Improved search for two body muon decay μ+→e+XH

Abstract

Charged lepton flavor violating muon decay μ þ → e þ X H , where X H is a massive neutral boson, was sought by searching for extra peaks in the muon decay μ þ → e þ νν energy spectrum in the m X H mass region 47.8-95.1 MeV=c 2. No signal was found and 90% confidence level upper limits were set on the branching ratio Γðμ þ → e þ X H Þ=Γðμ þ → e þ ννÞ at the level of 10 −5 for this region.

Looking forward to lepton-flavor-violating ALPs

“…For higher masses the positron becomes too soft to be efficiently triggered on at TWIST. In this region older searches by Derenzo et al [65] and Bilger et al [66] and the recent PIENU result [67] complement the high mass coverage. We refer to the PIENU paper [67] for a summary plot focused on the high mass region.…”

A search for a cosmologically-relevant boson in muon decay

“…Minor penalties for signal loss to preamplifier resets, and for the simulated probability (∼89 %) of double 511 keV gamma escape are considered, and applied. Two main factors contribute to the excellent sensitivity foreseen: an improvement by more than three orders of magnitude in detector energy resolution with respect to previous searches using large scintillator calorimeters [20], and the modest background rate expected. A tiny detector size, and the small fraction of Michel decays at low E e (∼ 10 −5 for E e < 1 MeV) synergistically combine towards this last advantage.…”

“…In these models, the exotic muon decays mentioned above are usually heavily suppressed, necessitating a switch in focus to the µ → eX 0 channel [15], where X 0 is a new neutral boson. From an experimental point of view, this possibility has been explored in the two-body decay µ + → e + X 0 , by studying the (Michel) positron energy spectrum of decays from conventional µ + → e + νe ν µ , inspecting it for a superimposed anomalous monochromatic peak determined in its position by the mass of the new boson, m X (see [16][17][18][19][20], with a massless boson search at the spectral endpoint in [21]). The sensitivity of these searches is restricted to BR 10 −5 , due to the limited energy resolution of the large calorimeters employed, and to the background imposed by ever-present Michel positrons.…”

“…The top panel in Fig. 1 shows the commonly used representation of µ + → e + X 0 sensitivity in BR vs. m X phase space [16][17][18][19][20]. In view of it, one might be tempted to devise new ways to improve on BR, but it would be hard to justify pushing m X any closer to the kinematic limit, based on this portrayal alone.…”

“…In view of it, one might be tempted to devise new ways to improve on BR, but it would be hard to justify pushing m X any closer to the kinematic limit, based on this portrayal alone. An alternative visualization of the present experimental situation can be reached using the kinematics of two-body decay at rest [20]:…”