PHOTON STRUCTURE a

The photon is one of the fundamental gauge bosons of the Standard Model without self-couplings and without intrinsic structure. It couples to any kind of charged particle, which allows it to fluctuate directly into fermion-antifermion pairs and into bound states, vector mesons, which have the same spin-parity (J = 1−−) as the photon. Photon-photon interactions therefore become possible through these quantum fluctuations. At low energies photon-photon interactions can be studied e.g. in Delbrück scattering (elastic scattering of photons in the electric field of atoms) or in the elastic scattering process γγ → γγ where existing experimental limits using lasers are still 18 orders of magnitude above QED predictions . The knowledge of hadronic vacuum polarisations also still give the largest systematic error in the QED predictions for g-2 and for the running of the QED coupling constant . The electron and positron beams at LEP and HERA can also be viewed as a copious source of high energy quasi-real and virtual photons. Interactions of photons are the hadronic processes with the largest cross-section at these experiments. At these high energies fluctuation times are longer than typical hadronic interaction time allowing the photon to develop a ‘structure’. The interactions of high energy photons can be described using structure functions and parton distributions of the photon in analogy to the interactions of real hadrons like the proton. Why is it interesting to study photon structure ? Of course, it is simply important to understand the high energy scattering of a fundamental particle like the photon. Photon interactions are also sensitive to the quantum structure of the theory and can therefore be used to study QED and QCD. We can ask questions like: How similar to the proton is the photon ? How are the quarks and gluons distributed in the photon ? What happens if the photon becomes virtual ? At future linear colliders it will be very important to understand the