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4.1 Hadron Physics
Figure 6. The glueball spectrum as predicted by lattice
calculations. Exotic quantum numbers are indicated by colour.
far been inconclusive. An obvious problem is the sheer
number of states, often overlapping in width and mixing,
which makes their identification and classification difficult.
Therefore the glueball candidate f 0 (1500), although
experimentally clearly seen, cannot be unambiguously
identified as such. While lattice simulations can make
crisp predictions for the glueball spectrum in the puregauge
theory without quarks, the mixing of states in the
presence of light quarks remains to be a challenge. In
terms of hybrid searches, two states, the π 1 (1400) and
the π 1 (1600) have been seen by several experiments.
Even though no firm claim about their hybrid character
can be made, the exotic quantum numbers show that
states with degrees of freedom beyond a quark-antiquark
pair are now appearing in meson spectroscopy. More
progress in this field is also expected from experiments
with hadron beams, in particular the meson spectroscopy
programme with COMPASS at CERN.
There has also been considerable progress in the
spectroscopy of baryons. The constituent quark model
was accepted for many years as the starting point for
this field. It can explain much of the regularity in the
spectrum but it also has some obvious failures. The
most obvious one is the different ordering of the positive
and negative parity states, in particular the difficulty in
reproducing the low masses of the N*(1440) and Λ(1405)
resonances. A second puzzle is the effective suppression
of the expected large spin-orbit interaction required to
fit the baryon spectrum.
With the advent of high-duty-factor electron machines
(such as ELSA, JLab and MAMI), baryon resonances with
higher masses can be produced. A striking result is that
the constituent quark model predicts far more states than
are actually observed for masses larger than 1.9 GeV.
Either these “missing resonances”’ are not observed,
because they couple primarily to channels (for example
two-meson- or vector-meson-production) which have not
been well-studied experimentally or the quark model is
not a complete description and other degrees of freedom
might become relevant.
A further complication is that certain resonances
might be generated as meson-baryon molecules through
channel couplings and threshold effects. A particular
well-studied example is the hard-to-model Λ(1405) that
has been suggested might be a superposition of two
distinct S-matrix poles. Reaction calculations need to
be confronted with precision experiments to test such
scenarios.
At high energies, charmed baryons can be produced
and offer the possibility to study pairs of light quarks
in the presence of a heavy quark. The ground-state
charmed baryon is now well established and a series
of excited states has been reported although determinations
of their spin-parities require further investigations.
Doubly charmed baryons remain a promise for the future
(with the exception of one candidate Ξ + cc
reported at
3519 MeV by the SELEX experiment at Fermilab, but
unconfirmed by BaBar or BELLE).
Recent theoretical studies of baryons have focused
largely on LQCD and effective models based on hadronic
degrees of freedom. In lattice simulations, significant
progress has been made in understanding the masses
of the ground-state octet and decuplet baryons. The
key issues for baryon excitations are finding a suitable
basis of operators that allows the various excitations in a
given channel to be identified and developing methods to
extract the width of these instable states. Much progress
has also been made in describing the spectrum and
dynamics of excited baryonic states through coupledchannel
studies based on effective hadronic Lagrangians
constrained by QCD symmetries. These approaches
are still the main tools for analysing the tremendous
amount of new data generated at JLab, ELSA, MAMI
and SPring-8. Other theoretical approaches to describing
baryons include quark-diquark models based on the
Bethe-Salpeter equation and the string-theory-motivated
AdS/QCD model, describing strong interactions in terms
of a dual gravitational theory.
70 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010