The design of the MEG II experiment

The MEG experiment, designed to search for the $${\mu ^+ \rightarrow \hbox {e}^+ \gamma }$$ decay, completed data-taking in 2013 reaching a sensitivity level of $${5.3\times 10^{-13}}$$ for the branching ratio. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of $$6\times 10^{-14}$$ , 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.