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?

e+e-e+e-

Examples of Models• Quantum Electrodynamics

– specifies the rules for calculating probabilities– can be represented diagramatically:

time

space

Remember, we don’t observe

the photon... it’s virtual.

Initial state

Final state

Other Models

• The Electroweak model:– Similar to quantum electrodynamics, except

with extra heavy photons: W§, Z0

– Includes QED– Also explains nuclear β-decay: npe- ν

e+

e-

μ+

μ-Z0

Testing the Electroweak Model

Energy of e+e- collisions

“Probability” of producing

W+W-

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?

x-rays: Roentgen, 1895

positron production threshold

pion production thresholdkaon production threshold

anti-proton production threshold

W§/Z0 bosonstop quark

charm and bottom quarks

Higgs? Supersymmetry?

Particle Accelerators

• Classical kinetic energy:• To get high energies, make large:

• Almost always use electromagnetic forces to accelerate particles.

• Prefer to work with stable particles: electrons and protons, but also heavy ions

acceleation

force = (mass) x (acceleration)

Particle Acceleration

• Like charges repel:

• Electric field:

• E can be static or change with time

+Q+q

E

+q

First Particle Accelerators

- +

e-

Electric field

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:

Magnetic field in Gauss

(104 Gauss = 1 Tesla)Momentum in MeV/c

(E2 = m2c4 + p2c2)

Radius in centimeters

Divide r by 2 if the particle has charge 2e...

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

Cyclotrons

• Classic description:

Lawrence’s Cyclotron (c. 1930)

“...discoveries of unexpected character

and of tremendous importance.”

Cyclotrons: the start of Big Science

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...

• 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...

Luminosity and Cross Section

Experiments with Particle Beams

Beam particle Target particleInteraction Decay products

FIXED TARGET

Beam particle Beam particle

COLLIDING BEAMS

To conserve momentum, the decay products carry away some of the initial energy

In the centre-of-mass system, all of the initial energy can be used to produce new particles

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

• Hydrogen ion source• 400 MeV Linac• 8 GeV synchrotron• 150 GeV synchrotron• Antiproton storage ring• 2 TeV collider• Two detectors

The Fermilab Accelerator Complex

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