# Electrons and X-rays to Muon Pairs (EXMP)
## 6 Conclusions  
We described a muon source based on electron-photon collisions at ultra-high luminosity, capable of reaching muon fluxes up to a few \(10^{11}\) muon pairs per second at an outstanding normalised transverse emittance of a few nm rad, with muon beam energies peaking at 50-100 GeV. The electron-photon collider is based on a primary electron LINAC with energy in the 200-500 GeV range and a FEL LINAC driver with FEL photon energy in the 60-150 keV range. Extremely large beam power (both electron and photon beams) are needed to achieve the ultra-high luminosity, in excess of \(10^{41}\) cm\({}^{-2}\)s\({}^{-1}\). Since the electron-photon collisions transfer only a very small amount of power from the primary beams into the secondary beams, an efficient energy recovery must be implemented in the scenario, so as to reduce the amount of beam power loss down to the level of hundreds of MW, from 100 GW of beam power stored in the primary beams at collision. This is the main challenge of such a muon source, together with challenging beam collision spot sizes, in the range of a few tens of nanometers, and handling of an extremely large FEL photon beam power. The analysed scenario is based on a twin array of LINACs arranged face-to-face, providing both the primary electron beam and the FEL driving beam. It is of paramount importance to find a strategy of beam energy recovery with counterpropagating electron beams in the same LINAC. This was the original ERL layout conceived by [PERSON] a long time ago (see reference [20]), subject to R&D on twin super conducting cavities accommodating counterpropagating beams, but not yet demonstrated [21; 22]. Further studies on the feasibility of these scenarios are necessary to assess the achievable luminosity of a muon collider based on this muon source, depending on the kind of accumulator scheme to combine with such a muon source scenario. The \"promise\" is to achieve the requested luminosity using much lower muon beam currents with respect to the other schemes, thanks to the very low emittances, which would significantly alleviate the issue of the muon beam-induced background. Additional potentialities are also to be studied and carefully analysed, e.g., the simultaneous acceleration of muon beams and primary electron beams in the main LINACs, possible polarization of muon beams if polarised primary electron beams are used (FEL photons are naturally polarized), parasitic use of intense GeV-class positron beams (up to \(10^{16}\) pairs per second) generated in the electron-photon collisions, and of 100-GeV-class intense (in excess of \(10^{12}\) per second) monochromatic photons. We are aware that the evaluations illustrated in this paper do not prove the feasibility of EXMP; they should be intended as guidelines-showing a roadmap towards a future feasibility study, followed by a conceptual design study.  
Conceptualization, investigation, writing-original draft preparation, writing-review and editing, C.C. and L.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.