The Los Alamos crystal box experiment: A search for μ → eγ, μ → eγγ, and μ → eee

A search has been performed for the decays μ → eee, μ → eγ, and μ → eγγ with a sensitivity in the branching ratios at the level of 10−10. The experiment used a surface muon beam at LAMPF with an average intensity of 300 kHz. A total of ≳1011 muon decays was examined for the present result. The detector for the experiment is the Crystal Box which consists of a cylindrical drift chamber surrounded by 396 NaI(T1) crystals. A layer of scintillation counters in front of the crystals provided timing for electrons and a veto for photons. The energy resolution for electrons and photons is ∼6% (FWHM). The position resolution of the drift chamber is 350 μm leading to a vertex cut with a rejection of 103 for μ → eee. The timing resolution is ∼400 ps from the scintillators and ∼1 ns from the crystals. From these values, the eee and eγγ limits are expected to be background free.