AAC Abstract (50 word limit)
Use the direct annihilation of high energy positrons and atomic electrons in the reaction, e+e→μ+μ- to produce low emittance muon beams for injection into a multi-TeV muon collider. Several material targets and three positron beam energies are considered. Luminosity estimates are made for each positron beam energy and compared to the design luminosity of a 6 TeV muon collider using a proton source.
AAC Summary (300 wd. Limit)
Direct annihilation of positrons with atomic electrons producing two muons is a potential source for a muon collider. Even though the muon rate from direct positron annihilation is 5-7 orders of magnitude smaller than the muon rate from a proton source, the emittance of the muon beam is lower and the much higher energy muon beam at the source saves muon cooling and early acceleration stages.
The fixed target threshold is a 43.7 GeV positron beam to produce two muons at rest in the center of mass. To achieve the smallest possible muon beam emittance it is important, as much as possible, that the emittance will only be determined by the positron beam size and the maximum muon production angle. In general this determines the thickness of the target to limit multiple coulomb scattering (MCS) of the incoming positron beam and outgoing muon beam.
For a positron beam near the muon production threshold the target thickness is also limited by beam degradation from radiative Bhabha scattering, e+e-→e+e-γ and bremsstrahlung. For equivalent thickness, nt ≥ 1024 e-/cm2, and beam energy near production threshold, a substantial fraction of the beam positrons begin to fall below the production threshold. For targets greater than about one radiation length, the muon production is dominated by the two- stage Bethe-Heitler process, e+→γ→μ+μ-, which gives a much larger angular spread as well as more MCS. Previous work on this subject is briefly discussed as well as the possibility of using an electron plasma as the muon production target.
Three positron beam energies are considered: 44.5 GeV (near threshold), 60 GeV (the peak of the muon production cross section), and 250 GeV (the design energy of the ILC).
In addition the muon rate and emittance from several solid targets is estimated. These results are compared to the design luminosity of a 6 TeV CM muon collider.