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coupling can be applied. A key tool to extract hadron
structure information in this non-perturbative regime
is the property of factorisation. It makes it possible to
separate cleanly the high-momentum (perturbative) and
the low-momentum (non-perturbative) aspects of the
interaction.
In this way, hard reactions such as inclusive deepinelastic
scattering (DIS), semi-inclusive DIS, or hard
exclusive reactions are routinely used nowadays as
probes of hadron structure. It is the hard scale involved
in these processes, which allows a perturbative QCD
description of the reaction and which selects well-defined
operators in terms of quarks or gluons. By measuring the
matrix elements of such operators in the hadron, we can
then extract the soft part of the amplitude, which lies in
the realm of non-perturbative QCD. Depending on the
operator selected, key questions on hardon structure
can be addressed, such as:
• How are quarks spatially distributed inside the proton
• Can we understand the energy-momentum and angular-momentum
dependent aspects of the structure of
hadrons from first principles
• Is there a connection between orbital motion of quarks
and gluons, their spin and the spin of the proton
• How does the internal structure of baryons and
mesons emerge from the dynamics of quarks and
gluons
• What are the contributions of different quark flavours
to hadron structure
Recent Achievements,
Current State-of-the-Art
To address the above questions, hadrons are explored
experimentally by studying their response to high-precision
probes at various energies. Electroweak probes
offer versatile and well-understood tools to access the
internal quark-gluon structure of hadrons. With the relatively
weak coupling strength of the electromagnetic
inter action, resulting dominantly in one photon exchange,
a clean separation of the probe and the system under
investigation is possible. This is true for processes with
photons or charged leptons in the initial or in the final
state. Typical examples are electron scattering or the
observation of lepton pairs from decays or Drell-Yan type
processes. Equally important is the fact that the virtuality
of the exchanged photon can be tuned. This allows
the spatial structure of hadrons to be resolved and the
distributions of their constituents to be studied.
Figure 2. Different projectile energies test different properties of the nucleon structure.
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 63