Abstract
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.
Highlights
A search for the Charged Lepton Flavour Violating (CLFV) decay μ+ → e+γ, the MEG experiment is in progress at the Paul Scherrer Institut (PSI) in Switzerland
As with all phase-space measurements performed at high rate, a scanning technique was used: either one using a small cylindrical “pill” scintillator of 2 mm diameter and 2 mm thickness coupled to a miniature PMT [8], or one using a thick depletion layer Avalanche PhotoDiode (APD), without a scintillator, in the case of measurements in a magnetic field
After the subtraction of these effects, the γ -ray conversion time is obtained by combining the timings of those PMTs which contain more than 50 photoelectrons and calculating the minimum value of χ2 defined as a sum of the squared difference between each calculated and reconstructed time
Summary
The Drift CHamber (DCH) system of the MEG experiment [15] is designed to ensure precision measurement of positrons from μ+ → e+γ decays. It must fulfil several stringent requirements: cope with a huge number of Michel positrons due to a very high muon stopping rate up to 3 × 107 μ+/s, be a low-mass tracker as the momentum resolution is limited by multiple Coulomb scattering and in order to minimise the accidental γ -ray background by positron annihilation-in-flight, and provide excellent resolution in the measurement of the radial coordinate as well as in the z coordinate. The two detector planes are enclosed by the so-called hood cathode. The middle as well as the hood cathodes are made of a 12.5 μm-thick polyamide foil with an aluminium deposition of 2500 Å
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