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 AdventureThe Particle Adventure –Particle Physics Education SitesParticle Physics Education Sites –Encyclopedia BritanicaEncyclopedia 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-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 §, Z 0 –Includes QED –Also explains nuclear β-decay: n  pe - ν e+e+ e-e- μ+μ+ μ-μ- Z0Z0

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 = mc 2 –Example: e + e -  Z 0, pp  H 0 (Higgs boson) We need high intensities to do precision studies, or look for rare events –Example: K 0  π 0 ν ν (KOPIO experiment)KOPIO experiment –Probability might be about 2x –Better odds playing the lottery (once) –Make K 0 particles... you might seen 20.

How Much Energy? x-rays: Roentgen, 1895 positron production threshold pion production threshold kaon production threshold anti-proton production threshold W § /Z 0 bosons top 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

First Particle Accelerators - + e-e- Electric field V That’s why we measure energy in electron volts

First Particle 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 (10 4 Gauss = 1 Tesla) Momentum in MeV/c (E 2 = m 2 c 4 + p 2 c 2 ) Radius in centimeters Divide r by 2 if the particle has charge 2e... Example: Fermilab Tevatron ring: p≈2 TeV/c = 10 6 MeV/c, superconducting magnets produce B=4.2 Tesla = Gauss  r = 79,365 cm = 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 TRIUMFTRIUMF –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 Widrer öe (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: 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 300x10 9 protons, beams are about 25 μm in diameter...

Quantum mechanics: calculates probabilities of producing, say, a pair of top quarks We measure “probabilities” in cm 2 so that Example: But we only find about 1% of them... Luminosity and Cross Section

Experiments with Particle Beams Beam particle Target particle Interaction Decay products FIXED TARGET 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