Perspectives of Nuclear Physics in Europe - European Science ...

4.1 Hadron Physics
and the ability to detect the hadronic decay modes.
The FAIR antiproton facility and PANDA detector were
designed with such requirements in mind. Since this
facility is capable of addressing the hot topics for meson
spectroscopy, exotics and glueballs, and also the study
of charmed baryons, the highest priority should be to
build and develop the HESR accelerator and the PANDA
detector with the proposed specifications.
A second important thread in hadron spectroscopy is
the study of baryon resonances using electromagnetic
excitation. Here, a major technical is the availability of
polarised targets at facilities where they can be combined
with polarised beams and detectors that can
measure recoil polarization. Measurements of complete
sets of spin photo-production amplitudes in the region
of specific baryon resonances have started at ELSA
and JLab and will begin soon at MAMI. This accumulation
of new data must be accompanied by intensive
analysis efforts. Models developed at Bonn, Giessen,
GWU, Jülich, JLab, Mainz, and other places are being
continuously improved and extended for this. Multichannel
methods can provide a comprehensive view
of very different reactions and offer the best chance of
discovering new states.
There is also considerable progress on the horizon
for ab initio LQCD calculations of excited hadrons.
Resonances are much harder than ground states to
study in these simulations since they can decay and thus
much larger Fock spaces are needed to describe them.
The study of excited states on the lattice is therefore a
formidable task, which requires small pion masses so
that resonances indeed decay, and large volumes, since
resonances leave their mark in the volume dependence
of energy levels. Furthermore, the effects of channel
coupling must be accounted for, which calls for new
theoretical tools (such as multi-channel extensions of
the Lüscher formalism). All this demands large investments
in high-performance computing and in manpower.
The potential pay-off is, however, tremendous, both in
terms of detailed information on the wave functions of
resonances, and also for understanding the emergence
of patterns in the spectrum controlled by the symmetries
of QCD.
Effective field theories (EFTs) will also have to be developed
further to get a deeper understanding of how QCD
can give rise to weakly bound states such as hadronic
molecules. These theories can provide a systematic
framework for clarifying the origin of states such as the
X(3872) and determining the parameters controlling them.
This will form a vital complement to LQCD studies, as
loosely bound -- and thus very extended -- states are
likely to be beyond the reach of lattice methods for a
long time.
In addition, EFTs incorporating chiral, isospin and
heavy-quark symmetries should be developed beyond
leading order to elucidate the inner workings of
heavy-light systems. Similarly, unitarisation and coupledchannel
approaches need to be refined and applied to
as much data as possible from a range of very different
processes including both hadron and electromagnetic
production.
The interplay between experiment and theory in
hadron physics is of particular importance for the field.
It has led to much progress already and will continue
to play an important role in the future. The success of
the new generation of experiments studying the fundamental
spectroscopy of hadrons depends critically on
the robust information that can be extracted out of the
data. These experiments, with unprecedented statistics,
excellent resolution and measurements of polarization
observables, demand an unrivalled degree of detail in
theoretical understanding if new discoveries and insights
into the hadron spectrum are to result. A close collaboration
between theory and experiment seems therefore
mandatory and has in many cases already been established
inside the experimental collaborations. Also, large
theoretical networks like the “hadron physics theory” in
the EU FP6 and “QCDnet” in the FP7 have led to much
new and fruitful collaboration.
European Perspective
As a result of its dedicated accelerators for hadron physics,
Europe has taken the leading role in this field. Results
from experiments using hadron beams and from others
with lepton or photon beams often give complimentary
information which is essential for understanding the
hadron spectrum. Most of the detector systems at the
existing facilties have been upgraded and are now capable
of taking data with the necessary precision. The next
step forward will require the HESR antiproton facility at
FAIR and the PANDA experiment to be completed in a
timely fashion and according to specifications. In the
meantime, to ensure continuity in this challenging field,
it is absolutely necessary that dedicated programmes
at ELSA, MAMI and COSY should continue to produce
world-class data in their energy domains.
In support of these experimental efforts, European
physicists are also playing leading roles in the theoretical
aspects of hadron physics. The groups working in
the most prominent domains, effective field theories
and lattice gauge calculations, are highly respected and
their track record of innovative developments should
guarantee progress in this field.
72 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010