Welcome to the Homepage of the Tόbingen GDH group
Our GDH group is a part of the international GDH collaboration , which is performing a novel class of polarized photohadronic experiments at the electron accelerator facilities MAMI B (A2) and ELSA. These measurements represent the first step into a new aera of experiments, which are using polarization degrees of freedom when probing nuclear properties with the electromagnetic interaction.
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The scientific objectives
The GDH sum rule
The GDH experiment
Current images from GDH at A2
Internal A2-services |
setup of the pilot experiment
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DAPHNE
Bonn GDH detector
Pilot experiment |
Click for your way to more information about the GDH Experiments and to some members of the GDH Collaboration:
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list of members (HTML)
and also available as PS file
MAinzer MIcrotron MAMI B
Institut für Kernphysik, Universität Mainz
A2-Collaboration at MAMI B
ELSA Group Physikalisches Institut, Universität Bonn
Polarized Target Group
Physikalisches Institut, Universität Bonn
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The model independent derivation of the GDH sum rule is based on very fundamental physical principles and the widely accepted No-subtraction-hypothesis. Therfore an experimental test of the sum rule also represents an experimental check of the underlying assumptions used in its derivation, if the integrand can be determined over the whole energy range.
Apart from the measurements of the polarized total cross sections, which are needed for a test of the GDH sumrule, the determination of the polarized partial cross sections of all one- and two-pion photoproduction channels on nucleons is a outstanding goal of our experiments. The polarised cross sections of the Np(p) channels allows one to test predictions of various nucleon models in a very detailed way. This certainly will lead to improved models of the nucleon and to an growing understanding about the structure of the nucleon and its resonances.
The GDH sum rule relates the total cross sections of circularly polarized photons on longitudinally polarized nucleons with static properties of the nucleon, i.e. its mass m and its anomalous magnetic moment k.
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The sum rule was already derived in 1966 by Gerasimov, Drell and Hearn [Ger66,Dre66], but will be tested for the first time, because experimental preconditions regarding the polarized photon beam and the polarized nucleon target were not yet available and thus the determination of s3/2 and/or s1/2 was not possible. Nowadays both, polarized beams and polarized targets are well established tools.
The GDH experiment stands for a extensive experimental programme which includes:
To cover a wide range of photon energies, the experiments are carried out at the accelerator facilities MAMI B at Mainz (Germany) and ELSA at Bonn (Germany). MAMI B covers the energy range 140 £ n £ 800 MeV, ELSA covers the energy range 500 £ n £ 3000 MeV. Both facilities provides polarized electron beams of high quality and degree of polarization which are converted to circularly polarized, real photons by the helicity transfer in the bremsstrahlungs process of longitudinally polarized electrons on an amorphous radiator.
Energy and flux of the collimated photon beam are determined by the established tagging method. The degree of polarisation of the photons are determined by employing Møller- and Compton-polarimeters.
The longitudinally polarized nucleons are supplied by the specially designed Frozen Spin Target BICEPS [Dut94] which is using butanol (C4H9OH) as material for the proton target and deuterated butanol (C4D9OD) as material for the neutron target. Also the employment of a polarized 3He gas target [Eck92],[Hei94] to substitute as neutron target is planned.
The reaction products are detected with detector setups, which covers almost 4p solid angle each and offer high detection efficiencies for charged and uncharged particles. At MAMI B the well established detector DAPHNE [Aud91] and at ELSA the new BONN-GDH-Detector [Hel93],[Hel97],[Sau98] are employed in the two GDH experiments. Both setups are well suited for that kind of measurements and offers an effective supression of the electromagnetic background as well as a good coverage of small forward angles.
