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

opportunity to determine the moduli of the complex FFs
in the time-like domain over a wide range of momentum
transfers from antiproton annihilation reactions, with
expected statistical errors 20 to 50 times smaller than
those on present data.
In addition to the measurement of electromagnetic FFs
there has been an extended experimental programme
on measuring the weak form factors of the nucleon by
parity violating electron scattering experiments at JLab
and at MAMI. Due to the electroweak mixing, a measurement
of parity violating asymmetries of order parts
per million allows a very clean study of the contribution
of the strange sea quarks to the electromagnetic FFs
of the nucleon.
Parton Distributions
The internal quark-gluon structure of hadrons, which
is accessed in inclusive DIS of high-energy leptons off
nucleons, is encoded in a well-defined hierarchy of correlation
functions. The simples of these are the unpolarised
and polarised parton distribution functions (PDFs), which
give the number density of partons of type q inside a
proton, carrying a momentum fraction x. Similar information,
although less detailed, has been obtained about
the number density of longitudinally polarised partons
inside longitudinally polarised protons, the helicity parton
distribution. The successful prediction of the scale
dependence of the PDFs has been one of the great triumphs
of QCD.
Figure 3. Gluon polarization results from SMC, HERMES, and
COMPASS, in comparison with theoretical fits
A long-standing puzzle is the fact that only about one
third of the spin of the proton is carried by the spins of
the quarks. This has been addressed in several ways over
the last decade. One suggestion is that the quark spin
contribution is masked by a very large gluon polarization,
contributing via the axial anomaly. One way to test
this is to measure production of single hadrons or hadron
pairs in semi-inclusive DIS, which provides access
to the gluon polarization through photon-gluon fusion.
Recent data from COMPASS and HERMES, shown in
Figure 3, point to a rather small value for this polarization,
at least in the accessible range of x, now making
this scenario unlikely.
The measurement of open charm production instead
of that of light hadrons in DIS reduces the model dependence
of the analysis due to the absence of charmed
quarks in the nucleon. Recent data from COMPASS allow
a first determination of the contribution to the nucleon’s
spin from the gluon helicity distribution, also shown in
Figure 3. The data for this quantity are compatible with
zero, however the errors are still large and the accessed
x-range is limited.
Recent data from semi-inclusive DIS at HERMES point
towards a substantially smaller polarised strange quark
sea than previously assumed, but this is still controversial
in relation to global fits to DIS data.
Transverse-Momentum-Dependent
Parton Distributions
A fast moving proton can be viewed as a bunch of collinearly
moving quarks and gluons. Its inclusive reactions
can provide only limited information on the relative
motion of these partons. More details of this intrinsic
motion are encoded in the tyransverse-momentumdependent
distribution functions (TMDs). These include
spin-dependent correlation functions that link the parton
spin to the parent proton spin and to the parton intrinsic
motion. The 8 leading-twist, parity invariant TMDs
include the unpolarized, the helicity and the transversity
distributions, which are the only ones to survive
in the collinear limit. Similar correlations between spin
and transverse motion can occur in the fragmentation
process of a transversely polarised quark into a (noncollinear)
hadron. The fragmentation function gives the
number density of hadrons resulting from the hadronisation
of a parton.
A systematic attempt to study TMDs started a decade
ago, with both dedicated experiments and new theoretical
ideas. In this context, the crucial innovation is the
study of physical observables that are sensitive to the
transverse-momentum-dependent distributions. These
are sensitive to the transverse polarisation structure of
the nucleon and can be extracted from semi-inclusive
deep-inelastic scattering (SIDIS). The measured hadron
in the process results from fragmentation of a struck
quark and “remembers” the original transverse motion of
that quark. New information on this is obtained by analysing
the cross-section data as convolutions of TMDs
and fragmentation functions.
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 65