Particle accelerators Charged particles can be accelerated by

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 2 E in a collider but only m 2 E for a fixed target ( E = energy, m = mass of the particles, E » m, c=1). • The LHC collides protons with m 1 Ge. V, E = 7 Te. V. It produces the same center of mass energy as a proton with E = 105 Te. V hitting a proton at rest. • Cosmic rays enter the atmosphere with energies well beyond that achievable by accelerators ( up to E = 108 Te. V ). But they are scarce and are detected with a fixed target. 12 e. V • Fri. 1 Dec Te. V = 1000 Ge. V = 10 3 Phy 107 Lecture 34

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 Me. V can be reached , which are typical for nuclear physics. Fri. Dec 3 Phy 107 Lecture 34

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 Ge. V. Linear accelerators have been abandoned in favor of circular accelerators (synchrotrons). These are more compact and use only a. Phy 107 few. Lecture microwave cavities. Fri. Dec 3 34

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 Ge. V electrons vs. positrons. Discovered the W+, W -, Z bosons of the weak interaction. • Tevatron at Fermilab (Chicago): 1000 Ge. V = 1 Te. V protons vs. antiprotons. Discovered the top quark, the last missing quark. • LHC at CERN (Geneva): 7 Te. V protons vs. protons. Discovered the Higgs boson, the last missing particle of the Standard Model. Fri. Dec 3 Phy 107 Lecture 34

Fermilab Fri. Dec 3 Phy 107 Lecture 34

CERN LHC (large hadron collider) Highest energy worldwide. Found the Higgs boson. Still looking for supersymmetric particles and candidates for dark matter. 27 km Fri. Dec 3 Phy 107 Lecture 34

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 particles produced at high energy is enormous. There is so much information that most of the data have to be preselected automatically. A “trigger” committee decides on the algorithm for that. The number of scientists in a collaboration is reaching 3000 at the LHC. Fri. Dec 3 Phy 107 Lecture 34

Fri. Dec 3 Lecture 34 LHCPhy 107 Detector

One high energy event Fri. Dec 3 Phy 107 Lecture 34

Cosmic rays (mainly protons) produce 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. Fri. Dec 3 Phy 107 Lecture 34

The Auger cosmic ray detector in Argentina Cosmic rays are observed with energies of more than 1020 e. V, 100 million times greater than the energy reached with our accelerators. This is the energy of a 90 mph tennis ball compressed 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). Fri. Dec 3 Phy 107 Lecture 34

One of 1600 ground detectors Fri. Dec 3 Phy 107 Lecture 34

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 Ge. V), while neutrinos from the Sun and from radioactivity have only energies in the Me. V 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. Fri. Dec 3 Phy 107 Lecture 34

The South Pole Fri. Dec 3 Phy 107 Lecture 34

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. Fri. Dec 3 Phy 107 Lecture 34

A photon detector Fri. Dec 3 Phy 107 Lecture 34

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 e. V Cosmic rays: 3 1020 e. V The LHC: 7 1012 e. V It has been estimated that one would need an accelerator the size of the universe to reach the Planck energy. Fri. Dec 3 Phy 107 Lecture 34

Particle accelerators are time machines Make the particle energy equal to thermal energy soon after the Big Bang. Atoms form Fri. Dec 3 Phy 107 Lecture 34 400 000 Years