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An entirely new chapter in studies of hadron physics could unfold with the advent of a polarized-antiproton beam provided an efficient method for polarizing antiprotons can be demonstrated. The present JRA aims at a feasibility study for the production of polarized antiprotons. Polarized-antiproton beams could be employed at the European Facility for Antiproton and Ion Research (FAIR) and at the European Organization for Nuclear Research (CERN).

A cornerstone of the hadron physics program at FAIR is the collection, cooling, storing and acceleration of antiprotons in the 15 GeV High Energy Storage Ring (HESR). The PAX Collaboration has suggested to convert HESR into an asymmetric collider between polarized protons and polarized antiprotons and to study doubly polarized antiproton-proton reactions. The corresponding physics program has been repeatedly given the highest ratings by various scientific committees. Different theory groups are providing the basis and help to further develop the physics programme. At CERN, antiprotons are stored, cooled and decelerated from 3.57 GeV/c down to 100 MeV/c in the Antiproton-Decelerator (AD) complex.

There are major technological challenges to be met in order to produce beams of polarized antiprotons. Foremost is the question of how to polarize antiprotons effectively. The present JRA faces directly the issue by investigating the feasibility of the only known viable method that could yield a beam of polarized antiprotons, namely spin-filtering of a stored beam by selective loss and selective spin-flip through repetitive interaction with a polarized internal target. The proof-of-principle experiment has been carried out with protons at the Test Storage Ring (TSR, Heidelberg) in 1992; in this test-experiment, only small polarizations of 1-2% have been achieved, but this is already a demonstration of the feasibility of the method. The early attempts at CERN/LEAR to polarize antiprotons by hadronic scattering did not work, while the production of polarized antiprotons by weak decay of anti-lambda hyperons, as it was done at the Fermi National Accelerator Laboratory , yields impractically low beam intensities.

Although the TSR experiment clearly established that the spin-filtering technique works, there is now no unique interpretation of its result. Under discussion is the contribution of the electrons of the polarized atoms inside the target to the polarization build-up. In order to make spin filtering a workable method for polarizing antiprotons, preparatory experiments with both protons and antiprotons are required.

It is the aim of the present JRA to realize a prototype of the first ever facility for the production of polarized antiprotons.

In order to accomplish this goal, a proper experimental setup and a theoretical structure have to be implemented to support a series of dedicated experimental investigations.

Two sets of measurements are foreseen at the COSY-ring in Jülich:

            I.    Depolarization measurements of a stored polarized proton beam passing through an unpolarized target.

         II.    Spin-filtering tests with the proton beam.

The depolarization and spin-filtering experiments with protons at COSY will help to disentangle the hadronic and electromagnetic contributions to the polarization build-up. Two different interpretations of the TSR result exist at the moment, the first invoking the contribution from the electromagnetic interaction of the polarized proton beam with the polarized electrons of the polarized hydrogen target, the second making use of the hadronic interactions with the target protons only. The measurements at COSY will aim at a complete understanding of the spin-filtering process and to demonstrate that a significant polarization build-up can be achieved in a storage ring.

Once the principle of spin-filtering firmly established with protons, future measurements with antiprotons will be planned at the AD-ring at CERN. Only preparatory simulations concerning these future spin-filtering tests with the antiproton beam of AD-CERN are included in the present JRA. The AD at CERN is currently the only machine worldwide which provides the required experimental conditions to perform such spin-filtering experiments with antiprotons.

Conceptual and technological innovation

The major goals of the measurements proposed in this JRA will provide the ultimate tests towards the production of the first ever beam of polarized antiprotons. The main goals are:

-   A complete understanding of the mechanism underlying the spin-filtering process with protons, disentangling the contributions from electromagnetic and hadronic interactions.

-   At the same time, the measurement with protons will aim to demonstrate that with a proper setting of the ring parameters, a high level of polarization (>20 %) can be reached with the spin-filtering method.

Implementation plan of the Joint Research Activity

Building on the experience from spin-physics experiments at storage rings at TSR (FILTEX collaboration, Heidelberg, Germany), HERA (HERMES collaboration, Hamburg, Germany), IUCF (PINTEX collaboration, Bloomington, IN, USA), and COSY (ANKE collaboration, FZJ, Germany), the commissioning of a spin-filtering experiment in a storage ring requires the design, production and installation of three major dedicated components:

                    i.            a low-beta section,

                  ii.            a Polarized Internal Target using a storage cell, and

                iii.            beam and target polarimeters.

For the operation of the experimental setup, the implementation of a Data Acquisition System and Slow Control System will be required.

The design of the elements has to take into account their use both in the COSY ring environment for the experiment with protons and in the AD-ring for the subsequent experiment with antiprotons. In the latter case, dedicated modifications of the ring have also to be foreseen.

The finalization of the detector geometry and the analysis of the acquired data will require the development of dedicated software for the experiment.

The present theoretical understanding of the spin-filtering process is unsatisfactory and it is the goal of the proposed measurements to experimentally solve this issue. Additional theoretical investigation will be required both as predictions and for the interpretation of the results of the measurements.

T1 – Management

The objective of this task is the management of the present JRA. FZJ is the leading institution and, assisted by INFN-FE, will manage the JRA.

T2 – Depolarization measurements at COSY

A key element to disentangle electromagnetic and hadronic effects in the interaction of polarized (anti)protons with a polarized target is the study of the depolarization effect of a proton beam stored at COSY injection energy of T_p = 45 MeV by unpolarized electrons in a deuterium cluster jet target (or ⁴He storage cell target), as in effect inverse to a polarization build-up by polarized electrons as predicted by Meyer and Horowitz. These studies will be carried out at the COSY/ANKE Interaction Point.

The beam depolarization study will combine the experience of the COSY machine group of FZJ and the expertise of the ANKE collaboration. PAX collaborators from the former PINTEX and HERMES experiments, as well as the experimental groups of INFN-FE, UMainz, and GSI, will be involved.

T3 – Polarized Internal Target

The Spin Filtering method is based on spin-selective scattering of a circulating proton or antiproton beam off a polarized internal gas target. The aim of this task is to set up the polarized target consisting of:

·         An Atomic Beam Source (ABS) to produce the polarized atomic beam;

·         A so-called Breit-Rabi Polarimeter (BRP) to measure the polarization of the target gas;

·         An openable storage cell implemented into a target chamber.

Besides FZJ, the main involvement of FAU and INFN-FE is in T3. The groups from FAU and INFN-FE possess large experience in spin physics, ranging from polarized sources and targets to the leadership role in the HERMES experiment at HERA/DESY, exploring the structure of the nucleon. After its successful running in the HERMES experiment, the HERMES target has been transferred to FZJ and it will be put back into operation after proper modifications. An openable storage cell has to be designed and produced.

T4 – Detector and Data acquisition

The detector for the spin-filtering experiments constitutes a multipurpose device which should work as beam polarimeter and recoil detector for the measurement of the spin-dependent cross-section, cope with a broad range of beam energies and fit into the lattices of the different accelerators (COSY and AD). The objective of this task is the development of dedicated software for the Silicon Tracking Telescope (STT) tests, calibration and read-out procedures. It includes the development of the software needed to control front-end, trigger and read-out electronics of the STTs and a Graphical User Interface to control run status and front-end, read-out and trigger electronics.

The silicon-based detector systems presently utilized at the ANKE experiment at COSY constitute a completed recent development of FZJ. In addition, INFN-FE, GSI and UMainz will join this effort with their expertise in detection systems.

T5 – Simulation and data analysis for COSY and AD

The objective of this task is to perform dedicated simulations of the detector design and its optimization to efficiently detect proton-proton elastic scattering events at the COSY ring (see also T4). At the same time the possibility has to be foreseen to use the same detector (probably with minor modifications) with the antiproton beam of the AD ring.

The groups of INFN-FE, SU, and SINS will take responsibility for this task.

T6 – Design and installation of the low-β section

The objective of this task is the implementation of low-β sections into the lattices of the COSY and AD rings. It will cover the preliminary study of the optics and the operation scheme of the rings with low-β sections, the design of a system of quadrupoles for the realization of the low-β section to be used at COSY and AD, the design of compact, high-gradient superconducting quadrupole magnets, and finally the production, characterization and installation of these magnets. In order to perform experiments with a longitudinal polarized beam experiments, the operation of the rings with spin rotators (Siberian snakes) will be investigated as well.

The technical development of the low-β section gathers the expertise in designing and building systems of superconducting magnets of the groups from FZJ, INFN-FE, FAU and GSI.

T7 – Polarization build-up studies at COSY

The objective of this task is to perform of polarization build-up experiments at the COSY ring. The build-up measurements will complement the depolarization measurements and contribute to the complete understanding of the spin-filtering mechanism. At the same time an important goal will be the demonstration that a high-degree of polarization can be achieved in a storage ring by means of the spin-filtering technique. This will be a fundamental step beyond the feasibility test experiment at TSR (FILTEX). The measurements at COSY will also serve as a commissioning experiment of the experimental apparatus for the subsequent measurements at the AD ring.

The polarization buildup studies at COSY require close collaboration of groups involved in spin physics with few nucleon systems of FZJ, INFN-FE, GSI, and UMainz.

T8 – Theoretical Investigations

The main task of the theory team is to provide theoretical support for the spin-filtering experiments with protons at COSY, and antiprotons at AD.

The theoretical investigations involved in this JRA will be heavily supported by the expert theory groups of FZJ and Trinity College Dublin.

1.      Prototype of an open-cell prototype for the polarized internal target (delivery month from start date: 7)

3.      Final report on beam depolarization studies (delivery month from start date: 12)

5.      Final report on target commissioning (delivery month from start date: 18)

6.      Final report on detector and data acquisition system (delivery month from start date: 18)

10. Final report on spin-filtering studies (delivery month from start date: 30)

The possible use of polarized antiproton beams has been evaluated with highest rankings in the different meetings of the international Physics Advisory Committee (QCD-PAC) for the FAIR facility. The physics case for experiments with polarized antiprotons is outstanding and concerns:

-   doubly polarized antiproton-proton Drell-Yan reactions which are entirely dominated by the annihilation of valence quarks in the proton with the valence antiquarks in the antiproton. The measured double spin-asymmetry A_TT will allow the first direct measurement of transversity. No other existing or planed future facility will be able to directly measure the transversity in a competitive way;

-   measurements of the moduli and absolute phases of the proton electromagnetic form-factors in the time-like region;

-   measurements of doubly polarized proton-antiproton hard scattering to be compared to, and to shed light on the interpretation of, the analogue pp measurement, for which the asymmetry is the largest ever measured in hadron physics;

-   hadron-spectroscopy studies and searches for exotic new states like glueballs or hybrids, which will definitely benefit from polarization of beam and/or target particles, because the initial spin state of the system can be prepared at will.

The QCD-PAC and the FAIR Scientific and Technical Issues Committee explicitly requested from the PAX Collaboration to perform all the necessary efforts to produce the first ever beam of polarized antiprotons. The prospective polarized antiproton facility can be used at the European FAIR facility and will be open to European physicists. The possible use at CERN of the polarized antiproton facility would open this entirely new field of hadron physics with polarized antiprotons to the European high-energy physics community.

The actuality of the subject of the JRA and its foreseeable impact on the scientific community triggered a strong involvement of the participating national institutions which offered local structures (e.g. Workshops) and various setups from decommissioned experiments (e.g. the target of the HERMES experiment, and the HERMES Silicon recoil detector). In gathering the very critical components of internal polarized target and beams, and the expert teams on the subject, this JRA will significantly enhance the worldwide competence and leadership of Europe in this frontier field in science, technology and accelerator design.