EU-STRONG-2020

There are central questions that remain to be answered within the Standard Model. Examples are how the properties of strongly interacting particles can be understood from their fundamental constituents (quarks and gluons) or why these constituents are confined to these particles. Spin, a fundamental property of a particle, plays an important role in trying to understand these unresolved subjects. An ambitious spin program is therefore underway at the infrastructures ELSA (Bonn) and MAMI (Mainz), where double polarization experiments with polarized beams on polarized protons and deuterons (neutrons) targets are used to disentangle the complicated resonance spectra to learn how the elementary particles form matter. The PANDA experiment at the FAIR facility does experiments with the aim of understanding confinement, not just as a phenomenon but to comprehend it quantitatively from the theory of the strong force. New experiments exploiting high-energy antiproton and ion beams will also elucidate the generation of hadronic masses and by the use of an internal transverse polarized gas target the facility is well suited to measure single or double spin asymmetries to determine nucleon structure observables. At high energies the COMPASS experiment at CERN investigate, how the different constituents (quark flavors, gluons, sea-quarks, angular momentum) contribute to the spin of the nucleons.

Technically, the polarized nucleons for these experiments are provided by polarized solid state targets, using the method of Dynamic Nuclear Polarization (DNP) or internal Atomic Beam Source (ABS) polarized gas targets. The combination of large angular acceptance nonmagnetic detector arrangements and frozen spin polarized solid state targets equipped with internal superconducting holding coils, has led to a large number of measured double polarization observables at ELSA, MAMI and JLAB. However, the frozen spin technique has some systematic disadvantages such as the limited beam intensity they can sustain, the time loss for repolarization and the need of a sophisticated moving system for the external superconducting magnet for DNP and the detection system. To improve this class of experiments and to overcome the drawbacks of the frozen-spin polarized target, especially to get rid of the external DNP magnet, we will optimize the small low mass internal LTS (low temperature superconducting) coils to strengthen the magnetic field for permanent DNP (“4π-DNP continuous mode” target) while keeping the excellent angular acceptance and to gain the luminosity of the existing target schemes. To get access to new polarization observables it is foreseen to add additional small low mass superconducting coil schemes for individual polarization directions. In contrast to the above mentioned detector and polarized target concepts, the PANDA detector will operate with a strong longitudinal magnetic field to provide the measurement of charged particle trajectories with high resolution. To align the spins of the target nucleons in other directions than the spectrometer field orientation, it is therefore necessary to shield the polarized gas target from the magnetic field of the spectrometer coil. The development of a low mass HTS (high temperature superconducting) active or passive shielding is the first step towards a transverse polarized gas target in PANDA. A polarization or magnetic field direction off the beam axis then has to be provided by additional small superconducting coils inside the shielding tubes. In spite of the different approaches, the superconducting structures of both subtasks have similar outer dimensions and have to be as thin as possible to minimize the radiation length for penetrating particles. Our aim within the JRA is to increase the field and homogeneity of the LTS DNP-coils up to 2.5 T and to reduce the thickness of the HTS-structures towards operational prototypes and to implement for both schemes small superconducting coils for individual polarization directions under perpetration of the overall minimized thickness. State-of-the-art frozen spin targets for detection systems with full angular acceptance running at ELSA, MAMI and JLAB use a thin, superconducting coil inside a 3He/4He dilution refrigerator either to maintain the polarization with a relaxation time in the order of several hundred hours or in future, to build up the polarization during data taking. The outgoing particles have to punch through this coil to be detected. In spite of the minimized wall thickness of the internal magnets, this limits kinematically accessible region, especially for low momentum particles. The best technical option to overcome this drawback is to detect them inside the target material itself, using a so called ’active target’. The scintillating target material has to be polarized at cryogenic temperatures and the light has to be detected with dedicated electronics adopted to these extreme conditions. A first functional prototype demonstrated in a test experiment in Mainz under beam conditions the feasibility of this technique. The third task of our JRA is the development of active polarized targets at cryogenic temperatures and the further implementation of this technology with new, improved prototypes. The overall objective of the JRA is hence to develop future key technologies for new and innovative polarization experiments using polarized targets in Europe.