New constraint on the existence of the mu+-> e+ gamma decay

The analysis of a combined data set, totaling 3.6 \times 10^14 stopped muons on target, in the search for the lepton flavour violating decay mu^+ -> e^+ gamma 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 \times 10^-13 (90% confidence level). This represents a four times more stringent limit than the previous world best limit set by MEG.Comment: 5 pages, 3 figures, a version accepted in Phys. Rev. Let

The MEG detector for μ+→e+γ{\mu}+\to e+{\gamma} 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 \meg\ 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.Comment: 59 pages, 90 figure

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

A limit for the mu -> e gamma decay from the MEG experiment

A search for the decay mu -> e gamma, performed at PSI and based on data from the initial three months of operation of the MEG experiment, yields an upper limit on the branching ratio of BR(mu -> e gamma) < 2.8 x 10**-11 (90% C.L.). This corresponds to the measurement of positrons and photons from ~ 10**14 stopped mu-decays by means of a superconducting positron spectrometer and a 900 litre liquid xenon photon detector.Comment: 13 pages, 9 figures. v2: improved estimate of photon reconstruction efficienc

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)