The MEG detector for μ+→e+γ decay search

The MEG (Mu to Electron Gamma) experiment has been running at the Paul Scherrer Institut (PSI), Switzerland since 2008 to search for the decay mu(+) -> e(+)gamma by using one of the most intense continuous mu(+) beams in the world. This paper presents the MEG components: the positron spectrometer, including a thin target, a superconducting magnet, a set of drift chambers for measuring the muon decay vertex and the positron momentum, a timing counter for measuring the positron time, and a liquid xenon detector for measuring the photon energy, position and time. The trigger system, the read-out electronics and the data acquisition system are also presented in detail. The paper is completed with a description of the equipment and techniques developed for the calibration in time and energy and the simulation of the whole apparatus

Search for the lepton flavour violating decay mu(+) -> e(+) gamma with the full dataset of the MEG experiment

The final results of the search for the lepton flavour violating decay μ+→e+γ based on the full dataset collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013 and totalling 7.5×1014 stopped muons on target are presented. No significant excess of events is observed in the dataset with respect to the expected background and a new upper limit on the branching ratio of this decay of B(μ+→e+γ)<4.2×10−13 (90 % confidence level) is established, which represents the most stringent limit on the existence of this decay to date

Calibration and monitoring of the MEG experiment by a proton beam from a Cockcroft-Walton accelerator

The MEG experiment at PSI searches for the decay mu -> e gamma at a level of approximate to 10(-13) on the branching ratio BR(mu -> e gamma/mu -> tot), well beyond the present experimental limit (BR <= 1.2 x 10(-11)) and is sensitive to the predictions of SUSY-GUT theories. To reach this goal the experiment uses one of the most intense continuous surface muon beams available (approximate to 10(8) mu/s) and relies on advanced technology (LXe calorimetry, a gradient-field superconducting spectrometer as well as flexible and powerful trigger and acquisition systems). In order to maintain the highest possible energy, time and spatial resolutions for such detector, frequent calibration and monitoring, using a Cockcroft-Walton proton accelerator, are required. The proton beam is brought to the centre of MEG by a special bellows insertion system and travels in a direction opposite to the one of the normal mu-beam. Protons interact with a lithium tetraborate (Li(2)B(4)O(7)) nuclear target and produce one gamma (17.6 MeV) from the reaction (7)(3)Li(p,gamma)(4)(8)Be or two coincident gamma s (11.67 and 4.4 MeV) from the reaction (11)(5)B(P,gamma(1))(6)(12)C*. The 17.6 MeV gamma is used for calibrating and monitoring the LXe calorimeter (sigma(E gamma)/E(gamma) = 3.85 +/- 0.15% at 17.6 MeV) while the coincident 11.67 and 4.4 MeV gamma s are used to measure the relative timing of the calorimeter and the spectrometer timing counters (sigma(Delta t) = 0.450 +/- 0.015 ns)