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Spin and Orbital Angular Momentum of Quarks and Gluons in the Nucleon
A major controversy has arisen in QCD as to how to split the total angular momentum of a nucleon into separate quark and gluon components; an issue of importance for understanding the internal structure of the nucleon. Deep questions have arisen as to how precisely to define these components and as to their measurability. Underlying this are issues about the role of gauge invariance and the validity of the age old textbook claim that it is impossible to separate the photon (gluon) angular momentum into a spin part and an orbital part in a gauge-invariant way. This is a matter of concern to the many experimentalists who believe they are measuring the polarization of the gluon in a polarized nucleon! There have been a host of papers on this subject in the last few years.
Controversial arguments have been given as to whether a physically measurable quantity really needs to correspond to a gauge-invariant operator. New sum rules have been derived and novel suggestions have been made for measuring some of the terms in them, either experimentally or on a lattice. And it has turned out that orbital angular momentum is now recognized as a key concept in hadron structure and in the theory of high momentum transfer polarized reactions. We believe it would be very fruitful to have a high level discussion of
these issues at this point.
|Mauro Anselmino||University of Turin (I)|
|E. Leader||Imperial College|
|C. Lorcé||IPN Orsay & Univ. Liège|