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The analysis of a combined data set, totaling 3.6 × 10(14) stopped muons on target, in the search for the lepton flavor violating decay μ(+) → e(+)γ is presented. The data collected by the MEG experiment at the Paul Scherrer Institut show no excess of events compared to background expectations and yield a new upper limit on the branching ratio of this decay of 5.7 × 10(-13) (90% confidence level). This represents a four times more stringent limit than the previous world best limit set by MEG.

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.

Magnetic susceptibility, high-field magnetization and inelastic neutron scattering experiments are used to study the magnetic properties of a new S = 1 quasi-1-dimensional antiferromagnet PbNi2V2O8. Inter-chain interactions are shown to be almost, but not quite, strong enough to destroy the nonmagnetic singlet ground state and the energy gap in the magnetic excitation spectrum. Substituting nonmagnetic Mg 2+ (S = 0) ions for Ni 2+ (S = 1) induces a magnetically ordered state at low temperatures. To our knowledge this is the first observation of doping-induced long-range order in a Haldane-gap system.

We discovered a first-order phase transition between dimerzed-antiferromagnetic and uniform-antiferromagnetic phases in impurity-doped spin-Peierls (SP) cuprate Cu1−xMgxGeO3. As impurity concentration increases, linear reduction of SP transition temperature (TSP ) and linear increase of Néel temperature (TN ) are observed up to x ≃ 0.023. At this critical concentration (xc) SP transition suddenly disappears and TN jumps discontinuously. The peak of the susceptibility at xc around TN is not so sharp as those at other concentrations, which indicates the phase separation of low and high concentration phases. These results indicate the existence of the first-order phase transition between dimerizedantiferromagnetic and uniform-antiferromagnetic long-range orders. 75.10.Jm, 75.30.Kz, 75.50.Ee