The Mu to E Gamma (MEG) is a particle physics experiment dedicated to measuring the decay of the
muon into an
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
and a
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
, a decay mode which is heavily suppressed in the
Standard Model by
lepton flavour conservation, but enhanced in
supersymmetry and
grand unified theories
A Grand Unified Theory (GUT) is a model in particle physics in which, at high energies, the three gauge interactions of the Standard Model comprising the electromagnetic, weak, and strong forces are merged into a single force. Although this u ...
. It is located at the
Paul Scherrer Institute
The Paul Scherrer Institute (PSI) is a multi-disciplinary research institute for natural and engineering sciences in Switzerland. It is located in the Canton of Aargau in the municipalities Villigen and Würenlingen on either side of the River ...
and began taking data September 2008.
Results
In May 2016 the MEG experiment published the world's leading upper limit on the branching ratio of this decay:
:
at 90% confidence level,
based on data collected in 2009–2013. This improved the MEG limit from the prior MEGA experiment by a factor of about 28.
Apparatus
MEG uses a continuous muon beam (3 × 10
7/s) incident on a plastic target. The decay is reconstructed to look for a back-to-back
positron and monochromatic photon (52.8 MeV). A liquid xenon
scintillator with
photomultiplier tubes measure the photon energy, and a
drift chamber
A wire chamber or multi-wire proportional chamber is a type of proportional counter that detects charged particles and photons and can give positional information on their trajectory, by tracking the trails of gaseous ionization. was located via ...
in a magnetic field detects the positrons.
The MEG collaboration presented upgrade plans for MEG-II at the Particles and Nuclei International Conference 2014, with one order of magnitude greater sensitivity, and increased muon production, to begin data taking in 2017.
More experiments are planned to explore rare muon transitions, such as
Mu2e
Mu2e, or the Muon-to-Electron Conversion Experiment, is a particle physics experiment at Fermilab in the US. The goal of the experiment is to identify physics beyond the Standard Model, namely, the conversion of muons to electrons without the e ...
and
Mu3e.
References
External links
MEG-II experiment recordon
INSPIRE-HEP
*
Particle experiments
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