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 Emeritat
 Emeritat
QCD Spin Physics
Our knowledge of the nucleon spin structure has greatly improved over the last twenty
years or so, but still many fundamental questions remain unsolved. This means that one
has to fully understand it, in terms of the basic partonic constituents of the nucleon,
and simultaneously, the spin sector of the underlying dynamical theory, i .e. perturbative
QCD, must be tested at the highest possible precision level.
Our current activities in QCD spin physics involve, in particular, the phenomenological
description of parton distributions in the framework of a statistical approach, which
was first proposed in 2002. It can be applied equally well to unpolarized and polarized
data, a rather unique situation, and it is valid in a wide kinematical range in x and
Q2. It has, so far, a very good predictive power, for both Deep Inelastic Scattering and
hadronic collisions recent results. The transverse momentum dependence of the parton
distributions has been also included, because its relevance has been realized
recently, in connection with transverse spin effects and orbital angular momentum contributions.
We are also closely associate to the physics programme
of the polarized pp collider at RHIC-BNL, operating since 2001, for the measurement
of double helicity and single transverse spin asymmetries. It will soon reach
the c.m. energy of 500GeV, allowing the gauge bosons W± and Z production and the measurement
of the parity-violating asymmetries. Finally, let us mention a related topic, namely the
systematic study of positivity conditions for spin observables, which reduces the allowed
domain of values, as first observed in 1995 for the case of the quark transversity distributions
(see Physics Reports 470,1-92 (2009)).
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