The Mu to E Gamma (MEG) is a particle physics experiment dedicated to measuring the decay of the

muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of ''ħ'', but with a much greater mass. It is classified as a ...

into an

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

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 particles that can ...

, a decay mode which is heavily suppressed in the

Standard Model The Standard Model of particle physics is the Scientific theory, theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions – excluding gravity) in the unive ...

by lepton flavour conservation, but enhanced in

supersymmetry Supersymmetry is a Theory, theoretical framework in physics that suggests the existence of a symmetry between Particle physics, particles with integer Spin (physics), spin (''bosons'') and particles with half-integer spin (''fermions''). It propo ...

and grand unified theories. 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 Ri ...

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: :

\Beta ( \mu^+ \to e^+ \gamma) < 4.2 \times 10^

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 × 10⁷/s) incident on a plastic target. The decay is reconstructed to look for a back-to-back

positron The positron or antielectron is the particle with an electric charge of +1''elementary charge, e'', a Spin (physics), spin of 1/2 (the same as the electron), and the same Electron rest mass, mass as an electron. It is the antiparticle (antimatt ...

and monochromatic photon (52.8 MeV). A liquid xenon

scintillator A scintillator ( ) is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the ab ...

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 Comet (experiment), Mu2e and Mu3e.

References

External links

MEG-II experiment record
on

INSPIRE-HEP INSPIRE-HEP is an open access digital library for the field of high energy physics (HEP). It is the successor of the Stanford Physics Information Retrieval System (SPIRES) database, the main literature database for high energy physics since the 1 ...

* Particle experiments {{particle-stub