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A collider is a machine used for research in particle physics. A collider uses electromagnetic fields to accelerate charged elementary particles either in a ring or in a straight line in particle beams that collide either by two beams head-on against each other or by one beam against a stationary target.


  • The discipline of collider physics involves going from the direct collider observables to the underlying lagrangian of the theory. One of the simplest questions one can ask is how to recognize the presence of new particles. In colliders the answer to this question is simple, one collects groups of particles (pairs, for example) and one plots the invariant mass. If a bump is seen in this distribution, one says that there is a new particle. One can also look at the angular distribution of the particles and read off the spin of the new particle.
  • Very high energy collisions occur naturally in cosmic ray interactions; they also occurred in the early moments of our universe according to big-bang cosmology. Both these sources provide useful information but they cannot compare with systematic experimentations in accelerator laboratories when this is possible.
  • Stephen Hawking and I wrote an essay about future colliders that is relevant to both the CEPC and the FCC. We were encouraged by others to chime in because of discussions that arose in China about physics and economic and cultural issues surrounding building a future collider. The theoretical arguments for building a larger collider with several times the energy of the LHC are thus very strong, particularly in regard to solving the hierarchy problem. What would happen without data? Someone may get or even already have the solution, but no one will be convinced. With data pointing to the solution, we may be able to move on and obtain consensus about a comprehensive theory that incorporates the standard models of particle physics and cosmology and a quantum theory of general relativity, giving us a profound understanding of our universe.
  • There is virtually no chance that we will be able to do experiments involving processes at particle energies like 1016 GeV. With current technology the diameter of an accelerator is proportional to the energy given to the accelerated particles. To accelerate particles to an energy of 1016 GeV would require an accelerator a few light-years across.

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