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Foundations of Perturbative QCD by John Collins

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13

TMD factorization

An appealing interpretation of a parton density is that it is a number density of partons in a target hadron. As we saw in Sec. 6.7, a parton density in a simple theory is an expectation value of a light-front number operator, integrated over transverse momentum. A similar interpretation applies to fragmentation functions: Sec. 12.4.

As explained in Secs. 6.8 and 12.4, it is equally natural to define unintegrated, or transverse-momentum-dependent (TMD), parton densities and fragmentation functions, simply by omitting the integral over transverse momentum. In a sense, the TMD functions are more fundamental and present more information on non-perturbative phenomena than do the ordinary integrated functions. Therefore it is ...

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