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'''Mu3e''' is a [[particle physics]] experiment at the [[Paul Scherrer Institute]], searching for decays of [[Antiparticle|anti]]-[[muon]]s (''Mu'') to an [[electron]] and two [[positron]]s (''3e'').<ref>{{cite journal |doi= 10.1016/j.phpro.2011.06.035|title=A Novel Experiment to Search for the Decay μ→ eee|journal=Physics Procedia|volume=17|pages=181–190|last1=  Schöning|first1= A.|last2= Bachman| first2= S.| last3= Narayan| first3= R.|year=2011|bibcode=2011PhPro..17..181S|doi-access=free}}</ref><ref>{{cite arXiv|eprint=1301.6113|last1= Blondel|first1= A.|title= Research Proposal for an Experiment to Search for the Decay μ -> eee|last2=  Bravar|first2= A.|last3=  Pohl|first3= M.|last4=  Bachmann|first4= S.|last5=  Berger|first5= N.|last6=  Kiehn|first6= M.|last7=  Schöning|first7= A.|last8=  Wiedner|first8= D.|last9=  Windelband|first9= B.|last10=  Eckert|first10= P.|last11=  Schultz-Coulon|first11= H. -C.|last12=  Shen|first12= W.|last13=  Fischer|first13= P.|last14=  Perić|first14= I.|last15=  Hildebrandt|first15= M.|last16=  Kettle|first16= P. -R.|last17=  Papa|first17= A.|last18=  Ritt|first18= S.|last19=  Stoykov|first19= A.|last20=  Dissertori|first20= G.|last21=  Grab|first21= C.|last22=  Wallny|first22= R.|last23=  Gredig|first23= R.|last24=  Robmann|first24= P.|last25=  Straumann|first25= U.|class= physics.ins-det|year= 2013}}</ref> This decay is extremely unlikely in the [[Standard Model]] of particle physics, as it changes the [[lepton number]]. Several new theories, especially [[supersymmetry|supersymmetric]] ones, predict a much more frequent decay. Searching for this decay allows a test of these theories, even if they cannot be tested directly in other experiments like at the [[Large Hadron Collider|LHC]]. It has also been shown the experiment is sensitive to probe new light dark sector particles such as dark photon.<ref>{{cite journal |doi= 10.1007/JHEP01(2015)113|title=Projections for Dark Photon Searches at Mu3e|journal=Journal of High Energy Physics|volume=01|last1=  Echenard|first1= B.|last2= Essig| first2= R.| last3= Zhong| first3= Y.-M.|year=2015|page=113|arxiv=1411.1770|bibcode=2015JHEP...01..113E|doi-access= free}}</ref> 

Mu3e is constructed at the Paul Scherrer Institute. It is planned to create the world's most intense muon beam which will allow to analyze two billion decays per second. This rate is necessary to study more than 10<sup>16</sup> muon decays in total. Important backgrounds are the decays <math>\mu^+ \to e^+e^-e^+\bar{\nu_\mu}\nu_e</math> and <math>\mu^+ \to e^+\bar{\nu_\mu}\nu_e</math>. To distinguish between signal and background, the detector has a spatial resolution better than 200&nbsp;[[micrometre|μm]], a time resolution better than 100&nbsp;[[picosecond|ps]] and an energy resolution better than 0.5&nbsp;[[Electronvolt|MeV]] for the individual electrons. To minimize [[multiple scattering]], the detector is built as light as possible. [[Semiconductor detector]]s<ref>{{Cite journal|arxiv=1309.7896|last1=Berger|first1=Niklaus|title=A Tracker for the Mu3e Experiment based on High-Voltage Monolithic Active Pixel Sensors|journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment| volume=732|issue=2013|pages=61–65|last2=Augustin|first2=Heiko|last3=Bachmann|first3=Sebastian|last4=Kiehn|first4=Moritz|last5=Perić|first5=Ivan|last6=Perrevoort|first6=Ann-Kathrin|last7=Philipp|first7=Raphael|last8=Schöning|first8=André|last9=Stumpf|first9=Kevin|last10=Wiedner|first10=Dirk|last11=Windelband|first11=Bernd|last12=Zimmermann|first12=Marco|year=2013|doi=10.1016/j.nima.2013.05.035|bibcode=2013NIMPA.732...61B|s2cid=119183835}}</ref> are used for the spatial and energy resolution, [[Scintillation counter|scintillator]] fibers provide a good timing resolution. The whole experiment is in a [[magnetic field]] of 1&nbsp;[[Tesla (unit)|Tesla]] to determine the energy of the particles based on their curvature radius.

{{As of|November 2021||df=}} data taking is expected to begin in 2024. The full rate of two billion muons per second will not be reached before 2028.<ref>{{cite web|url=https://inspirehep.net/literature/1965010|title=Searching for the Muon Decay to Three Electrons with the Mu3e Experiment|author=Cristina Martin , Luigi Vigani}}</ref> The experiment is expected to either find the decay or to set an upper limit of 10<sup>−16</sup> on the [[branching fraction]], a factor 10,000 better than previous experiments.<ref>SINDRUM-Kollaboration: [https://inspirehep.net/record/251865 Search for the Decay mu+ ---> e+ e+ e-]</ref>

== See also ==
* [[Mu to E Gamma]]
* [[Mu2e]]

== External links ==
* [http://www.psi.ch/mu3e/introduction Description of the experiment at psi.ch]
* [http://www.physi.uni-heidelberg.de/Forschung/he/mu3e/documents/ResearchProposal.pdf Research Proposal]
* [https://inspirehep.net/experiments/1114537 Mu3e experiment record] on [[INSPIRE-HEP]]

== References ==
<references />

[[Category:Particle experiments]]