Particle accelerators • Charged particles can be accelerated by anelectricfield. • Colliders produce head-oncollisionswhicharemuchmore energeticthan hitting a fixed target. The center of mass energy is 2E in a collider butonly m2E forafixedtarget (E=energy,m=mass ofthe particles, E»m, c=1). • The LHCcollides protons with m1GeV,E=7TeV. It pro- duces the same center of mass energy as a proton with E=105 TeVhitting a proton at rest. • Cosmic raysenter the atmosphere with energies well be-yond that achievable by accelerators (up to E=108 TeV). But they are scarce and are detected with a fixed target. • 1 TeV = 1000 GeV = 1012 eV
Electrostatic accelerators • Anelectrostatic vandeGraaf accelerator uses highvoltage for acceleration, which is obtained by mechanical transferofelectrons from one material to another. • Energies of 10 MeV can be reached, which are typical for nuclear physics.
Linear accelerator • Particles gain energy by surfing an electromagnetic wave. • Thishappensinamicrowavecavity.High energy is reached by having a long series of cavities. The 3 km long Stanford Linear Accelerator (SLAC) accelerates electrons to 50GeV. Linear accelerators have been abandoned in favor of circular accelerators (synchrotrons). These are more compact and use only a few microwave cavities.
Circular accelerators (synchrotrons) • Use again a microwave cavity for acceleration,except that the particles keep coming around in a big circle. They are accelerated each time they pass the cavity. • LEP at CERN (Geneva): 115GeV electronsvs.positrons. Discoveredthe W+,W-,Z bosonsoftheweakinteraction. • Tevatron at Fermilab (Chicago): 1000GeV=1TeVprotons vs. antiprotons. Discovered the top quark,the last missing quark. • LHC at CERN (Geneva): 7TeVprotons vs. protons. Discoveredthe Higgs boson,the last missing particle of the Standard Model.
CERN LHC (large hadron collider) Highest energy worldwide. Found the Higgs boson. Still looking for super-symmetric particles and candidatesfordarkmatter. 27 km
Particle detectors • As the energy of the incident particles increases,there is more and more energy available for producing other particles.Feynmancomparedthistoshootingtwo Swiss precision watches against each other and tryingtofind out from the debris how a watch is built. • Detectors have become larger, and the number of par-ticles produced at high energy is enormous.Thereis so much information that mostofthedata have to be pre-selected automatically. A “trigger” committee decides on the algorithm for that.The numberof scientists ina collaboration is reaching 3000 at the LHC.
Cosmic rays Cosmic rays (mainly protons) pro-duce a shower of particles when they strike anucleus in the upper atmosphere. The shower spreads out over miles. We don’t know where cosmicrays are accelerated.Galaxies with a huge black hole at the center can emit particle jets over distances aslargeasagalaxy.Suchjetsmay act as huge linear accelerators.
The Auger cosmic ray detector in Argentina Cosmic rays are observed with energies of more than 1020eV,100 million timesgreater than the energy reached with our accelerators. This is the energy of a 90mph tennis ball com-pressed into a single proton! A particle shower (red line) isobserved in a group of grounddetectors(orange) and in four light detectors,which look up into the sky (blue and green).
Neutrino detectors • Neutrinos are verydifficulttodetect,because they don’t possess electric or strong charge. They can only interact via the weak interaction. • The weak interaction is transmitted by the W,Z bosons. Both have very high masses(approaching 100GeV),while neutrinos from the Sun and from radioactivity have only energies in the MeV range. They have to emit short-lived ‘virtual’ W or Z bosons, and that happens rarely. • Therefore,aneutrinodetector needstohave a very large detection volume, such as the Kamiokande detector in a mine inJapan orthe IceCube detector at the South Pole, where acubickilometer of clear ice serves as detector.
IceCube detector at the South Pole Strings of photon detectors are lowered into the ice along cables. Particles are tracked by the emitted light and by timing.
Particle accelerators are microscopes The uncertainty relation requiresa large momentum range p tofocusonto a small spot x. Large momentum implies large kinetic energy --therefore the need for high energy accelerators. Togetdown tothe Plancklength (thesmallestlengthscale)onewouldneedthe Planck energy (largestparticleenergy): Planck energy: 1028 eV Cosmic rays: 31020 eV The LHC: 71012 eV It has been estimated that one would need an accelerator the size of the universe to reach the Planck energy.
Atoms form 400000 Years Particle accelerators are time machines Make the particle energy equal to the thermal energy soon after the Big Bang. Phy107 Lecture 34