
into smaller bunches
• Intensity is referred to as luminosity:
• Example: the Tevatron has 36 bunches,
each with 300x109 protons, beams are
about 25 μm in diameter...
Luminosity and Cross Section
• Quantum mechanics: calculates probabilities of
producing, say, a pair of top quarks
• We measure “probabilities” in cm2 so that
• Example:
• But we only find about 1% of them...
Experiments with Particle Beams
In the centre-of-mass system,
all of the initial energy can be
used to produce new particles
Beam
Beamparticle
particle
To conserve momentum, the
decay products carry away
some of the initial energy
Target
particle
Interaction
FIXED TARGET
COLLIDING
BEAMS
Beam
particle
Decay
products
Particle Colliders
• Positive and negative particles bend in
opposite directions
– They can use the same set of magnets and
the same beam pipe
• Works for e+e- (LEP) and p p (Tevatron)
• The LHC is a p p collider: beams circulate
in separate pipes
HEP Laboratories
Large Electron Positron Collider
Fermilab Tevatron Collider
The Fermilab Accelerator Complex
•
•
•
•
•
•
•
Hydrogen ion source
400 MeV Linac
8 GeV synchrotron
150 GeV synchrotron
Antiproton storage ring
2 TeV
collider
Two detectors
Future Accelerators
• Large Hadron Collider: 14 TeV (2007?)
• Super LHC: much higher beam intensity
• International Linear Collider:
– 500 GeV to 1 TeV energy
– Currently being designed
– No site selected yet
• Ideas that are either crazy or brilliant:
– Muon colliders: accelerate and collide muons
before they decay
– Very Large Hadron Collider
Very Large Hadron Collider?
Summary
• Interesting history
• Many technical challenges have been met...
many remain
• Lots of spin-off technology:
– Medical applications (therapy, isotopes)
– Material structure studies (advanced photon sources)
• Fewer and fewer cutting edge facilities:
– Tevatron, LHC (energy)
– Several others with low energy but record intensity
• The future:
– Linear collider? Super LHC?
– More low energy, high intensity machines