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

4.1 Hadron Physics
4.1.1 Introduction
During the last century, Maxwell’s electro-dynamics
was combined with quantum theory into a field theory
called quantum electrodynamics (QED). As a host of
new particles were discovered experimentally, the
determination of their properties helped to develop the
theoretical framework further and there emerged a common
understanding of the weak and the electromagnetic
interactions. Built on these successes, the theory of the
strong interaction, Quantum Chromodynamics (QCD),
was developed on the exact colour symmetry SU(3) for
the fundamental particles called quarks and the carriers
of the strong force, the gluons. The Standard Model of
particle physics was born.
In the Standard Model, all forces or interactions show
basically the same behaviour, with a force law proportional
to the inverse-square of distance. The proper sets
of theories are called gauge theories. At this point, one
can ask the question: where does hadron physics enter
into this framework and what specific role does it play
in our understanding of physics
While high-energy physics works to identify the fundamental
particles of nature, hadron physics seeks to
understand the nature of composite particles, i.e., those
composed of the fundamental quarks and gluons. In fact,
it is only such composite particles that are observed in
nature; the quarks and gluons themselves have never
been seen in isolation. Two or three quarks together
form a hadron, which therefore represents the first level
of complexity in nature. The main problem of hadron
physics is to understand this complexity in terms of
elementary degrees of freedom – quarks and gluons –
and their underlying non-abelian gauge theory of QCD.
This problem is of utmost importance to the whole of
contemporary physics, since on the one hand it paves
the road to an ab initio understanding of even more complex
strongly interacting systems such as nuclei, while on
the other hand, it pushes forward the precision frontier
in high-energy physics, where analyses in many cases
are limited by the hadron physics input. Even the purely
mathematical implications of the challenge posed by
hadron physics are remarkable and worthy of one of the
Millennium Prizes put forward by the Clay Institute.
When we try to understand strongly interacting composite
particles at rest or at lower energies, it seems that
the relevant degrees of freedom of the QCD Lagrangian
are not the relevant ones for hadrons. We have two different
pictures or scenarios evolving, depending on the
scale under scrutiny:
The perturbative regime is explored in high-momentum
processes such as deep-inelastic scattering (DIS) of
leptons on hadrons. Here the observable properties are
directly related to the degrees of freedom that appear
in the QCD Lagrangian (so-called current quarks and
gluons). The momentum distributions of these particles,
known as partons, within hadrons have been determined
in great detail. It should be stressed, though, that at
very low Bjørken-x” and high virtuality, gluon saturation
sets in, forming the non-perturbative colour glass
condensate.
The nonperturbative regime is relevant to the structure
and interactions of hadrons at low momenta. The
degrees of freedom needed to describe low-energy
experiments, and hence quantities such as charge radii
or magnetic moments, are not current quarks and gluons.
Here first principles calculations can be performed with
numerical simulations of QCD on a space-time lattice.
Also, effective field theories based on hadron degrees
of freedom can provide import information on how the
symmetries of QCD constrain the interactions of these
particles. In many cases, models based on “constituent”
quarks with phenomenological masses can give better
descriptions than one might expect, but these still lack
any microscopic connection to the underlying theory
of QCD. Detailed experiments on the spectroscopy of
hadrons are needed to reveal the appropriate degrees
of freedom.
Understanding the physics of hadrons requires a large
variety of complementary experiments and theoretical
tools. On the experimental side, electromagnetic and
hadronic probes can be used to investigate various
aspects of hadron structure and dynamics at different
length scales. Here, the antiproton programme at
the upcoming FAIR facility will play a very prominent
role for the physics with charm and light quarks. On the
theoretical side, recent progress in lattice simulations
and effective field theories (or combinations thereof) has
allowed ab initio calculations of hadron properties and
interactions for the first time. The particularly important
interplay between experiment and theory has already led
to much progress in the field and will also play a central
role in the future. Furthermore, many methods developed
originally in hadron physics can be applied fruitfully in
other fields of physics, leading to connections between
very different areas of research.
After an introduction to the basic properties of QCD,
the theory underlying all of hadron physics, this chapter
is subsequently split into three topics that emerge
naturally in modern hadron physics: hadron structure,
hadron spectroscopy and hadronic interactions. At the
end, we present a list of recommendations for the future
development of the field of hadron physics.
60 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010