MadSci Network: Physics

Re: Has photon-photon scattering been observed.

Date: Wed Feb 24 13:23:56 1999
Posted By: Samuel Silverstein, faculty, physics, Stockholm University
Area of science: Physics
ID: 919290652.Ph


Two-photon scattering processes are regularly observed at high energy particle colliders which study electroweak interactions. Charged particles (electron-positron, electron-proton, etc...) are brought together in head-on collisions inside large detectors. Two-photon scattering is observed at such facilities when the approaching particles both emit photons, which then interact with each other via a charged fermion/antifermion pair.

Notice that I said "fermion", and not just "electron". A photon can fluctuate into any pair of charged fermions, either leptons or quarks! That makes the photon very exciting for particle physicists, because it means that if you probe the photon at high enough energies, you can find any sort of charged fermion at some level. The relative "abundance" of different particles "inside" the photon at different interaction energies is often referred to as the photon "structure".

One facility which is actively studying two-photon physics is the Large Electron Positron collider (LEP) at the CERN laboratory outside Geneva, Switzerland. I found this nice tutorial on two-photon physics which was put together by a researcher on OPAL, one of the four detectors on the LEP ring. The HERA electron-proton collider in Hamburg, Germany has also produced important photon structure function results. For more references than you would ever desire, try searching through the SLAC SPIRES preprint database, looking for titles with "Photon Structure" in them.

Current particle colliders rely on chance for two charged particles to create a photon-photon collision. However, plans are underway for the next generation of high-energy particle colliders to include electron-photon and photon-photon collisions! This will be done by firing two beams of electrons towards each other. About a centimeter away from the interaction point, a pulse of light from a high-powered laser meets the electrons head-on. The electrons transfer their energy to the photons, which in turn collide at the interaction point at extremely high energies. Such photon-photon colliders will give us the opportunity to observe collisions between many different combinations of quarks and leptons which are not possible at current facilities. Here is a nice web page with information on one proposed two-photon collider.

I hope this helps. Please send me an email at if you need more information. Cheers, Sam Silverstein

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