The MEG experiment, designed to search for the [equation] decay, completed data-taking in 2013 reaching a sensitivity level of [equation]for the branching ratio. In order to increase the sensitivity...

The MEG experiment took data at the Paul Scherrer Institute in the years 2009–2013 to test the violation of the lepton flavor conservation law, which originates from an accidental symmetry that the Standard Model of elementary particle physics has, and published the most stringent limit on the charged lepton flavor violating decay ${\mathsf{\mu }}^{+}\to {\mathrm{e}}^{+}\mathsf{\gamma }$: BR(${\mathsf{\mu }}^{+}\to {\mathrm{e}}^{+}\mathsf{\gamma }$) $<4.2\times {10}^{-13}$ at $90\%$ confidence level. The MEG detector has been upgraded in order to reach a sensitivity of $6\times {10}^{-14}$. The basic principle of MEG II is to achieve the highest possible sensitivity using the full muon beam intensity at the Paul Scherrer Institute ($7\times {10}^7$ muons/s) with an upgraded detector. The main improvements are better rate capability of all sub-detectors and improved resolutions while keeping the same detector concept. In this paper, we present the current status of the preparation, integration and commissioning of the MEG II detector in the recent engineering runs.

Electric dipole moments (EDM) of fundamental particles inherently violate time-reversal (T) and the combined charge-conjugation and parity symmetry (CP). We aim to measure the EDM of the muon using the frozen-spin technique within a compact storage trap. This method exploits the high effective electric field, $$E\approx {165\,\mathrm{MV/m}}$$ E ≈ 165 MV / m , experienced in the rest frame of the muon with a momentum of about $${23\,\mathrm{MeV/}}c$$ 23 MeV / c when it passes through a solenoidal magnetic field of $$|\vec {B}|={2.5\,\textrm{T}}$$ | B → | = 2.5 T . In this paper, we outline the fundamental considerations for a muon EDM search and present a conceptual design for a demonstration experiment to be conducted at secondary muon beamlines of the Paul Scherrer Institute in Switzerland. In Phase I, with an anticipated data acquisition period of 200 days, the expected sensitivity to a muon EDM is $$\sigma (d)\le {4E-21\,\mathrm{{\text {e}}\!\cdot \!\text {cm}}}$$ σ ( d ) ≤ 4 E - 21 e · cm . In a subsequent phase, Phase II, we propose to improve the sensitivity to $$\sigma (d)\le {6E-23\,\mathrm{{\text {e}}\!\cdot \!\text {cm}}}$$ σ ( d ) ≤ 6 E - 23 e · cm using a dedicated instrument installed on a different beamline that produces muons of momentum 125 $$\textrm{MeV}\!/\!{ c}$$ MeV / c .

The MEG experiment represents the state of the art in the search for the Charged Lepton Flavour Violating ${\mathsf{\mu }}^{+}\to {\mathrm{e}}^{+}\mathsf{\gamma }$ decay. With its first phase of operations at the Paul Scherrer Institut (PSI), MEG set the most stringent upper limit on the BR $({\mathsf{\mu }}^{+}\to {\mathrm{e}}^{+}\mathsf{\gamma })\le 4.2\times {10}^{-13}$ at $90\%$ confidence level, imposing one of the tightest constraints on models predicting LFV-enhancements through new physics beyond the Standard Model. An upgrade of the MEG experiment, MEG II, was designed and it is presently in the commissioning phase, aiming at a sensitivity level of $6\times {10}^{-14}$. The MEG II experiment relies on a series of upgrades, which include an improvement of the photon detector resolutions, brand new detectors on the positron side with better acceptance, efficiency and performances and new and optimized trigger and DAQ electronics to exploit a muon beam intensity twice as high as that of MEG ($7\times {10}^7$ ${\mathsf{\mu }}^{+}$/s). This paper presents a complete overview of the MEG II experimental apparatus and the current status of the detector commissioning in view of the physics data taking in the upcoming three years.