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 mesons.

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 experts on If you are interested in the experiment, please contact us by email:

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Last modified: Thu Jul 10 16:18:56 1997 by
Michael Düren.