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Elementary Particles

Unification Theories

strange quark, antiparticles, Mesons, antiprotons, positrons

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>  Grand Unified Theories and Beyond

When energetic particles collide, the energy released in the collision can convert to matter and produce new particles. The more energy produced in the collision, the heavier the new particles can be. Particle accelerators produce heavier elementary particles by accelerating beams of electrons, protons, or their antiparticles to very high energies. Once the accelerated particles reach the desired energy, scientists steer them into a collision. The particles can collide with a stationary object (in a fixed target experiment) or with another beam of accelerated particles (in a collider experiment).

Particle accelerators come in two basic types—linear accelerators and circular accelerators. Devices that accelerate particles in a straight line are called linear accelerators. They use electric fields to speed up charged particles. Traditional (not flat screen) television sets and computer monitors use this method to accelerate electrons toward the screen (Television: Picture Tube). Linear accelerators have two main uses: They can produce a beam of particles for a fixed target experiment, or they can feed particles into a circular accelerator.

Circular accelerators, or synchrotrons (pronounced SIN-krow-trons), use magnetic fields to accelerate charged particles in a circle. The particles can circle many times, gaining energy each time they travel around the circle. Thus synchrotrons can accelerate particles to extremely high energies. Synchrotrons can be used in fixed target experiments, or they can accelerate two beams simultaneously for use in a collider experiment.

Positively charged particles bend a different way in a magnetic field than do negatively charged particles, so a synchrotron can accelerate electrons in one direction and positrons in the other. A synchrotron can also accelerate protons in one direction and antiprotons in the other. Scientists are even considering building a synchrotron to accelerate less stable particles, such as muons and antimuons.

Once particles reach the desired energy, experimenters slightly change the magnetic field controlling the particles, bringing the two beams into a collision. The particles and antiparticles annihilate each other. The resulting energy produces numerous other particles for the scientists to study.

Electroweak Unification

Many great discoveries in particle physics have been made by looking to the heavens. The universe is a natural particle accelerator, and particles from outer space continually bombard Earth’s atmosphere. Extraterrestrial particles called cosmic rays—and their collisions with other particles in the atmosphere—produce many unusual and unstable particles. Scientists first discovered the muon and the pion in cosmic rays, as well as the positron. Mesons made up of the strange quark were also first spotted in cosmic ray experiments before modern large accelerator facilities were built.

Neutrinos stream to Earth from cosmic sources. Nuclear reactions in the Sun produce incredibly large numbers of electron neutrinos that can then be detected on Earth. Experiments studying these solar neutrinos suggest that the mass of the neutrino is very small but that it is not zero.

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