Low-initial-energy muon acceleration in beam-driven plasma wakefield using a plasma density down-ramp

The muon plays a key role in the field of particle physics and applied physics. To build the neutrino factories or muon colliders, high-quality muon sources are needed. At present, we can only get the low-flux cosmic-ray muons and low-energy accelerator-generated muons. The key issue about accelerating a low-initial-energy muon beam in the plasma wakefield driven by an electron beam is the phase matching between muons and a wakefield. A plasma density down-ramp is considered as an effective method for accelerating a low-initial-energy muon beam, and the decreasing phase velocity at the back edge of the wakefield can lower the muon trapped energy threshold. A 100 MeV muon beam can be accelerated to 6.21 GeV in the plasma wakefield based on a negative plasma density gradient. The trapping and accelerating process can be controlled by adjusting the parameters of the density down-ramp.