The MEG experiment, designed to search for the µ + → e + γ decay at a 10 −13 sensitivity level, completed datataking in 2013. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of 6 × 10 −14 for the branching ratio, a total upgrade, involving substantial changes to the experiment, has been undertaken, known as MEG II. We present both the motivation for the upgrade and a detailed overview of the design of the experiment and of the expected detector performance.

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 μ+→e+γ: BR(μ+→e+γ) <4.2×10−13 at 90% confidence level. The MEG detector has been upgraded in order to reach a sensitivity of 6×10−14. The basic principle of MEG II is to achieve the highest possible sensitivity using the full muon beam intensity at the Paul Scherrer Institute (7×107 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.

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We propose and demonstrate a novel scheme to reduce the recovery time of optical nonlinearity in a semiconductor laser optical amplifier driven under a loss condition for the signal light pulse. Additional light, which is set at a transparency wavelength in the active layer, promotes stimulated recombination of excess carriers induced by the absorption of the signal light. This scheme excludes any additional carrier transport mechanism and nonradiative recombination and hence generation of heat. The principle of operation is experimentally verified by measuring time-domain transmission variance using a pump–probe method. A drastic reduction of the excess-carrier lifetime to less than 70 ps was confirmed.

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