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Hard Exclusive Reactions are the key to the present and future stages in the exploration of the structure of hadrons. Technological progress in accelerator performance and detector design over the last years together with progress in the theory of hadron structure have made it possible that a 3-dimensional picture of the proton is now within reach.

The description of hadron structure in terms of Generalised Parton Distributions (GPDs) allows an approach that will simultaneously reveal the distributions of partons inside hadrons in one dimension of longitudinal momentum space and two dimensions of transverse impact parameters space. On the experimental side this requires fixed target kinematics with a hermetic coverage of the entire acceptance, and detection and identification of all reaction products. In addition to the forward spectrometer typical for fixed target experiments, the target must therefore be surrounded by a detector for target remnants, a recoil detector. It may even be required that the target itself is instrumented with detectors. In terms of data analysis and corresponding theoretical description of the data it is crucial to find ways to integrate all available world-data into a global analysis. In most measurements, Generalised Parton Distributions are not direct observables. It is necessary to build GPD models that fulfil all theoretical requirements and that are restricted by existing data on form factors, parton distribution functions and hard exclusive processes. These models can then be used to make predictions for further experiments, be refined in the process and so on – the typical process of model building in science. Finally, sufficiently well known GPDs will yield a 3-dimensional picture of hadrons and a host of additional detailed information about hadron structure –including the transversity structure through the extraction of chiral-odd GPDs and the solution of the spin puzzle via the so-called Ji sum rule. Existing data on form factors have already been used by collaborators of this proposal to restrict GPD models, with quite spectacular results on the distribution of u- and d-quarks inside the (polarised) nucleon. However, this is just the very beginning. While a lot of data already exists on form factors and parton distribution functions, Hard Exclusive Reactions are only in the early stages of exploration. This JRA targets an integrated approach to the analysis of all available and upcoming data on Hard Exclusive Reactions worldwide, R&D for key experiments in the near future and the preparation of future experiments.

The JRA consists of 5 sub-projects, each of which has a leading institute that is responsible for the coordination and execution of the tasks. The following list presents the sub-projects and tasks together with the participating institutes (short names) and the leading institute (bold face):

1.      COMPASS Recoil Detector and Calorimetry; UBO, ALU-FR, UMainz, CEA, SINS

COMPASS is an experimental facility located at CERN which was designed and built to measure, inter alia, the gluon contribution to the nucleon spin. Due to the unique availability of both positive and negative muon beams, COMPASS is especially suited to measure the beam-charge asymmetry in Deeply Virtual Compton Scattering. This requires a series of upgrades and extensions of the existing spectrometer, in particular a recoil detector to surround a new 2.5 m long cryogenic target and an extension and upgrading of the calorimetry. The general lines of such a program have been presented in an Expression of Interest in 2005 [CERN-SPSC-2005-007, SPSC-EOI-005].

• Design of an entirely new ECAL0 calorimeter at larger angle using wavelength-shifting fibres and Avalanche Micro-Pixel Photodiodes. The development of new and cheap AMPD’s is challenging in that it has to allow low threshold detection in a high magnetic field and high rate environment.
• Development of electronics for new ECAL1 modules to measure both amplitude and time signals in order to measure energy with a good resolution.
• Development of a reliable simulation of the extended apparatus including the new recoil detector based on a detailed study of the data which are presently being taken.
• Development of new electronics to register data at a frequency of 1GHz without dead-time.

2.      CLAS12 Central Detector; CNRS/IN2P3/LPC, INFN-LNF, INFN-GE, UGlasgow, CNRS/IN2P3/LPSC, CNRS/IN2P3/IPNO, CEA

JLab will increase its beam energy from currently 6 GeV to 12 GeV by 2013. This requires an upgrade of the CLAS detector, called CLAS12. The present design of CLAS12 consists of a two-part detector: a forward spectrometer and the Central Detector. The Central Detector is essentially a recoil detector and it will detect particles at large angles and lower energies. The effort to design and build this detector is lead and mainly carried by European groups and will maintain the leading role European groups are playing at JLab in this part of the physics programme.

Tasks: Design and prototyping of:

• Central Tracker: two technologies are under consideration, Silicon strip detectors and Micromegas (MM); a bulk technology opens the way to cylindrical geometries for MM, while the operation in a magnetic field of 5 Tesla is a new challenge for this kind of detector; Micromegas are also being considered as first detectors in the forward direction.
• Central Time-of-Flight scintillators: the challenge here is to collect the light and convert it to an electric signal in the high magnetic field environment with a time resolution better than 100 ps.
• Central Calorimeter: this electromagnetic calorimeter mainly will detect photons and neutrons; design options include pressurised-tungsten powder plus scintillating fibres, a ‘spaghetti calorimeter’ or a material optimized for neutron detection.

3.      PAX Target Region; INFN-BA, FAU, INFN-FE, GSI

The PAX Collaboration is proposing to perform experiments with polarised antiprotons at the new FAIR facility. Polarised antiproton-proton collisions will generate double and single spin asymmetries through which unique access will be given to different physical quantities. The present proposal refers to the initial, fixed-target stage of the experiment. The main physics topics addressed by PAX will be electromagnetic form factors, hard polarized pbar-p scattering and transversity measured using the Drell-Yan process.

·         Study of a toroidal magnetic field configuration for the spectrometer compatible with a polarised internal gaseous target.

·         Storage cell design and prototyping, using a thin Teflon foil of only 5 mm thickness instead of the Aluminium foils of typically 50-75 mm used nowadays.

·         Design and prototyping of the High Temperature Superconductor (HTSC) coils for the 0.3 Tesla target holding field. The use of this technology does not require a cryostat around the coils, as a “cold-finger” is sufficient to keep the magnets in temperature. This technology is not yet commercially available, but some companies are already producing cables which can be used for coil production (e.g. Bi-2223 in a AgAu matrix from THRITOR-Germany)

4.      Global Analysis of Hard Exclusive Reactions; INFN-BA, RUB, UBO, CNRS/IN2P3/LPC, DESY, FAU, INFN-FE, INFN-LNF, ALU-FR, JLU, UGlasgow, CNRS/IN2P3/LPSC, UMainz, CNRS/IN2P3/IPNO, INFN-PV, UREG, INFN-RM1, CEA, TAU, UVEG, SINS

The ultimate goal to determine GPDs requires the inclusion of all existing experimental data on Hard Exclusive Reactions, the development of improved GPD models and a fitting procedure. It is already clear that such a procedure has to be set up in next-to-leading order, since radiative corrections are crucial to describe the normalization of cross sections in various hard-exclusive processes. Extensive existing data sets on DVCS and Hard Exclusive Meson Production from the HERMES experiment at DESY (with Recoil Detector) and from JLab at 6 GeV are to be analysed.

• Development of a global fitting procedure for GPDs based on a flexible parameterization of GPDs motivated by model and lattice calculations and a very fast and numerically reliable code that is based on a next-to-leading order framework, including evolution.
• Analysis of existing and near future DVCS data from HERMES, COMPASS, JLab (CLAS and Hall A).
• Extraction of limits on the total angular momentum of u- and d-quarks inside the nucleon.
• Analysis of existing and near future data on hard exclusive vector meson production from HERMES, COMPASS and JLab.
• Development of GPD-based models for hard exclusive vector meson production.

5.      Preparation of Future Experiments at PANDA; RUB, CNRS/IN2P3/LPC, INFN-GE, JLU, UGlasgow, GSI, CNRS/IN2P3/IPNO, CNRS/CPHT, INFN-PV, TAU, UVEG

The antiproton-proton interactions at FAIR will allow a unique access to a number of new fundamental physics observables, both in the unpolarised and polarised beam cases. The form factors of hadrons will be measured in the time-like domain and therefore will be complementary to those undertaken with electron beams at JLab. The exclusive annihilation reactions (antiproton + proton → lepton pair + meson) in the forward region will yield new observables, called proton to meson transition distribution amplitudes (TDAs), which can be related to GPDs.

• Simulation of GPD and Transition Distribution Amplitude (TDA) related processes for the PANDA physics book.
• Analysis of two-photon effects in form factor measurements and modelling of different proton-to-meson TDAs.
• Development of a GPD based meson cloud model.
• Detailed phenomenological study of hadronic two-body reactions to quantify the pion background in PANDA as a function of the 4-momentum transfer.
• Evaluation and computation of radiative corrections.

The deliverables correspond to the objectives of the individual sub-projects and are therefore listed in an analogous way.

The project is based around the research infrastructures GSI (sub-projects 3 and 5) and COMPASS@CERN (sub-project 1), as well as the European involvement in the current and future facilities at JLab (sub-project 2). A particular emphasis lies on FAIR as a major future European Research Infrastructure. FAIR, including the PANDA and PAX experiments, has the highest priority in the NuPECC roadmap for nuclear physics research infrastructures in Europe. The global analysis of Hard Exclusive Reactions in sub-project 4 includes the analysis of data from further research infrastructures, especially HERMES@DESY (which was a research infrastructure in FP6), JLab and MAMI, the latter through the constraints given by nucleon form factor data. The project includes practically the entire research community working on Hard Exclusive Reactions in Europe and is expected to lead to a further integration of this community.

European groups and scientists have played a leading role in both the theoretical and experimental development of the field of Hard Exclusive Reactions. Several pioneering experiments were carried out at HERMES@DESY, which also featured the first dedicated recoil detector for exclusive measurements. The first data-constrained GPD models and GPD fits were also developed in Europe. For the duration of Framework 7, COMPASS@CERN will be the only remaining dedicated experimental facility in this field in Europe. With the end of data taking at HERA in June 2007 there is now no active high-energy electron accelerator in Europe and the experiments at FAIR will come online only from 2014/15, i.e. after the present Framework 7. In this context, the engagement of European groups at JLab is crucial for the future development of the field in Europe. It is one of the main goals of this JRA to maintain the leading role European groups are playing in this essential part of hadron physics.

A series of innovative technologies will be developed within the framework of this JRA that are likely to lead to new or improved instruments or to a new critical technology. The potential exploitation of these new technologies lies in future nuclear and particle physics experiments, but also in the field of medical imaging technology. The developments that are expected to have the biggest impact are:

• Sub-project 1, COMPASS: Calorimeter readout with wavelength-shifting fibres and Avalanche Micro-Pixel Photodiodes; low threshold readout in a high magnetic field.
• Sub-project 2, CLAS12: Cylindrical Micromegas detectors operating in a magnetic field of 5 Tesla.
• Sub-project 3, PAX: Target coils manufactured from high temperature superconductor cable; Teflon target cell with only 5 μm wall thickness.

Finally, the success of sub-project 4 in the form of a first global fit to hard exclusive data in terms of GPDs, would represent a significant step in the understanding of the structure of hadrons. It will open the way to a 3-dimensional picture of hadrons as well as to a solution of the spin puzzle via the Ji sum rule.