Author: Jean-Marc Richard
Title: Positivity constraints on spin observables
Abstract:
Spin observables are very useful to improve our insight into the microsocopic dynamics of exclusive and inclusive reactions, and parton distributions.
Each spin observable is typically normalised to vary between -1 and +1. Pairs of observables of the same reaction are often constrained to a sub-domain of the square [-1,+1]^2, such as the unit disk or a triangle. For triples of observables, the domain inside the cube [-1,+1]^3 can assume a variety of shapes: sphere, cone, pyramid, tetrahedron, etc.
The corresponding inequalities can be derived by purely algebraic methods, or from the positivity of the density matrices describing the initial or final spin states of the reaction and its crossed channels.
Examples will be given for some inclusive and exclusive reactions, in
particular the photoproduction of pseudoscalar mesons.
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