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parameterised in structure functions f, g and h, where f
refers to unpolarised quark distributions, g to longitudinally
polarised quarks and h to transverse polarisation.
In addition, the new generalised parton distributions
(GPDs), which are measured in exclusive reactions, allow
at the same time a correlated tomography of the nucleon
in the impact parameter plane as they have a direct connection
to the question of the angular momentum of the
quarks in the nucleon. The gluon spin contribution has
been confined by recent results from COMPASS and
HERMES to be small, but remains not understood due
to the large experimental errors. In addition, transverse
momentum dependent structure functions, measured in
semi inclusive scattering, give connections to angular
momentum of quarks as well as transverse polarisation
in the nucleon.
Besides these main topics, the process of helicity
dependent fragmentation is needed for the interpretation
of deep inelastic scattering. All recent theory developments
described above are based on factorisation
theorems. Any experimental study on these structure
functions requires first a large enough value of Q 2 and, in
addition, a large lever arm in Q 2 to show that factorisation
holds. This can naturally be achieved at a collider.
The electromagnetic probe yields the required precision,
but requires high luminosity. In addition, the collider
kinematics allow a complete reconstruction of the final
state, which is difficult for a fixed target experiment and
of utmost importance for the detection of exclusive scattering
processes.
The quest for the realisation of a high luminosity electron
nucleon collider with a centre-of-mass energy s 1/2
above 10 GeV has been made substantially easier by the
FAIR project. The HESR tunnel of the antiproton complex
with the PANDA experiment has been designed to store
antiprotons up to a momentum of 15 GeV/c.
We propose to extend the HESR storage ring facility –
presently under construction at FAIR – by an additional
electron ring. The project is called Electron Nucleon
Collider at FAIR (ENC@FAIR). It yields an opportunity
to create a doubly polarised collider on a comparatively
short timescale with very reasonable investment.
The figure above shows the acceptance of such a collider
project. It demonstrates the substantial extension
of the kinematical range to ensure a sufficiently large
lever arm in Q 2 for factorisation to hold.
Extensive physics simulations are in progress to
explore the response of the PANDA detector, which has
been designed for fixed target mode, for the collider
case. One expects for doubly polarised channels a gain
in the figure of merit of a factor 20 – 100 as compared
to fixed target experiments. This arises partly from the
missing fixed target dilution factors, but also from higher
efficiencies for the detection of final states in collider
mode, e.g. for the detection of D-mesons.
The accelerator physics aspects of ENC@FAIR are
presently studied by a working group consisting of accelerator
physicists from German universities (Mainz, Bonn,
Dortmund) and several research centers (GSI, FZJ, DESY
and BNL).
The HESR storage ring will provide ion beams with a
maximum momentum of 15 GeV/c. A 3.3 GeV electron
ring will be integrated into the HESR tunnel, yielding a
c.m. energy of s 1/2 =13.5 GeV in head on collisions with
protons.
The production and acceleration of polarised electrons
has been a standard feature at many electron accelerator
facilities. A sufficient electron polarisation lifetime in the
collider mode can be achieved by careful arrangement
of the electron ring lattice. Peak currents will be in the
region of 25 Amps, which does not exceed the limits
explored by existing storage rings.
The production and transport of polarised protons or
deuterons through the FAIR injector chain is also feasible,
and so is the spin stable operation of the HESR, since its
cooler solenoid can be operated as a full Siberian snake
with only moderate additional effort. We therefore believe
that sufficiently stable beams, with high polarisation
(P e =P ion ~0.8) and arbitrary direction of the spin in the
interaction region are achievable in proton operation.
The use of the PANDA detector (which is foreseen
for fixed target antiproton experiments at HESR) is an
attractive and especially cost-effective solution, since
many of its components can also be utilised in collider
mode. A design for the interaction region has already
been achieved. It is compliant with the needs of efficient
particle detection and stable collider operation.
This ‘conservative’ design is limited to luminosities of
1.1x10 32 . A more aggressive approach, which requires
advanced – but already demonstrated – techniques like
crab crossing, is under investigation. An increase of the
bunch number to 200 (collision rate 104 MHz) could then
allow for a luminosity of >4x10 32 .
ENC@FAIR has to make use of the electron cooler at
HESR (foreseen for antiproton cooling in PANDA high
luminosity mode) in order to conserve transverse and
longitudinal emittance during collider operation and to
achieve sufficiently small emittance during acceleration.
Therefore, for proton operation, the HESR cooler
must be upgraded to 8.2 MeV at d.c. currents of several
amps. The simultaneous achievement of these cooler
parameters has not been demonstrated yet, but does
not seem unrealistic.
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 51