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QCD forms the foundation of nuclear
physics, and our group has had high visibility in the community that
strives to find implications of quark-gluon dynamics in nuclei. The
starting point to tackle QCD at nuclear scales is quantum field theory
in the non-perturbative regime. In asymptotic theory, it is known that
non-perturbative phenomena can be linked to the behavior of the
associated perturbative series, but it is much less understood how
this link may be performed in quantum field theory. We have studied this
link in simple models by using the recent mathematical techniques of
hyperasymptotic theory. In the more realistic case of QCD, a difficult
but essential constraint to implement is color gauge invariance. Some of
our research has in fact focused on a gauge-invariant formulation of
quark dynamics. Gauge-invariant quark Green’s functions are being
studied in the limit of a large number of colors and might explain the
non-observability of free quarks. Despite such progress, the most
successful method to calculate properties of the QCD spectrum and
interactions at large distances continues to be simulations on a
Euclidean space-time lattice.

QCD simplifies in the heavy quark limit,
where quarks are non-relativistic and an approximate global symmetry
related to flavor and spin emerges. The large energy scale introduced by
the heavy quark mass also makes perturbative computations possible. We
have developed an in-depth understanding of quarkonium production in
proton-proton collisions at RHIC, the Tevatron and the LHC. A
fixed-target experiment (AFTER) using the LHC proton and nuclear beams
extracted by a bent crystal has been proposed, which would result in a
significant increase of the heavy-flavor and quarkonium production
yields compared

to RHIC.

In nuclear physics it is the light
quarks that play a dominant role, and another approximate symmetry of
QCD, chiral symmetry, becomes important. It manifests itself through the
interactions of the pseudo-Goldstone bosons of its spontaneous breaking
(pions, kaons and eta). Much of the effort of the group has been
dedicated to the dynamics of pions at both quark and hadronic levels. At the quark
level, we have proposed a general framework for modeling
nucleon-to-pion transition distribution amplitudes, an extension of the
concept of generalized parton distributions, and extracting them from
data.

At much lower energies, that is, momenta
comparable to the pion mass, an effective field theory (EFT) — Chiral
Perturbation Theory (ChPT) — exists that systematically describes
hadronic reactions in a controlled expansion in momenta. The available
data on the strong and electromagnetic width of the Delta resonance was
used at third order in the expansion to provide parameter-free
predictions for various aspects of the spin structure of the nucleon,
such as polarizabilities and moments of structure functions. The extension of ChPT to higher energies can
be accomplished with the use of dispersion relations. We have carried
out a dispersive analysis of the scalar and vector form factors of the
long-lived kaon state using the KTeV data on its decays into pions and
leptons, testing predictions of the Standard Model. Particularly
important is the s-wave pion-pion interaction, where structure
associated with a light scalar meson appears at relatively low energies.
We have used chiral constraints in the Omnes method to extract
information about light scalars, work that has shown how a systematic
application of analyticity allows the extraction of the couplings of
scalar mesons to quark-gluon operators from experimental data on the
scattering on pseudo-Goldstone mesons. In addition, we have developed
the tools to disentangle the final-state interaction from the Belle data
on two-photon fusion into two pions.

Hadron dynamics can in principle be extracted from lattice
QCD calculations as well as experimental data. EFT can be used as a
tool to extrapolate lattice results to quark masses too low to be
handled in such simulations, and as a result EFT low-energy constants
can be determined. In the last few years we have looked at a number of
processes where EFT can be matched to lattice data, with an emphasis on
the properties of resonances and on the convergence of the theory in the
strangeness sector.

Institut de Physique Nucléaire Orsay - 15 rue Georges CLEMENCEAU - 91406 ORSAY (FRANCE) |
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