Particle accelerators Charged particles can be accelerated by an electric field. Colliders produce head-on collisions which are much more energetic than hitting a fixed target. The center of mass energy is 2E in a collider but only m2E for a fixed target ( E = energy, m = mass of the particles, E » m, c=1). The LHC collides protons with m  1 GeV, E = 7 TeV. It pro- duces the same center of mass energy as a proton with E = 105 TeV hitting a proton at rest. Cosmic rays enter 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 An electrostatic van de Graaf accelerator uses high voltage for acceleration, which is obtained by mechanical transfer of electrons 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. This happens in a microwave cavity. High energy is reached by having a long series of cavities. The 3 km long Stanford Linear Accelerator (SLAC) accelerates electrons to 50 GeV. 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): 115 GeV electrons vs. positrons. Discovered the W+,W -, Z bosons of the weak interaction. Tevatron at Fermilab (Chicago): 1000 GeV = 1 TeV protons vs. antiprotons. Discovered the top quark, the last missing quark. LHC at CERN (Geneva): 7 TeV protons vs. protons . Discovered the Higgs boson, the last missing particle of the Standard Model.

Fermilab

CERN LHC (large hadron collider) Highest energy worldwide. Found the Higgs boson. Still looking for super-symmetric particles and candidates for dark matter. 27 km

Particle detectors As the energy of the incident particles increases, there is more and more energy available for producing other particles. Feynman compared this to shooting two Swiss precision watches against each other and trying to find 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. There is so much information that most of the data have to be pre-selected automatically. A “trigger” committee decides on the algorithm for that. The number of scientists in a collaboration is reaching 3000 at the LHC.

LHC Detector

One high energy event

Cosmic rays Cosmic rays (mainly protons) pro-duce a shower of particles when they strike a nucleus in the upper atmosphere . The shower spreads out over miles. We don’t know where cosmic rays are accelerated . Galaxies with a huge black hole at the center can emit particle jets over distances as large as a galaxy. Such jets may act as huge linear accelerators.

The Auger cosmic ray detector in Argentina Cosmic rays are observed with energies of more than 1020 eV, 100 million times greater than the energy reached with our accelerators. This is the energy of a 90 mph tennis ball com-pressed into a single proton ! A particle shower (red line) is observed in a group of ground detectors (orange) and in four light detectors, which look up into the sky (blue and green).

One of 1600 ground detectors

Neutrino detectors Neutrinos are very difficult to detect, 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 100 GeV), 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, a neutrino detector needs to have a very large detection volume, such as the Kamiokande detector in a mine in Japan or the Ice Cube detector at the South Pole, where a cubic kilometer of clear ice serves as detector.

The South Pole

Ice Cube 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.

A photon detector

Particle accelerators are microscopes The uncertainty relation requires a large momentum range p to focus onto a small spot x . Large momentum implies large kinetic energy -- therefore the need for high energy accelerators. To get down to the Planck length (the smallest length scale) one would need the Planck energy (largest particle energy): Planck energy: 1028 eV Cosmic rays: 31020 eV The LHC: 71012 eV It has been estimated that one would need an accelerator the size of the universe to reach the Planck energy.

Particle accelerators are time machines Make the particle energy equal to the thermal energy soon after the Big Bang. 400 000 Years Atoms form Fri. Dec 3 Phy107 Lecture 34