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.

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.

An automatic target monitoring method based on photographs taken by a CMOS photo-camera has been developed for the MEG II detector. The technique could be adapted for other fixed-target experiments requiring good knowledge of their target position to avoid biases and systematic errors in measuring the trajectories of the outcoming particles. A CMOS-based, high resolution, high radiation tolerant, and high magnetic field resistant photo-camera was mounted inside the MEG II detector at the Paul Scherrer Institute (Switzerland). MEG II is used to search for lepton flavor violation in muon decays. The photogrammetric method’s challenges, affecting measurements of low momentum particles’ tracks, are the high magnetic field of the spectrometer, high radiation levels, tight space constraints, and the need to limit the material budget in the tracking volume. The camera is focused on the dot pattern drawn on the thin MEG II target, about 1 m away from the detector endcaps where the photo-camera is placed. Target movements and deformations are monitored by comparing images of the dots taken at various times during the measurement. The images are acquired with a Raspberry board and analyzed using custom software. Global alignment to the spectrometer is guaranteed by corner cubes placed on the target support. As a result, the target monitoring fulfills the needs of the experiment.