Electron-positron linear colliders (and options using gamma gamma, e-gamma, e-e-) should be evaluated in terms of the physics potential that they offer, and in terms of the accelerator issues that will guide their evolution. This group should work in close collaboration with its counterpart, M3: Linear Colliders, and should examine the impact of a very large circular electron-positron collider discussed in M2: Electron-Positron Circular Colliders.
 

Charge:
 

1. The eeLC group should coordinate with the physics groups to help compile and critically examine the case for an initial phase of the ee collider at a cm energy of up to about 500 GeV, depending on the results from prior experiments at the Tevatron and LHC. For some representative physics scenarios, what is a reasonable goal for integrated luminosity at various cm energies, beam polarizations and beam particles? is there a compelling initial physics program at a luminosity of a few x 10³³ cm^-2s^-1? Are there particular advantages or challenges to experimentation raised by the different running conditions in the TESLA and NLC/JLC designs? 
2. The eeLC group should review the case for and feasibility of special options for LC operations: 
1. Catalogue the physics needs that may require positron polarization, gamma gamma collisions or e-e- collisions. Compare the capabilities of an electron-positron collider and a gamma-gamma collider for making detailed measurements of the properties of Higgs bosons, and for discovering Higgs bosons. What are the R&D issues remaining for each option? What are the requirements on the initial design to allow any of these to be added after the initial phase? 
2. Examine the case for high-luminosity operation at the Z pole. What are the benefits and drawbacks from the design of a special beam delivery system for low energy collisions? Should there be a special detector devoted to operating below 500 GeV? 
3. What special requirements are imposed if a free electron laser program is added to the high energy physics facility? What should the HEP community do to facilitate the potential for a FEL program? 
3. Evaluate the scientific case for an initial-phase "Higgs factory" at an energy of about 300 GeV. 
4. What new physics landmarks come into view as the energy of a linear collider is raised to 1 TeV; to 1.5 TeV; to 2 TeV; to 5 TeV? What luminosity and other performance characteristics would be required to maximize the scientific output? 
5. Are there particular issues that detector R&D must address to guarantee the productivity of a linear collider? 
6. What are the beam physics limits and accelerator limits imposed on LC performance, and what are the primary outstanding R&D issues that are critical to study in the next several years? 
7. The eeLC group should assume that a technical review panel will likely be established within the next year to evaluate the superconducting L-band and warm rf X- or C-band accelerator proposals. That review, conducted under the auspices of some worldwide body, would examine the performance parameters of the machines, the technical risks, needed R&D, comparative costs and upgradability. Without undertaking the work that such a panel would do, the eeLC group should work to sharpen the questions that this review panel should examine, and consider the way in which the panel should operate. 
8. What are the paths for upgrade of an initial LC, both in energy and in luminosity? What extensions in energy using the original TESLA or X-band LC designs are feasible? What R&D issues should be given priority? What is the possibility of upgrading either TESLA or X-band LC using two-beam drive power sources? What are the critical R&D issues? What constraints on the initial phase would ultimate conversion to two-beam drive impose? 
9. How can full international collaboration on a LC project be realized? Is it feasible to assign full responsibility for design, construction, commissioning, test and operation of major subsystems to different portions of the world community while maintaining effective overall project management?