682 research outputs found
Key features of the TMD soft-factor structure
We show that the geometry of the Wilson lines, entering the operator
definition of the transverse-momentum dependent parton distributions and that
of the soft factor, follows from the kinematics of the underlying physical
process in conjunction with the gauge invariance of the QCD Lagrangian. We
demonstrate our method in terms of concrete examples and determine the paths of
the associated Wilson lines. The validation of the factorization theorem in our
approach is postponed to future work.Comment: 10 pages, 2 figures. Invited contribution presented by the first
author at the Lightcone 2013+ Conference, Skiathos, Greece, 20-24 May, 2013.
Matches version to appear in Few Body System
Tagging the pion quark structure in QCD
We combine the constraints on the pion quark structure available from
perturbative QCD, nonperturbative QCD (nonlocal QCD sum rules and light cone
sum rules) with the analysis of current data on F_{\pi\gamma\gamma^*}(Q^2),
including recent high-precision lattice calculations of the second moment of
the pion's distribution amplitude. We supplement these constraints with those
extracted from the renormalon approach by means of the twist-four contributions
to the pion distribution amplitude in order to further increase stability with
respect to related theoretical uncertainties. We show which regions in the
space of the first two non-trivial Gegenbauer coefficients a_2 and a_4 of all
these constraints overlap, tagging this way the pion structure to the highest
degree possible at present.Comment: V1: 6 pages, 2 figures, 1 table. V2: Two references added with
corresponding insertions in the text. Matches version published in PR
Polarized and unpolarized -pair meson-induced Drell--Yan production and the pion distribution amplitude
We present a detailed analysis of meson-induced massive lepton (muon)
Drell--Yan production for the process , considering
both an unpolarized nucleon target and longitudinally polarized protons. Using
a QCD framework, we focus on the angular distribution of , which is
sensitive to the shape of the pion distribution amplitude, the goal being to
test corresponding results against available experimental data. Predictions are
made, employing various pion distribution amplitudes, for the azimuthal angle
dependence of the distribution in the polarized case, relevant for
the planned COMPASS experiment. QCD evolution is given particular attention in
both considered cases.Comment: V1: 18 pages, 10 figures (48 eps files), 2 tables. V2: one wrong
reference removed, missing reference in text inserted; matches version to
appear in Phys. Rev.
Improved estimates of the pion-photon transition form factor in the ~GeV range and their theoretical uncertainties
We consider the pion-photon transition form factor at low to intermediate
spacelike momenta within the theoretical framework of light-cone sum rules. We
derive predictions which take into account all currently known contributions
stemming from QCD perturbation theory up to the next-to-next-to-leading order
(NNLO) and by including all twist terms up to order six. In order to enable a
more detailed comparison with forthcoming high-precision data, we also estimate
the main systematic theoretical uncertainties, stemming from various sources,
and discuss their influence on the calculations --- in particular the dominant
one related to the still uncalculated part of the NNLO contribution. The
analysis addresses, in broad terms, also the role of the twist-two pion
distribution amplitude derived with different approaches.Comment: 10 pages, 3 figures, Based on the talk given at the XIIth
International Conference "Quark Confinement and the Hadronic Spectrum"
(Thessaloniki, Greece, 28 August - 4 September 2016
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