MEG: Muon to Electron and Gamma

The possible existence of the \mu \rightarrow {e} \gammaμ→eγ decay predicted by many new physics scenarios is investigated by stopping positive muons in a very thin target and measuring emitted photons and positrons with the best possible resolutions. Photons are measured by a 2.7 ton ultra pure liquid xenon detector while positron trajectories are measured in a specially designed gradient magnetic field by low-mass drift chambers and precisely timed by scintillation counters. A first phase of the experiment (MEG) ended in 2016, and excluded the existence of the decay with branching ratios larger than 4.2 \times 10^{-13}4.2×10−13 (90% C.L.). This provides approximately 30~times stronger constraints on a variety of new physics models than previous experiments. In the second phase (MEG II), most of the detectors have been upgraded by adopting up-to-date technologies to improve the search sensitivity by another order of magnitude down to \mathcal{O}(10^{-14})𝒪(10−14). MEG II will perform a search for physics beyond the Standard Model complementary to high energy collider experiments with compatible or even higher sensitivity.

Gas-filled detectors

Detectors that record charged particles through their ionisation of gases are found in many experiments of nuclear and particle physics. By conversion of the charges created along a track into electrical signals, particle trajectories can be measured with these detectors in large volumes, also inside magnetic fields. The operation principles of gaseous detectors are explained, which include charge generation, gas amplification, operation modes and gas mixtures. Different detector types are described in some detail, starting with ionisation chambers without gas amplification, proceeding to those with gas amplification like spark and streamer chambers, parallel plate arrangements, multi-wire proportional chambers, chambers with microstructured electrodes, drift chambers, and ending with time-projection chambers. The chapter closes with an overview of aging effects in gaseous detectors which cause negative alterations of the detector performance.

Semiconductor detectors

Already since the early 1960s semiconductor detectors have been employed in nuclear physics, in particular for gamma ray energy measurement. This chapter concentrates on position sensitive semiconductor detectors which have been developed in particle physics since the 1980s and which feature position resolutions in the range of 50–100 μ‎m by structuring the electrodes, thus reaching the best position resolutions of electronic detectors. For the first time this made the electronic measurement of secondary vertices and therewith the lifetime of heavy fermions possible. The chapter first conveys the basics of semiconductor physics, of semiconductor and metal-semiconductor junctions used in electronics and detector applications as well as particle detection with semiconductor detectors. It follows the description of different detector types, like strip and pixel detectors, silicon drift chambers and charged-coupled devices. New developments are addressed in the sections on ‘Monolithic pixel detectors’ and on ‘Precision timing with silicon detectors’. In the last sections detector deterioration by radiation damage is described and an overview of other semiconductor detector materials but silicon is given.