Transverse polarization in deep inelastic collisions

An introduction to transverse polarization phenomena in deep inelastic processes is given. A transversely polarized state of a massive or massless particle is defined as a coherent superposition of different helicity states; for a quark, it corresponds to transverse spin; for a gluon, to linear polarization. A simple toy model shows that quarks could have a sizeable transverse polarization in a transversely polarized baryon. A transverse polarization asymmetry corresponds to the exchange of a parton-antiparton pair of nonzero total helicity in the t-channel of the unitarity diagram. Conservation of t-channel helicity is used to select experiments which are sensitive to transverse polarization to leading order in αs and 1/Q. The transversely polarized quark and gluon densities have independent evolutions with Q2