What is APOLLON?
APOLLON is a proposal for a new experiment at the HERA electron beam
at DESY. The aim of the experiment is to measure the gluon spin
distribution of the nucleon by polarised photoproduction of J/Psi
Why is the gluon spin distribution so important?
Since the spin crisis in 1987 we know that our understanding of the
spin structure of the nucleon in terms of the quark-parton model is
very incomplete. Several experiments have been done in the meantime to
understand the spin puzzle. A lot of data have been collected to
measure the quark spin distributions of the nucleon. The missing link
in the understanding of the spin puzzle is the gluon spin which is not
easily accessible. APOLLON was invented to do the world's first direct
measurement of the gluon spin distribution.
How does APOLLON measure the gluon spin distribution?
In the APOLLON experiment, polarised high energy photons are scattered
of polarised nucleons. When a high energy photon hits a nucleon, it
is usually absorbed by a quark. Occasionally it reacts with a gluon
inside the nucleon along with the creation of a charmed
quark-antiquark pair. With a certain probability, the charmed quarks
form either a J/Psi meson or other charmed hadrons. The APOLLON
detector is able to identify the charmed hadrons.
As the charmed hadrons are created from gluons in the nucleon, the
spin asymmetry of the charm production is a direct measure of the spin
asymmetry of the gluons in the nucleon. Experimentally the spin
asymmetry is measured by keeping the polarisation of the photons fixed
and inverting the polarisation of the target nucleons.
How does APOLLON create the high energy polarised photon beam?
The beam of real high energy photons is generated by bringing a
ultraviolett laser beam in collision with the high energy positron
beam at HERA. The laser photons will be backscattered by Compton
scattering. Due to the high positron energy, the backscattered photons
will have energies up to 18 GeV. The laser wavelength is in the UV
range in order to maximize the photon energy. This is necessary as the
photn energy has to be well above the charm quark production
threshold. A so-called positron tagger will measure the energy of the
Compton scattered beam positron. This information is used to determine
the energy of each individual Compton photon. To increase the
intensity of the photon beam, the laser beam intentsity will be
magnified in an external cavity. The polarisatrion of the high energy
photons is achieved by polarising the laser photons.
When will the experiment run and how can I join it?
Currently, we are updating our proposal and discussing it internally
at DESY. The hope is to start running the experiment in 1999 or
2000. We are still looking for collaborators. The experiment needs
If you are interested in the experiment, please contact us by email:
- laser cavities
- polarised targets
- muon detectors
- polarised charm production theory.
Back to the APOLLON homepage.
Back to DESY.
Back to the HERMES experiment.
Last modified: Thu Jul 10 16:18:56 1997