Jefferson Lab > Spectator Tagging Project
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The Spectator Tagging Project develops the capabilities for high-energy electron scattering experiments with polarized light ions (deuteron 2H, 3He) and detection of spectator nucleons (protons, neutrons) at a future Electron-Ion Collider (EIC). Such experiments address basic questions of nuclear and strong interaction physics:

Polarized high-energy deuteron beams become available with the JLab medium-energy EIC design (MEIC). The detection of spectator protons ("tagging") identifies the active neutron and controls its quantum state during the scattering process, greatly improving the precision and control compared to conventional measurements. The MEIC forward detector provides the necessary coverage and resolution and is being optimized for the purpose. R&D objectives include:

The project is carried out as a collaborative effort of theorists and experimentalists. It is supported by JLab's Laboratory-Directed Research and Development (LDRD) program as FY14/15 Project "Physics potential of polarized light ions with EIC@JLab." The resources developed (theoretical models, computer codes, event generators) are publicly available for scientific applications and further development. The project aims to enable collaboration with laboratory users and other interested scientists to explore the physics potential of a future EIC.

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JLab LDRD collaborators:

External collaborators and users:

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Resources developed in the project include physics models for high-energy scattering processes on light ions with spectator nucleon tagging, event generators for Monte-Carlo simulations, and analysis tools for pseudodata.

Physics models represent theoretical models of the differential cross sections for high-energy processes on light ions with spectator nucleon tagging and their implementation in computer codes. Implementations are presently available for the following channels:

The theoretical treatment of nuclear effects (nuclear binding, final-state interactions, polarization, coherence at small x) varies widely between the different channels and is the subject of on-going development. Information on the presently available models (codes, documentation) and instructions for downloading can be found here.

Event generators provide random sets of kinematic variables of final-state particles (momentum 4-vectors) for Monte-Carlo integration and detector simulations (tracking). Generators for EIC kinematics are available in different formulations (reference frames) and several output formats. The codes and documentation are maintained on the JLab github server and can be obtained here.

The documents and computer codes posted here are the result of on-going scientific research and development. They are made available with the sole intent to stimulate scientific discussion, invite feedback, and enable further physics simulations for the EIC. Users are free to use and modify the codes for scientific purposes. Any scientific results obtained with the help of these resources should be properly acknowledged (please contact the authors for instructions). No guarantee is implied regarding the correctness of results or the performance of the codes.

Internal documents are maintained on the Spectator Tagging Wiki.

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Selected results


Fig. 1. Neutron structure from deep-inelastic scattering on the deuteron with spectator proton tagging. Left: (a) Conditional DIS on the deuteron e + De'+p+X. (b) Neutron pole at t = MN2 arising from the impulse approximation diagram. Right: Simulated measurement of the recoil momentum dependence with MEIC and on-shell extrapolation t → MN2 . The plot shows the conditional structure function, with the pole factor removed, as a function of the off-shellness MN2- t calculated from the measured proton recoil momentum. The error bars indicate the expected statistical errors. The three sets correspond to measurements in different intervals of the recoil light-cone momentum fraction αR. Figures from [arXiv:1409.5768].

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Context and background

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