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4.1 Hadron Physics
heavy-meson decays, such as D→Kππ at FOCUS, can
provide access to the S-wave πK scattering lengths.
These could thus lead to detailed tests of chiral QCD
dynamics with strange quarks.
Similarly, there are other indications from from both
EFTs and lattice simulations that the strange quark may
not be light enough to be treated using the same tools
as work well for up and down quarks. Using unitarisation
methods, one can extend the range of applicability of
these methods for S-wave meson-meson scattering to
higher energies, at the expense of some of the theoretical
rigor. This will permit, in particular, studies of aspects of
the scalar sector of QCD, including the suppression of the
strange-quark condensate, the dynamics of the OZI rule
violation, and the possible mixing of scalar mesons with
glueballs. There is also rich experimental information from
various laboratories on the decays of D and B mesons
into light-quark multi-meson states. When tackled with
appropriate theoretical tools, these should allow a variety
of precision studies of meson-meson interactions.
There has also been remarkable progress in experimental
and theoretical studies of pion-nucleon and
other meson-baryon interactions. These interactions
can provide a variety of tests of chiral and flavour SU(3)
symmetries, related mostly to the determination of the
S-wave scattering lengths. Moreover, these fundamental
interactions are important as input into the description
of the strong force that binds nucleons and hyperons
in nuclei and hypernuclei. Calculations of pion-nucleon
scattering in chiral perturbation theory and of pionic
hydrogen and deuterium have now matched the superb
precision of the decade-long experiments on these systems
at PSI, see Figure 9.
The situation is much less clear for the interactions of
(anti)kaons with nucleons, which play a similarly important
role in strangeness nuclear physics. The extraction of the
scattering lengths from kaonic hydrogen measured at
DAΦNE is at odds with the values obtained from the analysis
of older scattering data. This poses a fundamental
puzzle that requires further theoretical investigations and
the measurement of kaonic hydrogen and deuterium with
SIDDHARTA. It is also reflected in on-going discussions
about the possible existence of deeply bound kaonic
clusters, states in which an antikaon (K – ) is strongly bound
to several nucleons. Experiments performed with FINUDA
at DAΦNE on K – stopping in light nuclei lead to various
correlated hadron pairs that might be indicative of such
clustering phenomena. Other evidence for such exotic
compounds comes from the reanalysis of older DISTO
data. In this field, there is a particularly strong collaboration
between experimentalists and theorists.
Outstanding progress has been made in the derivation
of nuclear forces from chiral EFT. Although there is still
Figure 9. Determination of the πN scattering lengths from pionic
hydrogen and deuterium.
an ongoing debate about the renormalisation of these
forces and the resulting expansion scheme, in practice
very successful potentials have been constructed using
Weinberg’s original scheme at next-to-next-to-next-toleading
order in the chiral expansion. These potentials
provide an accurate representation of two-body scattering
data and are currently being used as input to
ab initio nuclear structure calculations. It should also
be mentioned that first lattice calculations can reproduce
the main trends of the nuclear interaction, namely
the intermediate-range attraction and the short rangerepulsion.
In addition, EFT provides a consistent framework for
the construction of three- and four-body interactions and
of effective electroweak current operators. Calculations
of light nuclei show that these three-body forces are
essential for an accurate description of their binding
energies and are intimately related to threshold pion production
in proton-proton collisions, which are currently
being studied in experiments at COSY. An appropriately
tailored EFT has been developed and it demonstrates
that a consistent picture can link data on pion production,
on nucleon-deuteron scattering, and on tritium
beta-decay. A particular achievement is the extraction
of the strong contribution to the proton-neutron mass
difference from the isospin-violating forward-backward
asymmetry of np→dπ 0 , which had been measured earlier
at TRIUMF. This shows that we now have the means to
extract fundamental QCD parameters from interactions
involving few nucleons and pions.
These techniques are now being extended to
hyperon-nucleon and hyperon-hyperon interactions.
74 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010