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4.1.4 Hadron Spectroscopy
Recent Achievements
and Hot Topics
Hadron spectroscopy has been and still is a vital but
also mysterious field of QCD. Its fundamental degrees
of freedom, the quarks and the gluons, have never been
observed in isolation, but appear only in colour-neutral
states, the hadrons. Colour neutrality seems to be a very
restrictive criterion. Most simply it can be achieved by
combining one quark and one antiquark to form a meson
or three quarks to form a baryon. However, there is a
variety of other ways to satisfy the principle of colourneutrality,
such as tetra- and pentaquark sytems. The
former are mesons made from two quarks and two antiquarks,
while the latter are composed of four quarks and
one antiquark. In addition, due to their self-interaction,
the gluons can form bound states by themselves, the socalled
glueballs. Colour-neutral combinations of quarks
with gluons contributing to the overall properties are
called hybrids.
Both glueballs and hybrids can have quantum numbers
that are not allowed for the simplest qq – or 3-quark
combinations, providing a unique experimental signature.
However, as already pointed out by Dalitz and
collaborators many decades ago, yet another form of
Figure 5. The established charmonium spectrum is shown on the
left side. On the right side newly discovered X and Y states with
unusual properties and as yet unknown nature are plotted.
hadronic bound states can exist, the so-called hadronic
molecules. In the vicinity of a two-particle threshold or
between two close-by thresholds, attractive S-wave
interactions can bind a pair of mesons or a meson and
a baryon to form this type of resonance. Given this rich
set of of possibilities, it is one of the mysteries of QCD
that so far only quark-antiquark and three quark states
have firmly been established.
The question of “exotic” bound states has gained
prominence over the last few years as a result of the
observation of a number of unexpected states in the
charmonium spectrum, many of them close to two-particle
thresholds. In particular the discovery of open- and
hidden-charm mesons with unexpected properties has
challenged our understanding of the QCD bound state
spectrum. First, the charm-strange mesons D * s0 ( 2 317 )
and D s1 (2460) discovered by BaBar did not fit into the
expected quark model spectrum. Various interpretations
as tetraquark states or DK molecules have been
advocated, which can also explain their unusual decay
patterns. Subsequently, a variety of hidden-charm mesons,
nowadays referred to as X, Y, Z states (see Figure 5),
were discovered.
In most cases these states are associated with charmonium
since the decay products contain charm quarks
but their classification is far from obvious. It is not clear
why, despite being above the open-charm threshold,
strong decays into open-charm states are suppressed
and the states decay rather into a charmonium ground
state and light mesons. If some of them were hybrids this
could be an indication of the long lifetime and small width
of gluonic excitations. The states close to two-particle
thresholds are conjectured to be hadronic molecules,
the most prominent of these being the X(3872) which lies
within 0.5 MeV of the D *0 D — 0
threshold and which decays
into J/ψπ + π – and J/ψπ + π – π 0 with similar rates.
The Z + particles must be multiquark states containing
two lighter quarks together with the charm and anticharm
quarks, implying that new combinations of degrees of
freedom in the strong interaction have started to show
up. Current experiments cannot add much more information,
because the observed particles are produced
in a decay chain and so, for example, the knowledge
of their widths is limited by detector resolution. Where
tested, these states couple strongly to antiproton-proton
annihilations, as shown by results from experiments at
LEAR and also at Fermilab experiments. In most cases,
limited detector resolution will not be an issue at FAIR
due to the possibility of directly scanning the resonances
with a high-precision antiproton beam. This should allow
PANDA to clarify the nature of the X, Y and Z states.
The search for glueballs and hybrids in the mass
region where only the lightest quarks play a role has so
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 69